A NATURAL LABORATORY FOR HYDROGEOLOGIC INVESTIGATIONS IN THE WEST-CENTRAL PIEDMONT—THE UPPER WOLF ISLAND CREEK WATERSHED NEAR REIDSVILLE, ROCKINGHAM CO., NC

The upper Wolf Island Creek watershed near Reidsville, NC, is located in the central Milton terrane, where the rock types, structure, regolith, and landscape typify much of the Appalachian Piedmont. Geologic-map units include mica gneiss and schist, felsic gneiss, migmatitic biotite gneiss, white-mica schist, amphibolite, ultramafic rock, foliated granite, and mylonitic schist and gneiss of the Carroll Creek shear zone. Foliation is parallel to gneissic layering except in hinges of tight to isoclinal folds. These early folds are refolded by open, upright folds. Gently to moderately dipping foliation-parallel parting surfaces locally merge into subhorizontal sheeting joints and are commonly connected by steep crosscutting joints. Some steep joints are concentrated in hinges of open, upright folds and others were intruded by Jurassic diabase dikes. Steep fractures in felsic gneiss are locally confined between amphibolite layers. While fracture density varies according to rock type, fractures in the upper zone of weathered rock are mostly open, and the proportion of closed fractures increases with depth. The character of the bedrock-regolith transition zone varies by rock type, and in interlayered rocks, is complicated by alternating zones of saprolite and rocks more resistant to weathering. Coreholes and well clusters are located along two transects where gneissic foliation dips gently to moderately SSE. Land surfaces slope in the foliation-dip direction toward Wolf Island Creek along the south transect (dip slope) and opposite the dip direction toward Carroll Creek along the north transect (cut slope). Well clusters along each transect include deep (bedrock, <450 ft), intermediate (transition zone), and shallow (regolith) wells. Well yields along the transects and vertical hydraulic head gradients within well clusters suggest that foliation-parallel parting has a strong influence on ground-water flow. Nominal bedrock well yields increase from 0.5 to 7 gpm down the cut-slope transect and from 0.5 to 55 gpm down the dip-slope transect. Vertical head gradients in most cut-slope-transect well clusters show recharge conditions, whereas those in all dip-slope-transect clusters show discharge conditions. Water hosted by amphibolite appears to be chemically distinct (high in Ca, Mg, Fe, sulfate, etc.).