Lewis_etal_2021_Draining the landscape: How do nitrogen concentrations in riparian groundwater and stream water change following milldam removal?

Dam removals are on the increase across the US with Pennsylvania currently leading the nation. While most dam removals are driven by aquatic habitat and public safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals decrease the stream base level, which results in dewatering of the riparian zone. We hypothesized that this dewatering of the riparian zone would increase nitrification and decrease denitrification, and thus result in nitrogen (N) leakage from riparian zones. This hypothesis was tested for a 1.5 m high milldam removal. Stream, soil water, and groundwater N concentrations were monitored over two years. Soil N concentrations and process rates and δ15N values were also determined. Denitrification rates and soil δ15N values in riparian sediments decreased supporting our hypothesis but no significant changes in nitrification were observed. While surficial soil water nitrate-N concentrations were high (median 4.5 mgN L-1), riparian groundwater nitrate-N values were low (median 0.09 mgN L-1), indicating that nitrate-N leakage was minimal. We attribute the low groundwater nitrate-N to denitrification losses at the lower, more dynamic, groundwater interface and/or dissimilatory nitrate reduction to ammonium (DNRA). Stream water nitrate-N concentrations were high (median 7.6 mgN L-1) and contrary to our dam-removal hypothesis displayed a watershed-wide decline that was attributed to regional hydrologic changes. This study provided important first insights on how dam removals could affect N cycle processes in riparian zones and its implications for water quality and watershed management.