THE IMPACT OF A RETENTION POND ON SMALL STREAM GROUNDWATER-SURFACE WATER INTERACTIONS IN THE VIRGINIA COASTAL PLAIN

Urbanization is a major factor leading to changing groundwater-surface water interactions in streams. Development of urban areas increases impervious surfaces leading to more surface runoff to small, concentrated areas. Retention ponds are one solution that collects surface runoff; however, the release of stormwater from retention ponds into small streams can often lead to substantial changes to the stream’s structure and functioning. One impacted area is a retention pond located in Williamsburg, VA that discharges into Strawberry Creek. Due to the retention pond, there is massive erosion that has created an incision of 30 feet over the last 20 years. Surface water temperature, vertical hydraulic gradient, and streambed hydraulic conductivity were measured to understand the impact of a retention pond on Strawberry Creek. Results were compared with Pogonia Creek, a healthy, unimpacted stream. Along the stream, Strawberry Creek displayed an unstable surface water temperature profile of alternating sections of warming and cooling. Temperature was significantly warmer and had a larger temperature range in comparison to Pogonia Creek. Hydraulic conductivity is predicted to be lower at Strawberry Creek due to larger amounts of fine-grained sediment being deposited downstream. Vertical hydraulic gradient is predicted to be greater and more variable at Strawberry Creek because of substantial water level changes from the retention pond during storm events. Overall, the goal of this research is to better understand how and to what extent groundwater-surface water interactions are changing in urbanized watersheds.