6b. Transmissivity Maps
In northeast Illinois, the abundance of glacial sediments and carbonates makes fieldwork necessary for understanding how water moves in the subsurface. Illinois State Water Survey scientist, George Roadcap, conducted a study on the region's transmissivity in 1993 that we considered for our model.
Transmissivity map for the fractured dolomite in Will County from Roadcap 1993
Although only part of Roadcap's 1993 map shows our model domain, it is clear that transmissivity values associated with the dolomite will vary by orders of magnitude within a short distance of each other. Roadcap hypothesized that the highest values in the region (indicated by the bullseye targets) occur due to taking a sample near carbonate dissolution.
To create a map of the hydraulic conductivity based on Roadcap's 1993 transmissivity measurements, the transmissivity was first converted from units of gallons per day per foot to hydraulic conductivity ft/day then divided by 50, the assumed thickness of the dolomite. We then used a kriging function to turn the observational points into a smoothed surface.
The code can be found here: https://github.com/dbabrams/ShallowDolomite_Group/blob/master/Master_Code/Variable_T_Roadcap_update.ipynb
These hydraulic conductivity values are higher than what is typically associated with fractured dolomite by one order of magnitude. When looking at the list of values, most are less than 50 ft/day but since several measurements are in the thousands or ten thousands the regional hydraulic conductivity is raised when kriging is applied. Perhaps as Roadcap speculated, these values represent carbonate dissolution. It is also possible that the high conductivity values could be from locations with sandy layers.
A map of the spatial variation of transmissivity in the model domain is very useful for any analysis which the model is meant to provide. Therefore it was important to include such a map in our final report. Horizontal transmissivity (T) of the entire aquifer consists of the sum of the horizontal transmissivity of each layer. T is calculated from hydraulic conductivity (K) according to the following formula: T = K*b where b is the thickness of the layer. In order for this equation to apply to the entire model domain we assumed that each layer was fully saturated. Our transmissivity map is shown in the figure below.
Modeled Horizontal Transmissivity Map of Will County
The map shows that our simulation of the aquifer in this region of Will County has high transmissivity. This agrees with the conclusions of Roadcap (1993). The areas of highest transmissivity correlate to areas in the geologic model which had a greater percentage of coarse-grained material. The areas of lowest transmissivity correlate to the areas with a greater percentage of fine-grained material. However, these areas do not necessarily match up with the locations of fine- or coarse-grained material at land surface. It is apparent from the transmissivity map that the northern part of our model has high transmissivity even though this is all fine-grain material at land surface. The blue areas in the central and southern part of this map seem to align fairly well with coarse-grained material at the surface.
Our modeled transmissivity map agrees well with measurements and spatial trends observed by Roadcap (1993). There appears to be a general decrease in transmissivity values from the northeast corner of the model map to the southwest corner, just as shown in the map of hydraulic conductivity derived from Roadcap's report. The model transmissivity also reflects the large difference in values of transmissivity within short distances of each other measured by Roadcap (1993) as shown in the map above labeled Figure 9. The similarity between our model and the earlier report is a positive indication that these investigations approximate the reality of the groundwater system in the part of Will County.