Abstract
Hydraulic tomography based on geostatistics has proven to be robust in characterizing subsurface heterogeneity in hydraulic conductivity (K) and specific storage (Ss) through the joint inversion of drawdown records from multiple pumping tests. However, the spatially variable estimates can be smooth or even erroneous for areas where pumping/observation data densities are not high. Previous hydraulic tomography surveys conducted at the North Campus Research Site (NCRS) on the University of Waterloo campus in Waterloo, Canada, revealed that the estimated hydraulic parameters were smooth and the known aquitard was erroneously identified as a high K zone. This was likely the consequence of the site being highly heterogeneous, while only utilizing four pumping tests and not having measurable drawdowns in the low K aquitard for inverse modeling. Here, we investigate whether improved K and Ss estimates could be obtained through the inclusion of additional pumping test data by stressing both aquifer and aquitard zones for a sufficiently long period. Specifically, six additional pumping/injection tests were conducted at the site, and a transient hydraulic tomography analysis with 14 tests was completed. Results reveal that there is a significant improvement to the K and Ss tomograms in terms of the visual correspondence with various geologic units, including its connectivity. More importantly, with the availability of additional data, we found that the inverse model now can better capture the high and low K features for nine boreholes when compared with K values obtained from permeameter tests. The estimated K and Ss tomograms are then used for the forward simulation of one additional pumping test not used for model calibration, revealing reasonable predictions. While encouraging results are obtained by including a large number of pumping tests to the transient hydraulic tomography analysis, stratigraphic boundaries are still smoothed, which is a direct consequence of utilizing a geostatistics-based inversion approach that assumes stationarity in statistical properties. To capture such sharp boundaries, incorporation of additional data types, such as geological and geophysical information, may be necessary when data densities are not sufficiently high.
Highlights
The subsurface distributions of hydraulic conductivity (K) and specific storage (Ss ) control fluid, solute, and energy transport in geologic media
We plotted the vertical K profiles along nine In Berg and Illman [18], a 45 m by 45 m by 15 m numerical model was built, and pumping tests boreholes from the transient hydraulic tomography (THT) analysis of Berg and Illman [18], as shown in Figure 5, and provided conducted at ports PW1-3, PW3-3, PW4-3, and PW5-3 were used for their THT analysis
Comparing the K and Ss tomogram sections of current study (Figure 8g–j) to those from Berg and Illman [18] (Figure 8c–f), we observe that: (1) the high K zones located at the top and bottom In Berg and Illman [18], a 45 m by 45 m by 15 m numerical model was built, and pumping tests areas of Figure 8c,d are properly mapped as low K zones in Figure 8g,h, which correspond well conducted at ports PW1-3, PW3-3, PW4-3, and PW5-3 were used for their THT analysis
Summary
The subsurface distributions of hydraulic conductivity (K) and specific storage (Ss ) control fluid, solute, and energy transport in geologic media. Conventional hydrogeological characterization techniques, such as slug and single-hole tests, as well as laboratory permeameter tests and grain size analyses (e.g., [1,2]), are invasive and cost-intensive to obtain high-resolution information. Direct push techniques can provide high-resolution information on K during each push (e.g., [3]), but without interpolation, there is no information of K between push locations. While direct push techniques are useful, they do not provide information on Ss and have difficulty penetrating unconsolidated media when there are cobble size particles and hard clays. Hydraulic tomography (HT) has been proposed as an alternative site characterization method to obtain information on heterogeneity in groundwater flow parameters between boreholes
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