SPATIAL AND TEMPORAL PATTERNS OF NITROGEN LOADING FROM GROUNDWATER - AND THEIR RESPONSE TO MANAGEMENT ACTIONS - IN WATERSHEDS ACROSS COASTAL CONNECTICUT

Groundwater discharge is an important source of water and nutrients to streams and freshwater wetlands, but the spatial and temporal patterns of groundwater discharge are not well characterized in most areas. However, an understanding of the controls on delivery of nutrients via groundwater discharge, and how they respond to management practices, is vital to effective reduction of nutrient loading to sensitive freshwater and marine ecosystems. Elevated nitrogen loads are a pervasive issue in the Long Island Sound (LIS), a saltwater estuary that receives freshwater and nutrients from surface-water and groundwater discharge. Prior work on the northern shore of LIS (Connecticut and areas of New York and Rhode Island) suggested that groundwater travel times are relatively short (median < 2 years) and that decade-long nutrient legacies are not widespread, but that seasonal flow and nutrient transport dynamics may be important. In this study, a monthly MODFLOW groundwater flow model (modeled area of 9,300 km²) coupled with MODPATH particle tracking was used to better characterize spatial and temporal patterns of nitrogen loading from atmospheric deposition, septic systems, and fertilizers via the groundwater discharge pathway within the study area. We calculated monthly nitrogen loads and travel times by nitrogen source for each HUC12 watershed. Model calibration was done with an ensemble approach that provided a distribution of parameter values, thereby allowing us to quantify the uncertainty in the simulated flows and nitrogen loads. Next, we simulated multiple potential management scenarios, including septic system upgrades and reductions in turf-grass fertilizer use, and analyzed the temporal lag and likelihood of substantial load reductions from each intervention. Improved understanding of the spatial and temporal patterns of groundwater nitrogen loads, and their sensitivity to management actions, will provide essential management guidance for the reduction of nutrient loads to sensitive ecosystems.