Estimating Transmissivity Using Tidal Response Testing in Fractured Rock Aquifers

In coastal areas, tidal fluctuations influence groundwater levels dynamically. The response of the aquifer to a tidal signal can be used to estimate the diffusivity (D) of the aquifer. If an independent estimate of storativity (S) is available, the transmissivity (T) can be calculated. Few studies have explored the use of tidal response testing in fractured rock aquifers. The advantage of tidal response testing over pumping tests in fractured rock is that the aquifer is stressed over a much larger area, potentially yielding T values that reflect the network of interconnected fractures rather than discrete fractures that often dominate a pumping response. In this study, T values from tidal response testing in 24 wells are compared. The results are also compared with T values from pumping tests. The study area is the Gulf Islands region of British Columbia, Canada. The majority of the area consists of fractured sedimentary rock comprising sandstone-dominant and mudstone-dominant formations. A portion of the study area consists of fractured igneous rock. Diffusivity (T/S) was calculated using the amplitude method for tidal response testing. D ranged from 0.01 to 13 m²/s. T was then calculated using the average S from 19 pumping tests conducted in sedimentary rocks (3x10^-4) and 4 tests conducted in igneous rocks (1x10^-3). T values calculated from the tidal analysis for all rock types were within one order of magnitude of each other. Average T for the sandstone-dominant formations was 8x10^-4 m²/s, for the mudstone-dominant formations 4x10^-4 m²/s, and for the igneous rocks 8x10^-5 m²/s. T values from 105 pumping tests for these three rock types were 2x10^-5 m²/s (91 tests), 2 x10^-5 m²/s (10 tests) and 5 x10^-4 m²/s (4 tests), respectively, again all within one order of magnitude. In the sedimentary rocks, the T values from the pumping tests are slightly lower than those from the tidal response testing, while in the igneous rocks the opposite is observed. These results suggest that there is little difference in T both between rock types and for the two different testing methods.