Nutrient and suspended sediment concentrations in surface water are highest at west side sites. 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 by relatively dilute inputs from major east side tributaries. On the basis of size distribution and load calculations in the San Joaquin River and tributaries, most suspended sediment in the river comes from west side sources.
Nutrient and suspended sediment loads in the lower San Joaquin River were much greater in a wet year (1986) than in a critically dry year (1988). Ratios of 1986 to 1988 loads increased with the particulate fraction of each constituent. During water years 1986-1988, nonpoint sources accounted for at least 81 percent of the total nitrogen load and 68 percent of the total phosphorus load from the San Joaquin Basin. The overall transport of total nitrogen and total phosphorus from the basin during this time was 5 percent and 3 percent of the total sources, respectively.
Flow adjusted nitrate concentrations in the lower San Joaquin River have increased steadily since 1950. This can be attributed to many factors, including increases in subsurface agricultural drainage, fertilizer application, wastewater treatment plant effluent, and runoff from dairies. Since 1970, this increase has been due primarily to increases of mostly native soil nitrogen in subsurface agricultural drainage. Flow adjusted ammonia concentration have decreased during the 1980s at several sites. These decreases are probably related to improved regulation of domestic and dairy wastes.
Abstract
Introduction
Description of the Study Unit
Physiographic and Geologic Settings
Climate
Surface Water Hydrology
Population and Land Use
Water Use
Environmental Framework for Water Quality Assessment
Point Sources
Nonpoint Sources
Water Quality Problems Identified by the State of
California
Environmental Settings
Lower San Joaquin River Basin, 1951–1990
Sources of Data
Compilation of Data
Screening of Data
Quality Assurance and Quality Control
Description of Available Data
Timing and Location of Sampling
Streamflow at Time of Water Quality Sampling
Description of Constituent Concentrations by Environmental Setting
Differences in Constituent Concentrations Among
Environmental Settings
Concentrations of Constituents in the Lower San
Joaquin River
Relation to National Conditions
Relation of Nutrient and Suspended Sediment Concentrations to Streamflow
Load Estimates
Annual Stream Loads
Relation of Stream Loads to Upstream Conditions
Atmospheric Loads
Total Loads in the Lower San Joaquin River Basin
Trends in Constituent Concentrations
Summary and Conclusions
References Cited