EFFECTS OF DUAL POROSITY ON THE HYDRAULIC PROPERTIES OF THE BEDROCK SANDSTONE FORMATIONS IN NORTHEASTERN OHIO

In any aquifer, the porosity can be either primary or secondary or dual porosity. Primary porosity is the inter-granular porosity inherent to the rock. It is the porosity that was generated at the time of lithogenesis. Secondary porosity is the porosity created by post-genetic processes, e.g. fracturing or dissolution of the rock. Dual porosity is the combination of the two. Bedrock fracturing in Northeastern Ohio can be product of tectonic stress or unloading from the retreat of glaciers. Rates of ground water production from bedrock sandstone units in Northeastern Ohio are directly related to the presence and extent of secondary porosity. The goals of this study were to (1) test our hypothesis that there exist distinct zones of higher porosity due to secondary porosity; (2) locate their distribution; and (3) quantify their effect on the hydraulic properties governing ground water production rates. We analyzed ninety well logs of the single-home water wells drilled through the glacial sediments into the sandstone bedrock formations within Auburn and Bainbridge Townships in Geauga County, and Aurora and Mantua Townships in Portage County, Ohio. Aquifer thickness in each water-well was determined from the lithological profiles, while the specific capacity data from production tests were used to estimate the values of transmissivity for each well. Combination of the two parameters yielded mappable values of hydraulic conductivity. The resulting values of hydraulic conductivity were characterized by distinctly binary distribution, with the low values corresponding to the unfractured, and high values corresponding to the fractured zones with dual porosity. Once contoured on a map, these zones appeared clearly, with a transition between the areas of high and low hydraulic conductivity, i.e. high and low potential for ground water production, respectively.