Aquifer Types - epsc428.wustl.edu
Transcript of Aquifer Types - epsc428.wustl.edu
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Confined Aquifer: aquifer between two aquitards. = Artesian aquifer if the water level in a well rises above aquifer
= Flowing Artesian aquifer if the well level rises above the ground surface. e.g., Dakota Sandstone: east dipping K sst, from Black Hills- artesian)
Unconfined Aquifer: aquifer in which the water table forms upper boundary. “Water table aquifer” At the water table: Head h = z and P = 1 atm e.g., Missouri, Mississippi & Meramec River valleys Hi yields, good quality Ogalalla Aquifer (High Plains aquifer): CO KS NE NM OK SD QT Sands & gravels, alluvial apron off Rocky Mts.
Perched Aquifer: unconfined aquifer above main water table; Generally above a lens of low-k material.
Hydrostratigraphic Unit: e.g. MO, IL C-Ord sequence of dolostone & sandstone capped by Maquoketa shale
Aquifer Types
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after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm
Unconfined Aquifer Water Table: Subdued replica of the topography
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Cones of depression, Sacramento Valley Criss & Davisson 1996, after DWR 1986
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after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm
Perched Aquifer: unconfined aquifer above main water table
Typically above a lens of low-k material.
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after Driscoll, FG (1986) http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm
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after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm
Aquifer
Confined Aquifer: Aquifer between two aquitards. = Artesian aquifer if the water level in a well rises above aquifer
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Potentiometric Surface Map of the hydraulic head Imaginary surface of level to which water would rise in non-pumping well, or “piezometer”, cased to the aquifer
For an Unconfined Aquifer,
Potentiometric Sfc ~ Water Table
= Water Table for vertical equipotential planes (horizontal flow only)
For a Confined Aquifer; there is no “Water Table”
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Hubbert (1940)
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after Darton 1909
Potentiometric Surface, Dakota Aquifer
Black Hills
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Cedar Bog, OH
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Criss
Cedar Bog, OH
h
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Criss
h
Cedar Bog, OH Q
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Criss
h
Cedar Bog, OH
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after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm
Unconfined Aquifer Water Table: Subdued replica of the topography
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Fetter after MK Hubbert (1940) http://www.wda-consultants.com/java_frame.htm?page17
Flowlines Equipotential sfc
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GRADIENT LAWS!!!
Darcy’s Law!!!!
Fourier’s Law ! of Heat Flow!!!Fick’s Law ! of Diffusion!!
!! Negative sign: flow is down gradient
!q = !K "h"x
!h!t
= TS
"2h "x2
!J = !k "T
"x !T
!t= k
"c "
2T "x2
!F = !D "C
"x !C
!t= D "
2C "x2
“DIFFUSION” EQUATIONS!!
!Hydraulic!Diffusion!
!!
! !!! ! ! ! Heat !Diffusion!
!!
! ! ! ! Chemical!Diffusion!
!
!!
1-D
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GRADIENT LAWS!!!
Darcy’s Law!!!!
Fourier’s Law ! of Heat Flow!!!Fick’s Law ! of Diffusion!!
!!
Negative sign: flow is down gradient
!q = !K"h !h!t
= TS
"2h
!J = !k"T !T
!t= k
"c "2T
!F = !D"C !C
!t= D "2C
“DIFFUSION” EQUATIONS!!
!Hydraulic!Diffusion!
!!
! !!! ! ! ! Heat !Diffusion!
!!
! ! ! ! Chemical!Diffusion!
!
!!
3-D
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Storativity S Units: dimensionless
S = Volume water/unit area/unit head drop = "Storage Coefficient” e.g., Heath p. 28
Transmissivity T = K*m m = aquifer thickness Units m2/sec
= Rate of flow of water thru unit-wide vertical strip of aquifer under a unit hydraulic gradient
T ≥ 0.015 m2/s in a good aquifer
Hydraulic Diffusivity (D): Units m2/s Freeze & Cherry p. 61
D = T/S Transmissivity /Storativity
= K/Ss Hydraulic Conductivity/ Specific Storage for a confined aquifer
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Storativity Parameters
Ss = specific storage Units: 1/length = Volume H2O released from storage /unit vol. aquifer /unit head drop (F&C p. 58)
Ss = ρ g (B + φ β) where B= aquifer compressibility ~ 10-5 /m for sandy gravel β = water compressibility φ = porosity
Sy = Specific yield Units: dimensionless = storativity for an unconfined aquifer "unconfined storativity" = Vol of H2O drained from storage/total volume rock (D&S, p. 116)
= Vol of H2O released (grav. drained) from storage/unit area aquifer/unit head drop Sy = Vwd/VT Typically, Sy = 0.01 to 0.30 Sy increases with grain size, becoming large for gravels Fetter p. 79; F&C, p. 61
Specific retention Sr = Vwr/VT Sr decreases with grain size, becoming large for clays, with Sr >>Sy Fetter p. 79
φ = Sy + Sr + unconnected porosity
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Storativity S Units: dimensionless
S = Volume water/unit area/unit head drop = "Storage Coefficient"
S = Sy + m Ss unconfined; note Sy >> mSs = Sy + h Ss
S = m Ss confined aquifer
For typical unconfined aquifer, S = 0.01 to 0.3 ~ Sy For typical confined aquifer, S = 0.005 to 0.00005
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Confined Aquifer:!!1) Get large changes in pressure (head) with ~ no change in the thickness ! ! ! ! ! of the saturated column. Potentiometric sfc remains above the unit. !!2) Get large head drop for a given amount of production, as Ss is very small.!!3) Low Storativity S= Ss*m , dimensionless, where Ss = specific storage (units: m-1) !
! ! ! ! S ~ 0.005 to 0.0005 for typical confined aquifers!
Unconfined Aquifer: Water table aquifer 1) Get large production for a given head drop,
because the Specific Yield Sy is large (~0.25).
2) High Storativity S = Sy + Ss*h ≅ Sy, commonly Dimensionless (eq 4.33 Fetter)
3) Easily contaminated
4) Artesian flow possible
Aquifer Properties!