Assessment and prediction of groundwater nitrate contamination trends in the Salt River Project area, Arizona

Historical nitrate occurrence in groundwater in the Salt River Project water service area, Arizona and projected future conditions are examined in this study. An empirical analysis of historical data was conducted to identify time trends and correlations with generalized land use categories and land use history (e.g., desert, agriculture, urban). This involved the assembly of a Geographical Information System database containing data from a wide variety of sources. In brief, nitrate concentrations could be correlated with land use history, and the highest concentrations (>10 mg/L) are associated with areas having agricultural use and the lowest concentrations (<5 mg/L) are associated with desert areas. Temporal trends from the 1930s to the 1990s generally show high concentrations decreasing from the 1930s until the 1970s and then increasing again to the mid-1990s. This is then followed by decreases until the present time. The temporal behavior correlates well with both groundwater elevation fluctuations and historical trends in fertilizer loading rates. A regional-scale groundwater nitrate transport model was also developed using a near-surface nitrogen budget, a near-surface water budget, an advective-dominated vadose zone transport model, and a groundwater flow model. The Avenue programming language was used to program the modeling components and to integrate them together in the ArcView environment. Some grid layers are generated to represent the areal-distributed parameters of the modeling components. This model was tested with available historical data and then used to project possible future groundwater nitrate contamination trends.