Analysis
During Phase I, the total petroleum hydrocarbon (TPH) content of the soil dropped from 6% to about 3.8% a reduction of about 37%. As can be seen in Figure 4, it appears to have made that drop during the first few months, risen slightly in the winter months (probably a sampling aberration), and then dropped to its present level in the following summer. This lack of further TPH reduction may be due to the action of the microbes; initially they may have broken down the oil into other petroleum hydrocarbons and then eventually those products were further broken down to CO 2 and H2O. It is also possible that some constituents of the oil were not utilized by resident microbes. Another factor may be the lack of water; oilfield operators in the Denver Basin, include sprinkling contaminated soil with water to insure rapid breakdown of oil (Flynn, 2000).
The pH of both the test plot and control plot is similar. The conductivity, chloride, and sulfate are markedly elevated in the test plot, but are considered well within an acceptable range for barley and field peas (Ayers and Westcot, 1976).
In Figure 4, it is striking that the soil moisture in the test plot is about 50% less than the control plot. Intuitively, the water-repelling property of the oil and paraffin would cause this low soil moisture. Furthermore, it is possible that the successful crop growth in oily soils does not depend upon oxidation of the oil by microbes, but rather on getting sufficient water to the plants. After surfactant was applied to the oily soil, it was observed that primary cause of poor plant growth was a lack of moisture in the soil rather than toxicity of the soil.
Figure 5 displays both soil fertility analyses and the fertilizer applications. The most important observation is the nearly complete removal of nitrogen in the test plot during the first year. Following each of three applications of nitrogen, the control plot showed high levels of nitrogen, while the test plot showed very low levels of nitrogen. Furthermore, with application of nitrogen, one would expect to see a commensurate decrease in the TPH values, but this is not the case. It is likely that insufficient nitrogen was applied to the test plot to allow the microbes to oxidize the oil. Other workers (Gawel, 1995, McMillan, 1994) have suggested that the most efficient microbial activity will occur when the Carbon: Nitrogen: Phosphorus ratio is 100: 10: 2.5 to 5. If it is assumed that the oil contained 80% elemental carbon (1800 pounds), then 180 pounds of nitrogen should have been applied, but in fact only about 16 pounds were applied.
Because other studies suggested that oil loading rates of from 5 to 10% by weight could be applied to the soil with success (Kincannon, 1972), it was considered that the loading rate of 6% in Phase I would be acceptable. However, this may be more practical for non-cultivated lands. A field experiment in Alberta, Canada (Pojasok, et. al. 1992) showed that crops planted into soil with 0.5% freshly applied oil produced half the yield of untreated control plots. In both greenhouse and field trials, the 0.5% to 1% loading rate produced the fastest percentage of oil degradation. Two applications of 0.5% gave the fastest rate of degradation (Macyk, et. al., 1992). This rate approximates the loading rate of Phase II (0.6%), in which crops were successfully grown.
The biodegradation of the 0.6% to 0.14% (an average of the two test plots in Phase II) records a TPH reduction of 77%, a much higher rate than in Phase I. These results reinforce the importance of low TPH application rates. Not only does it appear to control the success of crops, but it may also control the rate of remediation of oil contaminated soil.
The addition of manure appears to have been very beneficial in promoting plant growth in the Phase I test plot in the first year after application of the tank bottoms. However, as the manure biodegraded, the plot reverted to its original hydrophobic state and crop growth was poor in the third year after application. It is likely that the manure added enough wet-able material to allow water infiltration and retention and thus allowed the crops to grow. The resident microbes may have preferred the manure to the oil, breaking it down and consuming less oil.
