SEDIMENT, NITRATE, AND AMMONIUM IN SURFACE RUNOFF FROM 2 TAHOE BASIN SOIL TYPES

Few studies have addressed the natural pollution potential of pristine subalpine forested watersheds on a site-specific basis. Consequently, specific source and amounts of nutrient discharge to tributaries of t he Tahoe Basin are difficult to identify. The sediment content and nit rate and ammonium levels in surface runoff from two soil types (Meeks and Umpa), four plot conditions (wooded natural and disturbed, open na tural and disturbed), and three slopes (gentle, moderate, and steep) w ere studied using rainfall simulation that applied a 9 cm h-1, 1-h eve nt. A significant (P < 0.005) two-way interaction between soil type an d plot condition affected runoff nitrate concentration. Runoff from na tural or disturbed open plots contained significantly (P = 0.05) great er nitrate than wooded plots. Peak concentrations of nitrate commonly occurred during early runoff, suggesting that peak nitrate discharge t o Lake Tahoe tributaries can be expected during early runoff from snow melt and summer precipitation events. The highest nitrate runoff conce ntration and 1-h cumulative loading from the 0.46 m2 plots were 6.7 mg L-1 (Umpa, open natural, 15-30 percent slope), and 0.7 mg (Umpa, open natural, > 30 percent slope), respectively. Ammonium in surface runof f was generally below detection limits (< 0.05 mug L-1). No statistica l relationship between runoff nitrate and sediment discharge was detec ted.