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Sources of Phosphorus to the Carson River Upstream from Lahontan Reservoir, Nevada and California, Water Years 2001-02

By Nancy L. Alvarez and Ralph L. Seiler

Report availability: Portable Document Format (PDF).

Abstract

Discharge of treated municipal-sewage effluent to the Carson River in western Nevada and eastern California ceased by 1987 and resulted in a substantial decrease in phosphorus concentrations in the Carson River. Nonetheless, concentrations of total phosphorus and suspended sediment still commonly exceed beneficial-use criteria established for the Carson River by the Nevada Division of Environmental Protection. Potential sources of phosphorus in the study area include natural inputs from undisturbed soils, erosion of soils and streambanks, construction of low-head dams and their destruction during floods, manure production and grazing by cattle along streambanks, drainage from fields irrigated with streamwater and treated municipal-sewage effluent, ground-water seepage, and urban runoff including inputs from golf courses. In 2000, the U.S. Geological Survey (USGS), in cooperation with Carson Water Subconservancy District, began an investigation with the overall purpose of providing managers and regulators with information necessary to develop and implement total maximum daily loads for the Carson River. Two specific goals of the investigation were (1) to identify those reaches of the Carson River upstream from Lahontan Reservoir where the greatest increases in phosphorus and suspended-sediment concentrations and loading occur, and (2) to identify the most important sources of phosphorus within the reaches of the Carson River where the greatest increases in concentration and loading occur.

Total-phosphorus concentrations in surface-water samples collected by USGS in the study area during water years 2001-02 ranged from <0.01 to 1.78 mg/L and dissolved-orthophosphate concentrations ranged from <0.01 to 1.81 mg/L as phosphorus. In streamflow entering Carson Valley from headwater areas in the East Fork Carson River, the majority of samples exceeding the total phosphorus water-quality standard of 0.1 mg/L occur during spring runoff (March, April, and May) when suspended-sediment concentrations are high. Downstream from Carson Valley, almost all samples exceed the water-quality standard, with the greatest concentrations observed during spring and summer months.

Estimated annual total-phosphorus loads ranged from 1.33 tons at the West Fork Carson River at Woodfords to 43.41 tons at the Carson River near Carson City during water years 2001-02. Loads are greatest during spring runoff, followed by fall and winter, and least during the summer, which corresponds to the amount of streamflow in the Carson River. The estimated average annual phosphorus load entering Carson Valley was 21.9 tons; whereas, the estimated average annual phosphorus load leaving Carson Valley was 37.8 tons, for an annual gain in load across Carson Valley of 15.9 tons. Thus, about 58 percent of the total-phosphorus load leaving Carson Valley on an annual basis could be attributed to headwater reaches upstream from Carson Valley. During spring and summer (April 1-September 30) an average of 85 percent of the total-phosphorus load leaving Carson Valley could be attributed to headwater reaches. During fall and winter (October 1-March 31) only 17 percent of the phosphorus load leaving Carson Valley could be attributed to headwater reaches.

The composition of the phosphorus changes during summer from particulate phosphorus entering Carson Valley to dissolved orthophosphate leaving Carson Valley. Particulate phosphorus entering Carson Valley could be settling out when water is applied to fields and be replaced by dissolved orthophosphate from other sources. Alternatively, the particulate phosphorus could be converted to dissolved orthophosphate as it travels across Carson Valley. Data collected during the study are not sufficient to distinguish between the two possibilities.

Eagle Valley and Dayton-Churchill Valleys may act as sinks for phosphorus. On an annual basis, during water years 2001-02, about 90 percent of the phosphorus entering Eagle Valley left the valley. Similarly, only about 85 percent of the phosphorus entering Dayton-Churchill Valleys was discharged from the valleys.

Total-phosphorus concentrations and load increased substantially between Brockliss Slough at Highway 88 and Brockliss Slough upstream from the confluence with the Carson River. Between 10 and 22 percent of the total-phosphorus load measured at Carson River near Genoa during summer could be attributed to this reach. During summer, all phosphorus loads contributed to the river by the West Fork Ditch originated in the 8.7 mi reach downstream of Highway 88 and accounted for 27-36 percent of the load measured at Carson River near Genoa. Similarly, between 11 and 20 percent of the total-phosphorus load at Carson River near Genoa during summer could be attributed to the 1.5 mi reach between the East Fork Carson River at Muller Lane and the East Fork Carson River at the confluence of the West Fork Carson River. The combined load from Ambrosetti Pond Outlet and Williams Slough contributed between 6.7 and 11 percent of the total-phosphorus load at the Carson River near Carson City site.

The relation between suspended-sediment concentrations and phosphorus concentrations in water indicates that phosphorus from particulate phosphorus alone can exceed the State phosphorus standard when suspended-sediment concentrations exceed about 50 mg/L.

Little change occurred in water quality in the East Fork Carson River between the Markleeville and Dresslerville sites, implying that phosphorus and sediment entering Carson Valley in the East Fork originates upstream of the Markleeville gage. Large increases in phosphorus loads occur in the East Fork, West Fork/Brockliss Slough, and West Fork Ditch systems during summer and are caused principally by increases in dissolved orthophosphate. The source of dissolved orthophosphate in these reaches likely is agricultural. Because treated municipal-sewage effluent contains elevated phosphorus concentrations, drainwater from fields irrigated with this effluent is a potentially large source of phosphorus. Ambrosetti Pond, which stores drainwater from irrigated fields, can be a source of substantial amounts of phosphorus in the Carson River, particularly during winter.


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