Modeling agriculturally driven groundwater nutrients in the Truckee River, Nevada.

摘要

Nutrient loading in the form of non-point source pollution is of great concern for watershed managers. Eutrophication due to increases in nutrient loads can be detrimental to aquatic ecosystems causing unsuitable habitat for aquatic life. Along the lower Truckee River in Nevada, there are several small farms set along the banks. These farms typically utilize flood irrigation for alfalfa production and livestock pastures. Since the area is located in a high desert, the practice of flood irrigation increases the amount of groundwater entering the river in the farmed areas as compared to unfarmed areas. With this excess groundwater also come nutrients, (i.e. nitrogen and phosphorus), from the farming practices.; Using the groundwater flow model MODFLOW and the contaminant transport model MT3D, discharge and loading analyses were performed on two small farms located on the Truckee River. The modeling results showed an addition of groundwater ranging between 1 and 2 cubic feet per second entering the river and nutrient loads averaging around 1.40 Kg day−1 for total nitrogen and 0.51 Kg day−1 for total phosphorus. Variations in the discharges and loadings were dependent upon the site and the season. Coupling the loading analysis with biomass data from periphyton samples for the Truckee River showed that the nutrients entering the river in the groundwater are enough to maintain the biomass at the sites. The amount of nutrients entering daily, although small compared to other point sources along the river, should be considered when making large watershed management decisions.