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

Observational, Experimental and Inferred Evidence for Solute Diffusion in Fractured Granite Aquifers: Examples from the Mirror Lake Watershed, Grafton County, New Hampshire

Abstract

The role of solute diffusion between ground water and granite in the Mirror Lake drainage area was evaluated by direct observation, experiment, and inference. The outcrops display ubiquitous Liesegang bands associated with fractures that clearly indicate the activity of diffusion in this system in the past. Laboratory experiments determined that the effective diffusion coefficient for ¹³⁷Cs was approximately 6 x 10^-13 m²/s in granite from Mirror Lake. The ¹³⁷Cs penetrated to a depth of approximately 7 mm in 101 days, demonstrating the potential for rapid diffusion in this system. Porosities of 32 granite samples averaged of 1.46 percent with a range of 1.07 to 2.32 percent. Measurements of carbon-isotope of ground water in the fractures suggest that calcite, identified in the granite, is dissolving and the bicarbonate generated is diffusing to the fractures; that is, a significant amount of the dissolved solids in the water in the fractures are derived from diffusion of weathering products from the rock matrix. These observations taken together are consistent with the interpretation that diffusion is a major process controlling solutes in this fractured granite aquifer.