SCANNING ELECTRON MICROSCOPY TO EXPLAIN CHANGE IN HYDRAULIC CONDUCTIVITY OF HYDROCARBON-CONTAMINATED AQUIFER SAND

A series of laboratory tests were conducted to study the effect of common organic contaminants on the hydraulic conductivity of aquifer sand. The experiment was designed to expose sand to gasoline, trichloroethylene (TCE), benzene, toluene, ethylbenzene, xylenes (BTEX), and isooctane for varying periods of time–between 1 and 32 weeks, at different levels of saturation: 25, 50 and 100% by volume of voids to simulate change in the hydraulic conductivity of the aquifer sand. Hydraulic conductivity was determined for both uncontaminated and contaminated sand samples. The results showed a decrease of up to 60% in the hydraulic conductivity of sand upon contamination. The value fluctuated during the first few weeks, but generally stabilized after 8 weeks of contamination. Grain size parameters of sand, along with its density, void ratio, and porosity, did not show any change after the duration of contamination. However, scanning electron microscope (SEM) study revealed that some sand grains had undergone fragmentation, along with indications of hydrocarbon coating around the grains. Size of the fragmented particles ranged between 2.5x10^-5 and 1x10^-4 mm, but its volume was insignificant and ranged between 0.25 and 0.56% of the original sand volume. Based on the preliminary SEM study, reduction in hydraulic conductivity seems to result from an increase in surface tension and other intragranular forces due to adsorption of hydrocarbons around sand grains, enhanced by large surface areas of fine fragments. Further work is in progress.