Biodegradation of aged residues of atrazine and alachlor in a mix-load site soil

A selected microbial consortium (SMC) capable of degrading two specific herbicides, alachlor (2-chloro-2′,6′-diethyl-N-methoxymethyl]-acetanilide; AL) and atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine; AT) was isolated from a pesticide-contaminated mix-load site soil. Evaluation of bioaugmentation as a feasible bioremediation strategy for this mix-load site soil (Site 5A) was initiated in standard laboratory biometer flasks utilizing the isolated SMC. The biometer flasks were monitored for CO₂ evolution and pesticide degradation. The total amount of CO₂ evolved from the treated biometer flasks was significantly different from the control flasks. The rate of CO₂ evolution was 2.6 times faster in the treated soil (0.0123 mM CO₂ d⁻¹ vs. 0.0048 mM CO₂ d⁻¹). The total net CO₂ produced in the treated biometer flasks was 0.9481 mM, representing mineralization of approximately 10% of the AT and AL initially present. Forty-eight percent of AT and 70% of AL was degraded in the inoculated biometer flasks. The first-order rate constants were 0.0064 d⁻¹ and 0.1331 d⁻¹ for AT and AL, respectively. The calculated half-life of AT was 108 d while a 50% decrease in AL occurred by Day 5. In just 2 d, 20% of the AT was degraded while only 10% of the AL disappeared. The initial fast degradation rate of AT was followed by a much slower, more gradual degradation rate period that lasted about 35 d. Alternatively, the rate of AL degradation increased after the second day resulting in 60% of the AL being transformed by the end of the first week. Alachlor degradation appeared to be dependent upon AT degradation especially during the first several days of the incubation period. Complete disappearance of the herbicides over the study time was not achieved.