EFFECTS OF STORMWATER RUNOFF AND OVERLAND FLOW ON RETENTION PONDS AT GEORGIA SOUTHERN UNIVERSITY

This study examines the effects of parking lot overland flow and roof runoff inputs on the water chemistry of two retention ponds located at the Recreational Activity Center on the Georgia Southern University campus. The primary input to the ponds is calcium-bicarbonate groundwater pumped from the Floridan Aquifer. Water samples were collected every two weeks from both ponds during the fall and summer of 2010. Samples of overland flow and roof runoff were collected during storm events. Temperature, dissolved oxygen, and specific electrical conductivity were measured onsite; samples were then taken to a campus lab and analyzed for major cations and anions using an ion chromatograph. Precipitation data were obtained from a weather station located approximately 13 km from the study area.

The overland flow and runoff were characterized as having relatively low specific conductivity (15-113 μS/cm), acidic pH (4.5 to 6.4), and high dissolved oxygen content (80-99%). The storm inputs were in marked contrast to the pond water itself, which had relatively high specific conductivity (157-240 μS/cm), alkaline pH (7.5 to 9.2), and dissolved oxygen content that ranged from undersaturated to supersaturated (59-157%). Trilinear (Piper) diagrams show that the ponds contain calcium bicarbonate type water, and while the dominate species in the overland flow and runoff inputs were calcium and bicarbonate, the stormwater had a more mixed chemistry and contained elevated levels of sodium, chloride, and sulfate. The overland flow and runoff also contained elevated levels of ammonium, nitrate, and phosphate relative to the ponds. Because the stormwater originated from asphalt surfaces, its water chemistry is believed to largely be influenced by atmospheric fallout.

Temporal variations in the general chemistry of the ponds do not appear to be influenced by stormwater inputs. This can be explained by the relatively small volume of stormwater compared to the pond volume, and by the general chemistry of the storm and pond waters being somewhat similar. In contrast, the overland flow and runoff inputs of nutrients did have a small impact on the nutrient concentration in the ponds, particularly for ammonium. This is attributed to the concentration of nutrients in the stormwater being significantly higher than the ponds.