2005: Water Quality in a Retention Pond During a Storm Event

Sarah Gruber (Biology) Mount Holyoke

The creation of impermeable surfaces in developed land causes rain water runoff to increase. If the runoff water is allowed to directly flow into natural rivers and streams natural water bodies will receive increased levels of sediments and nutrients that are found on the impermeable surfaces. To counteract the effects of development on water quality developers are required to create storm water management plans. Retention ponds are currently the most popular option for managing stormwater; however, the effectiveness of these ponds is highly variable. This study looked at the effectiveness of a retention pond in James City County, Virginia. The pond is located in Ironbound Village, a four year old development composed of single family houses and small office buildings. This pond has had major problems since it was first constructed in 2001. It was hypothesized that the water quality would decrease as the water passed through the pond.

ISCO automated water samplers were used to collect data at the inflow and outflow of the pond. The inflow machine was manually turned on once a storm was detected. The outflow machine was connected to a flow meter that would trigger it to begin sampling once the water level hit a certain height. After a storm the samples were brought back to the lab and analyzed for conductivity, fecal coliform, total suspended sediments (TSS), ammonium, phosphate and nitrate/nitrite concentrations.

The inflow and outflow TSS loads were found to have little lag time once the runoff water peaked at the inflow. The inflow brought in 11763 g of sediments into the pond while the outflow released 14610 g (32 lbs) of sediment into the stream increasing the TSS load by 2847 g. When looking at the nitrate/nitrite concentrations it was found that the outflow values vastly exceeded the inflow concentrations. Autotrophic bacteria are known to use ammonium as electron donors and then to release nitrate as their by product. When outflow concentrations for nitrate/nitrite and ammonium were plotted together they showed inverse oscillations, where nitrate/nitrite peaked when ammonium dropped and vice versa. The extra ammonium was most likely stuck to the sediments that were resuspended in the water during the storm. This then provided the autotrophic bacteria with a new source for electron donors and allowed them to increase their activity.

This study showed that Ironbound Village retention pond is currently not functioning properly. The pond is increasing the amount of sediments that are being released into the stream. These increased sediments allow autotrophic bacteria to cause nitrification of the pond during storm events; thus, giving the stream higher nitrate/nitrite concentrations along with increased suspended sediments.

For additional documentation Sarah Gruber provided a PowerPoint Presentation entitled "Water Quality in a Retention Pond During a Storm Event " provided here in PDF form.