Sources and concentrations of dissolved solids and selenium in the San Joaquin River and its tributaries, California, October 1985 to March 1987

Sources and concentrations of dissolved solids and selenium in the San Joaquin River and its tributaries, California, were assessed by a mass-balance approach to determine the effects of tile-drain water and irrigation-return flows on the river. The study included low-flow periods from October 1985 to mid-February 1986 and mid-May 1986 through March 1987, and a high-flow period from mid-February to mid-May 1985. During the combined low-flow period, the dissolved-solids load from eastside tributaries and the upper San Joaquin River accounted for only 18% of the total load at Vernalis, located farthest downstream, even though they accounted for 71% of the streamflow. Salt and Mud Sloughs contributed 40% of the dissolved-solids load but only 9% of streamflow. Unmeasured sources of dissolved solids contributed about 42% of the total load during low flow. In contrast, Salt and Mud Sloughs, which receive most of the tile-drain water that enters the river, contributed almost 80% of the total selenium load to the river, and loading of selenium concentrations were highest in Salt and Mud Sloughs and decreased downstream in the San Joaquin River with dilution from eastside tributaries. A State standard for dissolved solids of 500 mg/L was exceeded 11% of the time in the San Joaquin River at Vernalis. The U.S. Environmental Protection Agency 's 4-day average aquatic-life criterion of 5 micrograms/L of selenium was exceeded in more than 60% of the samples from the sloughs and in about 20% of the samples from the San Joaquin River, just downstream of the Merced River. (USGS)