| The primary objective of this study was to investigate the potential effects of hydrocarbons on the geotechnical properties of sand. A series of laboratory experiments were carried out to ascertain the influence of some common and widespread organic contaminants on the hydraulic conductivity of sand. Bulk sand samples from the Missouri River Valley aquifer--- a major source of groundwater supply in the U.S. Midwest--- were collected for this study. Sand samples, representing the aquifer and vadose zone, comprising -2.36 mm and -0.425 mm fractions respectively, prepared from the bulk samples, were used for conducting laboratory tests. Gasoline and some of its constituent chemicals---benzene, toluene, ethylbenzene, xylene (BTEX), isooctane--- as well as trichloroethylene (TCE) were used to contaminate sand samples at varying saturation levels for extended periods of time to simulate changes in geotechnical properties and hydraulic conductivities of sand caused by oil tank spills and leaking petroleum pipelines. Seventy-four medium grained sand samples were contaminated with TCE and gasoline at about 25%, 50% and 100% saturation levels for 2, 4, 8, 16, and 32 weeks. In addition, five sand samples were fully saturated with BTEX and isooctane for eight weeks to compare the results of their geotechnical properties and hydraulic conductivities with gasoline-contaminated sand. Also, 10 aquifer sand samples were exposed to aqueous solution contaminated with about 1, 2, 3, and up to 10% of TCE and gasoline for 8 weeks. Grain size parameters of the sand and its density, void ratio, porosity, and hydraulic conductivity were determined before and after the duration of exposure. All tests were performed in accordance with the American Society for Testing and Materials' standard test methods. The results showed an overall decrease in the geotechnical properties and up to 60% reduction in hydraulic conductivity of contaminated sand. Another set of experiments was carried out to determine the influence of evaporation rate of contaminating chemicals on hydraulic conductivity of sand. Results indicate that chemicals permeating sand grains have variable evaporation rates with a certain amount of chemicals left behind in sand voids. The inherent differences in the structure and nature of chemicals influenced hydraulic conductivity such that the observed decrease was greater for aliphatic than aromatic and chlorinated hydrocarbons. X-ray diffraction results show that the contaminating chemicals used in this study did not alter the mineralogy of both aquifer and upper layer sand, even after extended periods of exposure. Scanning electron microscopy of contaminated sand grains revealed highly irregular surfaces with presence of nano size grooves along with presence of prominent granules and grain fragments. This contributes to increase in surface area and decrease in porosity with attendant increase in surface tension. All of which resulted in a decrease in hydraulic conductivity of contaminated sand. |
