MAXIMIZING CONJUNCTIVE USE OF SURFACE AND GROUND-WATER UNDER SURFACE-WATER QUALITY CONSTRAINTS

A simulation/optimization (s/o) model is presented to address the increasingly common conflicts between water quantity and quality objectives. The model can assist water resources analysts in selecting compromise strategies for stream/aquifer systems in which the stream gains water from the aquifer. The water quantity objective is to maximize steady conjunctive use of groundwater and surface water resources. The water quality objective is to maximize waste loading from a sewage treatment plant (STP) to the stream without violating downstream water quality beyond acceptable limits. The STP discharge is proportional to human population. The two objectives conflict because an increase in groundwater extraction reduces dilution of the stream water contaminants. The result is a decrease in the STP waste loading to the stream and the waste-producing human population that can be supported. The trade-off between objectives is illustrated graphically via sets of noninferior solutions. The sets of noninferior solutions are prepared using the E-constraint method and assuming different upstream flow rates. The s/o model includes superposition expressions describing head and flow responses to decision variables (pumping, diversion, and loadings) and regression expressions describing contaminant concentration responses to these decision variables. Modeled contaminants include: 5-day biochemical oxygen demand, dissolved oxygen, nitrogen (organic, ammonia, nitrite, and nitrate), organic and dissolved phosphorus, total dissolved solids, and chlorophyll-a.