Study Examines Pollutant Loading in Watershed Stormwater Ponds

KCI partnered with Baltimore County, Towson University’s Urban Environmental Biogeochemistry Laboratory, and Chesapeake Environmental Management (CEM) to conduct a research study on the pollutant removal efficiencies of self-converted dry detention stormwater ponds, which have evolved over time to include shallow marsh and forested wetland systems. These facilities have not been well studied and may provide enhanced pollutant removal due to the development of wetland conditions and functions. The goal was to better understand how these stormwater best management practices (BMP) reduce specific pollutants and to more effectively prioritize restoration activities.

TMDL Pollutant Removal Efficiencies

In response to ongoing estuary-wide impairment, the Environmental Protection Agency (EPA) developed the Chesapeake Bay Total Maximum Daily Load (TMDL) for nitrogen, phosphorous and sediment in 2010. The Maryland Department of the Environment (MDE) calculates and records progress toward the program’s goals via application of credits, which quantify a municipality’s pollution reduction achievements based on what can be expected from different types of restoration and retrofit projects.

Although dry detention or dry extended detention ponds are currently considered to offer no reduction of pollutant loads, this study indicates that these facilities can provide increased removal efficiencies and should be eligible for credits.

Fields of Harvest
Ponds selected for this study range from those in highly commercial areas that drain acres of impervious area to facilities like this one in low density residential developments.

Dry detention pond BMPs were originally designed and installed to provide quantity control, with little to no water quality treatment. Stormwater is generally held for no longer than 24 hours and is slowly released to receiving waters, usually a stream or river. This short retention time allows for only limited settlement, microbial processing or vegetative uptake of pollutants.

To measure pollution reductions, KCI monitored storm flow and runoff quality at six ponds. Three had self-converted, and the others were conventional dry control ponds. Water quality sampling was performed at each pond during eight storm events spread over the course of 12 months.

Hunt Ridge Storm Sampling
During storm events, samples were taken from each inlet/outlet location. Discharge levels were recorded at the same time and at five to ten minute intervals during storm flow.

Scientists monitored storm and base flow (when present) by sampling water flowing into and out of each pond. Rain gauges collected continuous rainfall data, and pressure transducer level loggers and flow gauging devices documented continuous discharge at all inflow and outflow structures. Discrete water quality samples were collected at inlets/outlets and were analyzed in the laboratory to identify suspended solids and several varieties of nutrients including total nitrogen and total phosphorus.

Analysis of the data suggests that mature (decades-old) dry detention ponds provide greater removal efficiencies than the crediting previously provided, whether they are considered self-converted or unconverted. And the data suggests that the self-converted facilities are outperforming the unconverted ponds, which would make them less desirable to retrofit.

Pollutant Removal Efficiencies

The calculated removal efficiencies from this study provide further evidence for increased credit for these mature detention ponds as part of the Chesapeake Bay restoration framework and potentially a new classification for self-converted facilities. Results can also help the county demonstrate progress towards addressing the stormwater wasteload allocations for water quality pollutants under its NPDES MS4 permit, and enhance the ability to more effectively prioritize restoration activities for pollutant load reductions.

Sodium and Chloride Analysis

McCormick Warehouse
Continuous discharge was monitored for each site using data loggers paired with flow restriction devices.

Scientists also investigated the impact of salt related to deicing activities during winter weather events. While there are currently no water quality regulations for chloride in Maryland, this study indicates that stormwater management facilities could be acting as a long-term, year-round source into the ground- and surface water.

Although all six ponds provided volume reduction, there was no evidence of load reductions for sodium, calcium, or chlorine at any of the sites. Instead, the ponds were measured releasing higher levels of each chemical than they were taking in, suggesting that ions remain in the pond long after winter snow and ice events. Seasonal storms in the spring, summer and fall dissolve and transport the deposits into receiving streams causing year-round elevated concentrations.

“State and local governments have the difficult decisions with regard to keeping roads safe in the winter without over applying salt as we are seeing large spikes of sodium and chloride entering freshwater streams after snow events,” said Scientist Rob Owen. “Many sensitive fish and bug species in streams are being extirpated.”

Four of the six study ponds drain to tributaries that lead to nearby Loch Raven Reservoir, which is part of Baltimore City’s water supply system and already exceeds the EPA’s sodium water quality standard for safe drinking water. In addition, multiple effluent samples and several baseflow samples exceeded the EPA’s acute or chronic criteria for chloride. Although more study is needed, the results suggest that regulators and municipalities should consider residual sodium and chloride release in both their water quality compliance and storm preparedness strategies.

County Roads Salt Facts