6) Groundwater Storage

what happens when we pump unconfined aquifers?

drain the water

dewater the pores

water table is lower

what happens when we pump confined aquifers?

decreased water pressure, compacting aquifer causing ground cracks, fissures, subsidence

what are 2 things needed for land subsidence to happen?

compressible sediment

extraction of water out of aquifer

is groundwater a renewable resource?

depends on climate and geology

renewable: shallow aquifers that are actively recharged (rain, snowmelt, surface water)

nonrenewable: low annual recharge, old water, long residence time, isolated aquifers 

arid and permafrost regions don’t get much recharge

difference between aquiclude and aquitard

aquiclude are impermeable

aquitard have low permeability and restrict the flow of water but are not impermeable

in an unconfined aquifer, what does the total head at the water table represent

z, because pressure head at the water table is 0

in a confined aquifer, what does the potentiometric surface represent?

total head 

pressure + elevation

where does water come from in confined aquifer?

storage/release of H2O occurs through compressibility of fluid and medium

equal to specific yield (plus a bit of specific storage*thickness but its not that much)

where does water come form in unconfined aquifer?

storage/release of H2O occurs primarily through dewatering of pores

equal to thickness times specific storage

definition of specific yield

volume of water released by gravity drainage per unit surface are per unit decline in the water table

Sy = n-Sr = ∆Vw / (A*∆h)

unitless

definition of specific retention

ratio of volume of water a soil retains after gravity drainage to teh total volume of soil

Sr = Vwr / VT = n-Sy

unitless

the coarser the material ___ (Sy and n)

why

the closer Sy is to n

less is retained with coarser material

capillary forces mean some water is retained in the sediment after drainage

definition of storativity

volume of water released from storage per unit surface area of the aquifer per unit decline in hydraulic head

accounts for aquifer thickness (b)

S = SsB

unitless

definition of specific storage

volume of water released from storage per unit volume of the aquifer per unit decline in hydraulic head

SS = S/b

units 1/Length

if we lower the HH by an amount, how much water actually comes out in volume
goal is to calculate storativity

how can we add more water to the REV after it’s already saturated?

increase pore pressure, how?

water is slightly compressible, higher pressure means more mass per volume, higher density of water

sediments behave elastically (like a sponge), but may not be reversible, undergoing pressure, solid particles rearrange themselves to accommodate more water, resulting in increased porosity

info on force and pressure

pressure is force over area

N = kg*m/s²

details on compressibility of water (definition)

approx: 4.47×10^-10

change in water volume over volume related to pressure

Bw = -(∆Vw / Vw) / ∆P = -(1/Vw)*(dVw/dP)

units are 1/Pa

details on compressibility of solids

individual solid grains or rock matrix are highly incompressible
considered negligible

details on compressibility of aquifer matrix (definition)

aquifer matrix represents interconnected individual grains that form the void space

change in total volume/ total volume / change in effective stress

alpha = -(∆VT/VT)/∆oe = -(1/VT)*(dVt/doe)

oe is pressure or effective stress acting on the matrix

what 2 things support the weight of the column of soil and water?

what is effective stress

forces across grain-to-grain contacts (this is effective stress)
distributed forces

pore water pressure

details on total stress

fluid pressure + effective stress 
since total stress is constant, then a decrease in fluid pressure = increase in effective stress, and vice versa

oT = oe + P = oe + pgW = pgb

P = density of water * gravity * pressure head 

b=length

if head decreases, what happens to effective stress?

what effect would pumping have

increases

increase, removing water, decreasing head

what type of grain is the most compressible

clay

canyou get more water out of a confined or unconfined aquifer?

unconfined

what happens when you pump a confined aquifer

reduce pore water pressure

lowers potentiometric surface

increases effective stress (total stress unchanged)

if storativity is a large number it’s probably ___?

unconfined aquifer

what are acceptable K values for gravel, sand, silt, clay (in log)

gravel: -2

sand: -3 to -2

silt: -5 to -4

clay: -8 or smaller

what are acceptable storage ranges for confined and unconfined aquifers

and effective porosity estimates

specific storage confined: 0.0001 or less

specific yield unconfined: 0.05 to 0.2

effective porosity: 0.1 to 0.2

definition of transmissivity

amount of water that can be transmitted horizontally through a unit width by full saturated thickness of. theaquifer under a hydraulic gradient of 1

T = K*b

transmissivities greater than 0.015 m²/s are good aquifers for water extraction

Out of K, Ss, T, and S, which are material properties and which are aquifer properties?

K and Ss are material properties

T and S are aquifer properties