Preprint version. Published in Remediation Journal, Volume 13, Issue 4, Fall October 1, 2003, pages 61-78.
A pilot-scale land treatment unit (LTU) was constructed at the former Guadalupe oil production field with the purpose of investigating the effect of co-substrate addition on the bacterial community and the resulting rate and extent of total petroleum hydrocarbon (TPH) degradation. The TPH was a weathered mid-cut distillate (C10-C32) excavated from the subsurface and stockpiled before treatment. A control cell (Cell 1) in the LTU was amended with nitrogen and phosphorus while the experimental cell (Cell 2) was amended with additional complex co-substrate—corn steep liquor. During the pilot LTU operation, measurements were taken of TPH, nutrients, moisture, aerobic heterotrophic bacteria (AHB), and diesel oxidizing bacteria (DOB). The bacterial community was also assayed using community-level physiology profiles (CLPP) and 16S rDNA terminal restriction fragment (TRF) analysis. TPH degradation in both cells was characterized by a rapid phase of degradation that lasted for the first three weeks, followed by a slower degradation phase that continued through the remainder of the project. The initial rate of TPH-degradation in Cell 1 (−0.021 day−1) was slower than in Cell 2 (−0.035 day−1). During the slower phase, degradation rates in both cells were similar (−0.0026 and −0.0024 respectively). AHB and DOB counts were similar in both cells during the fast degradation phase. A second addition of co-substrate to Cell 2 at the beginning of the slow degradation phase resulted in an increased AHB population that lasted for the remainder of the project but did not affect TPH degradation rates. CLPP data showed that co-substrate addition altered the functional capacity of the bacterial community during both phases of the project. However, TRF data indicated that the phylogenetic composition of the community was not different in the two cells during the fast degradation phase. The bacterial phylogenetic structure in Cell 2 differed from Cell 1 after the second application of co-substrate, during the slow degradation phase. Thus, co-substrate addition appeared to enhance the functional capacity of the bacterial community during the fast degradation phase when the majority of TPH was bioavailable, resulting in increased degradation rates, but did not affect rates during the slow degradation phase when the remaining TPH may not have been bioavailable. These data show that co-substrate addition might prove most useful for applications such as land farming where TPH is regularly applied to the same soil and initial degradation rates are more important to the project goals.