HISTORICAL, PHYSICAL, AND CHEMICAL CHARACTERISTICS OF A STORM WATER RETENTION BASIN, BRYN MAWR, PA: PROJECTED-VS-ACTUAL OUTCOMES

Bryn Mawr College, located in southeastern Pennsylvania, built a storm water retention basin (Rhoads Pond) in 2001 to control regional erosion, flooding, and pollution. Historically, a creek ran through campus until 1900 when it was diverted underground to accommodate an athletic playing field. However, this field and the surrounding suburban area often flooded, representing a regional drainage problem, which the construction of Rhoads Pond attempts to resolve. The pond consists of a forebay (collecting initial water flow, including sediments and pollutants) and a main bay that are separated by a filtering gabion wall. Water enters through a culvert (~4’ diameter) and passes through a feeder delta before it reaches the forebay. Water level and exit water flow are manually controlled through a water flow box located in the main bay. Water then travels in underground pipes until its discharge into Mill Creek. Sediments collected across the feeder delta, potential provenance locations, and sites in Mill Creek were dried, sieved, and analyzed under the microscope to determine mineral content and grain size trends throughout the drainage basin. Ground Penetrating Radar (GPR) successfully imaged depth-to-sediment and depth-to-bedrock in the forebay, from which sedimentation rates will be calculated. GPR data will be collected in the main bay using a grid pattern, allowing for the analysis of the pond in three dimensions. Precipitation data from the past three years will be used to reconstruct water influx effects on sedimentation rate. Water samples were collected every two weeks and analyzed for field parameters and major ions. Acid extractions were performed on pond sediment samples and analyzed for copper, zinc and iron. Preliminary evaluation of pond water chemistry suggests that the water reflects a dolomitic signature. This agrees with an X-ray diffraction analysis of a sediment sample from the pond. Sediment-bound iron occurs on an order of magnitude of a few thousand mg/kg soil, copper a few hundred mg/kg soil, and zinc tens of mg/kg soil. All results will be compared to theoretical models created before the pond was built to determine the accuracy and success of the project.