The Effects of Pore Pressure on the Conductivity of Fractured Aquifers

Abstract In this study, we investigate the degree to which climate‐induced changes in hydraulic head can affect the stress regime and hydraulic conductivity of fractured aquifers. The effects of stress changes on conductivity are well‐known, but not usually considered in the analysis of ground water systems because of the misconception that the head change necessary to make a measurable impact is beyond the range of normal head fluctuations. An empirical model relating changes in hydraulic conductivity of fractured aquifers to vertical‐effective stress was used in analyzing the effects of pore pressure changes on conductivity. An analysis showed that for realistic head changes (20 to 100 m), conductivity in an unconfined system can change as much as 20 to 30%. The analysis also showed that for a given head increase, increases in conductivity are greatest at shallow depths and that conductivity is a function of both hydraulic head and the relative position of head to the land surface. Maximum conductivity changes occur when the initial pore pressure is a relatively large percentage of the total stress. Similar results were also obtained in confined systems. Field examples demonstrate that large head changes have occurred in bedrock aquifers due to climatic changes and pumping withdrawals. Thus, large head changes needed to make a measurable impact on hydraulic conductivity are quite plausible. Conductivity changes can in turn lead to changes in calculated ground water flow and solute transport rates. As a result, when significant head changes are expected, changes in conductivity should be considered in the analysis of fractured aquifer systems.