BIOMONITORING OF TOXICITY REDUCTION DURING IN-SITU BIOREMEDIATION OF MONOAROMATIC COMPOUNDS IN GROUNDWATER

Biomonitoring of toxicity, using the Ceriodaphnia dubia acute toxicity test, was performed on a gasoline-contaminated aquifer in San Diego, Calif. undergoing in situ bioremediation. The bioremediation approach here featured nitrate enrichment of extracted groundwater (with subseq uent reinfiltration back into the aquifer) to stimulate bacterial deni trification. Dentrification has been shown to support biodegradation o f the monoaromatic hydrocarbons-benzene, toluene, ethylbenzene, and th e isomers of xylene (BTEX). During the period between the start of bio remediation (October, 1989) and the end of nutrient enrichment(March, 1992), BTEX levels at the two most contaminated wells on site (MW1 and MW5) were reducted by about 81 and 99%, respectively. In acute bioass ays of groundwater from MW1 during the first 6 months of bioremediatio n, concentrations of groundwater from about 3 to 10% produced 50% mort ality of Ceriodaphnia after 24 h exposure, whereas groundwater concent rations of greater than 20% were required after remediation. When the 24-h LC50's for mortality were plotted as a function of BTEX concentra tion, the slope of the regression line was similar for both laboratory and field nitrate-enrichment studies. At MW5, where the BTEX level wa s dramatically reduced by 99% during bioremediation, the 24-h LC50 aft er bioremediation increased to 56% (from initial values of 12-15%), wh ile uncontaminated groundwater from a well up-gradient of the gasoline leak, showed an LC50 value of 74%. These results demonstrate that eve n after bioremediation of an aquifer (with an associated BTEX reductio n of 81-99%), the toxicity (as measured by Ceriodaphnia) of the ground waters may not be reduced to precontamination levels.