Nonpoint source pollution management models for regional groundwater quality control

Nonpoint source pollution threatens the quality of enormous reservoirs of groundwater. Limits on distributed source densities, effluent quality standards for sources, and regulations on land use activities can be effective components of strategies to protect groundwaters. To facilitate the formulation or evaluation of strategies to protect groundwaters from nonpoint source pollution, groundwater quality management models have been developed. Decision makers can use these models to obtain estimates of regional subsurface wasteload allocations than can in turn be used to obtain tangible estimates of desirable distributed source densities (i.e., septic tank densities) or identify land use patterns that will maintain acceptable subsurface water quality. Several steady-state groundwater quality management models useful for investigating regional groundwater wasteload allocation from nonpoint sources are presented. These management models are constructed as linear programming optimization models. Equations from a finite difference, steady-state, two-dimensional horizontal, unconfined, advective contaminant transport model are used as part of each optimization problem constraint set. The management models were applied over the Sole Source aquifer of Barnstable County, Massachusetts. Barnstable County is incurring widespread nitrate contamination from distributed septic systems which serve 88 percent of the population. The modeling approach requires general data normally available through state geological surveys, regional planning commissions, and the census bureau. The optimal regional nonpoint source groundwater wasteload allocations are generated from this data as are resultant contaminant distributions, boundaries of critical recharge areas, and the associated water quality tradeoffs for changes in existing and proposed land use (or source) management schemes. The optimal wasteload allocations were translated into estimates of distributed source densities and land use development patterns. The results of these groundwater quality management models, used in conjunction with additional field and planning information, provide additional insight that is essential for consideration of groundwater resource protection in the planning of regional land use activities