Evaluation of natural attenuation rate at a gasoline spill site

Contamination of groundwater by gasoline and other petroleum-derived hydroc arbons released from underground storage tanks (USTs) is a serious and wide spread environmental problem. Natural attenuation is a passive remedial app roach that depends upon natural processes to degrade and dissipate contamin ants in soil and groundwater. Currently, in situ column technique, microcos m, and computer modeling have been applied for the natural attenuation rate calculation. However, the subsurface heterogeneity reduces the applicabili ty of these techniques. In this study, a mass Aux approach was used to calc ulate the contaminant mass reduction and held-scale decay rate at a gasolin e spill site. The mass flux technique is a simplified mass balance procedur e, which is accomplished using the differences in total contaminant mass fl ux across two cross-sections of the contaminant plume. The mass flux calcul ation shows that up to 87% of the dissolved total benzene, toluene, ethylbe nzene, and xylene (BTEX) isomers removal was observed via natural attenuati on at this site. The efficiency of natural biodegradation was evaluated by the in situ tracer method, and the first-order decay model was applied for the natural attenuation/biodegradation rate calculation. Results reveal tha t natural biodegradation was the major cause of the BTEX mass reduction amo ng the natural attenuation processes, and approximately 88% of the BTEX rem oval was due to the natural biodegradation process. The calculated total BT EX first-order attenuation and biodegradation rates were 0.036 and 0.025% p er day, respectively. Results suggest that the natural attenuation mechanis ms can effectively contain the plume, and the mass flux method is useful in assessing the occurrence and efficiency of the natural attenuation process . (C) 2001 Elsevier Science B.V. All rights reserved.