SOURCES OF WATER TO WELLS FOR TRANSIENT CYCLIC SYSTEMS

Many state agencies are currently (1995) developing wellhead protectio n programs. The thrust of some of these programs is to protect water s upplies by determining the areas contributing recharge to water-supply wells and by specifying regulations to minimize the opportunity for c ontamination of the recharge water by activities at the land surface. The area contributing recharge to a discharging well is the surface ar ea at the water table through which the water flowing to the well ente red the ground-water system. In the analyses of ground-water flow syst ems, steady-state average conditions are commonly used to simplify the problem and make a solution tractable. However, recharge is usually c yclic in nature, with seasonal cycles and longer term climatic cycles. The effect of these cyclic stresses on the area contributing recharge to wells is quantitatively analyzed for a hypothetical alluvial valle y aquifer system that is representative of a large class of ground-wat er systems that are extensively developed for water supply. The analys is shows that, in many cases, these cyclic changes in the recharge rat es do not significantly affect the location and size of the areas cont ributing recharge to wells. The ratio of the mean travel time to the l ength of the cyclic stress period appears to be an indicator of whethe r the transient effects of the cyclic stress must be explicitly repres ented in the analysis of contributing areas to wells. For the cases ex amined, if the ratio of the mean travel time to the period of the cycl ic stress was much greater than one, then the transient area contribut ing recharge to wells was similar to the area calculated using an aver age steady-state condition. However, cyclic stresses on systems with r atios less than one do have an effect on the location and size of the areas contributing recharge to wells.