Groundwater

transcript

Groundwater

• Of the roughly 350 billion gallons of fresh water withdrawn per day in US, 24% from groundwater

• 16 million pumping water wells in US for public and private supply, irrigation, livestock, manufacturing, mining, thermoelectric power, and other purposes– 15.4 million private wells

• Largest # private wells in Michigan (1,121,075)• Pennsylvania 2nd, North Carolina 3rd, New York 4th, Florida

5th.– Irrigation is biggest volumetric use,

• Nebraska (69,583), California (64,423), Texas (59,955), Arkansas (35,574), Missouri (10,913), Kansas (19,124), Florida (16,376)

America’s Largest Well

• Kansas City Kansas

• Horizontal collector well 135 feet below the surface

• 40 MGD

• 90% induced surface water recharge

• 10% groundwater from aquifer

Washington Wells• Installation regulation by Dept. of Ecology• 72 hour notification• No water right needed, if <5000 GPD for

domestic or industrial supplies and will irrigate less than ½ acre of lawn or non-commercial garden

• Setbacks of 100 feet from contamination source (Septic tanks, privies, stockyards, etc.); 1000 feet for landfill

• Must be installed by licensed installer; general contractors’s registration and insurance bond with DLI also necessary

• Maybe additional local ordinances

Washington Well Laws and Regulations

• Chapter 18.104 RCW WATER WELL CONSTRUCTION

• Chapter 90.36 RCW ARTESIAN WELLS• Chapter 90.44 RCW REGULATION OF PUBLIC

GROUND WATERS• Chapter 173-160 WAC, Minimum Standards For

Construction And Maintenance Of Wells • Chapter 173-162 WAC, Regulation And

Licensing Of Well - Contractors And Operators

Subsurface Terminology:

• Aquifer– Perched– Confined– Unconfined

• Regolith• Unsaturated Zone

(Vadose)• Soil• Saturated Zone (Phreatic)• Water Table

• Capillary Fringe• Infiltration• Recharge• Evapotranspiration• Aquitard• Aquiclude• Clay lense

The Subsurface Environment:

Every aquifer has a different make-up but the general components are the same. Components of an aquifer may include the unsaturated zone, capillary fringe/zone, groundwater table, saturated zone, and semi-confining or confining layers.

Unsaturated zone (vadose zone or zone of aeration)

The zone between the land surface and the water table. It includes the root zone, intermediate zone, and capillary fringe.The pore spaces contain water at less than atmospheric pressure, as well as air and other gases. Saturated bodies, such as perched groundwater, may exist in the unsaturated zone (Fetter 1988).

Capillary fringe

The zone immediately above the water table, where water is drawn upward by capillary attraction. The capillary fringe can be considered the "line" between the unsaturated and saturated portions of the aquifer. The thickness of this area varies as the water table moves up or down. However, the portion of the capillary fringe located close to where the 100% water saturation is reach is called the capillary zone.

Capillary Action

Capillarity

Water Table (Phreatic Surface)

• The water table, or phreatic surface, is an imaginary surface that bounds the saturated zone from above. It is defined as the surface at every point of which the water pressure is atmospheric.

• The level of the groundwater table is typically the level to which water will rise in a well.

• Rigorously, the well should be screened, i.e., hydraulically connected to the saturated zone at the phreatic surface.

Saturated zone (Phreatic Zone)

• The saturated portion of the ground that lies in the water table. The zone in which the voids in the rock or soil are filled with water at a pressure greater than atmospheric. The water table is the top of the saturated zone in an unconfined aquifer.

Confined aquifer

• Groundwater that becomes trapped under impermeable soil or rock may be under pressure. This is called a confined or artesian aquifer.

• A well that pierces a confined aquifer is known as an artesian well . Water pressure in the confined aquifer will cause water in the well to rise above the aquifer level.

• The maximum level that the water in the well will rise to is known as the potentiometric surface (Piezometric), or potential water level. If this is higher than the top of the well, the well will overflow.

Hydraulic Head (h):

• h = z + p/ρg + v2/2g– z is the elevation head, height above a

reference elevation– p is the gauge pore water pressure– g is acceleration due to gravity– v is velocity of water

z = elevation at point of measurement (relative to arbitrary datum z0)

= elevation head

= p/g = pressure head

Hydraulic Head

Ground surface

Datum (usually at z=0)

Point ofmeasurement

Hydraulic head example

Aquifer/ geological characteristics

Observed– Aquifer type (consolidated/unconsolidated)– texture– homogeneity / sorting

Measured/Calculated– porosity– anisotropy– conductivity– storativity

Consolidated vs. Unconsolidated Aquifers

Particle Sizes:

Particle Size Distribution:

Porosity ()

• Determines the amount of water that can be stored in a saturated aquifer.

Volume Total

Voids of Volume

Range of Values of Porosity

Unconsolidated Deposit Gravel Sand Silt ClayRocks Fractured basalt Karst limestone Sandstone Limestone, dolomite Shale Fractured crystalline rock Dense crystalline rock

25-4025-5035-5040-70

5-505-505-300-200-100-100-5

More pores, more water! But………..

Anisotropy: non-uniform flow resistance

Heterogeneity: changes in geological structures over small distances

Anisotropy and isotropy

Hydraulic Conductivities:

Determining Hydraulic Conductivity:

• Solve Darcy’s equation for K– K = -q/(dh/dl)

• Determine in lab using permeameter

• Determine in field with pumping tests or slug tests

Specific Yield and Retention

• Specific Yield (Sy)is the water that can be pumped from a well (or would drain under gravity) (Effective Porosity)

• Specific Retention (Sr) is the portion of water which cannot be pumped out of well and is left as film on rock surfaces

Storativity and Specific Yield

• Unconfined aquifer– Depth of water stored/unit decline in hydraulic head =

Specific yield, Sy ~ porosity, n

• Confined aquifer– Storativity, S, corresponds to Specific yield– Function of compressibility and proportional to aquifer

thickness

– Specific storage, Ss , = S/b• b= aquifer thickness

Types of Pumping Wells

• Dug (wide, typically only 10-30 feet deep)• Driven (1.25 to 12 inches diameter, 30-50 feet deep)

– Typically in areas with thick sand or gravel deposits and water table within 15 feet of surface)

• Drilled (most 100-400 feet deep)– Cable tool

• Percussion (5-10 feet section progresses)– (4-18 inches diameter up to 1000 feet deep; upper section +4” for

grouting; depthe depends on geologic formation)• Rotary

– (3-24 inches diameter, up to 1000 feet deep; upper section at least 2” wider)

– Jetted (2-12” diameter, up to 50 feet deep)• (Casing for at least 25 feet and grouted to 10 feet)

– Bored (2-30” diameter, up to 125 ft deep)

Well Components

• Well Cap• Well Casing• Well Sea• Well Slab• Pitless Adaptor• Water Service

• Pressure Tank• Drop Pipe• Well Screen• Gravel Pack• Pump

Well Construction

• Drilling Operation

• Installing the Casing and Screen

• Installing the Artificial Filter Pack

• Grouting/Sealing the Well

• Developing the Well

• Installing Permanent Pump

Types of Wells

• Drilled Wells– Deep (at least 100 feet, up to 1000 feet)– Most expensive to install– Installed with Rotary or Percussion drilling rigs

• Bored or Dug Wells– <50 feet deep (bored wells may exceed 100ft)– Only a few feet into water table (up to 10 ft for bored)– Wider diameter (dug 2-30 feet; bored 2-30 inches)

• Jetted Wells– Up to 100 feet– 2-12 inch diameter

Well Caps• Vented• At least 6-8 inches

above ground (generally 12” or more)

• Snug Fit• Aluminum or

Thermoplastic

UnacceptableProper

Well Casing

• Carbon Steel, PVC, or Stainless Steel– Determined by water chemistry and geology

• Diameter affects well cost and efficiency• Deep wells and wells >4 inches casing chosen on basis

of pump accommodation (nominally 2 sizes larger than pump; can be smaller below pump if screened much lower); for wells 4” or less 1 size larger than pump is adequate

• Purpose to keep well from filling in• Three forces:

– Tensile forces– Column loading forces– Collapse pressure

Drop Pipe• PE Pipe (generally single piece)

– Not rated for hot water– Must no be kinked

• Actual conveyance of water

Well Screen

• Slotted openings; preferably continuous and uninterrupted around circumference

• V-shaped widening inward

• Single metal construction or plastic to avoid galvanic corrosion

• Maximum open construction consistent with strength requirements

PITLESS ADAPTOR:

WELL GROUTING:

Pressure Tanks

• Help preserve pump life• Help maintain adequate pressure at tap• Tanks typically constructed of lined metal

– Older unlined tanks more susceptible to corrosion

• Newer tanks have diaphragm to minimize water logging– Old tanks would become water logged and

had to be drained and pumped with air

How pumps work

• Suction pumps– Think of a drinking straw– Limited by physics; at sea level atmospheric pressure only 14.7

PSI or 34 feet of water– Practical lift 22-25 feet at sea level; decrease by 1 foot for every

1000 feet elevation

• Jet Pumps (~32% U.S. homes, 8% double jet)– Shoot stream of water through small nozzle into venturi tube

creates partial vacuum

• Submersible Pumps (~60% U.S. homes)– Push water using series of impellers or turbines, rather than pull

• Air-lift pumps (uncommon)– Uses bubbling of air to cause water to rise

• Shallow Well Pumps (typically <30 feet)– Piston Pump– Rotary Pump– Centrifugal Pump– Self-priming Pump– Single Jet pump

• Deep Well Pumps– Double Jet Pump (production from up to 75-85 feet)– Submersible Pumps (up to 1000 feet)

• Turbine, Rotary, Jet Pump

– Deep Piston (up to 600 feet)

Pump Placement

• Never directly at bottom of well

• Typically 10-20 feet up from bottom of well

• Placing inside recharge zone may create cascading water situation leading to sediment buildup in the well

Well Development

• Mechanical surging (plunger)

• High velocity jetting (focused in small area)

• Backwashing (reversal of flow)

• Rawhiding (lift and drop; re-circulate for shallow wells)

• Hydrofracking (air or water)

Calculating Capacity

Water Quantity

• Insufficient supply of water leads to unsafe conditions:– Fire protection– Adequate supply drinking water– Interruption of flow leading to back siphonage

• Goal:– Sufficient supply at high enough pressure to assure

flow of water at any tap at any plumbing fixture on demand

Average Day Use in Gallons (gal)• Survival 2 quarts/day

• Basic needs 40-50 gallons/capita/day

• Average City Consumption 150gpcd–Domestic 65 gpcd–Commercial 28 gpcd–Industrial 37 gpcd–Public Use 20 gpcd

• Residence: 400gal/day (600gal/day)

DISTRIBUTION CAPACITY ISSUES:

When designing capacity of small water systems:

• include domestic use and typical lawn and garden space

• Assume residence use ultra low plumbing fixtures

• Adjust numbers based on anticipation of higher usage rates, or service by other irrigation source

DISTRIBUTION CAPACITY ISSUES:When designing capacity of small

water systems, you must consider:• Average daily use per residence• Peak daily use for each residence

– 1250 gallons/day (Eastern Washington)– 750 gallons/day (Western Washington)

• Maximum instantaneous demand (MID)– GPM needed to meet any expected short-term

demand within a system– Total depends on number of residences/uses;

first residence 15-20gpm, then 2 to 2.1 for each additional service

• Fireflow

Fireflow

• 10 GPM minimum– ¼ inch nozzle deliver 30 feet stream

• 25 GPM – 3/8 inch or 2 ¼ nozzles 40 feet vertical or 65

feet horizontal, respectively

Groundwater Flow

Henry Darcy’s experiment (1856)

q specific discharge Q / A

where K = hydraulic conductivity h/l = hydraulic gradient

Alternatively, Q = -K A (h/ l)

Darcy’s Law:

• q = -K • dh/dl– q is the specific discharge (darcy flux); the

amount of water flowing across a unit area perpendicular to the flow per unit time (Q/A)

– K is hydraulic conductivity– dh/dl is the hydraulic gradient or head

gradient; change in head over a distance l– Darcy’s law does not apply for turbulences

Flow velocity

Q = A q and q = Q/A = K dh/dL = specific discharge

Much of the cross-sectional area is made up of solids, actual velocity v >> q

Seepage Velocity

Q= q A = v A’

v = q A/A’ = q AL / A’L = q * V / void volume = q / = specific discharge / porosity= K (dh/dL) / = average linear velocity= seepage velocity

Introducing an arbitrary length of aquifer

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Ground Water Hydraulics

Datum, e.g., sea level

aquifer

confining layer

groundwater flow

Ground surface

Water table

Darcy’s Law for Groundwater Flow

hhAKQ 12

Q = flow rate (m3 / day)K = hydraulic conductivity or coefficient of permeability (m / day)A = cross-sectional area (m2) in vertical plane perpendicular to flow

Impact of Well on Groundwater Flow

Radial Flow to Well

Q=2πr (-Qr)

Qr = -dФ/dr

Darcy’s Law Applied to Wells

• Darcy’s Law – q = -K • dh/dx (or q = -K dh/dx)

• Qw = -K(dh/dr) • 2πrb = K(ds/dr) • 2πrb

– h = water table height– r = radius– b = thickness of the aquifer (confined)– s = drawdown

Drawdown

• Steady State Drawdowns1 – s2 = (Qw/2πKb) •ln (r2/r1)

• Radius of Influence (R)R = b • sqrt(K/2N)

• N = annual recharge by precipitation

Drawdown and Radius of Influence

Well Capture Zone

ZOI and ZOCZOI- Zone of Influence

ZOC- Zone of Contribution

Locating of Wells

• Away from septic tanks and sewage disposal

• Increased separation in highly permeable areas

• Run drainage away from well

• Locate well higher in grade than contamination sources

• Locate well far enough from buildings to allow access

• Locate away from flooding sites

• Locate away from neighboring wells

• Locate away from property line

• Be aware of overhead lines

granularmediaflow

No dispersion

Effect ofdispersion

With retardation

column

Contaminant Movement in Ground Water

Model system

porousplate

groundwater flow direction

Lines of equal contaminant concentration

Plan View

Vertical Section View

Point Source of Contaminant

Contaminant plume

NAPLs:

• DNAPLs-Dense Non-Aqueous Phase Liquids

• LNAPLs-Light Non-Aqueous Phase Liquids

Subsurface NAPL

Free-Phase (Mobile)

Residual (Trapped)

NAPLs:

Groundwater

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