Improving Regional and Local Groundwater Quality Through Groundwater Recharge Management: A Focus on Nitrate Trends in the Tulare Lake Basin and Salinas Valley

Agricultural production has increased substantially in the past 60 years as we try to feed an increasing population. Water extraction technologies combined with the ability to synthesize fertilizer have facilitated this expansion. Although the increase in food production has provided necessary sustenance, it has had a negative impact on groundwater resources in agriculturally intensive areas of the world. Groundwater recharge from agricultural sources is often elevated in total dissolved salts and minerals (TDS) and contains concentrations of nitrate from fertilizer that exceed drinking water standards. As this source of recharge travels deeper into aquifer systems it can reach depths where supply wells extract drinking water, contaminating the source of drinking water. Research has shown that water withdrawn from a well originates from several sources, rather than a single source, and the time of travel for the groundwater between these sources and a single well can range from years to centuries. Due to the delayed response from source to well screen, contaminated sources for wells from fertilizer use for which the travel time exceeds 50 to 60 years have not yet contaminated that well. As time moves forward, however, the fraction of water originating from contaminated sources will increase, and the potential exists for higher contaminant concentrations in the extracted water during the coming decades to centuries. While many studies have documented facets of the problem at various scales such as mechanisms of transport, biological and chemical transformation, spatial distributions and frequencies of detection at the regional scale, and aquifer vulnerabilities at the basin scale, few studies have provided evidence of long-term nitrate concentration trends at the basin scale. An apparent upward trending of nitrate concentrations has been noted in many studies, but the sample sizes and time periods tend to be small.;In this study we use a regional statistical test for trend on nitrate data from 1949--2011 in the Tulare Lake Basin and Salinas Valley agricultural basins of California. Results show statistically significant, increasing nitrate concentrations in California's Tulare Lake Basin. Similar results were found in confined portions of the Salinas Valley. Additionally, we investigate two areas where groundwater recharge management practices have inadvertently reversed local and regional scale nitrate contamination because of the addition of low nitrate water to the subsurface. First, groundwater quality in proximity to an artificial recharge basin in the Fresno urban area is examined, where water from the San Joaquin and Kings Rivers has been used to enhance groundwater recharge for more than 40 years. Second, we look at the unconfined aquifers of the Salinas Valley. In this basin, reservoir construction (1957 and 1965) and reservoir release management has increased the amount of recharge to the subsurface by maintaining perennial flow in the Salinas River. The area has also experienced a significant shift to more efficient irrigation technology such as drip irrigation. In both circumstances it appears that low nitrate recharge has resulted in beneficial impacts on groundwater nitrate concentrations.