Assessment of Groundwater Nitrate Pollution Potential in Central Valley Aquifer Using Geodetector-Based Frequency Ratio (GFR) and Optimized-DRASTIC Methods

Anil Shrestha,Wei Luo

doi:10.3390/ijgi7060211

Abstract

Groundwater nitrate contamination in the Central Valley (CV) aquifer of California is widespread throughout the valley because of excess nitrogen fertilizer leaching down into the aquifer. The percolation of nitrate depends on several hydrogeological conditions of the valley. Groundwater contamination vulnerability mapping uses hydrogeologic conditions to predict vulnerable areas. This paper presents a new Geodetector-based Frequency Ratio (GFR) method and an optimized-DRASTIC method to generate nitrate vulnerability index values for the CV. The optimized-DRASTIC method combined the individual weights and rating values for Depth to water, Recharge rate, Aquifer media, Soil media, Topography, Impact of vadose zone, and Hydraulic conductivity. The GFR method incorporated the Frequency-Ratio (FR) method to derive rating values and the Geodetector method to derive relative Power of Determinant (PD) values as weights to generate nitrate susceptibility index map. The optimized-DRASTIC method generated very-high to high index values in the eastern part of the CV. The GFR method showed very-high index values in most part of the San Joaquin and Tulare basin. The quantitatively derived rating values and weights in the GFR method improved the vulnerability index and showed better consistency with the observed nitrate contamination pattern than optimized-DRASTIC index, suggesting that GFR is a better method for groundwater contamination vulnerability mapping in the CV aquifer.

Highlights

The Central Valley (CV) aquifer lies under one of the most productive areas of the United States
The Central Valley aquifer of California is a highly productive land, where 75% of the land is used for agriculture
The heavy application of fertilizer and changing hydrogeological condition caused by irrigation has exacerbated the groundwater nitrate contamination over last several decades in the CV

Summary

Introduction

The Central Valley (CV) aquifer lies under one of the most productive areas of the United States. It accounts for 17% of the total national irrigated land used for agriculture and 75% of the irrigated land of the state of California [1]. In the CV, for the past several decades huge amount of chemical nitrogen fertilizer has been applied to increase the crop productivity [2]. The excess nitrogen fertilizer, not absorbed by crops, as a chemical contaminant, dissolves in water and leaches into the aquifer to pollute the groundwater [4]. Harter [4] estimated more than 80 pounds of nitrogen per acre per year leaching into the groundwater. Several studies have attributed the groundwater nitrate concentration level to agricultural activities of the region [7,8,9]

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