GetWET Field Trip - student handout

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Colorado State University, Fort Collins *

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101C

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Geology

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Jan 9, 2024

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docx

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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 .

GEOL 121 Measuring the Hydrosphere Lab Depiction of the water table. Source: USGS https://water.usgs.gov/edu/earthgwaquifer.html 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 . 2

GEOL 121 Measuring the Hydrosphere Lab 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 3

GEOL 121 Measuring the Hydrosphere Lab 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 Siltstone 1x10 -11 to 1x10 -8 Shale 0-10% 1x10 -13 to 2x10 -9 Fractured crystalline rocks 8x10 -9 to 2x10 -2 Massive crystalline rocks 3x10 -14 to 2x10 -10 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 ?( 4

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