Groundwater Under the Pacific Northwest • November 2-3, 2005 • Stevenson, Washington

ABSTRACT

Use of a Transport Simulation Model with Optimization Methods to Manage Nitrate Loading from On-Site Wastewater Treatment Systems
to a Shallow Aquifer near La Pine, Oregon

Presenter: David S. Morgan, U.S. Geological Survey
Co-Authors: Stephen R. Hinkle, Barbara J. Rich, Rodney J. Weick

This study developed and demonstrated a method to estimate optimal nitrate-loading rates from decentralized wastewater treatment systems to a shallow aquifer near La Pine, Oregon. The method utilizes 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 method was demonstrated in conjunction with the National Demonstration Project (NDP) in the community of La Pine in southern Deschutes County, Oregon. The La Pine NDP has developed an extensive knowledge base on the hydrogeology of the shallow ground-water system, dynamics of nitrogen fate and transport, and performance of standard and new technologies for on-site wastewater treatment in this setting. One of the many products of the NDP was a nitrate fate and transport simulation model that could be used to demonstrate the simulation-optimization approach.

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 0.74 mi², 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 on-site systems in each management area that 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 on the incremental cost of loading reductions needed to meet more stringent 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 involving pollution credit trading and incentive programs to encourage the use of advanced treatment technology on-site systems.