Nutrient sampling sites on streams of the Trinity River Basin
during 1993-95. The greatest density of sites is in agricultural
and urban areas. Nutrients (nitrogen and phosphorus compounds)
in surface water are essential for aquatic plant and animal
life; but large concentrations of nutrients can have adverse
ecological effects. Fertilizer, manure, plant decay, and
atmospheric fallout are major nonpoint sources of nitrogen and
phosphorus. Wastewater-treatment plant effluent is a major point
source. The EPA (1996) maximum contaminant level (MCL) for
nitrate (as nitrogen) is 10 milligrams per liter. Excessive
nitrate can restrict oxygen transport in the bloodstream.
Ammonia in surface water is naturally converted to nitrate. This
conversion, in addition to increasing nitrate concentration,
removes oxygen from the water, which also can adversely affect
fish and invertebrates. The EPA recommends that total phosphorus
concentration (as phosphorus) not exceed 0.10 milligram per
liter in streams not discharging dire