Case Studies - PTTC Rockies

Abstract

Tank bottoms from a Williston Basin oilfield were applied to test plots in which crops were subsequently planted. Naturally occurring microbes reduced the 6% total petroleum hydrocarbon (TPH) concentration to 3.8% in a few months (a 37% reduction), but reduced it no further, possibly due to an insufficient amount of nitrogen or water or both. For the first three years of the study, the 6% TPH test plot did not grow crops. It was apparent that the high (6%) application rate of this high paraffin oil seriously restricted the infiltration of water into the soil; this is considered to be the primary cause of crop failure, rather than toxicity of the tank bottoms. After manure was applied in the fall of the third year, crops were successfully grown the following season. Two years after that, when the manure had degraded, crop growth was again very poor. The lack of water may have also affected the process of microbial oil degradation.

The second phase of the study examined the addition of straw and large amounts of nitrogen and phosphate fertilizer to new test plots. A 0.6% TPH concentration was applied to two test plots which had been previously planted to spring wheat. Because there had been no rain, the crop was poor, and there was concern that the application of oil plus tilling would kill the crop. When it did rain later in the summer, the seed left in the ground germinated and successfully produced a crop. The addition of straw did not increase the chances of crop growth; rather, it reduced the yield of the crop significantly, even with a higher rate of fertilizer application. The original 0.6% TPH concentration was reduced to 0.14% in one year, a 77% reduction, suggesting that lower application rates may remediate faster, in addition to allowing crops to grow. This study suggests that application of low concentrations of tank bottoms on agricultural land may be possible, but additional research is needed to discover how to control the hydrophobic effects of this disposal method. The addition of manure (rather than straw) to land spread with tank bottoms appears to be favorable to plant growth by increasing water infiltration and retention.