CONSEQUENCES OF WELL PUMPAGE UNDER CAPTURE-CONSTRAINING BOUNDARY CONDITIONS

A natural consequence of groundwater pumpage is the removal of water from subsurface storage, as well as increased recharge and decreased discharge (combined as “capture”), as described in the classic 1940 paper by C.V. Theis. Most capture translates into streamflow depletion, which can detrimentally impact ecosystems. This study assesses the partitioning of sources of pumpage into parts derived from storage depletion and capture, and how this partitioning changes with time under the influence of boundary conditions. Storage depletion and capture can be measured in terms of nondimensional fractions relative to pumpage. These measures can be computed from either flow rates or cumulative volumes. The former yields more gradually changing values reflecting long-term responses, but may not accurately indicate system status at any particular time. The fractions change exponentially with time, even for a constant rate of withdrawal. The fractional values of complementary depletion and capture, based on flow rates, should eventually reach 0.0 and 1.0, respectively. At this point, the aquifer system attains a new equilibrium and storage changes cease. However, if aquifer boundary conditions change in a way that limits further changes in capture, then a new equilibrium might not be attained and the rate of change in depletion and capture fractions will be lessened. Numerical simulation of a hypothetical groundwater basin is used to further illustrate some of Theis’ 1940 principles, particularly when capture is constrained by insufficient available water for capture. Most prior studies of depletion and capture have assumed that boundary conditions remain unconstrained and yield linear responses. When capture has induced so much stream infiltration that a reach of a stream goes dry, recharge from the stream can no longer increase. However, capture can continue to grow, though at a reduced rate, until downstream discharge of groundwater to the stream is substantially reduced or eliminated. At that time, base flow in the stream approaches zero, and groundwater storage depletion (groundwater mining) alone will continue to balance pumpage until such time that drawdown reduces well yields and pumpage is itself constrained. That is, groundwater depletion itself is inherently unsustainable.