USING SEDIMENT CORE ANALYSES FROM A MILLPOND TRIBUTARY DELTA TO MEASURE DOWNSTREAM IMPACTS OF A STORMWATER RETENTION POND

Urbanization leads to reduced infiltration, which magnifies stormwater runoff in catchments. Several Best Management Practices (BMPs) exist to mitigate these effects, including retention ponds that are designed to delay peak flow. However, retention ponds' failure to reduce discharge during storm events can cause high erosion and downstream sedimentation, which has negative implications for ecosystem health. Here we study the effects of a 1999 retention pond on stormwater management in a creek delta of Lake Matoaka, a historic 325 year-old millpond in Virginia’s Coastal Plain. To assess the effects of urbanization and the BMP on the creek delta, two sediment cores were taken and analyzed for fallout radionuclides ²¹⁰Pb and ¹³⁷Cs profiles to develop a chronology. There are remarkably high sedimentation rates in both core sites, at ~1cm/year for the creek delta core and ~0.63 cm/year lower millpond branch core. Sediment grain size distribution and organic matter composition was also analyzed to quantify the retention pond’s alteration of the creek’s hydraulic dynamics, changing which sediments are suspended and carried downstream. Grain-size analysis and loss-on-ignition measurements indicate that most of the sediments deposited in the lake’s delta are ~90% inorganic sands and silts. The evaluation and continued maintenance of retention ponds after installation is critical to protecting downstream ecosystems.