Mineral Grain Surface Observations At A Hydrocarbon-Contaminated Aquifer: Implications For The Geoelectrical Properties Of Soils

Abstract

A recent geoelectrical model suggests that the high electrical conductivities measured for soils contaminated by hydrocarbons result from enhanced mineral weathering accompanying microbial degradation of hydrocarbons. However, a preliminary study shows that the relationship between pore water electrical conductivity and electrical conductivity of soils contaminated with hydrocarbon is poor. It is possible that variations in lithology or modifications of the soil matrix by biogeochemical reactions accompanying hydrocarbon degradation may be responsible for this poor relationship. In this study, we use scanning electron microscopy (SEM) to examine the morphology of the surfaces of quartz and feldspars from discrete depth intervals associated with vertical electrical conductivity zonation at a site contaminated with hydrocarbons. The composition of precipitates on the mineral surfaces was determined using electron dispersion spectroscopy (EDS). In addition, X-ray diffraction (XRD) was used to determine the bulk mineralogy of the sediments. The goal of the study was to determine if vertical electrical conductivity anomalies observed in soils were related to changes in the soil matrix resulting indirectly from biodegradation of hydrocarbons. XRD analyses suggest that the mineralogy of the soils was dominated by quartz with minor amounts of carbonates occurring below the water table. SEM analyses showed minimal pitting on mineral grains in the upper vadose zone relative to deeper intervals. Mineral grains at depths <50 cm had precipitates on their surfaces that we attribute to precipitation related to soil forming processes. At locations where soils were contaminated with residual phase hydrocarbons, the surfaces of mineral grains were heavily etched and pitted compared to similar depths for soils in uncontaminated locations. In the saturated zone, limited etching/pitting and moderate amounts of precipitates were observed on mineral surfaces. EDS analyses suggest a composition of the precipitates consistent with the mineralogy of clays in soils above the water table and mostly carbonates and iron oxides in the saturated zone. Iron oxide precipitation was greater than 30% in uncontaminated soils but was less than 10% at hydrocarbon contaminated locations. Our results show that microbial degradation of hydrocarbon in soils cause changes in the surface grain morphology of minerals that can potentially impact the electrical properties of hydrocarbon contaminated soils. Introduction Groundwater in many sites across the United States is contaminated with hydrocarbons. Because of the health risks associated with hydrocarbon contamination, many of these sites need to be remediated. Natural attenuation of hydrocarbons by indigenous microorganisms is gaining wide acceptance as a method for remediating these sites (Suthersan, 1997). This method takes advantage of the ability of naturally occurring bacteria and geochemical processes to degrade the hydrocarbon. However, extensive monitoring is required to ensure that the plume is not spreading and is, in fact, degrading.