Jason Gurdak - Scale Dependence of Controls on Groundwater Vulnerability to Nitrate Contamination

Scale Dependence of Controls on Groundwater Vulnerability to Non-Point-Source Nitrate Contamination, California Coastal Basin Aquifer System Jason J. Gurdak1, San Francisco State University, Dept. of Earth & Climate Sciences, jgurdak@sfsu.edu, CC +1-415-338-6869 Claudia R. Corona5, San Francisco State University Gabriela E. Geyer2, San Francisco State University, Dept. of Earth & Climate Science Leora Nanus3, San Francisco State University, Dept. of Geography & Climate Science Makoto Taniguchi4, Research Institute for Humanity and Nature Understanding the controls on nonpoint-source (NPS) nitrate (NO3–) contamination in groundwater is motivated by the widespread detection of NO3–, implications for human health and aquatic ecosystems, groundwater sustainability, and a growing realization that such understanding across spatial scales promotes management and policy choices that optimize the Water-Energy-Food (WEF) Nexus. In the Coastal California basin aquifer system (CCB), the conflicts in the WEF Nexus are apparent because of intensive agricultural practices that have contributed to chronic NO3– loading to groundwater. Here we evaluate the scale-dependent relations among source, transport, and attenuation (STA) factors that control NPS NO3– contamination in recently recharged CCB groundwater. We present univariate and multivariate logistic regression models for two spatial scales common to management and policy decisions: sub-regional scale (the north, central, and south CCB sub-regions) and regional scale (the entire CCB region). Here we define STA scale dependence if such factors are statistically significant in either sub-regional or regional models and STA scale invariance if such factors are statistically significant in both sub-regional and regional models. We find that dissolved oxygen (DO) (attenuation) in groundwater is often the most significant STA factor in all model domains, indicating that DO is an important, scale-invariant factor controlling NPS NO3– contamination. During the collection of water quality data for this study, we found that DO is not regularly collected during routine groundwater sampling. Considering the importance of DO on denitrification and the relative inexpensive tools used to measure it, DO should be regularly monitored. Farm fertilizer (source) is also a significant scale-invariant factor, while many of the transport factors are scale-dependent factors. We present vulnerability maps that illustrate the spatial patterns of predicted probability of detected elevated NO3–. Findings here improve knowledge about the scale dependence of STA factors, which help decision makers develop best management strategies and policies that advances groundwater sustainability and optimizes the WEF Nexus.