Solute Movement through Quartz‐Diorite Saprolite Containing Quartz Veins and Biological Macropores

Abstract Saprolite is not permitted for use in on‐site wastewater disposal in North Carolina if quartz veins occur within 60 cm (2 ft) of the septic trench bottom because such features might transmit raw sewage quickly to groundwater. This study evaluated the time of travel (TOT) of a Br− solute through quartz veins in saprolite. Two 150 by 150 cm drainfields were constructed over saprolite containing quartz veins. Following saturation of the saprolite, a Br− tracer and dye were applied for a specific time period. The drainfields were then excavated to 90 cm, the dye pattern mapped, and soil samples collected for Br− analysis. There was no signilicant difference (α = 0.10) in depth of Br− penetration in saprolite with and without quartz veins. On the other hand, mean depth of Br− penetration increased from 14 cm for saprolite without macropores to 40 cm in saprolite with macropores (root channels) that extended to depths between 40 to 90 cm. A simple time of travel model predicted maximum depth of solute movement accurately where macropores were not present. To improve predictions for macropores, effective saturated hydraulic conductivities for saprolite with and without macropores were calculated and then used to estimate different rates of solute movement through saprolite when macropores were and were not present.