This study demonstrated a method to estimate optimal nitrate-loading rates from decentralized wastewater treatment systems to a shallow aquifer near La Pine, Oregon. The method uses a simulation-optimization approach in which a nitrate fate and transport simulation model is linked to an optimization model. Using this method, maximum sustainable loading rates that meet constraints on ground-water quality and nitrate loading to streams via ground-water discharge can be determined. This method enhances the value of a simulation model as a decision-support tool in developing land-use and wastewater treatment policies that will protect the quality of water resources.

The La Pine nitrate loading management model (NLMM) was developed by linking the simulation model to an optimization model using the response-matrix technique. The NLMM was used to determine the minimum nitrate loading reductions that would be required in 97 management areas, ranging in size from 0.25 to 0.75 sq mi, to meet specified constraints on ground-water nitrate concentration and nitrate loading to streams from ground-water discharge. Constraints can also be set on maximum or minimum loading reductions in management areas. The NLMM determines the minimum loading reductions for existing and future onsite systems in each management area, which are required to meet the specified constraints. Cost factors also can be applied to the optimization if the cost of reducing loading favors reductions for existing or future homes. The NLMM was used to perform trade-off analyses that illustrate the sensitivity of optimal loading to water quality constraints. The La Pine NLMM is currently being used by Deschutes County to support decisions on maximum nitrate loading rates and local onsite wastewater system performance standards as they implement a ground-water protection plan.