This data release contains hydraulic conductivity data from single well (slug) tests conducted from the slag-filled material of the open interval of monitoring well 37N14E-13.3a1 (MW2, United States Geological Survey [USGS] site number 414121087335701), Big Marsh, Chicago, Illinois, to calculate groundwater discharge into nearby ponds. Six mechanical slug tests were conducted on October 27, 2016, in monitoring well 37N14E-13.3a1 (MW2, USGS site number 414121087335701) within the Big Marsh, in Chicago, Illinois, in the historic Calumet Area of Chicago. The tests were undertaken as part of a study to measure the horizontal hydraulic conductivity as an effort to calculate the flux and water-balance of Big Marsh. Three falling-head (slug-in), and three rising-head (slug-out) tests were completed in the well. Of the 6 tests, the slug-out tests were determined to have adequate slug response curves for the analysis of the data. Analytical results can be found in the cited companion Scientific Investigations Report.