Draining the landscape: how does the drop in groundwater level following dam removal affect leaching of nitrogen (N) from riparian zones?
Date
2021
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Dam removals are on the increase, 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 result in a drop in stream water level, which results in drainage or dewatering of the riparian zone. It is 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. This hypothesis was tested for the 1.5 m high Krady milldam on Chiques Creek, PA which was removed on July 9, 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 surficial 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. This is the first study that has investigated the effects of dam removal on riparian groundwater N. Initial results from this study will help watershed managers and agencies make informed decisions with regard to dam removals and their impacts on water quality, particularly as it relates to nitrogen cycling in the riparian zone.
Description
Keywords
Dams, Denitrification, Nitrification, Nitrogen, Riparian zones