Groundwater Sensitivity and Solid Waste Disposal in Minnesota · 4.1 Unsaturated Flow ... 4.3.1...

$Page 1: Groundwater Sensitivity and Solid Waste Disposal in Minnesota · 4.1 Unsaturated Flow ... 4.3.1 Fractured Rock Aquifers ... Contaminant transport through fractures and solution channels$
GROUNDWATER SENSITIVITY AND SOLID WASTE DISPOSAL IN MINNESOTA Stuart Grubb, PGStuart Grubb, PG E. Calvin Alexander, Jr. E. Calvin Alexander, Jr. PhDPhD University of MinnesotaUniversity of Minnesota May 7, 2009May 7, 2009 Final Draft Approved for general distribution May be cited with attribution

© Friends of Washington County 2009

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Groundwater Sensitivity and Solid Waste Disposal 2 ©Friends of Washington County 2009

Groundwater Sensitivity and Solid Waste Disposal in Minnesota

Contents

Executive Summary ........................................................................................................................................... 4

1 Introduction and Background................................................................................................................... 9

2 Definitions.................................................................................................................................................. 9

2.1 Groundwater and Aquifers ................................................................................................................ 9

2.2 Groundwater Quality and Groundwater Contamination .............................................................12

2.3 Sensitivity .........................................................................................................................................12

3 Objectives .................................................................................................................................................13

4 Groundwater Flow....................................................................................................................................14

4.1 Unsaturated Flow.............................................................................................................................14

4.2 Saturated Flow through Porous Media Aquifers...........................................................................14

4.3 Fracture Flow Aquifers....................................................................................................................16

4.3.1 Fractured Rock Aquifers.........................................................................................................16

4.3.2 Karst Aquifers..........................................................................................................................16

5 Minnesota Geology and Groundwater Flow..........................................................................................16

5.1 General Aquifer Characteristics .....................................................................................................16

5.1.1 Igneous and Metamorphic .....................................................................................................17

5.1.2 Sedimentary .............................................................................................................................17

5.1.3 Karst .........................................................................................................................................18

5.1.4 Unconsolidated Glacial ...........................................................................................................19

5.1.5 Unconsolidated Non-Glacial ..................................................................................................19

5.2 Regional Groundwater .....................................................................................................................20

5.2.1 DNR Groundwater Provinces ................................................................................................20

5.2.2 Groundwater Profiles ..............................................................................................................20

5.2.3 Extent of Karst Areas ............................................................................................................25

6 Contaminant Transport .........................................................................................................................25

6.1 Advection, Dispersion, and Diffusion ..........................................................................................25

6.2 Sorption ............................................................................................................................................26

6.3 Contaminant Reactions and Degradation ....................................................................................26

7 Parameters, Measurements, and Testing..............................................................................................27

7.1 Groundwater Flow............................................................................................................................27

7.1.1 Unsaturated Flow.....................................................................................................................27


7.1.2 Saturated Porous Media..........................................................................................................27

7.1.3 Saturated Fracture Flow and Karst ........................................................................................29

7.1.4 Isotopes and Other Environmental Tracers .......................................................................30

7.2 Modeling Time of Travel ..............................................................................................................31

7.3 Contaminant Monitoring...............................................................................................................31

8 Groundwater Sensitivity Classifications................................................................................................32

8.1 Methodology....................................................................................................................................32

8.2 Range of Sensitivity ........................................................................................................................33

8.3 Evaluating Aquifer Materials .........................................................................................................33

8.4 Evaluating Travel Times................................................................................................................35

9 Excluding Landfill Sites Based on Hydrogeologic Conditions ...........................................................37

9.1 Methodology....................................................................................................................................37

9.2 Determining the Siting Region ......................................................................................................37

9.3 Identifying Target Areas ................................................................................................................38

9.4 Screening Candidate Sites ...............................................................................................................38

10 Conclusions and Recommendations ..................................................................................................39

11 References ............................................................................................................................................39

The Friends of Washington County 41

Figures

Figure 1. Minnesota Bedrock Aquifers.........................................................................................................16

Figure 2. Minnesota Karst Lands..................................................................................................................18

Figure 3. Minnesota Glacial Deposits...........................................................................................................19

Figure 4. Minnesota Groundwater Provinces ..............................................................................................20

Figure 5. Idealized Contaminant Plume.......................................................................................................27

Tables

Table 1. Groundwater Sensitivity Ranking..................................................................................................33

Table 2. Classification of Unconsolidated Materials .................................................................................34


Executive Summary

Introduction and Background The 2008 Legislature charged the MPCA to review current rules and policies, and to develop new rules that will provide the MPCA with a process to include groundwater sensitivity and financial assurance to reduce risks to groundwater. By January 15, 2010, the MPCA is to deliver a report to the Legislature on proposed rules.

This paper proposes new methods of hydrogeologic site investigation for siting a landfill. It proposes a fundamental shift in the methods for determining site suitability. Currently a site is selected, and then an investigation is conducted to determine if the site meets certain minimum characteristics. With the proposed methods, a regional landfill siting area is identified, and then the best landfill site is selected from the areas that are least sensitive to groundwater contamination.

Definitions MDNR (1991) defined groundwater sensitivity as “time of travel”, the time required for a contaminant to move vertically from the land surface to an aquifer This paper uses the same definition of sensitivity, with one key exception. MDNR (1991) and other publications consider only water flow vertically downward to the first aquifer. This paper also considers lateral migration of groundwater and contaminants to and in the saturated zone.

Objectives We have based our recommendations on the assumption that at least one landfill constructed in Minnesota in the future will release contaminants into the subsurface. Proactive siting of that landfill (and all others) in the best location available will minimize the threats to human health and the environment. The best location will have subsurface characteristics that slow the migration of contaminants to water supplies or to surface waters.

Groundwater Sensitivity Classifications The time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity. Time of travel is a good surrogate when accounting for other factors that might reduce contaminant concentrations and groundwater sensitivity, such as sorption and chemical reactions. In general, as time of travel increases due to the presence of silt and clay in the aquifer, the potential for sorption and favorable chemical reactions also increases.

Range of Sensitivity Table 1 shows a landfill site’s sensitivity to groundwater contamination based on hydrogeologic characteristics. The range of sensitivity is given on a scale of 10 (high sensitivity) to 1 (low sensitivity).


Table 1. Groundwater Sensitivity Ranking

Water Table

Aquifer -

Groundwater

Age Dating

Carbonate or

coarse clastic

overlain by a

karst surface

Carbonate or

coarse clastic

without a

karst surface

Other bedrock

with fracture

flow

Sand and

gravel

Sandy silt to

clayey silt

Lean clay and

fat clay

Other bedrock

without

fracture flow

Less than 1 year 10 9 8 7 6 5 1

1 to 10 years 9 8 7 6 5 4 1

10 to 100 years 8 7 6 5 4 3 1

More than 100

years 7 6 5 4 3 2 1

Water Table Aquifer - Most Sensitive Material

Evaluating Aquifer Materials For purposes of groundwater sensitivity classification, unconsolidated aquifer materials are grouped into three categories, as shown in Table 2..

Table 2. Classification of Unconsolidated Materials

Description Hydraulic conductivity

Unified Soil Classification System

High Permeability/ Sand and Gravel

>10-3 cm/s Clayey gravel (GC), Silty gravel (GM), Poorly graded gravel (GP), Well graded gravel (GW), Poorly graded sand (SP), Well graded sand (SW)

Moderate Permeability/ Sandy Loam and Silt

10-3 to 10-6 cm/s Silty sand (SM), Clayey sand (SC)

Low Permeability/ Clay

<10-6 cm/s Fat clay (CH), Lean clay (CL)

Four categories are proposed for consolidated bedrock aquifers. From most sensitive to least sensitive, they are:

� Carbonate and coarse clastic aquifers overlain by a karst surface. � Carbonate and coarse clastic aquifers without a karst surface. � Other bedrock with fracture flow. � Other bedrock without fracture flow.


These bedrock aquifer categories were selected on two basic premises:

1. Contaminant transport through fractures and solution channels is faster and therefore more significant than contaminant transport through porous media.

2. Solution cavities and karst features are common in carbonates, even if they are not encountered in a few boreholes at a site.

Evaluating Travel Times A key recommendation of this paper is that groundwater travel times from the surface or near surface to the water table aquifer must be directly measured using the methods described in Section 7.1.3 and 7.1.4. However, it may not be possible to directly measure the groundwater age at each of the many locations being considered for siting a particular landfill. Regulators and landfill proposers need to work together to develop conceptual models of travel time based on the geologic materials found in the area. The conceptual models would need to be calibrated and validated using actual field test results.

Excluding Landfill Sites Based on Hydrogeologic Conditions

Methodology MPCA should adopt a landfill siting method based on the premise that landfills should be sited in the most favorable locations with the lowest groundwater sensitivity within a designated search area. Landfills should be excluded from less favorable locations. That is, a potential landfill site with a higher groundwater sensitivity ranking (according to Table 1) should be excluded if a site with a lower groundwater sensitivity ranking exists within the search area.

MPCA should require tracer testing as described in Sections 7.1.3 and 7.1.4 for all sites being considered for landfill siting. While it is probably not possible to exclude sites based on certain values or concentrations of tracers, the data should be used to compare potential landfill sites. Landfill sites with tracer tests indicating older groundwater or slower recharge should be given preference over sites with younger, more sensitive groundwater.

Siting Regions If taken to an illogical extreme, the proposed method could lead to all landfills in Minnesota being sited within one very small area with optimal hydrogeologic conditions. To prevent this, each proposed landfill should be assigned a large “siting region” within which waste transport and disposal would be practical. The siting region would be different for various waste types and various parts of the state.

The proposed method is based solely on a site’s geologic and hydrogeologic conditions. The so-called “three-legged stool” concept that incorporates waste segregation, engineered barriers and systems, and hydrogeologic conditions is not relevant to the siting process. Waste segregation and engineered barriers and systems certainly have a role in landfill design, construction, operation, and closure, but not in site selection.

MPCA should also prohibit all landfills on all sites with high Groundwater Sensitivity Rankings shown on Table 1.. For example, landfills should be prohibited on sites with a groundwater sensitivity ranking of 8, 9, or 10.


Each proposed landfill would have its own siting region based on the location and the type of waste. The determination should be part of the scoping/EAW process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would delineate the siting region.

The size of the siting region would be partly based on type of waste and the environmental risk associated with disposal. Low-volume, high-risk wastes such as radioactive wastes would have the largest siting regions, possibly extending outside the state. Industrial waste and mixed municipal solid waste would have a siting region that might cover several counties. The siting region might also be extended to avoid karst areas. High-volume, low-risk wastes such as construction and demolition fill would have the smallest siting region.

Identifying Target Areas After the siting region has been established, the siting process should identify the areas within the siting region that have the best hydrogeologic characteristics (the “target areas”). First, areas with higher groundwater sensitivity, as indicated in Table 1, will be disqualified in favor of target areas with the lowest groundwater sensitivity.

The goal of this process is to identify the target areas within the siting region with the lowest groundwater sensitivity.

The “target areas” identification process will likely use existing regional hydrogeologic data to identify the target areas. It is important to use the most recent and best available data. The siting process also should exclude areas from the target area that do not meet current regulatory standards for landfill sites. Minnesota Rules (i.e. 7035.1600) and agency policies prohibit landfill siting near critical areas such as water resources, roads, occupied dwellings, etc.

Screening Candidate Sites The next step in the siting process is for the landfill proposer to identify and screen candidate sites within the acceptable target areas. The landfill proposer can select specific proposed sites based on secondary criteria such as land cost, existing infrastructure, transportation costs, construction and operating costs, etc.

The landfill proposer may submit one or more sites for regulatory permitting. Extensive site-specfic geologic and hydrogeologic testing will be required for the permits. This includes groundwater age dating and confirmation that the groundwater sensitivity ranking is valid.

Conclusions and Recommendations � MPCA should adopt a landfill siting method based on the premise that landfills should be sited

in the most favorable locations with the lowest groundwater sensitivity within a region considered for landfill siting.

� Determining the region considered for landfill siting should be part of the scoping/EAW process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would determine the region considered for landfill siting.

� The time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity.


� MPCA should prohibit all landfills on all sites with Groundwater Sensitivity Rankings of 8, 9, or 10 shown on Table 1..

� MPCA should require tracer testing as described in Sections 7.1.3 and 7.1.4 for all sites being considered for landfill siting.

� MPCA should develop standards for the type and number of tests required for a evaluating a potential landfill site under this system.


Groundwater Sensitivity and Solid Waste Disposal in Minnesota

1 Introduction and Background Minnesota Statutes 116.07 (Subdivision 4) states that:

“The rules for the disposal of solid waste shall include site-specific criteria to prohibit solid waste disposal based on the area's sensitivity to groundwater contamination, including site-specific testing.”

The 2008 Legislature also imposed a moratorium on the siting of most new landfills. It directed the MPCA to reexamine its methods for determining groundwater sensitivity to pollution from solid waste management facilities. Specifically, it charged the MPCA to review current rules and policies, and to develop new rules that will provide the MPCA with a process to include groundwater sensitivity and financial assurance to reduce risks to groundwater. By January 15, 2010, the MPCA is to deliver a report to the Legislature on proposed rules.

A report produced by Construction and Demolition and Industrial Solid Waste Landfill (CDIL) Work Group (MPCA Work Group, 2009) concluded:

“Current Minnesota Rules, part 7035.2815, subpart 3, require a detailed investigation to determine hydrogeologic characteristics for siting and expanding mixed municipal solid waste (MMSW) and combustor ash landfills. This investigation is used to develop a conceptual model of groundwater flow, which serves as the basis for design of a groundwater monitoring network to detect and intercept a contaminant release. These same rules are currently applied by policy for construction and demolition (C&D) and industrial landfill siting and expansion, although the scope of the investigation may be reduced based upon the presumed level of risk posed by the specific waste type to be landfilled.”

“Under current practice, results from hydrogeologic investigations at a site are used primarily for determining if a site is monitorable and remediable, and for design of the groundwater monitoring system. The hydrogeologic investigation is not explicitly used to determine general suitability of a landfill site or to establish engineered control requirements (except for depth to the water table). Guidance should be developed, as a transition to rules, that addresses the scope and use of a hydrogeologic site investigation in determining site suitability and engineered control requirements as part of a risk-based environmental performance evaluation.”

This paper proposes new methods of hydrogeologic site investigation for siting a landfill. It proposes a fundamental shift in the methods for determining site suitability. Currently a site is selected, and then an investigation is conducted to determine if the site meets certain minimum characteristics. With the proposed methods, a regional landfill siting area is identified, and then the best landfill site is selected from the areas that are least sensitive to groundwater contamination.


The purpose of this paper is to:

� Provide common definitions to groundwater-related terms and concepts to facilitate better communication

� Examine the concept of groundwater sensitivity in the context of landfill siting and Minnesota’s geology

� Report on advancements in investigation and analysis techniques that should be utilized in landfill siting

� Recommend a methodology for determining groundwater sensitivity when siting landfills

� Recommend criteria for excluding landfill sites from consideration based on hydrogeologic characteristics

2 Definitions

2.1 Groundwater and Aquifers The definition of groundwater is crucial in regulatory matters. The simplest definition, which is useful in many contexts, is “any water that is below ground surface”. When considering landfill siting, this definition leads to many practical problems that make it unusable. For example, water in the unsaturated zone just below the ground surface is found everywhere and will be highly sensitive to pollution.

The definition of groundwater often differs according to the problem being considered. When considering a detailed water budget for a site or a region, water in the unsaturated zone and the effects of evapotranspiration can be very significant. When considering how much water can be pumped from regional aquifers, groundwater in the unsaturated zone or small areas of perched groundwater are no longer included under the practical definition of groundwater.

The State of Minnesota defines groundwater different ways for different purposes:

� Minnesota Statutes, Section 115.01 Subd. 6 and Minnesota Rules Section 7035.0300 "Groundwater" means water contained below the surface of the earth in the saturated zone including, without limitation, all waters whether under confined, unconfined, or perched conditions, in near-surface unconsolidated sediment or regolith, or in rock formations deeper underground.

� Minnesota Administrative Rules, 6115.0630 Definitions, Subp. 11. "Groundwater" means

subsurface water in the saturated zone. The saturated zone may contain water under atmospheric pressure (water table condition), or greater than atmospheric pressure (artesian condition).


� Minnesota Administrative Rules 1505.3010 (Department of Agriculture, Pest and Disease Control). "Groundwater" means the water in the zone of saturation in which all of the pore spaces of the subsurface material are filled with water. The water that supplies springs and wells is groundwater.

� Minnesota Administrative Rules 4075.0100 (Department of Health, Wells and Borings). "Groundwater" has the meaning given in Minnesota Statutes, section 115.01, subdivision 6, and does not include water in an artificially created basin, such as a tank excavation, that is not hydrologically connected to the earth outside the basin.

� Minnesota Statutes, Section 103B.305 Subd. 4. "Groundwater systems" means the 14 principal aquifers of the state as defined by the United States Geological Survey in the Water-Resources Investigations 81-51, entitled "Designation of Principal Water Supply Aquifers in Minnesota" (August 1981), and its revisions.

When considering groundwater issues on a larger scale, the term “aquifer” is often more descriptive and useful. Minnesota has definitions for different types of aquifers:

� Minnesota Administrative Rules 4075.0100 (Department of Health, Wells and Borings) and 7035.0300 (Pollution Control Agency, Solid Waste). "Aquifer" means a stratum of saturated, permeable bedrock or unconsolidated material having a recognizable water table or potentiometric surface which is capable of producing water to supply a well.

� Minnesota Administrative Rules, 6115.0630 Definitions, Subp. 2. "Aquifer" means any water-bearing bed or stratum of earth or rock capable of yielding groundwater in sufficient quantities that can be extracted.

� Minnesota Administrative Rules, 6115.0630 Definitions, Subp. 4. “Artesian aquifer” or “confined aquifer” means a water body or overlain by a layer of material of less permeability than the aquifer. The water is under sufficient pressure so that when it is penetrated by a well, the water will rise above the top of the aquifer. A flowing artesian condition exists when the water flow is at or above the land surface.

� Minnesota Administrative Rules, 6115.0630 Definitions, Subp. 17. “Water table aquifer”

or “unconfined aquifer” means an aquifer where groundwater is under atmospheric pressure.

A perched aquifer (or perched groundwater) is usually defined as groundwater in a saturated zone with a confining layer and unsaturated material below. We did not find a regulatory definition. In Minnesota, some perched aquifers can be extensive, but many are only a few inches thick and have very limited extent.

We recommend using the following definitions for groundwater and aquifer:

� Groundwater - Subsurface water in the saturated zone(s). � Aquifer - A body of rock or sediment that is can yield significant or economic quantities

of groundwater to wells and springs.


The definition of aquifer is intended to exclude small areas of perched groundwater.

In this paper (and in most other similar publications and rules), the terms groundwater and aquifer are used interchangeably. In particular, when the terms “groundwater sensitivity” and “groundwater protection” are discussed, the reader may assume they mean the same as “aquifer sensitivity” and “aquifer protection”. Although that is a useful equivalence, “groundwater” is the water while “aquifer” is the container and that distinction can be significant.

2.2 Groundwater Quality and Groundwater Contamination Quantifying groundwater quality and groundwater contamination is beyond the scope of this paper. Any substance that emanates from a landfill to the subsurface is considered a contaminant. Any time one of these substances reaches the groundwater/aquifer, the groundwater/aquifer is considered to be degraded.

2.3 Sensitivity MDNR (1991) defined sensitivity as follows:

“The term ‘sensitivity’ is commonly used to describe the general potential for an aquifer to be contaminated. One aquifer is said to be more sensitive than another aquifer if it has a higher potential to be contaminated. However, this definition of sensitivity is unsatisfactory because ‘potential’ is vague and difficult, if not impossible to measure.”

“Instead of trying to use an unmeasurable term such as “potential” to define relative sensitivity, this report uses the concept of ‘time of travel’, the time required for a contaminant to move vertically from the land surface to an aquifer. This interpretation is preferred as being specific and measurable.”

“The factors mentioned above can make it very difficult to determine the travel time for a contaminant to reach an aquifer. Therefore certain simplifying assumptions have been adopted. In particular, any factors that may change over time, such as land use and seasonal effects, are not considered. Since contaminants are so variable in their behavior, contaminants are assumed to be inert and conservative and to behave the same as water….”

This paper uses the same definition of sensitivity, with one key exception. MDNR (1991) and other publications consider only water flow vertically downward to the first aquifer. This paper also considers lateral migration of groundwater and contaminants to and in the saturated zone. For example, one aquifer/landfill pair is more sensitive than a second aquifer/landfill pair if contaminants reaching the aquifer below the landfill at one site will migrate faster to the landfill boundary or a well than at the other site. In karst and fractured aquifers the contaminant advection in the aquifer can be significantly faster than the movement in the unsaturated zone.


3 Objectives Minnesota Statutes 103H.001 states the following degradation prevention goal: “It is the goal of the state that groundwater be maintained in its natural condition, free from any degradation caused by human activities. It is recognized that for some human activities this degradation prevention goal cannot be practicably achieved. However, where prevention is practicable, it is intended that it be achieved. Where it is not currently practicable, the development of methods and technology that will make prevention practicable is encouraged.”

We have based our recommendations on the assumption that at least one landfill constructed in Minnesota in the future will release contaminants into the subsurface. Proactive siting of that landfill (and all others) in the best location available will minimize the threats to human health and the environment. The best location will have subsurface characteristics that slow the migration of contaminants to water supplies or to surface waters. These optimal subsurface materials and characteristics will help achieve the following basic objectives:

1. Minimize contaminant release. Low-permeability materials will help contain contaminants that escape the landfill liner, leachate collection and other engineered protection systems.

2. Contain contamination. Once contaminants are released, low-permeability materials will restrict contaminant migration away from the landfill. Rapid and unpredictable flow conditions (i.e. fracture or conduit flow) should especially be avoided.

3. Remediate contamination. Contaminants that have not been contained must first be detected by a monitoring system. After a contaminant release has been detected, it may be possible to remediate the problem by removing the contaminants from the subsurface or degrading them in situ. It is generally easier to perform this type of monitoring and remediation in isotropic, homogeneous, high-permeability materials than in low-permeability or heterogeneous materials. Ironically, this could lead to the mistaken conclusion that landfills should be sited in areas with high-permeability subsurface materials, which would contradict the first two objectives above. Landfill remediation has proven to be extremely expensive and ineffective in many cases. Therefore, monitoring and remediation is a much less preferable approach than is minimizing and containing contamination. Facilitating monitoring and remediation should not be considered as a landfill siting criterion.

4. Minimize impacts. In the event that the first three objectives are not met, then is desirable to have the landfill sited in an area that will lead to minimal impacts to human health and the environment. This usually means picking locations where contaminants migrating from the landfill will not encounter a drinking water supply or surface waters.


4 Groundwater Flow Our understanding of groundwater flow continues to evolve. Some advancements in the past 10 to 20 years have a bearing on landfill siting, but have not yet been incorporated into solid waste regulations. The discussion below is a basic introduction to some groundwater flow concepts with an emphasis on landfill issues and some common misconceptions. More detailed discussion is presented in several textbooks including Fetter (2001) and Freeze and Cherry (1979).

4.1 Unsaturated Flow Unsaturated flow is the movement of water through pore spaces that are not completely water filled or saturated. In most applications, only vertical movement (both up and down) is considered but horizontal flows can be significant. In landfill studies, unsaturated flow describes the movement of water or contaminants from below the landfill liner to the water table. Unsaturated flow is also important in understanding recharge of the water table aquifer by rainfall or surface water away from the landfill.

Unsaturated flow is more complex and difficult to quantify than saturated flow. The factors that can significantly affect unsaturated flow include:

� Water content of the soil � Pressure head of water in the soil � Hydraulic conductivity (discussed more in the next section)

Water content and pressure head may not be continuous across the site. That is, both parameters can vary significantly over short distances and over time. Water content and pressure head are difficult to measure accurately in the field. Finally, the mathematical equations that describe unsaturated flow are more difficult to solve than the equations describing saturated flow.

Natural processes such as evaporation, transpiration, and capillary action affect unsaturated groundwater flow. Man-made activities including land use and changes to soil properties (i.e. compaction) must also be considered.

Because unsaturated flow is so complex, simplifying assumptions are often made for landfill engineering and design purposes. For example, monitoring systems may assume that leachate emanating from the landfill flows vertically downward and immediately enters the water table. Direct measurements of the travel times through the unsaturated zone with tracers are an efficient ways of determining the travel times directly, without having to explain and quantify all of the factors that influence unsaturated flow.

4.2 Saturated Flow through Porous Media Aquifers Porous media aquifers have interconnected pore spaces between grains that can transport groundwater. The porous media can be either consolidated (i.e. sandstone) or unconsolidated (i.e. glacial outwash).

Where a porous media aquifer is saturated, groundwater flow can be quantified and described using Darcy’s Law:

Q = K*i*A


Where:

Q = Volume of groundwater flow over time (i.e. gal/min) K = Hydraulic conductivity i = Hydraulic pressure gradient A = Cross-sectional area of the aquifer.

Hydraulic conductivity is an empirical parameter that incorporates many features of the porous medium. That is, hydraulic conductivity must be measured in laboratory or field tests rather than calculated from other physical parameters of the soil or rock. We know the simple relationship between some soil properties and hydraulic conductivity:

� Larger grain size → Higher hydraulic conductivity � Well rounded grains → Higher hydraulic conductivity � Variety of grain sizes mixed together → Lower hydraulic conductivity

Scientists and engineers also frequently make simplifying assumptions regarding hydraulic conductivity, including:

� Hydraulic conductivity does not change over time � Hydraulic conductivity does not change over the range of groundwater velocities considered.

(Darcy’s Law does not apply to rapid, turbulent flow.)

Hydraulic gradient is the change in pressure over distance, both horizontal and vertical. This is usually measured by installing a well in the aquifer and measuring how high the water in the well rises. Groundwater flows from areas of high hydraulic pressure to areas of lower hydraulic pressure. Pumping wells and natural groundwater discharge points such as springs or rivers can create areas of lower hydraulic pressure. In water table aquifers, a common simplifying assumption is that there is no hydraulic gradient in the vertical direction.

Quantifying and predicting changes to groundwater flow is often accomplished by applying mathematical models and equations. Mathematical models range from simple analytical solutions to complex numerical models solved using computer programs. All models require assumptions, and the simpler, more convenient models require more simplifying assumptions, such as:

� The aquifer is homogenous and isotropic (same hydraulic conductivity in all directions). � The aquifer is infinite in horizontal extent, or has impermeable no-flow boundaries. � There is no vertical flow across confining layers at the top or bottom of the aquifer.

The problem with this concept of groundwater flow is that it seldom occurs in nature. It is useful in some applications, such as designing a water supply well. But saturated porous media models often fail to accurately predict the direction of contaminant transport and underestimate contaminant transport velocities by factors of 10 to 10,000. The reason is that in most natural aquifers are not homogenous. Even subtle variations in seemingly homogenous materials can cause significant variations in hydraulic conductivity and hydraulic gradients. Preferential flow pathways can develop, and contaminants can be transported more quickly than predicted to various parts of the aquifer.


4.3 Fracture Flow Aquifers

4.3.1 Fractured Rock Aquifers Groundwater flow occurs through fractures, cracks, joints, faults, etc. in rock aquifers. The pore spaces between individual grains are the primary porosity. The porosity in fractures and cracks is called secondary porosity. Groundwater flow through the secondary porosity can be very rapid compared to groundwater flow through primary porosity. Darcy’s Law is often not valid. Fractures and cracks are typically only partially interconnected. Consequently, it can be very difficult or impossible to quantify and predict groundwater flow patterns through fractured rock aquifers.

An assumption that is often made for regional groundwater studies is that fractured rock behaves as a “equivalent porous medium” on a large scale. That is, if a large enough aquifer area is considered, flow through individual fractures is not so important, and the aquifer responds similarly to a saturated porous medium. At the scale of landfill siting and design, this assumption does not adequately describe the groundwater flow.

4.3.2 Karst Aquifers Karst aquifers are triple porosity aquifers. That is, they have porous matrix flow components, fracture flow components and conduit flow components in the same volume of aquifer. The conduits are self-organized into continuous flow systems by solution and mechanical erosion process. The conduits function to move water from recharge areas to discharge points (springs or wells). In Minnesota karst aquifer conduits, flow velocities of miles per day are common. Most of the water transport in karst aquifers is via the conduits.

The matrix flow components serve as the storage components of karst aquifers. Most of the water stored in karst aquifers between recharge events is in the matrix. Water moves short distances from the conduits to the matrix during recharge events and from the matrix to the conduits during recession periods between recharge events. The water in the matrix components can have very long residence times.

The fracture flow components function to connect the conduit and matrix components.

5 Minnesota Geology and Groundwater Flow Minnesota’s diverse geology and hydrogeology create challenges for developing state-wide environmental policy. Simple terms and concepts like “bedrock fracture” and “groundwater” can be very different in different parts of the state. The challenges are compounded by the fact that boundaries between different hydrogeologic regions cannot be precisely delineated, individual aquifers with different characteristics often overlap and interact, and the hydrogeologic boundaries never follow established political boundaries. This section provides context for later discussions about testing methods and groundwater classification.

5.1 General Aquifer Characteristics Bedrock aquifers can be separated into two groups – igneous and metamorphic aquifers and sedimentary aquifers. Karst aquifers develop in sedimentary aquifers and have different characteristics than other aquifers, so they are discussed separately and in more detail.


Unconsolidated aquifers have a broad spectrum of characteristics, but it is useful to consider two groups – glacial aquifers and non-glacial aquifers.

5.1.1 Igneous and Metamorphic The general areas where igneous and metamorphic aquifers are important sources of drinking water are shown on Figure 1. Igneous and metamorphic aquifers have very little primary porosity (interconnected voids between individual grains in the rock), but may have considerable secondary porosity in partings, joints, fractures, and dissolution features. As a result, groundwater flow is often very fast. Groundwater flow often does not follow a straight line, either horizontally or vertically. It instead follows a complex path of preferential flowpaths. Regional studies can often identify general aquifer characteristics and flow patterns (e.g. flow towards an open mine pit), but localized flow patterns can be unpredictable.

5.1.2 Sedimentary Sedimentary aquifers are found generally in the southeast third of the state (Figure 1). Sedimentary aquifers were traditionally grouped as sandstone, limestone, and shale aquifers. Recent work at the Minnesota Geological Survey (Runkel et al., 2003, RI 61) demonstrates that a more realistic and useful classification groups the aquifers as Coarse Clastic, Fine Clastic and Carbonate Aquifers:

� Coarse Clastics (some, but far from all, sandstones) often have considerable primary porosity. However, groundwater flow (both volume and velocity) is often much greater through secondary porosity features (partings, joints, fractures and dissolution features) than through primary porosity. Coarse Clastics in Minnesota function as aquifers.

� Fine Clastics have lower primary porosity and can function as aquitards.

� Carbonates (limestones and dolomites) in Minnesota typically have very low primary porosity but typically have significant secondary porosity and permeability via extensive joints and bedding plane partings that connect to well-developed solution conduits. Carbonates in Minnesota function as aquifers.

When considering regional groundwater resources, it is customary to model sedimentary aquifers as homogenous porous media, but these models and assumptions often do not adequately describe groundwater flow at individual landfill sites.

It is also important to recognize that sedimentary formations have different hydrogeologic characteristics in different areas. In the Twin Cities area, the Franconia Sandstone Formation is notorious for having very different hydraulic conductivity in different areas. The St. Lawrence Formation is often considered a confining layer when analyzing regional groundwater resources, but many residential wells are completed in the St. Lawrence Formation and dye traces have recently demonstrated rapid horizontal groundwater flows in the St. Lawrence Formation.

Figure 1. Minnesota Bedrock Aquifers


Finally, the Minnesota Geological Survey (Runkel et al., 2003, RI 61) has shown that the hydraulic characteristics of all of the sedimentary aquifers become much more transmissive when those rocks are within a few hundred feet vertically or horizontally of the present or former land surface. The impermeability of confining layers drops dramatically in the near surface environment. Almost all potential landfill sites will be in this surface or near surface zone.

5.1.3 Karst In Minnesota karst aquifers have developed in soluble rocks composed of, containing beds of, or containing cements of limestone and dolomite. Karst aquifers are also developed in sandstones. Major parts of the karst aquifers in Minnesota are covered by glacial deposits. It has proven useful to divide the karst aquifers into the regions with less than 50 feet of glacial sediment cover, 50-100 feet of glacial cover and more than 100 feet of glacial cover. The regional scale distribution of karst aquifers is shown in Figure 2.

Figure 2. Minnesota Karst Lands

The word “karst” was originally defined by geographers as a land surface developed over soluble bedrock, which is characterized by sinkholes, caves, springs, sinking streams, etc. Current usage of the phrase “karst aquifer” includes all aquifers in which solution porosity and permeability play a significant hydraulic role in the aquifer. All karst surfaces overly karst aquifers, but most karst aquifers in Minnesota do not have classically defined karst surfaces over them currently. Glacial on other processes have obscured or obliterated the karst surfaces but the karst aquifers remain.


A paucity or absence of surface karst feature is not evidence that the underlying aquifer can be correctly modeled, regulated, or managed as a porous media aquifer.

5.1.4 Unconsol idated Glacial All of Minnesota has been shaped by glacial activity at some time in the past. The lasting effects of the glaciation vary widely across the state. Glacial terrains can include:

� Areas where soils are thin or absent because glaciers removed them. � Thick, dense, clay-rich tills that originated at the base of large glaciers (basal tills). � Variable and unpredictable assemblages of sand, silt, and clay that create hummocky terrain

(lateral and terminal moraines). � Well-sorted deposits of sand and gravel that were deposited in cracks or rivers through the

glacial ice (kames and eskers). � Expansive areas of sand deposited by water running off a melting glacier (outwash deposits). � Expansive areas of silt deposited by wind blowing across a bare landscape after a glacier

recedes (loess). � Silt and sand deposited at the edge or in the middle of lakes that developed on top of or

adjacent to glaciers.

Glacial deposits of sand, silt, and clay can vary in thickness zero feet to over three hundred feet in thickness. The deposits can also vary significantly over horizontal distances of a few feet. The variations create the need for extensive and detailed characterizations of potential landfill sites. Glacial deposits in Minnesota are generally classified as clayey, sandy, and thin or absent, as shown on Figure 3.

Coarse-grained glacial deposits are often tapped for water supplies by residential, municipal, and industrial wells. Groundwater flow through some glacial aquifers can

sometimes be approximated as a homogenous and isotropic. However, care must taken to identify common areas of anisotropy (especially in the vertical direction) and preferential flowpaths.

5.1.5 Unconsol idated Non-Glacial Areas of sand and silt deposited by rivers are found throughout Minnesota. Many have limited areal extent, although some deposits associated with major rivers are significantly larger. The coarser deposits can be important water supplies, especially in areas where bedrock aquifers have low yields and poor water quality. Because of their limited extent, generally high hydraulic conductivity, and importance as local water supplies, river deposits should almost always be excluded as landfill sites.

Figure 3. Minnesota Glacial Deposits


5.2 Regional Groundwater DNR and MPCA have developed classification systems for groundwater throughout the state. They are very similar. Both have advantages for various different applications. Both systems are presented here to help with discussions about regional landfill issues. These classification systems and maps cannot be substituted for detailed site studies or other area investigations such as a county hydrogeologic atlas.

5.2.1 DNR Groundwater Provinces DNR developed a classification of six groundwater provinces based on the basic geologic information discussed above (MDNR, 2009). Combining the regions of the two general geology settings in Figure 1 (bedrock) and Figure 3 (glacial deposits) creates the groundwater provinces shown in Figure 4.

Provinces 1 and 4 (metro and central, respectively) are characterized by buried sand aquifers and relatively extensive surficial sand plains as part of a thick layer of unconsolidated sediments deposited by glaciers overlying the bedrock. Province 1 is underlain by sedimentary bedrock that has good aquifer properties, but in Province 4 the glacial sediments are thick, sand and gravel aquifers are common, and the deeper fractured bedrock is rarely used as an aquifer.

The unconsolidated glacial sediments of Provinces 2 and 5 (south-central and western, respectively) are typically clayey and may contain limited extent surficial and buried sand aquifers. In Province 2 the sedimentary bedrock aquifers are commonly used, but in Province 5 the fractured bedrock is usually buried deeply beneath

glacial sediments and is only locally used as an aquifer.

The unconsolidated sediments in Provinces 3 (southeastern) and 6 (arrowhead) are thin or absent and, therefore, not used or relatively unimportant, except in major river valleys where sediment thickness is greater. However Province 3 is underlain by productive bedrock aquifers, but Province 6 is underlain by hard fractured bedrock that typically has limited ground-water yield.

5.2.2 Groundwater Profiles An Interagency Ground Water Coordination Group developed profiles of nine groundwater regions (MPCA 1995). For landfill siting discussions, the regional profiles are often more useful than the DNR province classifications. The regions are smaller and are described by county, a governmental unit commonly involved in solid waste issues.

Figure 4. Minnesota Groundwater Provinces


5.2.2.1 South Central Region Location: Blue Earth, Faribault, LeSueur, Martin, Nicollet, Waseca, and Watonwan Counties

Hydrogeology:

� The distribution of aquifers in this area is transitional between those having abundant, high-quality groundwater to the east and those with groundwater that is scarce and poor in quality to the west.

� Glacial aquifers are limited in extent and yield.

Quantity Issues:

� Occasional well interference problems are noted.

Quality Issues

� Nitrate contamination may affect the Prairie du Chien-Jordan aquifer as well as surficial aquifers.

� Agricultural drainage wells, where present, pollute deeper ground water. � Thick, clay-rich till is fairly protective. There are no high-priority problems for public water

supplies in till areas. � Proper well construction practices are critical when penetrating confining layers.

5.2.2.2 Southwest Region Location: Brown, Cottonwood, Jackson, Lac Qui Parle, Lincoln, Lyon, Murray, Nobles, Pipestone, Redwood, Rock and Yellow Medicine Counties

Hydrogeology:

� Scattered, shallow alluvial sands and limited, buried sand aquifers are present.

� Low-yield crystalline bedrock is vulnerable to contamination at or near the surface.

� The highest yielding aquifers in this region are mostly narrow, channel outwash deposits.

Quantity Issues:

� Aquifers located here tend to be low yielding and not as well defined as elsewhere in the state. � The Sioux Quartzite aquifer is near the surface in much of the region and is known for its low

yield and high vulnerability to contamination.


Quality Issues

� Wells completed in the buried sand and gravel and Cretaceous aquifers often yield water of poor natural quality (high sulfate and total dissolved solids).

� Channel aquifers are highly susceptible to contaminants including nitrate from feedlots, agriculture, and human wastewater.

5.2.2.3 North Woods Region Location: Aitkin, Beltrami, Clearwater, Itasca, Kanabec, Koochiching, Lake of the Woods, Mahnomen, Mille Lacs and Pine Counties

Hydrogeology:

� The region is dominated by shallow glacial aquifers with numerous connections to wetlands.

� Availability of ground water varies because of low-yielding crystalline bedrock is present near the surface in many areas.

Quality Issues:

� The presence and level of “natural contaminants” (trace metals) should be better defined to determine long-term health impacts.

5.2.2.4 Central Sands Region Location: Benton, Cass, Crow Wing, Hubbard, Morrison, Stearns, Todd and Wadena Counties

Hydrogeology:

� Extensive high-yield, sand-plain aquifers characterize much of the region.

� Some areas with crystalline bedrock near the surface offer little potential as aquifers.

Quantity Issues:

� Some areas must rely on low-yielding, glacial and bedrock aquifers.

Quality Issues

� Sand aquifers are highly susceptible to contamination by land-use activity such as irrigated agriculture, septic systems, lakeshore development, and commercial and industrial development that lacks a proper sewer system.

� Deeper aquifers may have higher levels of dissolved solids and trace metals than water-table aquifers.


5.2.2.5 West Central Region Location: Becker, Big Stone, Chippewa, Douglas, Grant, Kandiyohi, McLeod, Meeker, Otter Tail, Pope, Renville, Sibley, Stevens, Swift and Traverse Counties

Hydrogeology:

� High-yield, surficial, sand aquifers are present in large parts of this area.

� Deeper glacial aquifers are limited in areal extent. � Bedrock does not yield large amounts of ground water in this

region.

Quantity Issues:

� Some well interference problems exist. There is adequate observation-well density. � Where present, surficial sands are susceptible to contamination.

Quality Issues

� Arsenic and other elevated trace metals are associated with the geology of the region. � Agricultural practices and domestic land uses may impact ground-water quality with the

increased presence of nitrates and dissolved solids.

5.2.2.6 Southeast Region Location: Dodge, Fillmore, Freeborn, Goodhue, Houston, Mower, Olmstead, Rice, Steele, Wabasha and Winona Counties

Hydrogeology:

� Layered sandstone and carbonate bedrock aquifer systems are highly productive and of high natural quality.

� Extensive near-surface karst areas result in aquifers that are highly vulnerable to contamination.

� Glacial aquifers are not widely present and are often of moderate to poor yield.

Quality Issues

� Near-surface, karst aquifers have potential for contamination and major problems with land-use management, including siting of industrial, municipal, and agricultural facilities.

� Pesticides occur in older, shallow wells constructed in karst aquifers. � Nitrate contamination in near-surface aquifers is widespread.


5.2.2.7 Red River Valley Region Location: Clay, Kittson, Marshall, Norman, Pennington, Polk, Red Lake, Roseau and Wilkin Counties

Hydrogeology:

� The area has a flat topography with clay-rich, poorly drained soils.

� Beach ridges are local recharge areas susceptible to contamination.

� Natural ground-water quality in deeper aquifers is frequently poor (high-dissolved solids).

Quality Issues

� Heavy pumping of overlying drift aquifer may cause the upward flow of lesser quality groundwater from the Cretaceous aquifer in some areas.

� Water-quality parameters of special concern in this area include: manganese, arsenic, chloride, sulfate, nitrate, and total dissolved solids.

�

5.2.2.8 Greater Metro Region Location: Anoka, Carver, Chisago, Dakota, Hennepin, Isanti, Ramsey, Scott, Sherburne, Washington and Wright Counties

Hydrogeology:

� High yielding bedrock aquifers form the Twin Cities Basin. � Buried sand and gravel recharges deeper bedrock aquifers. � Extensive sand-plain aquifers are present in the north, east

and southeast parts of the region. � Natural ground-water quality is good, but is susceptible to

contamination from the effects of urbanization.

Quantity Issues:

� There is a need to match urban development with groundwater availability.

� The continued long-term impact from pumpage, with regards to water levels in the Prairie du Chien-Jordan and Mt. Simon aquifers, is not well known.

� Irrigation-related well interference can be a problem under drought conditions, especially on portions of the Anoka sand plain.


Quality Issues

� Widespread, low-level contamination of upper aquifers has been observed from the effects of urbanization.

� Growth areas are served by ground water, but are often susceptible to geology and the lack of proper sewer systems in developing areas.

� Nitrate contamination of sand-plain aquifers is noted.

5.2.2.9 Arrowhead Region Location: Cook, Carleton, Lake, and St. Louis Counties

Hydrogeology:

� Glacial aquifers commonly are thin and limited in their extent and yield.

� Bedrock aquifers have limited yield, generally from fractures; ground-water movement is difficult to define.

Quantity Issues:

� Ground water is scarce in some areas and property values increase with the presence of a good well (e.g. North Shore).

� There are no large-scale regional aquifers.

Quality Issues

� High salinity (presence of salt) along the North Shore is poorly understood and highly variable. Elevated boron, fluoride, and trace metals are common.

� The impact of land use and ground-water recharge to mine pits that are used for public water supplies are an issue.

5.2.3 Extent of Karst Areas As shown in Fig (Minn. Karst Lands), karst aquifers underlie essentially all of the MPCA’s Southeast Region, major parts of the South Central region and major parts of the Greater Metro region. In addition a significant karst aquifer exists in the Hinckley Sandstone of central Pine County in the North Woods region.

6 Contaminant Transport The following discussion briefly introduces a few key concepts that are important when considering landfill siting.

6.1 Advection, Dispersion, and Diffusion Advection is the transport of contaminants being carried along by the moving groundwater. Advection occurs along the groundwater flowpath in the direction of the hydraulic gradient at the same velocity as groundwater flow.


Dispersion is the spreading of contaminants away from the main groundwater flowpath. When a contaminant or tracer is introduced into the subsurface, some of it will be transported faster or slower than the average rate of groundwater flow. Similarly, some of it will be transported laterally or vertically away from the main groundwater flowpath. The amount of dispersion over time can be measured and expressed as a dispersion coefficient. Dispersion is influenced by many factors, including the chemical properties of the contaminant, the homogeneity of aquifer materials, and changes to groundwater flow and direction over time.

Diffusion is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. Diffusion is typically slow, often negligibly slower, than advection or dispersion is aquifers with typical ground water flow velocities. However, if the advection and dispersion are naturally or artificially low (for example, in a landfill liner or aquitard) diffusion can dominate contaminant transport (Manning and Ingebritsen, 1999).

6.2 Sorption Sorption is the adherence of contaminants to aquifer materials. Absorption and adsorption are terms used to describe the degree of chemical and physical bonding of the contaminants to the aquifer material, and the term sorption refers to both processes taking place simultaneously.

Sorption is generally considered as a favorable property for aquifer materials because it can retard the transport of contaminants. In general, fine grained materials have greater sorption capacity than coarse grained materials. This is because fine grained materials have more surface area for sorption of contaminant molecules. Some fine grained materials such as clays have chemical properties that increase sorption.

6.3 Contaminant Reactions and Degradation Some contaminants undergo chemical reactions and degradation in the subsurface including oxidation and reduction, biological reactions, and radioactive decay. In general, these reactions decrease the concentration of the contaminants over time, and their products are less toxic than the original contaminants. Contaminants and other molecules that do not degrade are called conservative molecules.

In an idealized uniform porous aquifer, contaminants released from a landfill would form a contaminant “plume” in the subsurface. The concentration of contaminants at a monitoring point at any given time will depend on the effects of advection, dispersion, sorption and other chemical reactions, as illustrated in Figure 5.

In real aquifers heterogeneities in the contaminant characteristics, aquifer flow fields, aquifer parameters, recharge events, pumping wells, etc., produce much more complex patterns in three physical directions and in time.


7 Parameters, Measurements, and Testing

7.1 Groundwater Flow

7.1.1 Unsaturated Flow Unsaturated flow is very difficult to measure accurately. Many parameters influence the velocity of water flow and contaminant transport through the unsaturated zone. The measurement of all these individual parameters can be expensive and subject to significant errors in field data collection and laboratory analysis. Therefore, we recommend that quantifying unsaturated flow be minimized as a determining factor in determining groundwater sensitivity to pollution.

However, unsaturated flow is generally faster through unconsolidated deposits with higher hydraulic conductivity than through unconsolidated deposits with lower hydraulic conductivity. Therefore, the saturated hydraulic conductivity of a particular deposit can be used as a surrogate parameter for determining the relative velocity and quantity of flow through the unsaturated deposit. For example, unsaturated flow through high-conductivity sands will be greater than unsaturated flow through low-conductivity silt and clay.

We recommend that unsaturated deposits be evaluated using tracer tests described in Section 7.1.4. Additional data may be obtained with the laboratory analytical methods for determining saturated hydraulic conductivity described below (falling head and constant head tests.)

7.1.2 Saturated Porous Media The test methods listed below give a brief overview of some techniques that are used to characterize saturated porous media aquifers. The reader should refer to American Society of Testing and Materials (ASTM) standards or other textbooks for more specific information.

MONITORING POINT

CONTAMINANT SOURCE

GROUNDWATER FLOW AND ADVECTIVE TRANSORT

Figure 5. Idealized Contaminant Plume


7.1.2.1 Laboratory Tests 1. Grain size (ASTM WK11776).

a. Sieve analysis. Sieve analysis measures the distribution of particles larger than 75 microns. The soil samples is put through a series of progressively finer-meshed sieves. The particles retained by each sieve are weighed and reported as a percent of the whole sample.

b. Hydrometer analysis. Hydrometer testing measures the distribution of particles smaller than 75 microns. The soil sample is dispersed in water and the percent (by weight) of the particles that settle out over time is measured. It is important to do this test in conjunction with the sieve analysis because silt and clay-sized particles can play a key role in determining hydraulic conductivity and contaminant transport processes.

2. Falling head or constant head (ASTM D5084-03). Falling head and constant head tests measure the hydraulic conductivity of a soil sample by running water through it. The test results are of limited use in site investigations because they only measure a very small volume of the aquifer, and generally do not measure the influence of secondary porosity. The tests are often conducted because they are relatively inexpensive and do not require installation of a well.

3. Statistical analysis of multiple tests. Results of both laboratory and field tests will vary significantly across a site. Statistical tests are required to identify outliers and describe the “true” character of the site.

7.1.2.2 Field Tests 1. Slug test (ASTM D4044-96). Slug tests either inject or remove a known volume of water

into a well very rapidly, and then measure the time required for the well and the aquifer to return to static conditions. Slug tests measure the hydraulic conductivity of a very limited area surrounding the well. Slug tests have an advantage over laboratory tests in that they can measure groundwater flow through a larger volume of the aquifer and may show the influence of some larger structures such as cracks.

2. Pump test (ASTM D4050-96 and others, depending on the type of aquifer). Pump tests withdraw water from a pumping well and measure the response of water levels in the pumping well and nearby observation wells. They have several advantages over laboratory tests and slug tests:

� Pump tests measure the hydraulic conductivity of the entire aquifer between the pumping well and the observation wells.

� Pumping can be conducted on different schedules to obtain different types of data from the test.

� Careful interpretation of the drawdown observations over time (drawdown curve) can yield important information about the primary and secondary porosity of the aquifer.

3. Tracer tests are an important way of measuring the flow velocity to and through the aquifer underlying a landfill. Both natural and forced gradient tests yield definitive data.

MPCA should develop standards for the type and number of tests required for a evaluating a potential landfill site. Statistical methods must also be specified for analyzing multiple test results.


7.1.3 Saturated Fracture Flow and Karst Fracture flow and karst aquifers are difficult and expensive to adequately characterize. They cannot be adequately modeled using any of the conventional porous media models. Dual and triple porosity research models are currently being developed but none of them are yet in routine environmental hydrogeology practice. Fracture flow and karst sites are very difficult to monitor adequately at the site scale. Any site will be part of one or more larger karst or fracture flow systems. Groundwater flow from such sites is typically via one or more conduits and the probability of intersecting the relevant conduit with any rational number of monitoring wells is vanishingly small. Contaminants routinely are not detected in site monitor wells but show up in off site wells or springs long distances from the site. Karst and fracture flow aquifers drain to springs or major pumping wells which are typically off site and may be miles away. However, the springs may be inaccessible, for example, beneath lakes or rivers. Fracture flow and karst aquifers can be monitored – but such monitoring will be difficult, time consuming and expensive.

Near-surface karst and fracture flow aquifers are usually exceedingly “flashy”. The flow and contaminants in karst and fracture flow aquifers often change by factors of 10 or more on time-scales of minutes to hours after recharge events. Traditional quarterly monitoring schedules yield erroneous, under-estimates of the contaminant fluxes. Continuous monitoring with data loggers is required to accurately monitor such aquifers. Conventional water table potentiometric maps are notoriously ineffective in establishing flow direction and underestimate groundwater flow velocities by many orders of magnitude.

The conceptual models used to interpret any data or information about a site in karst or fracture flow environments are critical to successful site selection, facility design and construction, operation, monitoring, closure, and long-term maintenance efforts. Therefore, landfill designers and operators must make the most of every opportunity to gain information about the subsurface and the nature of groundwater flow below a site. Karst systems are heterogeneous, and critical features can go undetected even in well-designed subsurface boring investigations. The best opportunity to observe geologic details of the site is during the construction phase. Construction inspectors and regulators must be diligent to document unexpected features including fractures and solution cavities. Landfill constructors must also be prepared to address potentially problematic features, including abandoning the site if the features will threaten the future integrity of the landfill.

7.1.3.1 Artificial Tracer Testing Tracer tests are the established and preferred method of documenting groundwater flow velocities and directions in karst and fracture flow aquifers. A variety of tracers can be introduced into the groundwater flow at one point and the measured as those tracers move down a natural or induced gradient to detections points. Tracer test can be used to identify the discharge springs or pumping wells that drain a given site and yield direct measurements of the time-of-travel. Trace tests can also yield information about dispersion and retardation of the specific contaminant types in the aquifers. Tracer testing should be executed by individuals with documented experience and success in tracer techniques in the aquifers in question and should be interpreted in the relevant conceptual models. Tracer tests can and should be used to document that monitoring wells are hydraulically connected to specific sites. Such connections cannot be assumed. Water level changes in a monitoring well during pump tests are not adequate to establish flow connections.


7.1.3.2 Other Physical Testing Hydrophysical measurements of monitoring wells or water production wells are useful to document heterogeneities – but inherently yield point data that is not representative of karst or fracture flow aquifers. Downhole flow measurements under static or stressed conditions often yield instructive information about high transmisstivity zones in the aquifers. If several such measurements yield consistent data, they can be useful in isolating the significant flow zones for monitoring. Soil and bedrock borings can be useful if interpreted in relevant conceptual models but have vanishingly small probabilities of sampling the important conduits or fractures. Long backhoe trenches are useful for getting 2D views of the near-surface features at a potential site. As above, such observations need to be interpreted in the relevant conceptual models but can demonstrate the presence of near surface collapse features not visible on the surface. 2D and 3D geophysical measurements can identify problematic areas and are most useful when verified by backhoe trenches (preferred) or multiple borings. The problem is that the critical features may be smaller than the boring diameter. Again, such data are only useful if interpreted in relevant conceptual models of karst and fracture flow aquifers.

7.1.4 Isotopes and Other Environmental Tracers Natural and anthropogenic environmental tracers should be fully utilized in efforts to characterize potential landfill sites. Such tracers are both economical and can yield direct measurements of the residence-times of the groundwater on the year, decade, century and millennium time-scales. They should be routinely used.

7.1.4.1 Tritium, 3H – Year to Decade Time-Scales Enriched tritium, 3H, is a very useful, economical technique for measuring the residence time of the groundwater beneath a proposed site on the year to decade-time scale. “Tritium-Helium”, “3H-3He”, in principle yield more precise estimates but in most cases is not cost effective. Enriched versus unenriched tritium measurements are necessary to obtain a sufficiently low detection limit of <1 tritium unit (TU). The presence of detectable tritium in the water sample indicates the presence of, at least, a component of water that has recharged since the advent of atmospheric thermonuclear weapons testing in 1953.

7.1.4.2 Carbon 14, 14C – Century to Tens of Millennium Time-Scales If the groundwater under at a prospective landfill site contains low or no detectable 3H, an analysis for 14C in the dissolved inorganic carbon of the water (or also in the dissolved organic carbon if desired) can yield information on the groundwater residence time on the century to tens of millennium time-scale. A very old 14C age would indicate effective isolation of the groundwater from surface recharge.

7.1.4.3 CFCs, Chlorofluorocarbon Compounds – Years to Decades Time-Scales Measurements of the levels of CFCs can yield information on the residence times of groundwater on a time-scale comparable to that of tritium and can be an independent support of tritium ages.


7.1.4.4 Stable Isotopes (2H, deuterium and 18O) – Months to Decades Time-Scales Measurements of the stable isotopic composition of the water molecule (reported as �2H and �18O) yield information on the water molecules themselves rather than on material dissolved in them. In Minnesota there are significant variations in the stable isotopic composition of water between winter snows and summer rains. Stable isotope data can yield information indicating that seasonal recharge effects are reaching the near-surface aquifer. Such data can also identify significant lake –groundwater interactions.

7.1.4.5 Other Tracers – Decade Time-Scales. Several synthetic organic compounds, SOCs (Caffeine, pesticides, VOCs, pharmaceuticals, PFCs, MTBE, etc.) are being detected in groundwater. Many of these compounds were first introduced into the surface environment at known times in the past. The presence of such compounds in the groundwater under a prospective landfill site is a strong indication of a short time-scale connection to the surface. Inorganic chemicals can yield important information about the source of groundwater beneath a prospective landfill site. Examples include:

� Nitrate ions from a variety of anthropogenic sources. � Chloride to bromide ratios in the water, which can identify the source of the chloride in the

water. � The basic cation and anion chemistry of the groundwater, interpreted in terms of the

composition of local aquifer data can help identify the flow paths leading to and from a proposed site.

Microbiological sampling, identification and DNA characterization is a rapidly maturing technique for identifying the sources of surface inputs to groundwater.

7.2 Modeling Time of Travel Testing and observation are preferred over modeling. For documented (as opposed to assumed) porous media aquifers, models explicitly designed and calibrated for time-of travel calculation may provide useful insights for a proposed site. For all proposed sites over fractured and or carbonate aquifers, only dual or triple porosity models calibrated with local data and conceptual models have any chance of yielding relevant times of travel.

7.3 Contaminant Monitoring Detection of leaks from landfills generally involves monitoring contaminant concentrations at observation points established within the landfill liner and leachate collection systems or at monitoring wells near the landfill. Groundwater sample collection methods, frequency of monitoring, laboratory analysis, and the significance of results are all import topics for landfill operation, but they are beyond the scope of this paper.

Collecting and analyzing groundwater samples during the landfill siting process can yield useful information about groundwater flow. Natural chemical or contaminant concentrations can indicate the origins of groundwater prior to reaching the site. Background testing and analysis is required as part of the landfill permitting process.


8 Groundwater Sensitivity Classifications

8.1 Methodology As discussed in Section 2.3 above, the time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity. Time of travel is a good surrogate when accounting for other factors might reduce contaminant concentrations and groundwater sensitivity, such as sorption and chemical reactions. In general, as time of travel increases due to the presence of silt and clay in the aquifer, the potential for sorption and favorable chemical reactions also increases.

This paper agrees with the following parts of the methodology used by MDNR (1991):

“In addition, the method does not evaluate the effect of human related activities such as ground water withdrawals or improperly constructed, maintained or sealed wells on the movement of contaminants to or within an aquifer.”

“The only factor affecting sensitivity that is fundamental to contaminant movement, relatively well understood and stable over time is the geology of an area. The properties of various geologic materials are sufficiently known that reasonable estimates of contaminant time of travel form a source to an aquifer are possible. Since the time of travel estimate uses only geologic information, the evaluation is of geologic sensitivity, not some broader interpretation.”

This paper will build on the Time of Travel Sensitivity classification currently used by the DNR for the Sensitivity maps in their County Hydrogeologic Map series. That Time of Travel classification is based mainly on data from environmental tracers.

The objective of good landfill siting is to protect groundwater resources for at least many decades and ideally in perpetuity. So it is assumed that contaminants released into the subsurface will eventually reach underlying aquifers.

The first consideration in groundwater sensitivity should be the relative time of travel. That statement does not mean the human and natural history of a proposed site is not an important criterion for landfill siting. One unrecognized improperly abandoned well, one open fracture or joint, one paleocollapse reactivated by the construction or loading, one undetected collapse growing toward the surface can and has led to catastrophic failures of waste storage structures.

Hydraulic gradients should also be considered when comparing one potential landfill site against another. Contaminant transport is a function of the hydraulic gradient. Sites with high hydraulic gradients (either horizontally or vertically) are more susceptible to contaminant migration than sites with low hydraulic gradients. Sites with upward vertical gradients are preferred over sites with downward vertical gradients because upward gradients prevent contaminant migration to deeper aquifers.


Hydraulic gradients are not a primary consideration in determining groundwater sensitivity because they are likely to change over time. Constructing a large, impermeable landfill cover will alter gradients at the site. Short term and long term variations in climate will affect nearby water resources and groundwater gradients. Land use changes in the vicinity of the landfill can also affect groundwater recharge, groundwater discharge from wells, and hydraulic gradients. Predicted changes in gradients should be part of the hydrogeologic characterization of a candidate site.

8.2 Range of Sensitivity Table 1 shows a landfill site’s sensitivity to groundwater contamination based on hydrogeologic characteristics. The range of sensitivity is given on a scale of 10 (high sensitivity) to 1 (low sensitivity).

Table 1. Groundwater Sensitivity Ranking

Water Table

Aquifer -

Groundwater

Age Dating

Carbonate or

coarse clastic

overlain by a

karst surface

Carbonate or

coarse clastic

without a

karst surface

Other bedrock

with fracture

flow

Sand and

gravel

Sandy silt to

clayey silt

Lean clay and

fat clay

Other bedrock

without

fracture flow

Less than 1 year 10 9 8 7 6 5 1

1 to 10 years 9 8 7 6 5 4 1

10 to 100 years 8 7 6 5 4 3 1

More than 100

years 7 6 5 4 3 2 1

Water Table Aquifer - Most Sensitive Material

8.3 Evaluating Aquifer Materials For purposes of groundwater sensitivity classification, unconsolidated aquifer materials are grouped into three categories, such as shown in Table 2.


Table 2. Classification of Unconsolidated Materials

Description Hydraulic conductivity

Unified Soil Classification System

High Permeability/ Sand and Gravel

>10-3 cm/s Clayey gravel (GC), Silty gravel (GM), Poorly graded gravel (GP), Well graded gravel (GW), Poorly graded sand (SP), Well graded sand (SW)

Moderate Permeability/ Sandy Loam and Silt

10-3 to 10-6 cm/s Silty sand (SM), Clayey sand (SC)

Low Permeability/ Clay

<10-6 cm/s Fat clay (CH), Lean clay (CL)

One limitation of such ordering of unconsolidated materials is that the hydraulic conductivity data were measured on homogenized samples in the laboratory. Such homogenization removes an entire range of macropore structure that dominates the actual, “in place” hydraulic conductivity of unconsolidated sediments. Such macropores are ubiquitous in natural unconsolidated materials and dominate the hydraulic conductivity of the less permeable materials.

A second limitation is that the Unified Soil Classification System eliminates all information about the formation of the materials in question. That procedure discards what may be critical, known information about the permeability structure of the hydraulic conductivity.

Four categories are proposed for consolidated bedrock aquifers. From most sensitive to least sensitive, they are:

� Carbonate and coarse clastic aquifers overlain by a karst surface. � Carbonate and coarse clastic aquifers without a karst surface. � Other bedrock with fracture flow. � Other bedrock without fracture flow.

These bedrock aquifer categories were selected on two basic premises:

3. Contaminant transport through fractures and solution channels is faster and therefore more significant than contaminant transport through porous media.

4. Solution cavities and karst features are common in carbonates, even if they are not encountered in a few boreholes at a site.

We recognize that “Other bedrock without fracture flow” is actually not an aquifer at all, but it was added to make sure that the table was complete.


The “water table aquifer” is the uppermost aquifer at the site. Aquifers with more than one type of material not separated by a confining layer (see definition below) must be considered as having a sensitivity ranking of the most sensitive material. For example, if a silt aquifer overlies a fractured bedrock aquifer with no confining layer in between, then the aquifer is considered as a fractured bedrock aquifer for sensitivity ranking. The site could take credit for the silt as a low or moderate permeability unit.

“Confining layer” refers to soil or bedrock that might effectively prevent groundwater flow and protect the underlying aquifer. It must be demonstrated that the confining layer is at least 20 feet thick everywhere at the site. Materials less than 20 feet thick are prone to cracks or undetected thin areas (i.e. fensters) that lessen the effectiveness as a restrictive barrier (see RI 61). For unconsolidated material, a confining layer must have a hydraulic conductivity of less than 10-6 cm/s. For consolidated materials, a confining layer must have a hydraulic conductivity of less than 10-6 cm/s. To be a classified as a confining layer a geologic formation must be continuous over the entire site and groundwater basin and have multiple, measured, in situ, hydraulic conductivity of less 10-6 cm/sec on a large enough scale to include multiple examples of the inevitable joints and fractures.

8.4 Evaluating Travel Times A key recommendation of this paper is that groundwater travel times from the surface or near surface to the water table aquifer must be directly measured using the methods described in Section 7.1.3 and 7.1.4. However, it may not be possible to directly measure the groundwater age at each of the many locations being considered for siting a particular landfill. Regulators and landfill proposers need to work together to develop conceptual models of travel time based on the geologic materials found in the area. The conceptual models need to be calibrated and validated using actual field test results. The following conceptual models illustrate how groundwater travel times might be determined:

1. MDNR (1991) recognized four categories of vadose zone materials based on the presence of low or moderate permeability units. The associated travel times might be:

Vadose zone materials Predicted travel times

No low permeability units greater than 10 feet thick and no moderate permeability units greater than 20 ft thick.

Less than 1 year

A single moderate permeability unit a minimum of 20 feet thick.

1 to 10 years

An aggregate of low permeability units greater than or equal to 10 feet thick with no single unit greater than 10 feet thick.

10 to 100 years

At least one low permeability unit a minimum of 10 feet thick.

More than 100 years


2. Several decades of field data mapping sinkhole distributions, summarized in Figure 2 (Minnesota Karst Lands map) document the relevant and critical thicknesses of sediments over soluble bedrock. The associated travel times might be:

Vadose zone materials Predicted travel times

Less than 50 feet of sediment cover – macropores up to and including sinkholes commonly penetrate the entire sediment cover.

Less than 1 year

50 to 100 feet of sediment cover – macropores up to and including sinkholes penetrate the entire sediment cover only occasionally.

1 to 10 years

Greater than 100 feet of sediment cover – macropores up to and including sinkholes rarely penetrate the sediment cover.

10 to 100 years

3. The recently completed Regional Hydrogeologic Assessment, Traverse-Grant Area, West Central Minnesota (Berg 2008) presented a conceptual model of the groundwater sensitivity in buried aquifers. The model was based on multiple “recharge surfaces” and the presence of fine-grained protective layers over the aquifer. The associated travel times were:

Thickness of protective layer between the aquifer and the nearest overlying recharge surface

Predicted travel times

Less than 20 feet Less than 1 year

20 to 30 feet 1 to 10 years

30 to 40 feet 10 to 100 years

More than 40 feet More than 100 years


9 Excluding Landfill Sites Based on Hydrogeologic Conditions

9.1 Methodology MPCA should adopt a landfill siting method based on the premise that landfills should be sited in the most favorable locations with the lowest groundwater sensitivity within a designated search area. Landfills should be excluded from less favorable locations. That is, a potential landfill site with a higher groundwater sensitivity ranking (according to Table ) should be excluded if a site with a lower groundwater sensitivity ranking exists within the search area.

MPCA should require tracer testing as described in Sections 7.1.3 and 7.1.4 for all sites being considered for landfill siting. While it is probably not possible to exclude sites based on certain values or concentrations of tracers, the data should be used to compare potential landfill sites. Landfill sites with tracer tests indicating older groundwater or slower recharge should be given preference over sites with younger, more sensitive groundwater.

If taken to an illogical extreme, the proposed method could lead to all landfills in Minnesota being sited within one very small area with optimal hydrogeologic conditions. To prevent this, each proposed landfill should be assigned a large siting region within which waste transport and disposal would be practical. The siting region would be different for various waste types and various parts of the state.

The proposed method for siting a landfill can be summarized as follows:

“A landfill must be sited only in areas with the best hydrogeologic properties, and will be excluded from areas with poor hydrogeologic properties, within the region being considered for landfill siting.”

The proposed method is based solely on a site’s geologic and hydrogeologic conditions. The so-called “three-legged stool” concept that incorporates waste segregation, engineered barriers and systems, and hydrogeologic conditions is not relevant to the siting process. Waste segregation and engineered barriers and systems certainly have a role in landfill design, construction, operation, and closure, but not in site selection.

MPCA should also prohibit all landfills on all sites with high Groundwater Sensitivity Rankings shown on Table . For example, landfills should be prohibited on sites with a groundwater sensitivity rating of 8, 9, or 10. This would help resolve some inconsistencies in the current solid waste rules. Currently, municipal solid waste and demolition landfills are prohibited in karst areas, but industrial waste landfills are not prohibited.

9.2 Determining the Siting Region Determining the region considered for landfill siting (the “siting region”) is likely to be an inexact and political process. Each proposed landfill would have its own siting region based on the location and the type of waste. The determination should be part of the scoping/EAW process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would delineate the siting region.


The size of the siting region would be partly based on type of waste and the environmental risk associated with disposal. Low-volume, high-risk wastes such as radioactive wastes would have the largest siting regions, possibly extending outside the state. Industrial waste and mixed municipal solid waste would have a siting region that might cover more than one of the MPCA-designated groundwater regions described in Section 5.2. The siting region might also be extended to avoid karst areas. High-volume, low-risk wastes such as construction and demolition fill would have the smallest siting region.

9.3 Identifying Target Areas After the siting region has been established, the siting process should identify the areas within the siting region that have the best hydrogeologic characteristics (the “target areas”). First, areas with higher groundwater sensitivity, as indicated in Table 1, will be disqualified in favor of target areas with the lowest groundwater sensitivity.

The goal of this process is to identify the target areas within the siting region with the lowest groundwater sensitivity. This process may identify a small target area with exceptional hydrogeologic characteristics, but it is more likely to identify a larger target areas with similar hydrogeologic characteristics found throughout.

The “target areas” identification process will likely use existing regional hydrogeologic data to identify the target areas. Hydrogeologic assessments, county atlases, and other studies exist for much of the state. It is important to use the most recent and best available data. If groundwater sensitivity maps of the siting region do not exist, the landfill proposer should work with geologic experts (Minnesota Geological Survey, university faculty) and regulators (MPCA, DNR) to develop conceptual models of groundwater sensitivity based on available geologic data. It may be necessary to perform testing and gather groundwater age data at several sites to complete the hydrogeologic assessment.

Second, the siting process should exclude areas from the target area that do not meet current regulatory standards for landfill sites. Minnesota Rules (i.e. 7035.1600) and agency policies prohibit landfill siting near critical areas such as water resources, roads, occupied dwellings, etc.

9.4 Screening Candidate Sites The next step in the siting process is for the landfill proposer to identify and screen candidate sites within the acceptable target areas. The landfill proposer can select specific proposed sites based on secondary criteria such as land cost, existing infrastructure, transportation costs, construction and operating costs, etc.

The landfill proposer may submit one or more sites for regulatory permitting. Extensive geologic and hydrogeologic testing will be required for the permits. This includes groundwater age dating and confirmation that the groundwater sensitivity ranking is valid.


10 Conclusions and Recommendations � MPCA should adopt a landfill siting method based on the premise that landfills should be sited

in the most favorable locations with the lowest groundwater sensitivity within a region considered for landfill siting.

� Determining the region considered for landfill siting should be part of the scoping/EAW process, subject to public input and regulatory review. The Responsible Governmental Unit (RGU) would determine the region considered for landfill siting.

� The time of travel for contaminants moving through the vadose zone and aquifer is the primary factor that should be used to determine groundwater sensitivity.

� MPCA should prohibit all landfills on all sites with Groundwater Sensitivity Rankings of 8, 9, or 10 shown on Table 1.

� MPCA should require tracer testing as described in Sections 7.1.3 and 7.1.4 for all sites being considered for landfill siting.

� MPCA should develop standards for the type and number of tests required for a evaluating a potential landfill site under this system.

11 References Berg, J.A., 2008. “Regional Hydrogeologic Assessment, Grant-Traverse Area, West-Central Minnesota.” Minnesota Department of Natural Resources. RHA-6, Part B.

Fetter, C.W., 2001. Applied Hydrogeology. Prentice-Hall.

Freeze, A.R., and Cherry, J.A., 1979. Groundwater. Prentice-Hall.

Manning, C.E., and Ingebritsen, S.E., 1999, Permeability of the Continental Crust: Implications of Geothermal Data and Metamorphic Systems, Reviews of Geophysics, v. 37, n. 1, p. 127-150.

Minnesota Department of Natural Resources, 1991. “Criteria and Guidelines for Assessing Geologic Sensitivity of Ground Water Resources in Minnesota”. MDNR Division of Waters.

Minnesota Department of Natural Resources website, 2009. “Ground Water Provinces.” http://www.dnr.state.mn.us/groundwater/provinces/index.html.

Minnesota Pollution Control Agency website, 1995. “Regional Ground Water Profiles.” http://www.pca.state.mn.us/water/gwprofiles.html.

MPCA Work Group, 2009. “Report to Minnesota Legislature on Management of Industrial Solid Waste and Construction and Demolition Debris in Land Disposal Facilities.” Minnesota Pollution Control Agency January, 2009.


Runkel, A.C., Tipping, R.G., Alexander, E.C., Jr., Green, J., Mossler, J.H., and Alexander, S. (2003) Hydrogeology of the Paleozoic Bedrock in Southeastern Minnesota. Minn. Geol. Survey Report of Investigations 61, St. Paul, MN, 105 p. + 2 plates.ftp://mgssun6.mngs.umn.edu/pub3/ri-61/


The Friends of Washington County is a non-profit organization that encourages sustainable development practices and excellent stewardship of natural and cultural resources in Washington County, MN.

President Chuck Haas, Hugo

Vice President Mary Hauser, Birchwood Village

Secretary/Treasurer Barry Johnson, Woodbury

Directors Dave Engstrom, Afton

Dorian Grilley, Mahtomedi

David Junker, Stillwater

Stan Karwoski, Oakdale

Peg Larsen, Lakeland

John Leinen, May Township

Len Price, Woodbury

Roger Tomten, Stillwater

Executive Director Marc Hugunin, Grant

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