ASSESSMENT OF CONTAMINATED STORMWATER SEDIMENT IN A KARST LANDSCAPE

Typical stormwater discharge monitoring includes the chemical analysis of the stormwater and the total suspended solids (TSS). Little attention is paid to the chemical quality of the sediment in the stormwater. Over time, storm events can result in the discharge of hundreds of tons of sediment with unknown chemical concentrations into retention ponds, surface water bodies, and directly into drinking water aquifers.

A stormwater drainage ditch that receives runoff from a 824-acre site is located in a karst limestone valley in south-central Pennsylvania, and the soluble nature of the karst results in the many rocky sinkhole locations along its length. Stormwater runoff that enters the ditch during a typical rain event is captured in these sinkholes into the underlying aquifer. Only about six storms per year cause runoff to overflow the sinkholes and travel the length of the ditch to a perennial stream named Trindle Spring Run.

Stormwater and suspended sediment samples were taken from site outfalls, an industrial property outfall, sinkhole locations in the ditch, the confluence of the ditch with Trindle Spring Run, and local springs that are known to have a hydraulic connection to the sinkholes. Hundreds of gallons of stormwater were collected at each location and centrifuged to create separate samples for stormwater and suspended sediment. The samples were analyzed for contaminants of concern including polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAH), and metals.

By combining an existing annual water runoff budget with the sediment runoff budget, it was found that 63 of the 97 tons of suspended sediment that enter the ditch on an annual basis are captured by sinkholes and directed into the underlying aquifer. The average concentration of PCB and PAH in the sediment is 3 and 12 mg/kg, respectively. Of the remaining 34 tons of suspended sediment, 12 tons are deposited on the ditch floor and 22 tons are transported directly to Trindle Spring Run. In addition, it was found that the suspended sediment samples collected from the local springs contained diluted concentrations of PCB and PAH.