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

after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm

Unconfined Aquifer Water Table: Subdued replica of the topography

Cones of depression, Sacramento Valley Criss & Davisson 1996, after DWR 1986

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.

after Driscoll, FG (1986) http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm

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

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”

Hubbert (1940)

after Darton 1909

Potentiometric Surface, Dakota Aquifer

Black Hills

Cedar Bog, OH

Criss

Cedar Bog, OH

h

Criss

h

Cedar Bog, OH Q

Criss

h

Cedar Bog, OH

after Fetter http://www.uwsp.edu/water/portage/undrstnd/aquifer.htm

Unconfined Aquifer Water Table: Subdued replica of the topography

Fetter after MK Hubbert (1940) http://www.wda-consultants.com/java_frame.htm?page17

Flowlines Equipotential sfc

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

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

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

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

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

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!