July-August 2010

Sediment Contamination in Stormwater Detention Ponds

Accumulated pollutants can reach levels of ecological and human health concern.

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All pictures: John E. Weinstein

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Sediment Contamination in Stormwater Detention Ponds
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Since the early 1990s, the state of South Carolina has required approved stormwater management plans prior to land disturbance activities for all residential and commercial developments within one-half mile of a natural receiving water body in order to protect against flooding and degradation of water quality. One recommended best management practice (BMP) for meeting these regulatory requirements has been the design and construction of stormwater detention ponds. In the ensuing years, detention ponds have become quite popular, not only because they address state requirements for water-quality protection by acting as flood control structures and settling basins, but also because they provide fill material for development, provide open space for recreation, enhance property aesthetics, and increase property values. By 1999, Siewicki and his colleagues estimated that there were more than 8,000 stormwater ponds in coastal South Carolina (in the area east of US Highway 17), and this number appears to be increasing at a rate of 13% per year. If this rate of increase has stayed consistent over the past 10 years, there may now well be more than 27,000 stormwater ponds in coastal South Carolina.

Another, often unintended, benefit of stormwater detention ponds is that they serve as wildlife habitat. This is especially pertinent in coastal South Carolina, where there is a paucity of freshwater habitat. In fact, researchers at The Citadel have recently documented 70 species of birds directly using stormwater detention ponds for food, water, and breeding. Sediment-dwelling animals, such as worms, crustaceans, and mussels, as well as fish, reptiles, and mammals have also been documented in these ponds. Because the ponds are designed to retain stormwater, sediments, and any associated contaminants, wildlife inhabiting the ponds may be at risk to adverse effects resulting from exposure to the contaminants retained within.

Not only are wildlife at risk from exposure to polluted stormwater pond sediments, but humans may also be at risk as a result of recreational activities, such as fishing and swimming, and/or sediment disposal activities following maintenance dredging. As stormwater ponds age, the accumulation of sediment reduces their overall efficiency and jeopardizes their ability to protect nearby natural receiving waters. To keep ponds fully functional, periodic removal of accumulated sediment is recommended. Depending on state and local regulations, excavated pond sediments are typically treated as either solid waste or hazardous waste and transported to either landfills or hazardous waste facilities for disposal. However, according to a recent survey by Robert Polta for the Metropolitan Council Environmental Services, in at least 16 states, including South Carolina, onsite disposal of the excavated sediment is another option. Because sediment removal costs can be decreased by as much as 50% if an onsite disposal area is available, this option could prove to be popular among property owners and homeowner associations. However, onsite disposal, especially in residential areas where the material could be used as fill, may result in human exposure to the contaminants.

Detention ponds have become popular because they provide fill material for development and open space for recreation, enhance property aesthetics, and increase property values.

In an effort to understand the potential adverse risks that exposure to detention pond sediments pose to both wildlife and humans, we recently characterized the chemical contaminants in the sediments of 16 detention ponds located in Charleston, Beaufort, Georgetown, and Myrtle Beach, SC. Based upon information provided in the original state permit, all detention ponds were classified into the following broad land use classes: golf course (three ponds), residential (six ponds), and commercial (seven ponds). Pond ages ranged from four to 12 years old with a median age of 10 years old. In addition, two manmade ponds, not known to be receiving any point discharges of stormwater, were selected as reference ponds. Based upon a survey of property owners, none of the ponds had been excavated to remove accumulated sediments.

Not surprisingly, commercial ponds had the highest levels of overall chemical contamination. One common class of chemical contaminant, the polycyclic aromatic hydrocarbons, or PAHs, was found in commercial pond sediments at levels six times higher than in any other class of detention pond (Figure 1). In suburban areas, PAHs are commonly associated with the vehicular combustion of gasoline and diesel. Another source of PAHs, especially in the commercial ponds, may have been coal-tar-based pavement sealants used on parking lots. According to recent research by Mahler and her colleagues (2005), dust from these types of asphalt sealants can be a major contributor to overall PAH loadings in urban areas. Commercial ponds also had higher levels of lead and zinc than any other class of detention pond (Figure 2). Both lead and zinc in suburban areas have also been attributed to vehicular use, particularly the wearing of breaks and tires. Levels of copper tended to be higher in commercial, residential, and golf course ponds compared to the reference ponds. There was a strong correlation between sediment levels of copper and the reported use of copper-based algaecides in these detention ponds. In fact, detention ponds previously treated with copper-based algaecides had sediment levels of copper that, on average, were 250% higher than those ponds not previously treated with copper-based algaecides. Sediment levels of metals not commonly associated with suburban development, including aluminum, cadmium, and iron, were similar in all classes of detention ponds.

Only a few historical pesticides were frequently detected in detention pond sediments. The organophosphate chlorpyrifos, banned for residential use by the USEPA in 2001, was the most frequently detected pesticide, found in nine of the 16 detention ponds sampled. Less frequently detected were the organochlorides chlordane, DDD, and DDE, which were found in only two of the 16 ponds. Both DDD and DDE are degradation products of DDT. Overall, pesticide levels in pond sediments were not dependent upon current land use. Instead, the presence of certain pesticides, such as DDD and DDE, in detention pond sediments probably more closely reflects historic land uses (e.g., agriculture).

In addition to land use, we also investigated the relationship between watershed size and sediment contaminant levels in detention pond sediments. Watershed size was a significant factor, explaining 72.5%, 61.1%, and 59.0% of the variability in the PAH, lead, and zinc levels of sediment, respectively (Figure 3). Our findings suggest that ponds with larger watershed sizes have higher levels of contaminants. Underscoring these findings is that fact that the residential pond having the highest levels of PAHs (17,400 µg/kg), had a drainage area (6.8 hectares) that was more than twofold greater than the average drainage area for other suburban residential ponds (3.0 hectares).

In an effort to quantify the relative degree of potential risks for adverse effects from exposure to these contaminants in detention pond sediments, we conducted ecological and human health screening-level assessments. Screening-level assessments are usually the first step in a risk assessment and are typically performed by calculating hazard quotients. Hazard quotients are defined here as the concentration of a particular contaminant found in detention pond sediments divided by a sediment quality benchmark for that particular contaminant. For the ecological assessments, we used consensus-based probable effect concentrations determined by MacDonald and his colleagues (2000) as the sediment quality benchmarks. Using these benchmarks, a value greater than one would indicate that adverse effects are probably being experienced by sediment-dwelling organisms (e.g., worms, insects, crustaceans) due to the exposure to that particular contaminant.

Sediments of five stormwater ponds (four commercial ponds and that one residential pond with a large drainage area) had hazard quotients greater than one for several individual PAH analytes. Only occasionally did any of the detention ponds have hazard quotients greater than one for either metals or pesticides. An analysis of the average hazard indices (sum of hazard quotients of all contaminants within a pond) among the various land use classes demonstrated that PAHs are the contaminant of greatest potential ecological concern, especially in commercial detention pond sediments (Figure 4). Our screening-level results suggest that PAHs currently pose moderate to high risks of adverse effects to sediment-dwelling organisms in commercial pond sediments.

For the human health screening assessment, we used Regional Screening Levels, or RSLs, recently developed by the USEPA as our sediment benchmarks. RSL values, formerly known as Preliminary Remediation Goals (PRGs), represent a harmonization of similar-risk based screening levels used by EPA Regions 3, 6, and 9. RSLs combine current human health toxicity values with standard exposure factors to estimate contaminant concentrations in soil that are considered to be health protective of human exposures (including sensitive groups), following a lifetime of exposure. Specifically, RSLs are sediment concentrations that correspond to fixed levels of risk (i.e., either a one-in-one million [10^-6] target cancer risk or a noncarcinogenic hazard quotient of one). The use of RSL values in this study assumes that the detention pond sediment has been excavated and is being disposed of onsite.

Most states recommend sediment removal approximately every 10 years.

It is estimated that, by 1999, there were more than 8,000 stormwater ponds in coastal South Carolina, and this number appears to be increasing at a rate of 13% per year.

As ponds age, the risk of exposure to elevated PAH levels for wildlife increases.

Similar to our findings in the ecological assessment, PAHs were the contaminant of greatest potential concern to human health. Only occasionally did any of the detention ponds exceed RSL values for either metals or pesticides. By contrast, sediments from commercial ponds exceeded, on average, 35% of the RSL values for individual PAH analytes (Figure 5). The five detention ponds with the most-contaminated sediments (four commercial ponds and one residential pond with a large drainage area) specifically exceeded RSL values for five cancer-causing, or carcinogenic, PAH analytes (benzo[a]anthracene, benzo[b]fluoranthene, benzo[a]pyrene, dibenz[a,h]anthracene, and indeno[1,2,3,c,d]pyrene). These results suggest that the PAH levels in sediments from certain commercial and residential ponds have the potential to increase the carcinogenic target risk of one-in-one million individuals following sediment excavation, onsite disposal, and a lifetime of exposure. In fact, it is interesting to note that, in terms of disposal options, the excavated sediments from these five detention ponds do not meet the guidelines for onsite disposal in several states, including Florida, Pennsylvania, and New Jersey, specifically due to their levels of carcinogenic PAHs.

Although not explicitly addressed in the current study, there are two other considerations relevant to the overall assessment of adverse risk of PAHs in detention pond sediments: the age of the pond and the frequency with which the sediments are excavated. Previous studies suggest that pond age, in the absence of sediment excavation, is an important determinant of sediment PAH levels. For example, Fernandez and Hutchinson found levels of PAHs in sediments in Florida vary from near background levels in stormwater ponds that were less than one year old to greater than 7,000,000 µg/kg in 30-year-old ponds. Thus, as ponds age, the risk of exposure to elevated PAH levels for wildlife increases. Presumably, periodic sediment removal would reduce sediment levels of PAH and consequently the risk of adverse effects to wildlife from their exposure.

Although recommendations for the frequency of periodic sediment removal vary, most states, including South Carolina, recommend removal approximately every 10 years. However, recent surveys along the South Carolina coast of property owners and managers responsible for pond maintenance suggest that periodic sediment removal is not occurring as recommended. When the guidelines for periodic sediment removal are not followed, it is likely that the need for excavation will become apparent only many years later when the stormwater pond becomes very shallow and flooding occurs. As ponds age and PAH levels become elevated, onsite disposal of pond sediments, especially those sediments from commercial ponds, could pose increased risks of cancer to humans.

In summary, our results suggest that PAHs are a contaminant of potential ecological and human health concern in detention ponds of coastal South Carolina. Several commercial ponds, and one residential pond with a relatively large watershed, have sediment levels of PAHs that pose moderate to high risks of chronic adverse effects to sediment-dwelling organisms. If the sediment were to be excavated and used as fill material, these same sediments would have the potential to pose an increase in the carcinogenic target risk to exposed individuals following a lifetime of exposure. Based upon these results, we recommend that sediments from these types of stormwater ponds be tested prior to excavation to determine the appropriate method of disposal. Currently, in South Carolina, there is no requirement that excavated pond sediments be tested for chemical contaminants. We also recommend that local and state regulatory agencies increase awareness of, and enforce guidelines for, periodic sediment removal, as this should reduce risks to both wildlife and humans.