Quantification of petroleum hydrocarbon residual and biodegradation functional genes in rain garden field sites

Stormwater is known to convey oils, greases, and polycyclic aromatic hydrocarbons from impervious surfaces, and previous studies have indicated that bioretention is effective at removal of these pollutants. Concern has been expressed that such petrochemicals in stormwater could accumulate in the soil during infiltration and create "pollutant depots" in raingardens, resulting in environmental liability for the site owner. This research was performed to determine if petroleum hydrocarbon hotspots exist in bioretention areas, what factors influence petroleum hydrocarbon concentrations, and if bacteria capable of degrading petroleum hydrocarbons are present in raingardens. As a result, a field survey in the Minneapolis/St. Paul metropolitan area was conducted. Soil samples were collected from 56 raingardens and 4 upland locations and total petroleum hydrocarbons (TPH) and bacterial DNA were extracted and quantified. TPH was detected in many of the raingarden soil samples, but at low levels; upland samples were uniformly non-detect. TPH levels did not correlate to site characteristics such as catchment area or vegetation type. Functional genes levels in soil samples ranged from non-detect to 10 ¹⁰ copies/g soil. Overall, we observed that a substantial "toxic depot" effect did not occur, as TPH levels in raingardens were significantly below typical levels of concern. Furthermore, the ubiquity of genes indicative of petroleum hydrocarbon degradation capacity suggests that accumulation of TPH is not a major concern as petroleum hydrocarbons are likely to be biodegraded in raingardens.