Dept. of Environmental Science
Oregon Graduate Center
This study explores the groundwater transport of a series of chlorophenols and chlorophenoxyphenols from the chemical disposal site at Alkali Lake, Oregon. Since burial in 1976, the contaminants have moved up to 600 m in the shallow water table aquifer beneath the site. Groundwater movement in the area is controlled by a series of springs to the east of the site which maintains a west-ward hydraulic gradient around and through the site towards West Alkali Lake. Transport of the contaminants is facilitated by the presence of a very large number of fractures in the soil. These fractures probably had their origins as bedding planes in the lacustrine sediments of the playa on which the site is situated. The bedding planes may have been opened by the processes of dehydration and rehydration and/or by dissolution of carbonates. Groundwater velocities in the fractures may exceed 1 m/day, even under the moderate hydraulic gradient (0.001) present at the site. Solute transport is much slower than 1 m/day, however, because of the retarding effect of matrix diffusion. The average velocity of a non-sorbed tracer downgradient of the site is probably 0.1 m/day or less. The roles of the fractures and of matrix diffusion in transport have been successfully characterized by a series of field tracer tests. These tests provided direct evidence that the groundwater is flowing primarily through fractures and that matrix diffusion is important in retarding contaminant transport. The tests also made possible a quantitative evaluation of the average properties of the fracture system (fracture aperture and spacing, matrix diffusion coefficient, etc.) and the effective velocity and dispersion of the contaminants at the site. The current distributions of contaminants downgradient of the site indicate that sorption has been important in retarding the movement of some of the compounds. Because both sorbing and non-sorbing compounds were buried at the site, the movement of the sorbed compounds relative to the non-sorbed compounds could be used to investigate the processes which have led to the observed distributions. Retardation has also been predicted from laboratory measurements of batch equilibrium partitioning, bulk soil density and porosity. The retardation factors estimated from the observed contaminant distributions were much smaller than those predicted from the laboratory data. The primary reason for this is the local groundwater flow pattern which results in: 1)non-uniform flow through the site; 2) decreasing velocity with distance from the site; and 3) spreading of the contaminant plume beyond 150+/-50 m west of the site. Groundwater flow is not uniform through the CDS. Data suggest that the primary pathway of water movement is through the southern portion of the site, with a secondary pathway along the northern edge of the site. Groundwater flow through the center of the site may be reduced due to a decrease in hydraulic conductivity within the site as a result of the burial process. This flow pattern has a large impact on the shape of the contaminant distributions, causing concentrations to drop with distance from the CDS more rapidly than predicted by a one-dimensional model of the site. The effect of the slowing and spreading of the groundwater as it moves away from the site is to allow the sorbed compounds to "catch up" with the non-sorbed compounds, resulting in an apparent decrease in relative retardation of the sorbed compounds. The ultimate destination of the contaminants from the site is probably West Alkali Lake. There is a high probability that contaminants will reach ground surface in that area. There is also the possibility that seasonal fluctuations in the water table will bring contaminants to the surface at distances of from 200 to 400 m west of the CDS, a region where levels of contamination are currently quite high.
Johnson, Richard Lee, "The groundwater Transport of Chlorophenolics Through a Highly Fractured Soil at Alkali Lake, Oregon" (1984). Scholar Archive. 10.