Concentrations of Heavy Metals and PAHs in Ditch Cleaning Samples Collected Across Florida.

Drainage ditches help controlling the flow direction of stormwater and preventing flooding. To provide optimum water flow, these ditches are cleaned and maintained when necessary, which generate huge quantity of solid wastes in Florida. However, there are concerns about the presence of heavy metals and organic contaminants in the ditch cleanings. In this study we collected 78 representative ditch cleaning samples throughout the state and analyzed the concentrations of 503 metals (As, Cd, Cr, Cu, Pb, Mo, Ni, Se and Zn) and 16 PAHs. The average pH of the samples was 6.5, which is 1.5 to 2 units higher than typical Florida soils. The geometric mean concentrations of As (0.61 mg kg^-1), Cd (1.14 mg kg^-1), Cu (2.98 mg kg^-1), Mo (1.04 mg kg^-1), Se (1.19 mg kg^-1), and Zn (15.3 mg kg^-1) were all lower than their respective background soil concentrations. However, the geometric mean concentrations of Cr (2.05 mg kg^-1), Ni (0.50 mg kg^-1) and Pb (3.88 mg kg^-1) were higher than the background values.  Except for arsenic, metal concentrations were all lower than the Florida soil cleanup target levels for residential and industrial soils (SCTL-R and SCTL-I).  However, they were slightly higher than the background metal concentrations in Florida soils in some samples. Six samples (8%) had arsenic concentrations exceeding the Florida SCTL-R of 2.1 mg kg^-1. All 16 PAHs investigated were detected in measurable quantities in some of the 67 samples analyzed. Except for benzo(a)pyrene, levels of all other 15 PAHs were lower than their respective SCTL-R and SCTL-I. For benzo(a)pyrene, 11 samples exceeded the SCTL-R and two measured values (1.01 and 0.85 mg kg^-1) were 1.4 and 1.2 times higher than the SCTL-I. However, the geometric mean concentrations of benzo(a)pyrene (0.012 mg kg^-1) was lower than both SCTL-R and SCTL-I.  Thus the results suggest that the ditch cleaning samples were relatively clean with little contamination with heavy metals and PAHs.