Manual on Subsurface Investigations (2019) / Chapter Skim
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117 C H A P T E R 7 Hydrogeologic Characterization Introduction Hydrogeology plays a significant role in the geotechnical analysis, design, and performance of the subsurface features of the transportation infrastructure. Thus, a hydrogeologic characterization to gain an understanding of the distribution, thickness, composition, and continuity of the lithologic units and anthropogenic features (e.g., wells, pumps, vaults, pipelines, trenches)
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118 Aquifer Characteristics Aquifers are water-bearing soil or rock zones that transmit water easily. The following are the key characteristics of an aquifer that should be evaluated as part of a hydrogeologic characterization: • Hydraulic conductivity • Porosity • Permeability • Transmissivity • Storage coefficient (confined aquifers)
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119 = hydraulic conductivity = dynamic viscosity of the fluid = acceleration of gravity Intrinsic permeability has units of area. In general, hydraulic conductivity is measured in a site investigation.
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120 Direction and Gradient of Groundwater Flow Potentiometric information (from piezometers or monitoring wells with short well screens) and measurements of the hydraulic conductivity of aquifers are used to estimate the direction and gradient of groundwater flow.
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121 7.6.1.1 Existing Information Sources Existing information pertaining to hydrogeology is available from a variety of sources including (i) the United States Geological survey (USGS)
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122 Source: CalEPA (2014) Figure 7-1.
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123 To enable consistent and accurate water level elevation measurements, each monitoring well should have a surveyed elevation reference point that is consistently used when measuring water levels and total well depth. If monitoring wells have been installed and surveyed over time during successive phases of investigations at a site, it is recommended that a single elevation survey be conducted for the entire network to ensure that the reference point elevations are accurate.
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124 Source: Geosyntec Consultants, Inc. Figure 7-3.
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From page 125... ...
125 should always be complemented by direct observation of groundwater conditions by means of monitoring wells and piezometers. 7.6.2 Methods for Aquifer Characteristics Aquifer tests provide a means of determining the properties of water-bearing zones in the subsurface, including hydraulic conductivity, permeability, transmissivity, storage coefficient, and specific yield.
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126 Elapsed Time Measurement Frequency 1 to 2 hours 10 minutes 2 to 3 hours 20 minutes 3 to 15 hours 1 hour 15 to 60 hours 5 hours Source: ASTM D4050 Numerous methods are available to analyze data from pumping tests to calculate hydraulic conductivity, permeability, transmissivity, storage coefficient, and specific yield. The appropriate method depends on whether (i)
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127 Source: Geosyntec Consultants, Inc. Figure 7-4.
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128 Source: Geosyntec Consultants, Inc. Figure 7-5.
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From page 129... ...
129 Chapter 7 References Bouwer, H., 1989. "The Bouwer-Rice Slug Test -- An Update." Ground Water, Vol 27, No.
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