Impact of heterogeneous aquifer materials on sorption capacities and sorption dynamics of organic contaminants

Abstract

Abstract Physico-chemical properties as well as sorption and long term sorption kinetics of phenanthrene were measured in various size fractions and characteristic lithocomponents of heterogeneous aquifer material from the Horkheim Aquifer Test Site in southwest Germany. High contents of organic carbon as well as high equilibrium sorption capacities were found in the silt and the gravel fraction. The relatively high sorption capacities in the gravel sized fraction is due to high amounts of Triassic and Jurassic carbonate lithofragments which contain a significant amount of fossil organic matter (presumably kerogen). However, since sorption kinetics is slow in the coarse fraction, sorption equilibrium is not reached. Therefore under nonequilibrium conditions retardation factors will be orders of magnitudes lower than expected. INTRODUCTION Aquifer sediments are often a heterogeneous mixture of various lithocomponents which differ in size and properties. Fate and transport of contaminants depend on the relevant chemical and physical properties of specific lithocomponents of the aquifer sediments. These reactive lithocomponents may constitute only a relatively small fraction of the bulk material. Weber et al. (1992), for example, report that shale-like materials in soil samples showed sorption capacities of 1,2,4-trichlorobenzene approximately 100 times higher than the parent bulk material. The paramagnetic fraction of aquifer sediments from the Cape Cod Test Site in Massachusetts showed higher sorption of chlorobenzene (factor 3) and a much higher organic carbon content (factor 5) than the nonmagnetic fraction (Barber et al., 1992). Studies on the sorption kinetics of hydrophobic organic contaminants in various aquifer materials show that diffusive mass transfer was limiting the sorptive uptake and the desorption rates (Karickhoff & Morris, 1985; Wu & Gschwend, 1986; Steinberg et al., 1987; Ball & Roberts, 1991; Brusseau, 1991). In many cases very low rates were observed (Grathwohl & Reinhard, 1993 ; Farrell & Reinhard, 1994; Harmon & Roberts, 1994). The time periods required in order to reach sorption equilibrium or to desorb the pollutants from contaminated material ranged from several months to many years (Steinberg et al, 1987; Pignatello et al, 1993; Schuth & Grathwohl, 1994). If the sorption kinetics are slow compared to the advective contaminant transport in groundwater, retardation factors become velocity or time dependent (Roberts et al, 1986; Schuth & Grathwohl, 1994) and extended tailing in aquifer remediation