Estimating Non-Point-Source Loads and Associated Water Quality Impacts

Abstract

A water quality modeling project was undertaken to estimate non-point-source loading and associated water quality impacts for the Carson River in Nevada. As part of the modeling initiative, the WASP5 program was modified to simulate attached algae. Simulation results showed that a 48 reduction of total phosphorus (TP) load would be required to achieve the existing in-stream TP annual average standard of 0.10 mg-P/L. While this reduction decreased the predicted areal extent of water quality degradation, stream reaches immediately downstream of agricultural loading still experienced high algal populations and large concomitant diel dissolved oxygen fluctuations due to the fact that phosphorus is not rate limiting for algae growth at concentrations near 0.10 mg-P/L. A simulated 97 reduction of TP load was required to decrease significantly the population of attached algae, while only a 70 reduction of total non-point-source loading (both nitrogen and phosphorus) achieved similar results.