CONTAMINANT TRANSPORT AND PREDICTIVE MODELING IN THE UPPER SANTA CRUZ BASIN

The Santa Cruz River originates in the United States in the San Rafael Basin and flows south into Sonora, Mexico before flowing back into the United States near the twin cities of Nogales, Sonora and Nogales, Arizona. This semi-arid region has experienced population growth and industrial development that has resulted in increasing water demand and greater risk of groundwater contamination. Moreover, historic mining activities and ore deposits in the Patagonia Mountains have resulted in heavy-metal contamination of some tributaries to the Santa Cruz River. Increased water demand and subsequent pumping have lead to ephemeral flow in portions of the river, while discharges from the Nogales International Wastewater Treatment Plant (NIWTP) now sustain perennial flow in the downstream reach. In combination, these factors have altered the natural groundwater flow patterns in the basin. The objective of this study is to examine the impact of these changes on contaminant transport in the subsurface and human-health risk using a groundwater flow model developed for the portion of the aquifer downgradient of the NIWTP. Water-quality data from numerous sources (state agencies, consultant reports, USGS and EPA national databases, as well as published sources) are being collected and aggregated to identify major contaminants and their distribution. Numerical modeling is being conducted using particle tracking to simulate non-reactive transport under various scenarios, including changes in climate, such as decreasing precipitation, and changes in land-use activities, such as increasing development and urbanization and decreasing agriculture. Additional analyses will employ a reactive transport model. The results of this study will be used to help local communities evaluate the impact of land-use and climate change on groundwater contamination and potential human-health risks.