RETARDATION OF A LARGE ORGANIC CONTAMINANT PLUME IN FRACTURED SEDIMENTARY ROCK

The behavior of a large plume of chlorinated solvents and other organic contaminants, migrating horizontally in flat-lying sandstone, is being studied using a combination of the following methods: chemical analyses of closely spaced rock core samples, geochemical as well as isotopic groundwater analyses, packer testing, pumping tests, borehole geophysical logging and flow metering, and depth-discrete multi-level monitoring. The plume is defined by 234 sampling points, many of which have been sampled regularly for the past 12 years. The plume originates from a DNAPL (dense non aqueous phase liquid) source zone occurring in fractured sandstone between 43 m below ground surface (bgs) and 54 m bgs. Since the formation of the DNAPL source zone 40 to 50 years ago, the plume has traveled 3 km horizontally between the depths of 45 m bgs and 55 m bgs in fractured, bedded sandstone. The average annual migration rate of the plume-front, 70 m/yr, obtained by dividing the plume length by the plume migration time, is one-tenth the average linear groundwater velocity of 670 m/yr. The average linear groundwater velocity was determined using Darcy’s law in combination with potentiometric surface maps, bulk hydraulic conductivity measurements, and fracture observations. An independent estimate of the groundwater velocity obtained from travel times of a secondary contaminant slug occurring within the original plume agrees well with the average linear groundwater velocity calculated using Darcy’s law. The data obtained from rock core contaminant profiles, monitoring wells, multi-level systems, and pumping tests suggest that the contaminants are transported in a highly interconnected fracture network not dominated by any specific fracture or structural feature. The rock core chemical analyses show measurable diffusion haloes confirming that diffusion-driven contaminant mass transfer from the fractures into the rock matrix (matrix porosity of 10 to 15%) accounts for the strong plume-front retardation relative to the average linear groundwater velocity.