Abstract
As climate change has increased the frequency and severity of droughts in the American Southwest, California has become increasingly dependent on its groundwater resources. Due to the state’s dependence on groundwater for drinking-water supplies, nitrate contamination in groundwater is of high concern, especially in agricultural regions such as the Central Valley. Repeatably consuming water with nitrate above the maximum contaminant limit set by the United States Safe Drinking Water Act over long periods of time, can lead to adverse health effects. Prior studies have examined water-quality trends in broad depth zones (e.g. shallow and deep aquifers systems), but vertical transport dynamics of agriculturally-affected groundwater from shallow to deep portions of regional aquifer systems is unclear. This study examines well-construction depth trends for populations of public-supply wells in the Central Valley that are vulnerable to nitrate contamination. The primary hypothesis of this study is that wells in the Central Valley affected by elevated nitrate levels are getting deeper over time due to the vertical migration of nitrate-contaminated groundwater deeper into regional aquifer systems during the past 20 years.
This study splits the well-construction and water-quality data within the Central Valley into nine study areas and bins data into four 5-year time periods from 2000 to 2019. The population of wells with well-construction and water-quality data within a given time period was then subdivided into three groups: 1) all wells sampled in a time period, 2) all wells sampled in a time period that had less than or equal to five milligrams per liter of nitrate, 3) all wells sampled in a time period that had greater than five milligrams per liter of nitrate. Non-parametric statistical comparisons showed that throughout the Central Valley elevated nitrate wells tended to be shallower than low nitrate wells, with the exception of those in the Southern San Joaquin Valley, where this relationship was inverted. Temporal comparisons showed that although there were no significant differences in sampled well construction depths over time, the depths of wells that had nitrate concentrations over five milligrams per liter, particularly the depth to top of perforations, increased over time in the Central and Southeastern San Joaquin Valley study areas. In these areas the probability wells will have nitrate concentrations over five milligrams per liter increased by an average of 15 percent in the 200-to-400-foot depth zone. This study statistically characterizes changes in aquifer depth zones vulnerable to nitrate contamination over time and may help to predict depth zones in Central Valley aquifer systems that are vulnerable to contamination in the future.