IDENTIFYING SOURCES OF AGRICULTURAL GROUNDWATER CONTAMINATION BY USING CONCENTRATIONS OF MAJOR MINERALS, HERBICIDE RESIDUES AND DEGRADATES, AND STABLE ISOTOPE RATIOS

Groundwater contamination in intense agricultural regions is a critical issue. The primary cause is the release of agricultural chemicals into the environment, such as fertilizers, pesticides, and herbicides, and the discharge of waste waters generated by agricultural processing, municipalities, and confined animal facilities, such as diaries. Identifying possible sources of contamination is a basic requirement for any abatement actions to be effective. We focused on identifying the sources using stable isotope signatures, namely δ¹⁸O and δ¹⁵N of dissolved nitrate, together with the concentrations of major minerals, herbicide residues and degradates in groundwater samples. We conducted two studies in the eastern part of the San Joaquin Valley.

In our first study, we collected 52 samples from randomly selected domestic water wells in a mixed urban and agricultural area, approximately 8 x 3 Km, over an alluvial aquifer. It includes a municipal sewage treatment plant, a raisin processing plant, a meat processing plant, a turkey farm, various dairy operations, and agricultural fields. The results indicate that the ground water is primarily a mixture between the fresh water recharged from the mountains to the east and various components of irrigation and waste waters containing nutrients and other contaminants. We conclude that agricultural and industrial activities have significantly impacted the groundwater quality in the region. Furthermore, the δ¹⁸O and δ¹⁵N isotopic ratios and the concentrations of dissolved nitrate show that significant denitrification may have occurred locally.

In our second study, 69 water samples, all from agricultural water wells covering an area of 1,830 Km² of mostly farmland, were analyzed. Major mineral concentration data show trends consistent with nitrate contamination caused by the use of ammonium sulfate soil amendment. This interpretation is also supported by the isotopic signatures. However, unlike in the first study area, there is no evidence of significant denitrification. Dissolved nitrate concentrations show no correlation with the concentrations of herbicide residues, but instead, they do exhibit slight positive trends with herbicide degradates (DACT and ACET) concentrations. We conclude that most of the nitrate is from fertilizers.