4c. Cross Sections
The model domain was divided into 6 East-West and 5 North-South transects as shown in the image below.
Many of the transects follow major roads as references. ILWATER was used to find wells along the transects. From the well logs, we constructed profiles of the geologic material along each transect. We chose to classify our geologic material into four general categories: (1) clay/silt, (2) gravel/sand, (3) limestone/dolomite, and (4) shale. Any topsoil was lumped in with the next layer. Our preliminary transects are shown in a variety of ways, some hand-drawn and others made digitally:
From completing the transects, we found that the first 50 ft of depth include mostly silt and clay sediments with some gravel deposits. After this depth the dolomite unit begins and extends to up to 200 feet. We find that in the western part of our domain the dolomite is sometimes interrupted by a unit of shale.
Using the cross-sectional data from all the group members, the cross-sections were recreated and elevation data was added in python as shown below.
Map of Cross-sections Used:
Additional cross-sections were completed for a better comparison to the MODFLOW cross-sections (labeled on the map in pink):
North to South: cross-section from ILWATER data:
cross-section from MODFLOW:
West to East: cross-section from ILWATER data:
cross-section from MODFLOW:
The two cross-sections above were created with more detail on the layers overlying the bedrock. A 20% threshold was used where everything above this will be defined with course hydraulic conductivity (sand and gravel) and everything below this will be fine grained (clay and silt) for the model. The first cross-section was generated using logs from the Illinois State Geological Survey's Interactive Water Map (ILWATER). The second cross-section was generated through the model output using MODFLOW and well data from the Illinois State Water Survey.
Looking at the north-south cross-section, we see that clay becomes thicker with pockets of sand and gravel before reaching the Des Plaines River. Elevation of the bedrock and other above units also decrease steadily towards the south except for the thick clay unit towards the Des Plaines River. The ILWATER cross-section shows a continuous sand unit throughout while the model does not include that layer. The elevations seem to be very similar in both cross-sections and both show the clear sand and gravel unit within the thicker clay unit.
The east-west cross-section features a prominent thick clay unit in the middle. Both models show this, however, the model output shows an interbedded sand unit and the ILWATER data cross-section shows a thick sand unit on the bottom leading to the bedrock. To the west, both models show interbedded sand and clay units.