Draining the Landscape: Do milldam removals result in nitrogen leakage from riparian zones?

Abstract:

Dam removals are on the increase, with Pennsylvania currently leading the nation. While most dam removals are driven by habitat and safety considerations, we know little about how dam removals impact water quality and riparian zone processes. Dam removals cause a drop in stream water level, which results in drainage or dewatering of the riparian zone. We hypothesized that this drainage and drying of the riparian soils would increase nitrification (an oxic process) and decrease denitrification (an anoxic process), and thus result in nitrogen (N) leakage from riparian zones. We tested this hypothesis for the 5 foot high Krady milldam on Chiques Creek, PA which was removed in July 2018. Stream water, soil water, and groundwater was collected from the site and analyzed over two years for nitrogen species. Stream water NO₃-N concentrations varied between 2 -10 mg/L seasonally but did not reveal any long-term decline or increase. Groundwater NO₃-N concentrations remained low and ranged from of 0 - 4 mg/L. Shallow soil water NO₃-N concentrations were elevated and ranged between 1- 25 mg/L. Nitrification and denitrification rate measurements on riparian soils indicated that denitrification rates generally exceeded the nitrification values. We speculate that while dam removal and soil drainage have increased the rates of nitrification in these riparian soils, denitrification rates in groundwater continue to remain high enough to consume any excess NO₃-N. These results suggest that riparian systems are resilient to changes associated with dam removal or the changes that occur in riparian zone N budgets could take more time beyond the 2-year period of this study. Ultimately this study can provide guidance to scientists and watershed agencies on how dam removals can impact stream water quality, particularly as it relates to nitrogen cycling in the riparian zone.