RESUMO
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]