Relationship between geomorphology and contamination with weathered hydrocarbons in an old river levee/marsh association

A site evaluation was made for a low-lying property in an old river levee/marsh association, used marginally for cattle raising, which was contaminated due to historical practices. Contamination was related to geomorphology: the lowest areas contained 79% of the contaminated samples. Other contaminated samples were all rear access roads [probable intentional dumping]. There were no samples in the higher, levee areas away from roads with hydrocarbon concentrations above 3,000 mg/kg. Medium and high impact levels, corresponding to average hydrocarbon concentrations of 13,400 mg/kg and 109,700 mg/kg, respectively, were found in 28% and 27% of the property area. The medium impact level was predicted to have a reduced pasture production potential of- 50% while at the high impact level practically no pasture production was predicted. Toxicological risk to cattle is present in only 12% of the sample points although almost all of these are in very marshy areas not apt for cattle raising. Because the hydrocarbons at this tropical site are very weathered, acute toxicity was not related to hydrocarbon concentration. Although slightly more than half of the property has hydrocarbon concentrations above the permissible limit, only about one-third of the contaminated area is apt for cattle raising. This study, based on hydrocarbon distribution related to site geomorphology, soil toxicity bioassay, risk assessment to cattle and evaluation of pasture production allowed to made a more accurate assessment and definition of future use of the site. These criteria could be used for diagnostic studies of sites with similar tropical conditions

Evaluation of land farming and chemico-biological stabilization for treatment of heavily contaminated sediments in a tropical environment

Conventional and experimental methods were studied for the remediation of petroleum contaminated sediments from a dam previously used to collect acid run-off from a sulfur mine. The man-made lake had been neutralized, but bentonite rich sediments remained contaminated with very weathered hydrocarbons [sediments with 50,000-60,000 mg/kg Total Petroleum Hydrocarbons were used in this study]. Biostimulation, bioaugmentation [with native microorganisms] and chemico-biological stabilization, all resulted in similar reductions [14-16%] in the TPH concentration over a three month period. The land farming treatments resulted in variable reductions in toxicity, ranging from nil to complete, while the chemico-biological stabilization treatment, not only eliminated acute toxicity but also resulted in a slight stimulation [103-109%] of the test organism in the bioassay [Microtox]. All three treatments reduced polyaromatic hydrocarbons of probable carcinogenicity to below or nearly below the Mexican norms, reduced Toxic Characteristic Leaching Proceedure leachates to <1 mg/L, and left the material in a pH range of 7.0-7.8. The chemico-biological stabilization has the advantage of only requiring initial mixing of the chemical and organic reagents instead of daily aeration, thereby reducing operating costs. This method is also able to treat very difficult sites at low cost, relying on biological humification processes which are accelerated in a humid tropical and semitropical environment. The total unit cost of the chemico-biological stabilization treatment was estimated to be 60% of that for land farming in the southern Gulf of Mexico region

Water repellency in oil contaminated sandy and clayey soils

Two sites from a humid tropical environment were studied with respect to soil water repellency caused by hydrocarbon contamination. Samples were analyzed for water repellency [molarity ethanol droplet method], total petroleum hydrocarbons, acute toxicity [Microtox] and field capacity. At both sites, water absorption times were logarithmically related to the molarity ethanol drop value [R > 0.95]. In a sandy soil collected from an old separation battery which had been bioremediated, field capacity was strongly related to hydrocarbon concentration [R = 0.998]; and at 10,000 mg/kg the calculated field capacity was only 75% of the baseline. Water repellency was related to hydrocarbon concentration asymptotically and plant growth limiting values [severity > 3.0] were observed at low concentrations [2,400 mg/kg], even though toxicity was at, or below background levels. Bioremediated soil at this site had hydrocarbon concentrations only 1,300 ppm above background, but had extreme water repellency [severity = 4.6 - 4.7]. Soil water repellency was also measured in a clayey, organic rich floodable soil, in a multiple pipeline right-of-way colonized by water tolerant pasture and cattails. Water repellency was associated with total petroleum hydrocarbon concentration [R = 0.962], but was not related to field capacity or toxicity. In this low-lying site, the water repellency observed in the laboratory is probably not representative of field conditions: samples taken at the end of the ten week dry season [and only four days before the first rains] showed ample moisture [> 80% field capacity]