U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Spatial Variability of Metal-Ion Adsorption and Hydraulic Conductivity in a Sand and Gravel Aquifer, Cape Cod, Massachusetts

Abstract

The spatial variability of metal-ion adsorption and hydraulic conductivity was assessed from the results of laboratory experiments on 375 sediment samples collected from 14 boreholes in a glacial outwash, sand and gravel aquifer on Cape Cod, Massachusetts. Zinc and lead adsorption were measured in batch experiments. Hydraulic conductivity was estimated on the basis of grain-size distributions. Mean lead adsorption is greater than mean zinc adsorption; mean hydraulic conductivity is similar to that measured previously in this aquifer. All three properties vary significantly within the aquifer; variability in zinc adsorption is greater than variability in lead adsorption. A strong positive correlation is observed between the two metal-ion adsorptions. There is a statistically significant, but small, negative correlation between lead adsorption and hydraulic conductivity and a statistically insignificant negative correlation between zinc adsorption and hydraulic conductivity. Vertical correlation scales obtained by fitting an exponential model to experimental semivariograms equaled 0.10-0.26 meters for the hydraulic-conductivity and adsorption data sets. Similar vertical correlation scales were determined in an earlier study of the variability of hydraulic conductivity measured by use of permeameter and flowmeter tests. A horizontal correlation scale could not be identified in this study, possibly because the horizontal spacing between boreholes was large relative to the correlation scale, and the number of samples was small.