ESTIMATING NON-POINT-SOURCE LOADS AND ASSOCIATED WATER-QUALITY IMPACTS

A water quality modeling project was undertaken to estimate non-point- source loading and associated water quality impacts for the Carson Riv er in Nevada. As part of the modeling initiative, the WASPS program wa s modified to simulate attached algae. Simulation results showed that a 48% reduction of total phosphorus (TP) load would be required to ach ieve the existing in-stream TP annual average standard of 0.10 mg-P/L. While this reduction decreased the predicted areal extent of water qu ality degradation, stream reaches immediately downstream of agricultur al loading still experienced high a]gal populations and large concomit ant 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 sig nificantly the population of attached algae, while only a 70% reductio n of total non-point-source loading (both nitrogen and phosphorus) ach ieved similar results.