Aquifer Tests and Wells

Question 1

What type of solution does the Theis equation give for radial flow of ground water?

Answer 1

An exact analytical solution

Based on assumptions:
1. Constant Transmissivity
2. water withdrawn from

Question 2

Theis Equation for Transmissivity

Answer 2

T = 15.3 Q W(u) / s
Q - pumping rate
s - drawdown
W(u) - well function of u
u - 360 r^2 S / T t

ft^2 / day

Question 3

Theis Equation for Storativity

Answer 3

S = T t u / 360 r^2
t - time
r - distance from pumpin well to observation well
u - 360r^2 S / T t

Question 4

Time Drawdown Method/Jacob Method

Answer 4

Drawdown generated in pumping well is noted in observation well. Drawdown values plotted on semi-log paper as function of time. Do a straight line fit throught the points farthest from the pumping well because these points are more accurate. Aquifer parameters calculated using the slope of the straight line and the intercept of the line with the time axis when drawdown is zero.

Question 5

Time Drawdown Method Transmissivity Equation

Answer 5

T = 35 Q / ds
ds - the change in drawdown per one log cycle
Q - pumping rate

ft^2 / day

Question 6

Time Drawdown Method Storativity Equation

Answer 6

S = T to / 640 r^2
T - transmissivity
to - intercept of best fit line with time axis
r - distance from pumping well to observation well

Question 7

Distance Drawdown Method

Answer 7

Can be used if the drawdowns in three or more observation wells are measured simultaneously. The drawdown will vary with distance from pumping well. Drawdown plotted on arithmetic scale, distance plotted on log scale. Values read from graph are ds and ro after straight line fit.

Question 8

Distance Drawdown Method Transmissivity Equation

Answer 8

T = 70 Q / ds

ft^2 / day

Question 9

Distance Drawdown Method Storativity Equation

Answer 9

S = T t / 640 ro^2

Question 10

Well Efficiency

Answer 10

Ratio of the theoretical drawdown to the actual drawdown. Can be estimated from the distance drawdown graph.

Question 11

Well Efficiency Equation

Answer 11

W.E. = St / Sa x 100
St - theoretical drawdown
Sa - actual drawdown

Question 12

Common well efficiency values

Answer 12

Maximum of about 80%, 60% is realistic

Question 13

Specific Capacity

Answer 13

a measure of the pumping rate (gpm) divided by the drawdown (ft). If specific capacity is known, Drawdown at different pumping rates can be estimated.

100 gpm rate with 20 ft drawdown = 5 gpm/ft SC

Question 14

Water quality parameters that cause corrosion

Answer 14

pH < 7
TDS > 1000 mg/L
DO > 2 mg/L
CO2 > 50 mg/L
H2S @ 1 mg/L
Chloride > 500 mg/L

Question 15

Water quality parameters that cause passivation

Answer 15

pH > 7.5
Iron Precipitation > 0.5 mg/L
Oxygen > 0.2 mg/L (manganese precip)

Question 16

What does the Baylis Curve show?

Answer 16

the small range of values of pH and alkalinity that allow for quality drinking water.