Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/8/2011
Publication Date: N/A
Citation: Interpretive Summary: Utilization of stormwater runoff for replenishing municipal water supplies has increased in semi-arid regions where water resources are scarce and populations continue to grow. Urban land use can degrade runoff water quality and because this water is destined to recharge groundwater supplies, understanding the dynamics of water pollutant movement is clearly of importance. In 2009, we sampled groundwater wells throughout a recharge basin in Tucson, Arizona and used modern isotopic techniques to estimate water age. These methods allowed us identify areas where groundwater recharge was occurring most rapidly and to identify pollution sources. We found widespread contamination of groundwater with modern pollutants, indicating that surface water was not being cleansed by diffusing through soil pores. Rather, unimpeded flow of degraded surface water into groundwater was vastly increasing the possibility of groundwater contamination. These results will be incorporated into predictive models to identify threats to recharge waters and groundwater supplies, and municipal water managers will use the model results to identify best approaches to mitigating potential threats to drinking water quality.
Technical Abstract: The management of groundwater resources is paramount in semi-arid regions experiencing urban development. In the southwestern United States, enhancing recharge of urban storm runoff has been identified as a strategy for augmenting groundwater resources. An understanding of how urbanization may impact the timing of groundwater recharge and its quality is a prerequisite for mitigating water scarcity and identifying vulnerability to contamination. We sampled groundwater wells along the Rillito Creek in southern Arizona that had been previously analyzed for tritium in the late 1980s to early 1990s and analyzed samples for tritium (3H) and helium-3 (3H/3He) to evaluate changes in 3H and age date groundwaters. Groundwater samples were also analyzed for chlorofluorocarbons (CFCs) and basic water quality metrics. Substantial changes in 3H values from waters sampled in the early 1990s compared to 2009 were identified after accounting for radioactive decay and indicate areas of rapid recharge. 3H-3He groundwater ages ranged from 22 years before 2009 to modern recharge. CFC-11, -12 and -113 concentrations were anomalously high across the basin, and non-point source pollution in runoff and/or leaky infrastructure was identified as the most plausible source of this contamination. CFCs were strongly and positively correlated to nitrate (r2=0.77) and a mobile trace metal, nickel (r2=0.71) and negatively correlated to sulfate to chloride ratios in groundwater suggesting that solutes were sourced from a similar source. Findings from this study suggest new waters from urban non-point sources are contributing to groundwater recharge and adversely affecting water quality. Reducing delivery of contaminants to areas of focused recharge will be critical to protect future groundwater resources.