Abstract
Groundwater in California is very precious, yet what we can withdraw is often contaminated with natural and anthropogenic pollution sources. We have examined the Borrego Valley (BV) groundwater (N = 6 wells) in southern California to understand the source of arsenic and nitrate in some of its groundwater production wells. The results show that the arsenic values range from <2 ppb to 12.2 ppb and the nitrate values from <1 ppm to 10.2 ppm for different wells respectively. The results showed that the arsenic concentration increased 270% for the well # ID1-10 since 2004 and showed an increase of 63% since 2013 respectively. For other wells the results showed an increase of 147% and 72% since 2001. The nitrate concentration has jumped 42% in concentration since last year in one of the wells. The objective of this study is to understand the nature and source of arsenic and nitrate in BV groundwater aquifer as to how this change in arsenic and nitrate concentration occurs through the time. The arsenic retention in the sediments is highly variable and controlled by local processes as a result of natural weathering process of metamorphic bedrock. The second results from the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of arsenic from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to arsenic release. The high arsenic concentrations in some groundwater wells in Borrego Valley CA require the need for reconnaissance surveys in mineralized areas of fractured crystalline basement. Net groundwater extraction values are based on an irrigation efficiency of 78 percent with 14 to 22 percent irrigation return. We believe that the return flow from irrigational activity could be one of the major sources of nitrate transferring the agricultural contaminants such as nitrate to Borrego Valley aquifer.
Highlights
Groundwater is a major source of public drinking-water supply for many of the cities located in the Colorado Desert hydrogeologic province of California [1]
In terms of nitrate concentration, the nitrate values in the Borrego Valley (BV) study area groundwater wells were less than the health-based benchmark values (10 mg/l), but the results indicate the fluctuations of nitrate values possibly depending on the oxidation-reduction state of the groundwater [5]
Prior to the current study, the groundwater quality in the BV region was investigated by the US Geological Survey (USGS) from December 2008 to March 2010 and by Borrego Valley Water District (BVD) in the last 10 years
Summary
Groundwater is a major source of public drinking-water supply for many of the cities located in the Colorado Desert hydrogeologic province of California [1]. In terms of nitrate concentration, the nitrate values in the BV study area groundwater wells were less than the health-based benchmark values (10 mg/l), but the results indicate the fluctuations of nitrate values possibly depending on the oxidation-reduction state of the groundwater [5]. The understanding of groundwater chemistry, occurrence and distribution of chemical constituents in groundwater are valuable for identifying the natural and human factors affecting water quality [6]. Research studies indicate that the mobilization of arsenic and nitrate in sedimentary aquifers of these regions may be, in part, a result of changes in the geochemical environment due to agricultural irrigation [8]. Provides a baseline of groundwater quality in BV using the current ongoing research results and literature synthesis addressing the source and origin, trend, and pattern of contamination in BV aquifer. Processes causing changes in the aquifer geochemistry need to be considered since they directly affect the mobility of arsenic
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