Nitrogen and phosphorus are essential nutrients for aquatic plants. However, in high concentrations, they can cause excessive plant growth (eutrophication) and toxicity to infants ("blue baby syndrome" or methemoglobinemia from ingestion of nitrate). The USEPA has set criteria for the nitrate and ammonia forms of nitrogen, but not for phosphorus. The maximum contaminant level (MCL) for nitrate in drinking water is 10 milligrams per liter as nitrogen (mg/L as N) (U.S. Environmental Protection Agency, 1986).
The USEPA also has established criteria for maximum ammonia concentrations in surface water on the basis of chronic and acute exposure of aquatic organisms to un-ionized ammonia (U.S. Environmental Protection Agency, 1986). These criteria vary inversely with pH and temperature. The chronic criteria range from about 0.2 to 2 mg/L as N for the range of pH (7.5-8.5) and temperatures (5-25C) generally found in surface water in the San Joaquin Valley.
Nutrient concentrations in the lower San Joaquin River are determined primarily by relatively concentrated inputs from west-side agricultural drainage, east-side wastewater-treatment plants and runoff from dairies, and relatively dilute inputs from major east-side tributaries. Mud and Salt sloughs receive a part of their flow from subsurface drains that drain about 60,000 acres of agricultural land. Although the sloughs account for only about 10 percent of the streamflow in the San Joaquin River near Vernalis, the subsurface drainage is very high in nitrate (about 25 mg/L as N), and the sloughs contribute nearly one-half the nitrate (Kratzer and Shelton, in press). The nitrate transported in the San Joaquin River during a wet year (1986) was about 50 percent more than that transported in a dry year (1988).
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During both dry and wet years, much of the nitrate load in the San Joaquin River can be attributed to subsurface agricultural drainage discharged to Mud and Salt sloughs.
The nitrate MCL was exceeded in Spanish Grant Drain, Turlock Irrigation District lateral 5, and Orestimba Creek in 15, 11, and 9 percent, respectively, of samples collected between April 1993 and March 1995. However, these tributaries are not designated as drinking-water sources. The MCL was not exceeded during this period in the main stem of the San Joaquin River, a designated drinking-water source.
Increasing nitrate concentrations in the San Joaquin River could be attributed to several sources, including subsurface agricultural drainage, runoff from fertilizer applications, wastewater-treatment plant effluent, and runoff from dairies. The relative contribution of these sources was evaluated with estimates of nitrate loads and with trends in ammonia and phosphorus concentrations. Wastewater-treatment plant effluent and runoff from dairies have especially high concentrations of ammonia and phosphorus, yet concentrations of ammonia and phosphorus in the San Joaquin River generally declined or remained stable while nitrate concentrations steadily increased.
The flow-adjusted nitrate concentration in the San Joaquin River has increased from about 0.3 to 1.4 milligrams per liter (mg/L) over the last four decades.
The source of the nitrate increase during the 1950s was indeterminate. During the 1960s, runoff from fertilizer applications (primarily in east-side basins) and subsurface agricultural drainage were the probable sources of the increase. Since 1970, subsurface agricultural drainage has been the primary cause of the increasing nitrate trend (Kratzer and Shelton, in press). Other studies have determined that the nitrate in the subsurface agricultural drainage primarily comes from the leaching of native soil nitrogen and not from fertilizer application (Brown, 1975). Other sources of nitrate loads were especially important in the early 1980s because of the effect of an extremely wet year (1983) on the 5-year running averages. The unusually large inputs of nitrate in 1983 were probably from (1) inflow from the Tulare Basin through the Fresno Slough, (2) discharge from wastewater-treatment plants, (3) runoff from dairies, and (4) runoff from fertilizer applications west of the San Joaquin River (Kratzer and Shelton, in press). Despite this long-term increase in the San Joaquin River, nitrate concentrations are still well below the USEPA drinking-water standard.
Nitrate loads in the San Joaquin River from subsurface agricultural drains have increased steadily since the 1950s.
On the basis of monthly samples collected during 1985-88, ammonia concentrations in the San Joaquin River increased from Newman to Patterson, then declined from Patterson to Vernalis as a result of dilution by the Tuolumne and Stanislaus Rivers (Kratzer and Shelton, in press). The increase from Newman to Patterson is attributed to relatively concentrated inputs, such as Turlock Irrigation District lateral 5, Orestimba Creek, and Spanish Grant Drain. Most of the flow in Turlock Irrigation District lateral 5 is effluent from the Turlock wastewater-treatment plant, especially during the nonirrigation season. Ammonia concentrations exceeded the USEPA chronic criteria in 2 of the 51 samples collected at the San Joaquin River at Patterson during 1985-88.
Ammonia concentrations in Turlock Irrigation District lateral 5, Orestimba Creek, and Spanish Grant Drain exceeded the USEPA chronic criteria in 76, 14, and 5 percent, respectively, of samples collected between April 1993 and March 1995. None of the samples collected at the San Joaquin River at Patterson during April 1993 to March 1995 had ammonia concentrations that exceeded the USEPA chronic criteria, but some concentrations were just under the criteria.
Unlike nitrate, ammonia concentrations at the San Joaquin River near Vernalis did not increase from 1974 to 1990. Instead, ammonia concentrations increased until 1984, then declined. This decrease was probably due to a combination of factors, including conversion of ammonia to nitrate by improved wastewater treatment, reduced discharges from wastewater-treatment plants due to a sequence of dry years during the late 1980s and expanded use of land disposal, and reduced inputs from dairies during the sequence of dry years (Kratzer and Shelton, in press).
Nitrate concentration in the San Joaquin River have steadily increased while ammonia concentration has declined since 1984.