Entropy-Based Analysis for Spatio-Temporal Variability of Nitrate in Texas Aquifers Across Multiple Scales.

Nitrate in groundwater (GW) shows significant spatio-temporal variability across scales which comes from interaction among multiple geophysical factors such as irrigation pumping, rivers and aquifer interaction, types of aquifers, aquifer heterogeneity, geology, etc. For identifying dominant physical controls causing variability of nitrate-N in groundwater across scales, we developed an entropy based method to describe the spatio-temporal variability of nitrate-N at multiple scales. This study was carried out 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. A numerical study using Visual MODFLOW was conducted to verify the effect of different factors on spatial variability of nitrate-N at multiple scales. Results demonstrate that the spatial variability of aquifer nitrate-N is controlled by the GW pumping activities and hydraulic conductivity at the fine scale, the complex interactions between rivers and aquifers at the intermediate scale, and by lithology and geology at the coarse scale. There is maximum variability of nitrate-N contamination at the intermediate scale. In the Ogallala Aquifer, at all scales, overall mean nitrate-N has increased because of enhanced use of fertilizer since 1940. However, percent samples having higher nitrate-N (> 10 mg/L) have significantly decreased over the last seven decades. In the Trinity Aquifer, overall mean nitrate-N has declined with little change in the decadal variability from 1940 to 2008. Additionally, percent samples having nitrate-N> 10 mg/L did not change over time.