Groundwater Quality Monitoring Program for the Tunnel and Reservoir Plan (TARP) System

The Metropolitan Water Reclamation District of Greater Chicago (District) operates the Tunnel and Reservoir Plan (TARP) System, which is designed to transmit combined sewer flow (stormwater and wastewater) through tunnels in the lower part of the Silurian-aged bedrock to storage reservoirs during storms. After a precipitation event, water in the reservoirs is pumped to a water reclamation plant where it is treated before it is discharged to a nearby surface-water body. The District is committed to protecting and safeguarding the region's surface and groundwater quality.

The District implemented an extensive groundwater monitoring program during the construction of the TARP system. A series of groundwater monitoring and water level observational wells were installed adjacent to the tunnels and approximately 200 feet from the center line of the TARP tunnels. The number of wells increased as the tunnels were constructed. As of 2016, there are 120 groundwater monitoring and 34 observational wells which are being actively monitored. Monitoring wells are currently sampled at either six or three times per year for ammonia-N, chloride, electrical conductivity, fecal coliform, hardness, pH, sulfate, temperature, total dissolved solids, total dissolved organic carbon, and water elevation relative to City of Chicago datum.  The monitoring is done to assess the effect any potential leakage of combined sewer flow (CSF) from the TARP tunnels may have on water quality in the surrounding Silurian aquifer.

In 2014, the District established an intergovernmental agreement with the U.S. Geological Survey (USGS) to review and analyze historical TARP monitoring data and assess the underlying geology of the region. The USGS analyzed historical water quality monitoring data spanning the period 1995 - 2013 for the Mainstream, Calumet, Des Plaines, and Upper Des Plaines tunnel systems of TARP.  Overall, the USGS analysis showed that the TARP system had minimal impact on the surrounding Silurian aquifer.  A comprehensive report with detailed analysis of water quality monitoring data spanning from 1995 - 2013 is available at:

Kay, R.T., 2015, Hydrogeology and groundwater quality at monitoring wells installed for the Tunnel and Reservoir Plan System and nearby water-supply wells, Cook County, Illinois, 1995-2013: U.S. Geological Survey Scientific Investigations Report 2015-5186, 347 p.,

For additional information, please contact Dr. Guanglong Tian at or (708) 588-4201.