Reactive nitrogen (N) applied to land surfaces accumulates in aquifers, creating a source of legacy N that is later discharged from ground to surface waters. Focused groundwater discharges (preferential seepages) can act as legacy N point-sources and confound surface water quality management strategies that are based on reducing contemporary N application and sewage treatment discharges. Predicting the timing, magnitude, and spatial distribution of legacy N delivery to surface waters is complex because it is determined by both transport and processing along regional-scale groundwater flow paths and the local-scale reactivity of stream interface sediments at the terminal end of long groundwater flow paths. The overall objective of the proposed research is to develop a robust approach for mapping comprehensive spatial patterns of groundwater discharge and the role of stream interface sediments in attenuating legacy N at the watershed-scale. Our approach integrates spatial distributions of simulated groundwater discharge from existing regional models with emergent remote sensing technologies; we will conduct extensive thermal infrared (TIR) surveys at a cohesive spatial scale not before attempted across stream orders. Focused groundwater discharges identified during TIR surveys will be extensively characterized by fine-scale, near-stream measurements of groundwater discharge, N flux, and N removal rates, to quantify both the spatiotemporal distribution and drivers of legacy N processing at the stream-groundwater interface. Finally, we will refine river network models to integrate observed groundwater discharge patterns and to scale near-stream legacy N processing to the whole watershed.
This work is focused on the Farmington River watershed (1571 km2) located primarily in northwestern CT and southwestern MA. The Farmington discharges to the Connecticut River, which discharges to the Long Island Sound. Principal bedrock aquifers in the Farmington are the New England Crystalline-rock aquifer and the Mesozoic sandstone and basalt of the Newark Supergroup. The bedrock is overlain by glacial till across most of the watershed, with areas of valley fill stratified drift aquifers. The watershed has experienced substantial changes in land cover over the last several decades. Between 1973 and 2011 there was an approximate 40% reduction in agricultural and a 20% gain in urban land cover.
Our objectives are to 1) estimate the spatial distribution of groundwater discharge using a traditional groundwater modeling approach; 2) compare spatial patterns of modeled groundwater discharge to observed groundwater seeps over 10’s of km of river length; and 3) identify initial locations within the Farmington where stream interface sediments are potentially important filters or conduits of legacy reactive nitrogen from groundwater to surface water. This project landing page will serve all of the Science Base data releases associated with this work.