Earth-Tide Derived Aquifer Properties in Fractured Granite: Results from a Groundwater Monitoring Well Network in the Peninsular Ranges Batholith

Abstract:

Fractured rock aquifers are difficult to characterize because of the three dimensional spatial heterogeneity of the fracture networks. Aquifer properties cannot be determined from a single borehole and traditional aquifer tests are difficult to design and analyze without prior knowledge of the subsurface permeability distribution. Using passive monitoring of the water level responses to tidal strains in wells allows for characterization of the aquifer over greater spatial extent and can be used to guide the design and implementation of aquifer tests. In this study, the water level response to tidal strains measured in over 20 groundwater wells, spaced irregularly over an approximately 48 km² area, was used to estimate the specific storage and transmissivity of the surrounding granite aquifer. The water level data were corrected to remove barometric pressure effects before the amplitude and phase shifts for the O₁ and M₂ components of the tidal potential were calculated. Systematic differences in the calculated aquifer characteristics were observed. The differences correlate with the density of fractures observed in borehole geophysical logs. The aquifer properties derived from the earth-tide analysis were compared to those derived from aquifer tests conducted at two of the wells analyzed. The two methods yielded similar results.