Module 6

Question 1

Static GW level

Answer 1

Level water rises to in a well. Ground elev-Depth to Water table=Water Table Elevation. Table cannot be actively pumped while measuring.

Question 2

Distribution of water among all sources on earth

Answer 2

Ocean-97%
Ice caps-2.3%
Groundwater-0.6%
Lakes-0.09%
Rivers-0.009%
Atmosphere-0.001%

Question 3

What forms as a result of water table pumping

Answer 3

A zone of depression

Question 4

Where are springs and rivers located

Answer 4

Where surface intersects ground table.

Question 5

What forms when excess irrigation water enters the water table

Answer 5

Recharge Mound, where water table elevation is locally elevated.

Question 6

How to determine water flow vector on a water table topo map

Answer 6

Water flow vector is perpendicular to topo line (contour line)

Question 7

perennial vs ephemeral stream

Answer 7

Perennial-flowing throughout year
ephemeral-arid areas, flows sometimes throughout year

Question 8

Water table topography near rivers

Answer 8

> ’s upslope, similar to topography V.

Question 9

How does a geologist gather data to form a water topo map

Answer 9

Sets of 3 wells are used as data for 3 point problems which determines topo lines
TOC-Top of casing, Feet between surface and sea level
DTW-Depth to water, feet between surface and water table
WSE-Water Surface Elevation, feet between water table and sea level

Question 10

How do 3 point problems work generally

Answer 10

You go uphill, make a well, go downhill from that, make a well, then go halfway between then turn 90 degrees, walk out to reach a 3rd point such that you could make an equilateral triangle between them. Calculate Water Surface Elevation and find water contours. This will be a small scale interpretation, and many 3 point problems will be needed for a map.

Question 11

Aquifer

Answer 11

A body of water within sediment/rock with enough pores and is interconnected such that, if pumped through a well, would produce economically viable volumes of clean water.
This definition has economic, chemical and geologic themes.

Question 12

What is considered a typical reasonable rate of extraction that is considered economically viable.
What kind of porosity and hydraulic conductivity does a good yielding aquifer have?

Answer 12

1-2 gallons a minute.
High porosity (vol voids/vol total)
High Hydraulic Conductivity-what it sounds like. Denoted as K in Darcy’s Law

Question 13

Aquitard

Answer 13

A volume of sediment or rock that forms a barrier for water flow.
Examples include clay and granite.

Question 14

What deposits are considered the best aquifers

Answer 14

unconsolidated Sand and Gravel deposits, as well as limestone (Karst) and auto-brecciated volcanic rocks (basalt and fractured volcanic rocks)

Question 15

hydraulic barrier

Answer 15

A fault or dike with low hydraulic conductivity, hinders flow.

Question 16

confined vs unconfined water table

Answer 16

confined-upper boundary is water table
unconfined- upper boundary is an aquitard

Question 17

What is a common aquifer integrity/health issue you run into while drilling

Answer 17

Drilling past an aquifer with low quality water into another aquifer that has high quality water can result in cross contamination of aquifer. There are techniques to avoid this and contamination is malpractice.

Question 18

Artesian wells

Answer 18

Picture in folder.
Hydraulic pressure is contained by confined aquifers. This pressure can be used to bring water up a well without pumping. The height water comes up to, across multiple wells, forms a line called the piezometric surface. If the piezometric surface is above ground surface level, then you don’t need pumps, the aquifer does it for you.

Question 19

Is water quality high in confined or unconfined aquifers?

Answer 19

Confined.

Question 20

Darcy’s Law: What is the formula, what does it do

Answer 20

Q=-KA(dh/dl)
Q=discharge, volume of water measure in ft^3/day. Time unit can vary, not exclusively days. This water is moving through…
A=Cross sectional area, measured in ft^2. This is given a…
K=Hydraulic conductivity. Unit is Ft/day. This is determined by a table that shows the Hydraulic conductivity for each material. Finally we have
(dh/dl)=Hydraulic gradient. Unit is ft/ft
Note that Q and K share units. Make sure they are consistent (both ft/day) before calculating.
Note that dh/dl is a negative number, and will cancel with the -1 at the start of the formula.

Question 21

What is a section of land

Answer 21

1 square mile

Question 22

What is a gentle/steep gradient of water in terms of ft/ft

Answer 22

.00x ft/ft is gentle, 0.0x ft/ft is steep. Hundredths vs thousandths.
example is 0.005 ft/ft gentle, 0.02 ft/ft steep.

Question 23

Specific Discharge, when is it useful

Answer 23

q=Q/A
q=specific discharge
A=Area
Q=discharge
q is discharge over a unit area. It is useful when you are using Darcy’s law but don’t know the area through which the water is passing
Q=-KA(dh/dl)
q=Q/A=-K(dh/dl)

Question 24

Seepage velocity: Equation, what is it, how does it relate to darcy

Answer 24

Q=-KA(dh/dl)
q=Q/A=-K(dh/dl)
v=Q/(A*n)=q/n=-K(dh/dl)/n
n=Vvoid/Vtot
Q is discharge, q is specific discharge, v is seepage velocity, K is hydraulic conductivity, A is Area, dh/dl is hydraulic gradient.
Note that porosity has an inverse relationship with seepage velocity. That is because with more pores, the same volume of water is going to be distributed among a larger space. Thus, the seepage rate will be lower.

Question 25

Why does extracting water create a cone of depression

Answer 25

v is seepage velocity, K is hydraulic conductivity, n is porosity, and dh/dl is hydraulic gradient.
v=(-K/n)*(dh/dl)
When pumping water, the water is forced into a smaller and smaller cylinder around the well. However, the volume of the water stays the same. Thus, the water’s velocity must increase the closer it gets to the well. Looking at the equation, K n or dh/dl must increase. since K and n are constant, dh/dl is the only option. Thus, the gradient steepens as the water approaches the well.

Question 26

Specific Capacity

Answer 26

The ratio between Q discharge and drawdown (how low the water goes after pumping for 2-3 hours).
Note if you increase Q too much, the relationship will no longer be linear and drawdown will increase rapidly. It would be inefficient to power a pump to extract water this quickly.

Question 27

Optimum Pumping Rate

Answer 27

Qop=optimum pumping rate
The amount of discharged groundwater extracted before the drawdown begins dropping dramatically. This will be an efficient use of power while also extracting maximum groundwater.
Process
1. Record depth to water table.
2. Pump well for 2 hours at a discharge rate. Record rate.
3. Record new depth of water table. Calculate dropdown by subtracting new depth from initial depth.
4. Repeat steps 1-3 several times with different recharge rates. Plot discharge on y axis and dropdown on x axis.
5. Find Discharge rate where the relationship ceases to be linear. This is the optimum pumping rate.

Question 28

Why does the relationship between discharge rate and dropdown cease to be linear after a certain point? Why is it so inefficient?

Answer 28

The flow into the well becomes turbulent past a certain discharge. We want laminar (sheet-like) flow.

Question 29

Flow Net

Answer 29

Diagram or model of how water flows. Map view.