GEOL 121
Name _______________________________
Lab Section ___________
GetWET Field Trip
Learning Objectives
After completing this lab, you should be able to:
1) Measure the level of groundwater in wells.
2) Determine the direction and rate of groundwater flow.
3) Determine if a stream is gaining or losing water based on water levels of the stream and groundwater.
4) Measure and compare groundwater and surface water properties such as: temperature, pH, electrical
conductivity, and dissolved oxygen
Part 1: Measuring groundwater levels
Groundwater is the water that exists within sediment and rock underground, in the zone of saturation,
where all pore spaces in the rock or sediment are filled with water. The top of the zone of saturation is the
water table. Groundwater is derived from rain and snow melt that infiltrates the ground through fractures
and pores in the soil and bedrock where it flows underground through rock bodies called aquifers. An
aquifer is a permeable rock or sediment layer through which groundwater flows. Wells are holes dug or
drilled into the ground to extract groundwater. Usually wells are drilled into aquifers that are known for
their high porosity and permeability. Porosity refers to the volume of empty space in sediment or rock
relative to the entire volume of rock, expressed as a percentage. Higher porosity (or a higher percentage)
implies larger voids and a greater number of voids. Permeability refers to the interconnectedness of the
pore spaces, so that fluids can migrate or move from pore space to pore space. Permeable rock makes good
aquifers because the water can move easily through the material, like sandstone and some limestones.
Impermeable rock prevents the flow of water and forms a confining bed, such as a tightly compacted shale
layer.
Depiction of the water table. Source: USGS https://water.usgs.gov/edu/earthgwaquifer.html
GEOL 121
Measuring the Hydrosphere Lab
The water table is linked directly to water levels in nearby streams, rivers, and lakes. If the water table is
above the elevation of a stream (or lake), it is called a gaining stream (effluent stream), such as might
occur in humid environments. In such a situation, the stream will flow even when it is not raining because
of additions from groundwater. The source of the water in the stream is rain water (or snowmelt) that is
absorbed by the ground over the last several months and is discharging to the stream. If the water table is
below the elevation of the stream, it is called a losing stream (influent stream), and is common in arid
environments where the water table is deep. Rain water that hits the ground surface and washes into stream
channels will gradually soak into the streambed and help recharge the groundwater, thus raising the water
table. Northern Colorado has both types of rivers (effluent and influent) because we live in a semi-arid
environment.
Groundwater flows away from areas where the water table elevation is high toward areas where the water
table is low, because of differences in potential energy. The gradient or slope of the water table, therefore,
determines the direction of groundwater flow. Also, water in an aquifer is under pressure from overlying
water. Thus, the total energy, or hydraulic head, that drives groundwater motion is the sum of the potential
energy, related to elevation, and pressure energy, related to the weight of the overlying water.
A gaining stream is shown in A, with groundwater flowing into the stream. B shows a losing stream with
water flowing from the stream into the ground. C and D show contour maps of the water table. Note how
the contours are different around the stream depending on whether the stream is gaining or losing. Source:
Modified
from
USGS
http://pubs.usgs.gov/circ/circ1139/htdocs/natural_processes_of_ground.htm#interact
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GEOL 121
Measuring the Hydrosphere Lab
(W)
GW 6
BOTTOM
TOP
Run 1
BOTTOM
Not
recovered
5.85 m
TOP
BOTTOM
Run 3
MIDDLE
Run 3
TOP
TOP
5.85 m
BOTTOM
BOTTOM
+
Not
recovered
Not
recovered
5.85 m
+ 5.85 m
Note: elevations at the top of the well casing
Black
clay
: water table level
*: white space not recovered
+: shoe (sleeve extra if plotted at top of run)
MIDDLE TOP
MIDDLE
+
Run 4
4.62 m
Run 4
TOP
Not
recovered
5.85 m
Run 4
Not
recovered
+
Not
recovered
+
+
BOTTOM
Not
recovered
+
+
TOP
5.23 m
BOTTOM
Not
recovered
3.69 m
4.31 m
Run 4
BOTTOM
Run 4
Run 4
(Bottom 3”)
MIDDLE
TOP
BOTTOM
Run 3
Run 3
Not
recovered
4.31 m
5.85 m
+
Not
recovered
2.77 m
Not
recovered
5.23 m
TOP
TOP
BOTTOM
+
Not
recovered
*
Run 3
Run 3
Not
recovered
Run 2
2.15 m
+
*+
1.53 m
Run 2
Run 2
Not
recovered
1521.08 m
BOTTOM
Not
recovered
*
2.15 m
+
1521.22 m
BOTTOM MIDDLE TOP
+
Not
recovered
MIDDLE
TOP
Run 2
BOTTOM
Run 2
2.15 m
Not
recovered
(E)
GW 4
TOP
*
(N)
GW 3
1521.35 m
Run 1
TOP
+
Not
recovered
1.69 m
1521.16 m
BOTTOM
Run 1
Run 1
BOTTOM TOP
1521.27 m
Run 1
Elevation: 1521.29 m
GW 2
Run 1
GW 1
Run 2
(S)
GW 5
Sand
and
gravel
Weathered
bedrock
Bedrock
Cross sections of core collected from each well at the GetWET Observatory. Water levels during drilling
are shown by light gray line and accompanying measurement. Dashed lines indicate correlation of units
between wells. (modified from C. Spence, REU 2006)
Hydraulic conductivity (K) and porosity for various materials.
Material
Porosity
Hydraulic Conductivity (m/sec)
Unconsolidated deposits:
Gravel
30-50% (well sorted)
3x10-4 to 3x10-2
Coarse sand
25-50% (well sorted)
9x10-7 to 6x10-3
Fine sand
30-45%
2x10-7 to 2x10-4
Silt
30-60%
1x10-9 to 2x10-5
Clay
30-60%
1x10-11 to 5x10-9
Rocks:
Limestone
1-30%
1x10-9 to 2x10-2
Sandstone
3-30%
3x10-10 to 6x10-6
1x10-11 to 1x10-8
Siltstone
Shale
0-10%
1x10-13 to 2x10-9
Fractured crystalline rocks
8x10-9 to 2x10-2
Massive crystalline rocks
3x10-14 to 2x10-10
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GEOL 121
Measuring the Hydrosphere Lab
1) (1pt.) What are some reasons why we might want to know the level of groundwater beneath the surface
(in other words, the elevation of the water table)?
2) (4pts.) Create a map of the water table
Step 1: Measure the depth (in meters) to the water table in each of the six wells at the GetWET site.
Be sure to measure from the south side of the well at the black mark on the PVC casing and
record on the table below.
Step 2: Using the elevations of the top of the casing provided in the table, calculate the elevation of
the water table, and record in the column provided.
Step 3: Record the elevations of the water level of the wells and of Spring Creek on the map on the
next page.
Elevation data for groundwater wells and the gauge at Spring Creek.
Groundwater Well
Elevation at top of casing Depth to water from
(m)
top casing (m)
1
1521.27
2
1521.16
3
1521.35
4
1521.22
5
1521.29
6
1521.08
Spring Creek Gauge
Elevation of creek bed
at staff gauge (m)
1518.85
Depth of water at
staff gauge (m)
Elevation of the water
table (m)
Elevation of surface
water (m)
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GEOL 121
Measuring the Hydrosphere Lab
Locations of groundwater wells at the GetWET Observatory. Spring Creek runs from West to East across
the top of the map. The units are meters.
3) (1pt.) Groundwater flows from areas where the water table elevation is higher to areas where the water
table elevation is lower. Based on your measurements. What general direction do you think the
groundwater is flowing?
4) (2pts.) What is the hydraulic gradient of the water table?
Remember, hydraulic gradient is the slope of the water table; in other words, the change in height (Dh)
divided by the change in distance (Dx). Dh is your highest water table elevation minus your lowest water
table elevation, and Dx is the distance in meters between the two points. Your hydraulic gradient should be
measured in the same direction that you said the groundwater was flowing in Question 3. You can measure
the distance on your map. Be sure to show your calculations.
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GEOL 121
Measuring the Hydrosphere Lab
5) (2pts.) The rate of groundwater flow, q, with units [length/time], is the product of the hydraulic gradient
and the hydraulic conductivity, K. It is calculated as q=KDh/Dx. K is a constant that describes the ease
with which groundwater flow can occur through the subsurface and varies widely in different lithologies.
Calculate q using the hydraulic gradient you just calculated. Use 1x10-5 m/s as your value for K; this is
an approximate value based on the table on page 3 and the lithologies observed in the cores. Show all work
and include correct units with your answer.
6) (1pt.) Compare the value for q that you just calculated for the groundwater to the velocity of Spring
Creek next to the groundwater wells. How do they compare? What do you think explains the difference in
the rate of flow?
7) (2pts.) Using the diagram below construct a hydrogeologic cross-section of the subsurface between
Spring Creek and wells GW 2 and GW 3. Draw in: 1) the water level in Spring Creek, 2) the water
level at GW-2 and GW- 3, and 3) an arrow indicating groundwater flow direction.
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GEOL 121
Measuring the Hydrosphere Lab
8) (1pt.) Is Spring Creek a gaining or losing stream? How can you tell?
9) (1pt.) How do you think the level of Spring Creek might change seasonally? How does this relate to
the water table?
Part 2: Evaluate water quality of groundwater and surface water
10) (2pts.)
a. Collect one bailer full of water from one of the wells and deliver some water into a beaker.
b. Collect some water from Spring Creek into another beaker.
c. Measure temperature, pH, electrical conductivity (EC measures total dissolved solids (TDS) in
the water) and DO. Record your data here:
Temperature
pH
EC (in
TDS
DO (mg/L)
(oC)
microSiemens/cm)*
(mg/L)
Groundwater
Spring Creek
* Conductivity is measured as microSiemens per centimeter (mS/cm). To convert to TDS in mg/L,
multiply by 0.67, an average conversion factor.
11) (2pts.) How do these properties of groundwater compare to the surface water? What do you think
causes the differences you see?
12) (1pt.) What is still unclear about measuring groundwater? What questions do you have?
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