3. Results and discussion
3.1. Control and nutrient supplement microcosms
The changes of soil TPH concentrations in Groups A to F microcosms
are presented in Fig. 1a. Concentrations of TPH on Days
0 and 175 (end of operation) in the microcosms were applied to
determine the TPH removal percentage and efficiency. About 7.5%
of TPH removal was detected in A1 group during the operation. This
implies that the non-biological mechanisms did not have signifi-
cant impact on TPH removal, and this could be due to the
comparatively lower volatility characteristics of the lubricant and
diesel oils. Up to 26% of TPH removal by intrinsic microbial consortia
in soils was obtained in A2 group of microcosms.
Concentrations of TPH in Group B microcosms dropped from
4088 to 2577 mg/kg (37% removal) after 175 days of operation. TPH
degrading trend flatted off during the incubation period from 80 to
175 days. Decay rates were determined for two different decay
periods: (1) from days 0e80 (before the TPH decay trend flatted off)
(period 1), and (2) from days 0e175 (whole incubation period).
The TPH removal percentage and TPH decay rates for different
groups of microcosms are presented in Table 2. The decay rates for
A2 were 0.0039 and 0.0017 1/d for period 1 and whole period,
respectively. The decay rates for Group B microcosms were 0.0053
and 0.0026 1/d for period 1 and whole period, respectively. Results
demonstrate that higher TPH removal ratios and higher decay rates
were detected in Group B, which had 37% of TPH removal. Thus,
nutrient supplement could accelerate TPH removal effectively
compared to microcosms with natural attenuation mechanisms