Water Quality Monitoring of Stream Restoration for Legacy Sediments at Gramies Run, Maryland

Streambank legacy sediments are an important source of nutrients (nitrogen and phosphorus) in the Chesapeake Bay watershed. These sediments can also contribute high suspended sediment loads and increase water turbidity. Many jurisdictions holding Municipal Separate Storm Sewer System (MS4) permits are required to restore land to try to reduce pollutants to the Bay. Some are using stream restoration as a tool to receive credits and meet nutrient and sediment reduction targets. Credits are based on default restoration criteria despite differences in project designs and sites, and pre- and post-restoration monitoring is not common to assess if they achieve estimated reductions.

Gramies Run is a second order, Piedmont stream (drainage 2.96 mi²) located in Cecil County, Maryland. It is a tributary of Big Elk Creek, which drains into the Bay. A $4.3 million restoration project will grade vertical streambanks composed of legacy sediments to reduce erosion and add instream features to promote denitrification. The restoration will be completed in phases, for three stream reaches. Construction started at the lowest reach on June 15, 2018. Two other reaches will be restored by early 2020. This study seeks to determine how restoration impacts water quality and how measured sediment and nutrient reductions compare to design reductions.

Water quality monitoring is being conducted at three stream locations upstream (GR1) and downstream (GR3) of the restoration reaches and pre- (since July 2017), during- and post-restoration. In situ measurements using automated sondes will be taken every 30 minutes throughout the study period. Grab water samples will be collected weekly to measure total nitrogen, NO₃^-, PO₄^-, NH₄⁺, and total organic carbon. Storm samples will be collected to evaluate changes in suspended sediment concentration (SSC) during high flow. Pre-restoration dissolved NO₃^--N concentrations ranged from 0.729-1.984 mg/L at GR1 and 0.636-1.585 mg/L at GR3. SSC ranged from 6.5-29.6 mg/L at GR1 and 6.9-107.3 mg/L at GR3. Eventually, this study will also compare the Gramies Run restoration with other types of legacy sediment restoration practices and evaluate their role in meeting Chesapeake Bay water quality and TMDL goals.