The Principle of Capture: Clearing up Some Common Misconceptions

Nearly 70 years ago, C.V Theis explained that when a well is pumped, water initially is drawn from storage around the well. As pumping continues, the cone of depression spreads out and inflow to the aquifer (recharge) can increase and outflow from the aquifer (discharge) can decrease. The increased inflow and decreased outflow induced by ground water pumping is now referred to as “capture.” Since the original work by Theis, a number of authors have advanced the understanding of timing and ultimate effects of ground water withdrawals on connected surface water and phreatophytic plants. In spite of this work, certain concepts relating to capture are sometimes misunderstood by water managers and ground water professionals.  Some of the more common misconceptions seen in arguments related to capture include the following:
1. Total development of ground water resources in a river-aquifer system is “safe” or “sustainable” at rates up to the average rate of recharge.
2. Capture is dependent on the rate and direction of water movement in the aquifer.
3. The process of capture stops when pumping ceases.
4. Pumping ground water exclusively below a clay layer will eliminate the possibility of capture of surface water connected to the overlying ground water system.
Using principles of mass balance and numerical models, the expected capture response can be demonstrated, both for idealized hypothetical problems, as well as for complex real-world aquifer systems.