Nutrient dynamics in marsh sediments contaminated by an oil spill following a flood

Abstract

This research involves a study of the natural recovery of a brackish marsh impacted by an oil spill and fire in which the area was naturally enhanced with elevated nutrient levels. Flood waters during October, 1994, ruptured a group of pipelines which released gasoline, diesel fuel and crude oil into the San Jacinto River near Houston. This spill traveled downstream where it was ignited inside a flooded house and was allowed to bum for 7 days. A petroleum contaminated wetland was designated as a research area and received no cleanup during the spill response. Sediment samples, collected over a period of one year, were analyzed for nutrients and petroleum hydrocarbons. Natural levels were obtained by monitoring the nutrient values over a year after the event. Nutrients monitored include ammonium (plus ammonia), nitrate (plus nitrite), available phosphorus, total Kjeldahl nitrogen and total Kjeldahl phosphorus. Available nutrient concentrations were initially high and then declined to presumed background levels of approximately I 0.0 ppm P, 5.0 ppm N, and 0.5 ppm N for available phosphate, ammonium, and nitrate, respectively. Ammonium concentrations were as high as 40 ppm N and available phosphorus levels were as high as 75 ppm P during December, 1994. Available nutrient levels declined over the period of the study indicating that the system had been enriched. Theories about the source of enrichment include deposition of either nutrient rich sediments from erosion due to the flood or ashes from the fire. Also, the disturbance of the site from construction activities involved in building the sampling structures could have contributed to the perturbations observed. During the study, the sum of total petroleum target analytes decreased from approximately 160 ppm to less than 10 ppm. Correlation coefficients of 0.76, 0.75,-0.67, and-0.63 where found between the sum of target analytes and ammonium, available phosphorus, total Kjeldahl nitrogen, and total Kjeldahl phosphorus, respectively. These correlation coefficients suggest a strong interdependency between the nutrient levels and the degradation of the petroleum. The data suggest that the naturally elevated nutrient levels provided favorable conditions for the degradation of the oil.