Streamflow Depletion

Figure 1 : Aquifer-surface water system. Conceptual representation, plan view (top) and cross-section view (bottom).

In almost all streams, there is movement of water between the underlying groundwater and the surface water body. Because of this interaction, depletions to streams due to pumping can occur either by wells that intercept water that otherwise would have flowed to the surface water bodies or by causing water to move from the streams to the well. Pumping-induced reductions in base flow are not instantaneous nor do they occur only to streams closest to the well. There can be significant interaction among surface water bodies and it can take years or decades for depletions to be fully realized.

Within this context, you are asked to develop a model for the aquifer-surface water system shown in Figure 1 and determine the potential impact of a new water supply well. The system contains a river, a sensitive trout stream, and both a shallow and deep aquifer. Use the model to determine the impact of pumping:

• Determine where the pumped water comes from (the river or the stream);
• and if there is an adverse impact on the trout stream (i.e., does pumping cause more than a 10% depletion in stream flows in the trout stream? Note that pre-pumping flow in the stream is 5 cfs).

Hints/suggestions:

• Anisotropy ratio (K_x/K_z) for both aquifers: 10
• The river is fully connected to the aquifer (i.e., it can be represented as a constant head feature).
• The trout stream has a leakance (conductivity per unit thickness of the streambed) of 0.1 ft/day and a depth of 1m.
• The trout stream can give or receive water to/from the surficial aquifer depending on the hydraulic gradient between the stream and underlying aquifer (i.e., it is a two-way head-dependent feature).
• Use the ‘Distance Display’ capabilities when editing geometric features and/or the ‘Ruler’ tool to properly represent distances between model features.
• Make sure the ‘no-flow’ side boundaries are sufficiently far away so that they are outside the area of influence under maximum pumping conditions.
• Use a large grid size (NX=100).
• Use all relevant information from Figure 1 in your analysis.