Principles of Hydrogeology

Question 1

Base Flow

Answer 1

The amount of stream flow that can be attributed to groundwater inflow.

Question 2

Darcy

Answer 2

9.87 x 10^-9 cm² - Unit of intrinsic permeability

Question 3

Darcy’s Law

Answer 3

Q = KiA

Question 4

n_e

Answer 4

Effective Porosity
Fraction of the total volume of an aquifer material that consists of interconnected pore space, = specific yield

Question 5

Field Capacity

Answer 5

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.

Question 6

K

Answer 6

Hydraulic conductivity
Capacity of a porous medium to transmit water. Units (L/T)

Question 7

i

Answer 7

Hydraulic gradient,
Rate of change in total hydraulic head per unit distance, = (delta)h/L

Question 8

k_i (or just k)

Answer 8

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)

Question 9

Darcy’s law (permeability form)

Answer 9

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)

Question 10

Matric Potential

Answer 10

The pressure (tension) exerted on the pore water due to the soi-water attraction

Question 11

Meinzer

Answer 11

Unit of hydraulic conductivity measured in gpd/ft^2.

Question 12

n

Answer 12

Porosity
Fraction of the bulk volume of a rock or soil that is occupied by void space (unitless)

Question 13

S_r

Answer 13

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)

Question 14

S_s

Answer 14

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 = S_s 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)

Question 15

Q

Answer 15

Volumetric flow rate. Units L3/T

Question 16

S

Answer 16

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 = V_w / A · dh

S = S_s · b + S_y

Question 17

S_y

Answer 17

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, S_y = S

Question 18

Water table (pressure definition)

Answer 18

The surface within uncinfied groundwater at which the hydraulic pressure is equal to atmospheric pressure.

Question 19

Acres per square mile

Answer 19

640 acres/square mile

Question 20

Basin Storage Equation

Answer 20

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)

Question 21

Permeability of confining layer

Answer 21

Less than approx. 10^-2 darcy

Question 22

T

Answer 22

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

Question 23

K of clay

Answer 23

10^-5 to 10^-2 gpd/ft²

Question 24

K of clean sand

Answer 24

10¹ to 10⁵ gpd/ft²

Question 25

K of silt

Answer 25

10^-2 to 10² gpd/ft²

Question 26

K of glacial till

Answer 26

10^-5 to 10¹ gpd/ft²

Question 27

K of gravel

Answer 27

10⁴ to 10⁷ gpd/ft²

Question 28

K of silty sand

Answer 28

10⁰ to 10⁴ gpd/ft²

Question 29

Transmissivity form of Darcys Law

Answer 29

Q = T · i · w
where
T = transmissivity
i = hydraulic gradient
w = average width of aquifer

Question 30

Dupuit’s Law - formula and what is it for?

Answer 30

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

Question 31

V_d (with Darcy’s law formula)

Answer 31

Specific discharge or discharge velocity
V_d = K · i

Question 32

v_s

Answer 32

Seepage velocity
The velocity at which water moves through pore spaces in an aquifer
v_s = K i / n_e

Question 33

Saltwater Intrusion equation

Answer 33

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

Question 34

Darcy’s Law for cross-sectional flow into a flow net

Answer 34

Q = K · H · (n_f / n_d) · w
where
Q = Discharge rate (L3/T)
K = hydraulic conductivity (L/T)
H = total head loss through the system
n_f = number of flow tubes in the system
n_d = number of equipotential lines (head drops) in system
w = width of the feature into the cross-section

Question 35

K of unfractured igneous and metamorphic rocks

Answer 35

10^-7 to 10^-3

Question 36

K of sandstone

Answer 36

10^-3 to 10¹

Question 37

K of limestone and dolomite

Answer 37

10^-2 to 10¹

Question 38

K of fractured igneous and metamorphic rocks

Answer 38

10^-1 to 10³

Question 39

K of permeable basalt

Answer 39

10⁰ to 10⁵

Question 40

K of karst limestone

Answer 40

10¹ to 10⁵

Question 41

n_e of gravel

Answer 41

0.1-0.35

Question 42

n_e of clay

Answer 42

0.01 to 0.20

Question 43

n_e of sand - coarse, medium, fine

Answer 43

coarse: 0.20 to 0.35
medium: 0.15 to 0.30
fine: 0.10 to 0.30

Question 44

n_e of silt

Answer 44

0.01 to 0.30

Question 45

n_e of sandstone

Answer 45

0.10 to 0.35

Question 46

n_e of limestone

Answer 46

0.01 to 0.20

Question 47

n_e of unfractured crystalline rock

Answer 47

0.00 to 0.05

Question 48

n_e of fractured crystalline rock

Answer 48

0.00 to 0.10

Question 50

Density of water (imperial)

Answer 50

62.4 lbs / ft³

Question 51

Darcy flux or specific discharge

Answer 51

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.

Question 52

Bulk density relative to porosity

Answer 52

n = 1-(ρ_b/ρ_particle)
where
ρ_b = bulk density
ρ_particle = density of soil matrix material (for example, sand)

Question 53

Relationship between K (gpd/ft^2) and k (darcys = cm2)

Answer 53

Range of reasonable K (gpd/ft^2) is about 2 orders of magnitude higher than k (cm2 = darcy)