THE EFFECTS OF DETENTION BASINS ON NUTRIENT LOADS TO LAKE TAHOE

The efficiency of detention basins presently used around Lake Tahoe for reducing sediment and nutrient loads was uncertain. Cattlemans detention basin, situated adjacent to Cold Creek, a tributary to Lake Tahoe, was investigated. For more than 35 years, water clarity in Lake Tahoe has been decreasing about one foot per year and has prompted efforts to decrease sediment and nutrient loads from reaching the lake (Coats and others, 2002). As many of these basins were constructed near alpine streams, the potential exists for nutrients to be transported by ground water beneath the detention basins and possibly discharged to streams at some downstream point or to Lake Tahoe. Tracking the path of nutrients and understanding the shallow subsurface environment in which they travel is important for determining the effectiveness of these detention basins.

Complex ground-water/surface-water interactions were evaluated with ground-water and solute transport models. Models assume all water entering the basin infiltrates or is consumed by evapotranspiration, because very rarely did entering urban runoff exit as surface water. Preliminary modeling results indicate that the detention basin has altered the local ground-water flow system while efficiently reducing suspended sediments, although nutrients are not filtered out as readily. Nutrient destinations were tracked by calculating ground-water flow paths and endpoints. Between 10 and 15 percent of ground water originating from the detention basin discharges to Cold Creek just downstream from the detention basin. The remaining 85 to 90 percent of ground water discharges to evapotranspiration, to Cold Creek farther downstream, or to Lake Tahoe.