Aquifer Storage Properties CVEG 5243 Ground Water Hydrology T. Soerens

h0h0 h2h2 h1h1 well Darcy’s Law (K) describes the shape of the drawdown curve at equilibrium We need more information to describe the time-dependent (“transient”) behavior Q drawdown curve drawdown cone cone of depression h0h0 h 0 - h = drawdown Pump tests are used to determine aquifer parameters (hydraulic conductivity and storage properties)

Specific Yield, S y for unconfined aquifers –Volume of water released under gravity drainage from a volume of initially saturated material, divided by the total volume of the material m 3 1 m 3 Specific Retention, S r –Volume of water retained... Released + retained = total –S y + S r = n range of S y = 0.01 clay to 0.30 gravel Aquifer Storage Properties (p.58ff)

Specific storage, S s –Volume of water released per unit volume from storage under a unit decline in hydraulic head. –i.e., if head drops 1m while 0.01 m 3 is released 0.01 m 3 1 m 3

upper confining layer lower confining layer b = 3m Storativity, (storage coefficient) for confined aquifers –S = volume of water released or taken into storage per unit of aquifer storage area per unit change in head. S = S s. b (unitless) –S s = volume of water released per 1 m 3 of aquifer volume per 1m change in head example: S s = 0.01 m -1 (0.01 m 3 released per 1m 3 box) –Aquifer volume per 1 m 2 of aquifer area = aquifer thickness = b example: = 3m x 1m 2 / 1m 2 = 3m = b 1 m 2 well 1 m 3 S s = 0.01 m -1 range of S = to (10 -3 to ) 3 boxes, each 1 m 3

Transmissivity, T –T = K. b Pump test –assume confined –calculate T, S –if S ~ or lower  confined –if S >  unconfined Water released from –unconfined: drainage S = S y –confined: water expansion - minimal S ~ m -1 compression of aquifer

Assumptions –uniform hydraulic conductivity –aquifer not stratified –saturated thickness constant unconfined - before pumping confined - constant –well is 100% efficient –fully penetrating –no slope of water table or potentiometric surface –laminar flow –infinite extent Well terms –static water level (SWL) –pumping water level (PWL) –drawdown dif between initial and PWL s = h - h 0 –residual drawdown (during recovery) after pumping stops, water level comes up. –well yield discharge from a well (gpm) –specific capacity well yield per unit drawdown –varies with time –radius of influence horizontal distance from center of well to limit of cone of depression (no drawdown) –gets larger with time –larger in confined Unconfined Confined Equilibrium Well Equations