Abstract
Nitrate-N (NO3 -- N) is one of the most pervasive contaminants in groundwater. Nitrate in groundwater exhibits long-term behavior due to complex interactions at multiple scales among various geophysical factors, such as sources of nitrate-N, characteristics of the vadose zone and aquifer attributes. To minimize contamination of nitrate-N in groundwater, it is important to estimate hot spots (>10 mg/L of NO3 -- N), trends and persistence of nitrate-N in groundwater. To analyze the trends and persistence of nitrate-N in groundwater at multiple spatio-temporal scales, we developed and used an entropy-based method along with the Hurst exponent in two different hydrogeologic settings: the Trinity and Ogallala Aquifers in Texas at fine (2 km × 2 km), intermediate (10 km × 10 km) and coarse (100 km × 100 km) scales. Results show that nitrate-N exhibits long-term persistence at the intermediate and coarse scales. In the Trinity Aquifer, overall mean nitrate-N has declined with a slight increase in normalized marginal entropy (NME) over each decade from 1940 to 2008; however, the number of hot spots has increased over time. In the Ogallala Aquifer, overall mean nitrate-N has increased with slight moderation in NME since 1940; however, the number of hot spots has significantly decreased for the same period at all scales.
Highlights
More than 1.5 billion people rely on groundwater as their primary source of drinking water [1]
Overall mean nitrate-N has increased with slight moderation in normalized marginal entropy (N ME) since 1940; the number of hot spots has significantly decreased for the same period at all scales
We have developed an entropy-based approach jointly with the Hurst exponent to explore the trends and persistence of nitrate-N in groundwater to inform and develop optimal groundwater sampling and remediation strategies, in two different hydrogeologic settings: the Ogallala and Trinity Aquifers in Texas
Summary
More than 1.5 billion people rely on groundwater as their primary source of drinking water [1]. Nitrate-N is one of the most ubiquitous contaminants in groundwater. Nitrate-N can persist in groundwater for a long time (years to decades); it can further increase to high levels as more nitrogen is used at the land surface due to anthropogenic activities. Several studies have noted that high nitrate-N (>10 mg/L of NO3 - - N) concentration in drinking water is a threat to human health, for infants and pregnant women [3,4,5,6]. To mitigate health risks, cleaning up nitrate-contaminated groundwater is an expensive and infeasible solution, especially for large aquifers [7]. To minimize contamination by developing optimal management strategies, it is desirable to analyze hot spots and hot moments or the long-term behavior of nitrate in groundwater
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