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Groundwater -- Recharge and Discharge• Water is continually recycled through aquifer systems.• Groundwater recharge is any water added to the aquifer zone.• Processes that contribute to groundwater recharge includeprecipitation, streamflow, leakage (reservoirs, lakes,aqueducts), and artificial means (injection wells).• Groundwater discharge is any process that removes waterfrom an aquifer system. Natural springs and artificial wells areexamples of discharge processes.• Groundwater supplies 30% of the water present in ourstreams. Effluent streams act as discharge zones forgroundwater during dry seasons. This phenomenon is known asbase flow. Groundwater overdraft reduces the base flow, whichresults in the reduction of water supplied to our streams.S. Hughes, 2003

PerennialStream(effluent)(from Keller, 2000,Figure 10.5a)• Humid climate• Flows all year -- fed by groundwater base flow (1)• Discharges groundwaterS. Hughes, 2003

EphemeralStream(influent)(from Keller, 2000,Figure 10.5b)• Semiarid or arid climate• Flows only during wet periods (flashy runoff)• Recharges groundwaterS. Hughes, 2003

Groundwater -- Artesian Conditions•Water pressure in buildings ismaintained by a hydraulic head(h) and confinement of waterbeneath the pressure surface.•Natural artesian conditionsoccur when an aquifer isconfined by a saturated,impermeable clay layer (aquitardor aquiclude) below the slopingpressure surface.•An artesian well flowscontinually. It is produced when awell penetrates the clay layer andthe land surface is below thepressure surface.(from Keller, 2000, Figure 10.7)S. Hughes, 2003

Springs(from Keller, 2000, Figure 10.8)Discharge of groundwaterfrom a spring in California.Springs generally emerge atthe base of a hillslope.Some springs produce waterthat has traveled for manykilometers; while others emitwater that has traveled onlya few meters.Springs represent placeswhere the saturated zone(below the water table)comes in contact with theland surface.S. Hughes, 2003

Summary of Groundwater SystemsNOTE: Study each term, and the associated concepts andgeologic processes.(from Keller, 2000, Figure 10.9)S. Hughes, 2003

Groundwater Movement -- General ConceptsThe water table isactually a slopingsurface.Slope (gradient) isdetermined by thedifference in watertable elevation (h)over a specifieddistance (L).Direction of flow isdownslope.Flow rate depends onthe gradient and theproperties of theaquifer.(from Keller, 2000, Figure 10.6)S. Hughes, 2003

Groundwater MovementHYDRAULIC HEAD/ FLUID POTENTIAL = h (length units)• Measure of energy potential (essentially is a measure ofelevational/gravitational potential energy)• The driving force for groundwater flow• Water flows from high to low fluid potential or head (even if thismeans it may go "uphill"!)• Hydraulic head is used to determine the hydraulic gradientHydraulic head = the driving force that moves groundwater.The hydraulic head combines fluid pressure and gradient, andcan be though of as the standing elevation that water will rise toin a well allowed to come to equilibrium with the subsurface.Groundwater always moves from an area of higher hydraulichead to an area of lower hydraulic head. Therefore, groundwaternot only flows downward, it can also flow laterally or upward.S. Hughes, 2003

Groundwater MovementGeneral Concepts• Hydraulic gradient for anunconfined aquifer =approximately the slope ofthe water table.• Porosity = fraction (or %) of void space in rock or soil.• Permeability = Similar to hydraulic conductivity; a measure ofan earth material to transmit fluid, but only in terms of materialproperties, not fluid properties.• Hydraulic conductivity = ability of material to allow water tomove through it, expressed in terms of m/day (distance/time). Itis a function of the size and shape of particles, and the size,shape, and connectivity of pore spaces.S. Hughes, 2003

Groundwater MovementDetermine flow direction from well data:Well #1 4252’ elevdepth to WT = 120’Well #2 4315’ elev(WT elev = 4132’) depth to WT = 78’4180 4200 4220 4240(WT elev = 4237’)1. Calculate WT41404160418042003. Connectcontours ofequal elevation.4. Draw flow linesperpendicular tocontours.4220424042604280Well #3 4397’ elevdepth to WT = 95’(WT elev = 4302’)426042804300elevations.2. Interpolatecontourintervals.NS. Hughes, 2003

Groundwater Movement -- Cone of Depression(from Keller, 2000, Figure 10.10)Water tableflowflowCone of depressionPumping water from a well causes a cone of depression toform in the water table at the well site.S. Hughes, 2003

Groundwater MovementPOROSITY = Φ or n (units - fraction or %)= fraction of void space (empty space) in soil or rock.Represents the path water molecules can follow in thesubsurfacePrimary porosity - intergranularSecondary porosity - fractures, faults, cavities, etc.Porosity = volume of pore space relative to the total volume(rock and/or sediment + pore space). Primary porosity (%pore space) is the initial void space present (intergranular)when the rock formed. Secondary porosity (% added byopenings) develops later. It is the result of fracturing, faulting, ordissolution. Grain shape and cementation also affect porosity.S. Hughes, 2003

Groundwater MovementPERMEABILITY is the capability of a rock to allow the passageof fluids. Permeability is dependent on the size of pore spacesand to what degree the pore spaces are connected. Grain shape,grain packing, and cementation affect permeability.SPECIFIC YIELD (S y ) is the ratio of the volume of water drainedfrom a rock (due to gravity) to the total rock volume. Grain sizehas a definite effect on specific yield. Smaller grains have largersurface area/volume ratio, which means more surface tension.Fine-grained sediment will have a lower S y than coarse-grainedsediment.SPECIFIC RETENTION (S r ) is the ratio of the volume of water arock can retain (in spite of gravity) to the total volume of rock.Specific yield plus specific retention equals porosity (oftendesignated with the Greek letter phi):

Groundwater MovementMovement of groundwater depends on rock and sedimentproperties and the groundwater’s flow potential. Porosity,permeability, specific yield and specific retention are importantcomponents of hydraulic conductivity.HYDRAULIC CONDUCTIVITY = K (or P)units = length/time (m/day)Ability of a particular material to allow water to passthrough itThe definition of hydraulic conductivity (denoted "K" or "P" inhydrology formulas) is the rate at which water moves throughmaterial. Internal friction and the various paths water takes arefactors affecting hydraulic conductivity. Hydraulic conductivity isgenerally expressed in meters per day.S. Hughes, 2003

WELL SORTEDCoarse (sand-gravel)POORLY SORTEDCoarse - FineWELL SORTEDFine (silt-clay)Permeability and Hydraulic ConductivityHighLowSorting of material affects groundwater movement. Poorly sorted(well graded) material is less porous than well-sorted material.S. Hughes, 2003

Groundwater MovementTable 10.6 in textbook (Keller, 2000)Porosity and hydraulic conductivity of selected earth materialsHydraulicPorosity ConductivityMaterial (%) (m/day)UnconsolidatedClay 45 0.041Sand 35 32.8Gravel 25 205.0Gravel and sand 20 82.0RockSandstone 15 28.7Dense limestone or shale 5 0.041Granite 1 0.0041S. Hughes, 2003

Groundwater MovementThe tortuous path of groundwater molecules through anaquifer affects the hydraulic conductivity. How do the followingproperties contribute to the rate of water movement?• Clay content andadsorptive properties• Packing density• Friction• Surface tension• Preferred orientationof grains• Shape (angularity orroundness) of grains• Grain size• Hydraulic gradientS. Hughes, 2003

Groundwater Flow NetsWater table contour lines are similar to topographic lines on amap. They essentially represent "elevations" in the subsurface.These elevations are the hydraulic head mentioned above.Water table contour lines can be used to determine thedirection groundwater will flow in a given region. Many wellsare drilled and hydraulic head is measured in each one. Watertable contours (called equipotential lines) are constructed tojoin areas of equal head. Groundwater flow lines, whichrepresent the paths of groundwater downslope, are drawnperpendicular to the contour lines.A map of groundwater contour lines with groundwater flow linesis called a flow net.Remember: groundwater always moves from an area of higherhydraulic head to an area of lower hydraulic head, andperpendicular to equipotential lines.S. Hughes, 2003

Groundwater Flow Nets100 50QalA simple flow netCross-profile viewAquitard (granite)QalWTQalwellAquitard• Effect of aproducing well• Notice theapproximatediameter of thecone of depressionS. Hughes, 2003

Groundwater Flow NetsAquitard100 90 80Aquitard70QalWater table contoursWater is flowing from Qalto granite100908070QalWater is flowing fromgranite to QalDistorted contours mayoccur due to anisotropicconditions (changes inaquifer properties).Area of high permeability (high conductivity)S. Hughes, 2003

Groundwater Flow NetsWater table contours inDRAINAGEBASINWTcontoursFlow linesNdrainage basins roughlyfollow the surfacetopography, but dependgreatly on the properties ofrock and soil that composethe aquifer:• Variations in mineralogyand texture• Fractures and cavities• Impervious layers• ClimateDrainage basins are often used to collect clean,unpolluted water for domestic consumption.S. Hughes, 2003

Groundwater Flow Net414412410NWater Flow Lines408Water Table Contours406404Well402400

Groundwater Movement -- Darcy’s LawQ = KIA -- Henry Darcy, 1856, studied water flowing throughporous material. His equation describes groundwater flow.Darcy’s experiment:• Water is applied underpressure through endA, flows through thepipe, and discharges atend B.• Water pressure ismeasured usingpiezometer tubesHydraulic head = dh (change in height between A and B)Flow length = dL (distance between the two tubes)Hydraulic gradient (I) = dh / dLS. Hughes, 2003

Groundwater Movement -- Darcy’s LawThe velocity of groundwater is based on hydraulicconductivity (K), as well as the hydraulic head (I).The equation to describe the relations between subsurfacematerials and the movement of water through them isQ = KIAQ = Discharge = volumetric flow rate, volume of water flowingthrough an aquifer per unit time (m 3 /day)A = Area through which the groundwater is flowing, crosssectionalarea of flow (aquifer width x thickness, in m 2 )Rearrange the equation to Q/A = KI, known as the flux (v),which is an apparent velocityActual groundwater velocity is higher than that determinedby Darcy’s Law.S. Hughes, 2003

Groundwater Movement -- Darcy’s LawFLUX given by v = Q/A = KI is the IDEAL velocity ofgroundwater; it assumes that water molecules can flow in astraight line through the subsurface.NOTE: Flux doesn't account for the water molecules actuallyfollowing a tortuous path in and out of the pore spaces. Theytravel quite a bit farther and faster in reality than the flux wouldindicate.DARCY FLUX given by vx = Q/An = KI/n (m/sec) is theACTUAL velocity of groundwater, which DOES account fortortuosity of flow paths by including porosity (n) in thecalculation. Darcy velocity is higher than ideal velocity.Darcy’s Law is used extensively in groundwater studies. It canhelp answer important questions such as the direction apollution plume is moving in an aquifer, and how fast it istraveling.S. Hughes, 2003

Groundwater OverdraftOverpumping will have two effects:1. Changes the groundwater flowdirection.2. Lowers the water table, making itnecessary to dig a deeper well.• This is a leading cause fordesertification in some areas.• Original land users and land ownersoften spend lots of money to drill new,deeper wells.• Streams become permanently dry.S. Hughes, 2003

Groundwater Overdraft• Almost half the U.S. population uses groundwater as aprimary source for drinking water.• Groundwater accounts for ~20% of all water withdrawn forconsumption.• In many locations groundwater withdrawal exceeds naturalrecharge rates. This is known as overdraft.• In such areas, the water table is drawn down "permanently";therefore, groundwater is considered a nonrenewableresource.•The Ogallala aquifer underlies Midwestern states, includingTexas, Oklahoma, and New Mexico, while California, Arizonaand Nevada use the Colorado River as their primary watersource. All show serious groundwater overdraft.S. Hughes, 2003

Groundwater Overdraft in the Conterminous U.S.(from Keller, 2000, Figure 10.13a)S. Hughes, 2003

• Water-level changes in theTexas--Oklahoma-High Plainsarea.•The Ogallala aquifer --composed of water-bearingsands and gravel that underlieabout 400,000 km 2 .• Water is being used forirrigation at a rate up to 20times more than naturalrecharge by infiltration.• Water level (water table) inmany parts has declined andthe resource eventually maybe used up.Groundwater Overdraft(from Keller, 2000, Figure 10.13b)S. Hughes, 2003

artesian aquiferaquifercone of depressionconfined aquiferDarcy's Law (all terms)dischargeeffluent streamflow linesflow netgroundwaterhydraulic conductivityhydraulic gradienthydraulic headinfiltrationinfluent streamoverdraftGroundwater Termsoverland flowperched aquiferpermeabilityporesporosityrechargeresidence timesoil waterspecific retentionspecific yieldspringunconfined aquifervadose zonewater tablewater table contour linesS. Hughes, 2003