GROUNDWATER QUALITY AND ISOTOPIC STUDIES OF THE KINGS RIVER ALLUVIAL AQUIFER

Kings River alluvial fan is located on the east side of the San Joaquin Valley where groundwater quality is affected by intense agricultural and industrial activities as well as urban development. To understand how groundwater chemistry and quality vary within the alluvial aquifer, two study sites were selected for sampling and analysis, including d¹⁵N and d¹⁸O of dissolved nitrate. Two different study approaches were used.

The first study is a general survey in a mixed urban-agricultural setting, covering an extended area of 5 miles long by 1 to 2 miles wide, oriented along the general groundwater flow direction. This area includes a municipal waste water treatment facility, a raisin processing plant, a meat processing plant, a turkey farm, various dairy operations, and agricultural fields. Fifty-four randomly selected samples were collected from both domestic and monitoring wells, and analyzed for nutrients, major minerals, and nitrate isotopes.

The second is a more focused study. The area is a carefully designed 3-mile transect consisting of 20 monitoring wells of depths ranging from 70 to 270 feet, placed at 6 different sampling locations along the groundwater flow path. Vineyards and orchards are predominant land use. These wells are constructed as a part of the USGS Water Quality Assessment Program. Water samples are similarly analyzed. They provide an accurate snapshot of water quality data as a function of depth and location along the flow path.

The results indicate that the chemistry of the groundwater in the alluvial aquifer is primarily a product of mixing between the pristine Sierran water originated from the mountains and various components of irrigation and waste waters containing nutrients and other contaminants. Therefore, we conclude that agricultural and industrial activities have impacted the groundwater quality in the region. Furthermore, the d¹⁵N and d¹⁸O isotopes and the nitrate concentration data show that significant denitrification may have occurred locally in the first study area, but is absent from the second study area where the relatively oxic condition of the sandy soil does not provide an environment for denitrification to occur.

[These studies were performed with the collaboration of CA Department of Water Resources, USGS - Menlo Park, and USGS - Sacramento.]