Groundwater Learning objectives

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Groundwater Learning objectives Be able to quantify the flow of groundwater and evaluate the impacts of well pumping on groundwater levels Be able to quantify and calculate aquifer properties from groundwater information

Figure 8-6

Problem (Storage Coefficient) The average water table elevation has dropped 6 ft over a 50 mi2 area due to removal of 40,000 acre-ft by pumping from an aquifer. Determine the storage coefficient. Is this aquifer most likely confined or unconfined?

Drawdown around a well in a confined aquifer In a well that fully penetrates a confined aquifer, with thickness 100 ft, hydraulic conductivity 10 ft/day, pumping rate 50 gpm, well diameter 1 ft. Assume the drawdown at the well is 12 ft. Prepare a graph of drawdown versus distance from the well. Figure 8-14

Regional groundwater problem A confined aquifer is 75 ft thick and 1000 ft wide. Two observation wells are located 3000 ft apart in the direction of flow. Head in well 1 is 130 ft and in well 2 is 120 ft. Hydraulic conductivity is 10-3 cm/sec (Silty Sand – Table 8.3). What is the daily flow through the aquifer What is the height of the piezometric surface 2000 ft from well 1 and 1000 ft from well 2.

Dupoit assumptions for flow in an unconfined aquifer (p518) Water table is only slightly inclined Flow may be considered horizontal Slope of free surface (water table) and hydraulic gradient are equal Total flow over the depth of the saturated zone per unit width (ft2/day) 𝑞=𝐾ℎ 𝑑ℎ 𝑑𝑥

Drawdown around a well in an unconfined aquifer In a well that penetrates an unconfined aquifer, with thickness 100 ft, hydraulic conductivity 10 ft/day, pumping rate 50 gpm, well diameter 1 ft. Assume the drawdown at the well is 12 ft. Prepare a graph of drawdown versus distance from the well. Figure 8-15

Example 8-3 Two rivers located 1000 m apart fully penetrate an aquifer. K=0.5 m/day. Average rainfall 15 cm/yr, evaporation about 10 cm/yr. River elevations ho=20 m, hL=18 m. What is the location and height of the groundwater divide? What is the daily discharge into each river? Figure 8-13

K