Migration of contaminants in groundwater at a landfill: A case study: 4. A natural-gradient dispersion test

Abstract

A natural-gradient tracer test using a chloride solution with an initial injection volume of 0.7 m 3 was performed in the sandy aquifer at the Borden site. The solution was injected into five well points set ∼1 m below the water table in an uncontaminated zone situated above the contaminant plume at a location ∼450 m downflow from the landfill. Under conditions of natural groundwater flow, the tracer slug was monitored for a period of 4 months by withdrawing small-volume samples from points in a three-dimensional array of bundle-type multilevel samplers. Measurements of hydraulic head were obtained from a network of miniature piezometers. Soon after injection, the tracer slug gradually split into two halves, one half moving horizontally at an average velocity of 2.9 · 10 −6 ms −1 and the other horizontally at 8.2 · 10 −7 ms −1. Although the split has been attributed to local lateral heterogeneity, the nature of the heterogeneity and its influence on the hydraulic-head distribution were not clearly distinguishable in the field data obtained before, during or after the test. The chloride patterns for each of the two halves of the tracer slug evolved into Gaussian forms although the patterns demonstrated some irregularity at early time. The relatively smooth Gaussian forms were unexpected because the aquifer has numerous small-scale heterogeneities observed in vertical cores obtained from the tracer zone and because the glaciofluvial origin of the aquifer suggests that heterogeneities are not laterally continuous. Simulated chloride distributions from a three-dimensional analytical solution to the advection-dispersion equation were fitted to the field data to obtain best-fit estimates of the values of longitudinal, transverse-horizontal and transverse-vertical dispersivity at various travel distances for each of the two halves of the tracer zone. This is the first known field test that has permitted the estimation of three principal dispersion coefficients in layered media. The longitudinal dispersivity was found to increase from 0.01 m at a distance of 0.75 m from the source to 0.08 m at 11.0 m. The transverse-horizontal dispersivity increased also to a value of 0.03 m at 11.0 m. Transverse-vertical dispersion was very weak and was accounted for by molecular diffusion. The relative lack of vertical dispersion is consistent with the shape of the plume of leachate contamination from the landfill. It was concluded that the observed increase in dispersivity along the path of migration is likely caused by heterogeneities. Information on the dispersion-controlling heterogeneities is not yet available as practical field methodologies for their identification and description have not yet been developed. Until such information is incorporated into mass-transport models, a realistic solution of the dispersion problem in heterogeneous media will remain elusive.