ASSESSMENT OF NATURAL ATTENUATION OF AROMATIC-HYDROCARBONS IN GROUNDWATER NEAR A FORMER MANUFACTURED-GAS PLANT, SOUTH-CAROLINA, USA

Shallow, anaerobic groundwater near a former manufactured-gas plant (M GP) in Charleston, South Carolina, USA, contains mono- and polycyclic aromatic hydrocarbons (MAHs and PAHs, respectively). Between 1994 and 1997, a combination of field, laboratory, and numerical-flow and trans port-model investigations were made to assess natural attenuation proc esses affecting MAH and PAH distributions. This assessment included de termination of adsorption coefficients (K-ad) and first-order biodegra dation rate constants (K-bio) using aquifer material from the MGP site and adjacent properties. Naphthalene adsorption (K-ad = 1.35 x 10(-7) m(3)/mg) to aquifer sediments was higher than toluene adsorption (K-a d = 9 34 x 10(-10) m(3)/mg), suggesting preferential toluene transport relative to naphthalene. However, toluene and benzene distributions m easured in January 1994 were smaller than the naphthalene distribution . This scenario can be explained, in part, by the differences between biodegradation rates of the compounds. Aerobic first-order rate consta nts of C-14-toluene, C-14-benzene, and C-14-naphthalene degradation we re similar (-0.84, -0.03, and 0.88 day(-1), respectively), but anaerob ic rate constants were higher for toluene and benzene (-0.002 and -0.0 0014 day(-1), respectively) than for naphthalene (-0.000046 day(-1)). Both areal and cross-sectional numerical simulations were used to test the hypothesis suggested by these rate differences that MAH compounds will be contained relative to PAHs. Predictive simulations indicated that the distributions of toluene and benzene reach steady-state condi tions before groundwater flow lines discharge to an adjacent surface-w ater body, but do discharge low concentrations of naphthalene. Numeric al predictions were ''audited'' by measuring concentrations of naphtha lene, toluene, and benzene at the site in early 1997. Measured naphtha lene and toluene conentrations were substantially reduced and the area l extent of contamination smaller than was both observed in January 19 94 and predicted for 1997. Measured 1997 benzene concentrations and di stribution were shown to be relatively unchanged f rom those measured in 1994, and similar to predictions for 1997.