HYDRAULIC HEAD HYDROGRAPHS FROM DEPTH-DISCRETE HIGH RESOLUTION MULTILEVEL SYSTEMS FOR ESTIMATING LOADING EFFICIENCY AND SPECIFIC STORAGE IN THE SILURIAN DOLOSTONE

This study involves the application of the poroelastic theory of loading efficiency to a Silurian dual-porosity sedimentary fractured rock sequence at a 10 hectare study site within Guelph, Ontario. The dolostone sequence is 100m thick, covered by a thin layer of anthropogenic till and underlain by an extensive deposit of shale. The site is instrumented with 5 multilevel systems with 51 total pressure transducers. This study relies upon collected pore water and barometric time series datasets to determine depth-discrete loading efficiency profiles which are used to assess degree of confinement, and estimates of undrained uniaxial specific storage. The results are assessed for reasonableness against specific storage values previously collected on site with the same monitoring infrastructure using traditional methods.

The 5 loading efficiency profiles were found to follow the same general trend of was nearly 1 at the top of the sequence, underlain by a stepped transitional zone, 0.1 to 0.3 through the remainder of the dolostone sequence, and ~0.8 in the underlying shale. This indicated the spatial variability of the degree of confinement through the sequence. Inflections in the loading efficiency profiles were found to occur adjacent to multiple thin mechanical units with strong mechanical horizons and across the dolostone/shale boundary. The variability in elevation at which loading efficiency inflections occurred was attributed to where the thinly bedded deposition occurred overlying the undulating thickly bedded sequence. Uniaxial specific storage values were calculated for each monitoring port from the loading efficiency response and ranged from 1.47x10^-7 m^-1 to 2.71x10^-6 m^-1. These values were found to be half to one order of magnitude less than the values from the standard pumping test conducted at the site and may be an improved representation of the specific storage within the matrix. The poroelastic theory of loading efficiency improved the hydrogeolgocial characterization of the site by incorporating the heterogeneity of fractured dolostone rock aquifers in determining aquifer hydraulic properties.