Groundwater Mounding Groundwater Mounding and Minnesota ISTS and Minnesota ISTS

RulesRules

MPCA 2/21/06MPCA 2/21/06

What is GW mounding?What is GW mounding?

Subp. 4. Subp. 4. Groundwater moundGroundwater mound. . “Groundwater mound”, means “Groundwater mound”, means the rise in height of the the rise in height of the seasonally saturated soil or seasonally saturated soil or regional water table caused by regional water table caused by the addition of sewage effluent the addition of sewage effluent from a SSTS into the soil.from a SSTS into the soil.

What is GW mounding?What is GW mounding?

Simply, the rise in the Simply, the rise in the groundwater when water is added groundwater when water is added by man. by man.

What is GW mounding?What is GW mounding?

What is GW mounding?What is GW mounding?

Much More WaterMuch More Water

Natural recharge to the Natural recharge to the groundwater ranges from 3 to 11 groundwater ranges from 3 to 11 inches per year.inches per year.

The actual bottom loading to a The actual bottom loading to a SSTS SSTS (no inflated flow values)(no inflated flow values) ranges ranges from 6 to 30 feet per year. from 6 to 30 feet per year.

So What is the So What is the Problem?Problem? Reduce the unsaturated zone for Reduce the unsaturated zone for

pathogen removal pathogen removal

So What is the So What is the Problem?Problem? Impede oxygen transfer needed to Impede oxygen transfer needed to

breakdown the biomatbreakdown the biomat

oxygen

So What is the So What is the Problem?Problem? Breakout in downslope areasBreakout in downslope areas

So What is the So What is the Problem?Problem?

Reduced transmissivity?Reduced transmissivity?

Mounding and Mounding and ISTS

Mounding accounted for 7080 Mounding accounted for 7080 “cookbook” designs.“cookbook” designs.

Three-foot separation distance Three-foot separation distance required on all soil textures – required on all soil textures – balance between treatment and balance between treatment and mounding.mounding.

Mounding and ISTSMounding and ISTS

Low surface area

High surface area

Sandy Soil Heavy Soil

HistoryHistory

Agency recognized mounding Agency recognized mounding concerns since 1984 (HRSA concerns since 1984 (HRSA document).document).

With the greatly increased With the greatly increased number of LSTS and MSTS, new number of LSTS and MSTS, new concerns have been raised. concerns have been raised.

ConcernsConcerns

What is the relationship between What is the relationship between linear loading rate and mounding?linear loading rate and mounding?

Linear Loading RateLinear Loading Rate

ConcernsConcerns

Are GW mounding models Are GW mounding models appropriate for seasonally appropriate for seasonally saturated soil layers with slow saturated soil layers with slow permeability?permeability?

Model ApplicabilityModel Applicability

Regional Aquifer Permeable Materials

Model ApplicabilityModel Applicability

Seasonally Saturated Soil

Slowly Permeable Materials

ConcernsConcerns

Have the models proved accurate Have the models proved accurate in the past?in the past?

When should, and to what detail When should, and to what detail should a mounding analysis be should a mounding analysis be conducted.conducted.

ConcernsConcerns

What type and what frequency of What type and what frequency of field measurements should be field measurements should be used?used?

What operational monitoring What operational monitoring should be undertaken. should be undertaken.

Rules – MSTS Rules – MSTS (Chapter (Chapter 7081)7081)

– Preliminary groundwater evaluation Preliminary groundwater evaluation must be conducted for all MSTS must be conducted for all MSTS

– Evaluation = the size of the system, Evaluation = the size of the system, proposed loading rate, and system proposed loading rate, and system geometry must be determinedgeometry must be determined

– from the best available information: from the best available information: determine the general geology, shallow determine the general geology, shallow

groundwater setting, regional groundwater setting, regional groundwater setting, hydraulic groundwater setting, hydraulic conductivitiesconductivities

Rules – MSTS Rules – MSTS (Chapter (Chapter 7081)7081)

– The designer must consult with the The designer must consult with the local unit of government to local unit of government to determine whether the local unit of determine whether the local unit of government will require a field or government will require a field or further groundwater investigation further groundwater investigation and, if so, the extent of the and, if so, the extent of the investigation. investigation.

Rules – MSTS Rules – MSTS (Chapter (Chapter 7081)7081)

– The designer must consult with the The designer must consult with the local unit of government to local unit of government to determine if the local unit of determine if the local unit of government will require monitoring government will require monitoring and, if so, the extent of the and, if so, the extent of the monitoring. monitoring.

Rules – MSTS Rules – MSTS (Chapter (Chapter 7081)7081)

– An approximation of the rise in An approximation of the rise in groundwater from system operation groundwater from system operation must be determined must be determined

– A narrative evaluation of the A narrative evaluation of the accuracy of the approximation must accuracy of the approximation must be provided. be provided.

SDS Permit - LSTSSDS Permit - LSTS

The direction of groundwater flow The direction of groundwater flow The depth to the static The depth to the static

groundwater level groundwater level The rate of groundwater The rate of groundwater

movement (hydraulic movement (hydraulic conductivity)conductivity)

SDS Permit - LSTSSDS Permit - LSTS

The hydraulic gradient The hydraulic gradient The specific yieldThe specific yield To provide geologic cross-To provide geologic cross-

sectional analysis of the site sectional analysis of the site including the presence of deeper including the presence of deeper limiting layers, perched water limiting layers, perched water tables, bedrock, karst, etc.tables, bedrock, karst, etc.

SDS Permit - LSTSSDS Permit - LSTS

To characterize aquifers that may To characterize aquifers that may be impacted be impacted

SDS Permit - LSTSSDS Permit - LSTS

The items involve hydrogeologic field The items involve hydrogeologic field work operations. work operations.

This field work involves the drilling of This field work involves the drilling of deep borings at the proposed site deep borings at the proposed site

The borings are used to test for the The borings are used to test for the direction of groundwater flow, the direction of groundwater flow, the static water level, the hydraulic static water level, the hydraulic conductivity, the hydraulic gradient, conductivity, the hydraulic gradient, and the specific yield. and the specific yield.