SOLUTE MOVEMENT THROUGH QUARTZ-DIORITE SAPROLITE CONTAINING QUARTZ VEINS AND BIOLOGICAL MACROPORES

Saprolite is not permitted for use in on-site wastewater disposal in N orth 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 drainfiel ds 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, th e dye pattern mapped, and soil samples collected for Br- analysis. The re was no significant difference (a = 0.10) in depth of Br- penetratio n in saprolite with and without quartz veins. On the other hand, mean depth of Br- penetration increased from 14 cm for saprolite without ma cropores to 40 cm in saprolite with macropores (root channels) that ex tended to depths between 40 to 90 cm. A simple time of travel model pr edicted maximum depth of solute movement accurately where macropores w ere not present. To improve predictions for macropores, effective satu rated hydraulic conductivities for saprolite with and without macropor es were calculated and then used to estimate different rates of solute movement through saprolite when macropores were and were not present.