Predicting contaminant transport along quartz veins above the water table in a mica-schist saprolite

Abstract

Saprolite, thoroughly weathered bedrock, is a porous material that can be used treat wastewater. The inherited veins and fractures in saprolite have been suspected of being capable of transmitting the wastes to ground water. This study evaluated the time of travel of a Br − solute through a large quartz vein in a mica schist saprolite. Four 150 by 150 cm drainfields were constructed over a saprolite that contained a single quartz vein that was 45 cm wide and continuous across the site. Following saturation of the saprolite, a Br − tracer and blue-colored dye were applied for a period sufficient for the tracer to leach 30 or 40 cm. Saturated hydraulic conductivity was determined for each drainfield by measuring water intake and also by using the compact constant-head permeameter. The drainfields were then excavated to 90 cm, the dye pattern mapped and soil samples collected for Br − analysis. Dye patterns below the drainfields indicated that virtually no preferential flow along the vein occurred. Depth of dye penetration within the matrix and vein appeared similar. Deeper penetration did occur along root channels, which extended to 80 cm. There was no significant difference ( P=0.10) in depth of Br − penetration in saprolite with and without quartz veins. A simple time of travel model predicted maximum depth of solute movement accurately when no preferential flow occurred. These results showed that large quartz veins that contain clay or Fe–Mn oxides between the quartz gravels will conduct water at a rate that is similar to that found for the saprolite matrix. Such veins pose no apparent hazard to wastewater disposal.