Risk Assessment of Potential Groundwater Contamination by Agricultural Drainage Water in the Central Valley Watershed, California, USA

Risk assessment of potential groundwater contamination is essential for safeguarding public health, ensuring agricultural sustainability, and maintaining ecological balance. Groundwater, a primary source for drinking and irrigation, is threatened by pollutants. Proactive risk assessments prevent costly remediation and guide informed policy making and community actions, ensuring a clean and sustainable water source for current and future generations. The Central Valley aquifer in California had extensive groundwater nitrate (NO3) contamination due to excessive nitrogen fertilizer seeping into the aquifer. Numerous hydrogeological conditions of the region affect the percolation of NO3. Hydrogeologic conditions are used in groundwater contamination vulnerability mapping to identify susceptible regions. This chapter introduced DRASTIC and geodetector-based frequency ratio (GFR) methods to develop indices for NO3 vulnerability. In this study, seven different variables with different weights and rating values were combined using the DRASTIC method. The vulnerability index was improved by the quantitatively derived rating values and weights used in the GFR methods compared to the DRASTIC method. These values showed superior concordance with the observed NO3 contamination pattern to map the vulnerability to groundwater contamination, indicating that GFR was a superior method. Moreover, to create a NO3 sensitivity index map, the geodetector method (GED) was combined with the frequency ratio (FR) method to derive rating values and the geodetector method to derive relative power of determinant (PD) values as weights. Furthermore, geodetector (GED), principal component analysis (PCA), and geographically weighted regression (GWR) were used in a comparison study to find the most effective technique for identifying environmental elements influencing GW-NO3 concentration. The outcomes revealed that the San Joaquin and Tulare Lake Basin watersheds had notably higher proportions of wells with NO3 concentrations exceeding 5 mg/L compared to the Sacramento Valley. Overall, the GED method performed better in identifying the impact of explanatory variables on GW-NO3 contamination than the PCA and GWR methods.