Chemical and microbial evaluation of in-situ bioremediation of hydrocarbons in anoxic groundwater enriched with nutrients and nitrate.

In-situ bioremediation of benzene, toluene, ethylbenzene and the xylenes (BTEX) was carried out in an O2-poor (approx. 1 mg O2/l) fuel-contaminated aquifer. Extracted groundwater, enriched with ammonium polyphosphate (nutrients) and KNO3 (electron acceptor), was piped to an infiltration gallery over the contaminated site. Before, during and after infiltration, BTEX, nitrate and different populations of culturable bacteria were measured. BTEX declined by 78% in water from the monitoring well which was most contaminated initially and by nearly 99% in water from one of the extraction wells. These declines persisted after cessation of nutrient and nitrate addition. During the second half of the nutrient and nitrate addition period (weeks 107 to 160.5), nitrate appeared in the monitoring well, denitrifying bacteria increased about 50-fold and bacteria degrading benzene, toluene and xylenes (BTX) and phenanthrene (enumerated aerobically) increased 16- and 121-fold, respectively. At one of the extraction wells, down-gradient of the monitoring well, nitrate appeared in significant concentrations after week 124; this appearance coincided with a marked decline (> 90%) in BTEX concentration and 21- and 10-fold increases, respectively, in BTX- and phenanthrene-degrading bacteria. Low concentrations of BTEX and nitrate in down-gradient, off-site wells showed that water washing did not mobilize BTEX from the aquifer. The data indicate that the BTEX in this nitrate-enriched aquifer was biodegraded in-situ under denitrifying conditions.