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

A simulation/optimization (s/o) model is presented to address the incr easingly common conflicts between water quantity and quality objective s. The model can assist water resources analysts in selecting compromi se strategies for stream/aquifer systems in which the stream gains wat er from the aquifer. The water quantity objective is to maximize stead y conjunctive use of groundwater and surface water resources. The wate r quality objective is to maximize waste loading from a sewage treatme nt plant (STP) to the stream without violating downstream water qualit y beyond acceptable limits. The STP discharge is proportional to human population. The two objectives conflict because an increase in ground water extraction reduces dilution of the stream water contaminants. Th e 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 m odel includes superposition expressions describing head and flow respo nses to decision variables (pumping, diversion, and loadings) and regr ession expressions describing contaminant concentration responses to t hese decision variables. Modeled contaminants include: 5-day biochemic al oxygen demand, dissolved oxygen, nitrogen (organic, ammonia, nitrit e, and nitrate), organic and dissolved phosphorus, total dissolved sol ids, and chlorophyll-a.