Principles of Hydrogeology Flashcards
Base Flow
The amount of stream flow that can be attributed to groundwater inflow.
Darcy
9.87 x 10-9 cm2 - Unit of intrinsic permeability
Darcy’s Law
Q = KiA
ne
Effective Porosity
Fraction of the total volume of an aquifer material that consists of interconnected pore space, = specific yield
Field Capacity
amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. It is time dependent.
K
Hydraulic conductivity
Capacity of a porous medium to transmit water. Units (L/T)
i
Hydraulic gradient,
Rate of change in total hydraulic head per unit distance, = (delta)h/L
ki (or just k)
intrinsic permeability
property of the porous medium that measure the relative ease with which a fluid can be transmitted through it under a hydraulic gradient. Measured in darcys (units L2)
Darcy’s law (permeability form)
Q = (A · k · ΔP)/(μ · Δx)
Q = flow rate (cm3/sec) (L3/T)
μ = fluid viscosity (centipoise, cp, 0.01g/cm · s) (M/L · T)
k = permeability (Darcys, cm2) (L2)
A = cross-section area (cm2) (L2)
ΔP = change in pressure (atm) (M/L2)
Δx = distance (cm) (L)
Matric Potential
The pressure (tension) exerted on the pore water due to the soi-water attraction
Meinzer
Unit of hydraulic conductivity measured in gpd/ft^2.
n
Porosity
Fraction of the bulk volume of a rock or soil that is occupied by void space (unitless)
Sr
Specific Retention
Ratio of volume of water a soil or rock can retain against gravity drainage to the total volume of soil or rock (unitless)
Ss
Specific Storage
Coefficient relating the amount of water stored/released per unit head change in a fully confined (fully saturated aquifer)
For such an aquifer, S = Ss x b
Amount of water per unit volume of a saturated formation that is store or expelled from storage due to compressibility of the mineral skeleton and pore water per change in unit head (units L-1)
Q
Volumetric flow rate. Units L3/T
S
Storativity or Storage Coefficient
Volume of water that a permeable unit release from or takes into storage per unit surface area of the aquifer per unit change in head (unitless)
S = Vw / A · dh
S = Ss · b + Sy
Sy
Specific Yield
Ratio of the volume of water that drains from a saturated soil or rock by gravity to the total volume of the soil or rock (unitless)
In unconfined aquifer, Sy = S
Water table (pressure definition)
The surface within uncinfied groundwater at which the hydraulic pressure is equal to atmospheric pressure.
Acres per square mile
640 acres/square mile
Basin Storage Equation
dS = P - E ± R ± U
dS = change in storage
P = Precipitation
E = Evapotranspiration
R = Runoff (net change in SW storage)
U = Underflow (net change in GW storage)
Permeability of confining layer
Less than approx. 10-2 darcy
T
T = Transmissivity = K · b
T units gpd/ft (L3/T · L or L2/T)
where b = aquifer saturated thickness
Ability of an aquifer to transmit water
K of clay
10-5 to 10-2 gpd/ft2
K of clean sand
101 to 105 gpd/ft2
K of silt
10-2 to 102 gpd/ft2
K of glacial till
10-5 to 101 gpd/ft2
K of gravel
104 to 107 gpd/ft2
K of silty sand
100 to 104 gpd/ft2
Transmissivity form of Darcys Law
Q = T · i · w
where
T = transmissivity
i = hydraulic gradient
w = average width of aquifer
Dupuit’s Law - formula and what is it for?
Q = 1/2 · K · (H12 - H22)/l · w
where
H = height of water table at a well above an impermeable surface that serves as bottom of confined aquifer
l = distance between wells
w = aquifer width
For computing flow rate (Q) in unconfined aquifers
Vd (with Darcy’s law formula)
Specific discharge or discharge velocity
Vd = K · i
vs
Seepage velocity
The velocity at which water moves through pore spaces in an aquifer
vs = K i / ne
Saltwater Intrusion equation
Z = 40h
where, at a given location
Z = depth below sea level where the freshwater-salt water interface exists
h = the height above sea level of the potentiometric surface
Darcy’s Law for cross-sectional flow into a flow net
Q = K · H · (nf / nd) · w
where
Q = Discharge rate (L3/T)
K = hydraulic conductivity (L/T)
H = total head loss through the system
nf = number of flow tubes in the system
nd = number of equipotential lines (head drops) in system
w = width of the feature into the cross-section
K of unfractured igneous and metamorphic rocks
10-7 to 10-3
K of sandstone
10-3 to 101
K of limestone and dolomite
10-2 to 101
K of fractured igneous and metamorphic rocks
10-1 to 103
K of permeable basalt
100 to 105
K of karst limestone
101 to 105
ne of gravel
0.1-0.35
ne of clay
0.01 to 0.20
ne of sand - coarse, medium, fine
coarse: 0.20 to 0.35
medium: 0.15 to 0.30
fine: 0.10 to 0.30
ne of silt
0.01 to 0.30
ne of sandstone
0.10 to 0.35
ne of limestone
0.01 to 0.20
ne of unfractured crystalline rock
0.00 to 0.05
ne of fractured crystalline rock
0.00 to 0.10
Density of water (imperial)
62.4 lbs / ft3
Darcy flux or specific discharge
q = Q/A = K · i
Units (L3/T) / L2 (cubic feet / square foot x day) or just L/T (feet/day)
Also referred to as Darcy velocity or apparent velocity, but really isn’t a velocity.
Bulk density relative to porosity
n = 1-(ρb/ρparticle)
where
ρb = bulk density
ρparticle = density of soil matrix material (for example, sand)
Relationship between K (gpd/ft^2) and k (darcys = cm2)
Range of reasonable K (gpd/ft^2) is about 2 orders of magnitude higher than k (cm2 = darcy)
