The Hydro-Potential (HP) Value: A Rock Classification Technique for Evaluation of the Ground-Water Potential in Fractured Bedrock

Abstract

Ground-water development in mountainous terrain continues to present challenges where surficial unconsolidated materials have not been deposited, are thin, or have been largely removed by erosion leaving only fractured bedrock aquifers. To locate these bedrock ground-water sources, hydrogeologists employ a blend of investigative techniques that include terrain and fracture trace analysis coupled with structural mapping. Their goal is to locate large concentrations of open fractures that will convey ground water through bedrock. The hydro-potential (HP) value is a new rock-mass classification, semi-quantitative technique employed to evaluate the potential for developing ground water in bedrock. It is the potential for a rock mass to hydraulically transmit ground water. The method is a simple and quick technique used to evaluate rock outcrops surrounding and within the target basin. The technique is a modification of the engineering rock mass quality designation (Q) originally developed for evaluation of rock competency in tunnel design (Barton et al., 1974) and seismic rock fall susceptibility (Harp and Noble, 1993). The method describes six fracture characteristics of the rock mass and assigns a numerical value from reference tables based on their fracture properties. The equation for the hydro-potential value is as follows: where the fracture characteristics are: rock quality designation (RQD), joint number (J n ), joint roughness (J r ), joint hydraulic conductivity (J k ), joint aperture factor (J af ), and joint water factor (J w ). The numerical value of HP ranges from 1.33 × 10 −3 (for exceptionally poor quality rock) up to 800 (for exceptionally good quality, competent rock). During this investigation, the yields and specific capacities of thirty-seven (37) bedrock wells located in various lithologic terrains and flow regimes were compared to respective HP-values of surrounding rock outcrops. Contrasting the HP-values to yields and specific capacities of bedrock wells displayed a significant inverse exponential relationship with strong correlation coefficients (R) exceeding −0.80. Over 83 percent of the wells studied indicate higher yields and specific capacities correspond to lower HP-values for outcrops of similar lithology in the vicinity of the local bedrock wells. The same trend was true for over 63 percent of the regional bedrock wells. The data suggests the fractured rocks surrounding the basin mirror the fractured bedrock aquifer. The benefit of the HP-value, coupled with photo-terrain and fracture trace analysis and structural mapping, increases the probability of locating suitable test well sites in bedrock. The technique forces the investigator to look closely at the fracture characteristics of the rock mass near the proposed test well sites. Correlation curves generated from this investigation provide a useful tool to predict well yields and specific capacities of bedrock wells based on measured HP-values.