Geologic Substrata Infiltration Soils
Because rainwater is a significant input into the combined system, why not keep rainwater out of it? Are there places we can infiltrate water and keep it in the Lake Michigan watershed?
This excellent question led us to explore the potential to infiltrate water in Chicago, right in neighborhoods, right in industrial areas, and in streets that run through both. In looking back at the hydrologic history and the glacial soils, we located a series of glacial deposits at the surface and subsurface, composed of sands and gravels, capable of infiltrating water, and potentially creating recharge zones for Lake Michigan. Thus, the glaciated landscape of Chicago actually contains one of the keys to unlocking the system.
The U.S. Geologic Survey of 1902 mapped dune sand and sand & gravel soil types that exists in the coastal area, roughly 2-5 miles width of the shoreline. These soils may serve to infiltrate, wash and return rainwaters to the Lake. Local infiltration would further benefit those areas, by releasing waters flooding at the surface into the lower soil layers, assisting with passive irrigation for planting, and creating a healthier local microclimate by encouraging hydric and thermal exchange between surface and subsurface.
We can use this soil variability to re-structure the city accordingly, using the capacity of the geologic nature of the City as a robust performance system, managing water in a comprehensive way. By infiltrating water, we are also keeping water out of the combined system, which benefits the down-pipe conditions and both watersheds.
The image on the left is of sand, dune sand, and gravelly soils (cut from the USGS 1902 survey). The image at right combines a depiction of these same soil areas from a 2009 USGS map with other landscape networks as part of the Millennium Reserve and the glacial remnants of the Indiana coast area to the east.
To respond to water issues across the city, this research suggests that there is a network of urban lands where rainwater can be captured and conveyed through passive infiltration features - that together, will help to resolve the region's long term relationship with stormwater.
Part of this strategy will include creating a new interface between urban surface and geologic substrata permeability and transport, increasing the performance potential of both layers.