Ground-Water Nitrate and Organic Carbon Inputs to the Lower San Joaquin River Completed

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This proposal addresses drinking water and aquatic habitat issues associated with nitrate and organic carbon in the lower San Joaquin River (SJR). The primary drinking water issue with nitrate in the lower SJR is the role of nitrate in stimulating algal growth which affects the cost and effectiveness of water treatment downstream. Organic carbon is a drinking water issue because of the potential for forming disinfection byproducts when the water is treated. In addition to the drinking water issues, the nutrient inputs to the SJR and the resultant algal growth contribute to low dissolved oxygen concentrations in the SJR near Stockton which can be deleterious to Chinook salmon migration.

Based on previous work by the project team (Kratzer and others, 2004; Burow and others, 1998; Kratzer and Shelton, 1998; Phillips and others, 1991), it appears that the long-term increase (over 50 years) in nitrate concentrations in the SJR are currently mostly due to ground-water sources. In fact, the range of 15N and 18O values of nitrate in river samples collected in 2000 and 2001 suggest that animal waste or sewage was the most significant source of nitrate in the river at the time (Kratzer and others, 2004). There are several dairies close to the SJR on the east side that could contribute to the nitrate in the river. The goal of this study is to use three approaches to address the question of nitrate in ground-water accretions and the source of that nitrate. The three approaches are necessary to capture the different scales of spatial and temporal variability, as well as accurately quantify nitrate sources and loads.

The first approach involves doing a reconnaissance by boat to get a qualitative, semi-quantitative sense of significant ground-water inflow areas to the river by continuously measuring the water temperature, electrical conductivity (EC), and optical properties of water just above the streambed. Areas with significant changes in any of these parameters will be further evaluated and samples collected of the ground water inflow for stable isotopes of carbon, nitrogen, and oxygen, as well as other chemical indicators diagnostic of source. The second approach will be to re-visit three sites in the SJR where nested monitoring wells were installed in the late-1980s in the streambed and on both banks and also install three more sites in the river. The established sites were modeled during an extended dry period. The six sites will give a better spatial coverage of the study area and will allow a comparison of present conditions to the late-1980s. The third approach is to extend a synoptic sampling method recently used on the lower Merced River as part of the San Joaquin National Water-Quality Assessment (NAWQA) program to the SJR study area. The approach will be to sample at about 30 sites between the 6 permanent sites 4 times in one year (summer, fall, winter/spring, and summer again for comparison). At each site, a manometer with a drive-point will be used to evaluate hydraulic gradients below the streambed. Also, temperature differences between the river and below the streambed will be evaluated and samples will be collected from below the streambed in gaining reaches to sample for nitrate and organic carbon