USING GROUNDWATER MODELS TO UNDERSTAND THE EFFECTS OF FRACTURES ON THE TRANSPORT OF SOLUTES AT THE SHALE HILLS CRITICAL ZONE OBSERVATORY

The Shale Hills Critical Zone Observatory (CZO) is a small (approximately 8 hectare), forested catchment in central Pennsylvania underlain by fractured shale in the regolith and bedrock. The CZO is an interdisciplinary endeavor aimed at better understanding the function of the geochemical, hydrologic, biologic, and geomorphologic processes on soil formation from the depth of groundwater up to the outer limits of vegetation. Fractures play an important role in solute transport, but identifying hydrologically significant fractures is problematic. The introduction of new, non-invasive, geophysical methods along with tracer tests allows hydrogeologists to expand investigations and model the subsurface fracture framework.

A doublet tracer test at the site was conducted by the injection of tracer at the regolith/bedrock interface (5 to 6 m deep) to determine the path of groundwater flow via the fractured. Borehole resistivity was used to show the conductivity due to the addition of tracer. The resistivity data were collected across both the regolith and bedrock. One-dimensional modeling of transport in fractures has been completed for the field tracer test as well as core samples used for lab injection tracer tests. The cores were taken at intervals of 0.2 meters (m) to depths of 2.5 m. Optical televiewer logs were completed in the boreholes at the CZO. WellCAD was used to examine fracture spacing in the four borehole optical logs for depths of 3-16 m.

The optical televiewer logs indicated from 3 to 5 meters depth the average fracture spacing was 11 centimeters (cm), from 5 to 7 meters (regolith/bedrock interface) fracture spacing was 9 cm. Below 7 m the average fracture spacing increased to 41 cm. Modeling was used to determine fracture spacing, fracture aperture, and porosity. Transport modeling results indicated fracture spacing of 8 to 10 cm, which coincides with the shallow WellCad image logs. Apertures ranged between 0.08 and 0.1 cm and porosity ranged from 0.29 to 0.44. Matrix porosities of the Rose Hill formation were only 0.035 to 0.05, and the larger modeled values probably reflect the importance of the fracture transport system even in the matrix. Tracer modeling results, as well as optical televiewer logs, indicated an increase in fracture spacing with depth.