Pumping water from an aquifer typically results in a decrease in groundwater levels (drawdown). A decrease in the hydraulic head can cause a variety of issues, including decreased borehole yield, land subsidence, and boreholes running dry. The construction of a borehole in the vicinity of existing wells can lead to drawdown at the boreholes and in the surrounding areas. The magnitude of the drawdown is related to both aquifer properties (such as aquifer geometry, hydraulic properties, and recharge) and borehole properties (e.g. abstraction rate, penetration depth, screen length). At any given time, the drawdown at a borehole is the difference in head or water level from the initial head (before pumping began) at the same location. The drawdown can be calculated if the borehole and aquifer properties are known. This way, the long-term safe yield can be determined, avoiding the borehole from being pumped dry.
The widely used three-dimensional finite-difference groundwater model MODFLOW-2005 (Harbaugh, 2005) can be applied to represent the aquifer’s complexity accurately. However, if the well diameter is much smaller than the size of the grid-cells, the maximum drawdown near the well is underestimated. Refining the grid around the well-field would increase the run-time of the models, which is impractical for stochastic modelling. Therefore, analytical methods were used to calculate the maximum drawdown near the wells. In order to determine the specific drawdown at the location of the wells, the Theis equation is used (Theis, 1935), which describes the transient flow of water to a single pumping well in a confined aquifer.
Determining Drawdown Threshold in The wells to evaluate changes in groundwater levels
A drawdown threshold is evaluated to determine whether the calculated drawdown leads to wells drying, indicating changes in groundwater levels. In borehole design, the depth of the well-screen and the pump determines at what drawdown the well falls dry. This design can be altered according to the expected effects of the drawdown. At the time of writing, a well field design was not yet available. Therefore the threshold is based on common well design limitations, aquifer characteristics, and the considerations of the static water levels in wells. A pump is usually placed at the depth well screen in production boreholes, where the water passes from the aquifer into the borehole. The pump is placed at such a depth that it will not fall dry due to a drop in levels within the borehole or the aquifer. The advantage of placing the screen at a larger depth is a greater allowance for drawdown without the pump falling dry. The disadvantages are increased cost and risk of upconing. Commonly, the pump inlet is therefore installed at a depth several meters below the anticipated level drop in the boreholes.
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Harbaugh, A.W., 2005. MODFLOW-2005, the US Geological Survey modular groundwater model: The groundwater flow process. US Department of the Interior, US Geological Survey Reston, VA, USA.
Theis, C.V., 1935. The relation between the lowering of the piezometric surface and the rate and duration of discharge of a well using groundwater storage. US Department of the Interior, Geological Survey, Water Resources Division, Ground Water Branch.