In 2018, the City of Joliet assessed its long-term water supply and found that the city should change its water supply source, the deep Cambrian-Ordovician Sandstone aquifer, by 2030. A three-year follow-up study has been initiated with a collaboration among scientists from the Illinois State Water Survey and multiple communities and industries in the southwest suburbs of Chicago. This contract report summarizes the Year 1 findings.Withdrawals from the deep Cambrian-Ordovician aquifer system in northeastern Illinois have been unsustainable since the early 1900s. The sandstone aquifer system that supplies water to Will County contains the uppermost St. Peter Sandstone and the Ironton-Galesville Sandstone; this study frequently uses the terms “Cambrian-Ordovician Sandstone aquifers,” “sandstone aquifers,” or “sandstone” when referring to the St. Peter and Ironton-Galesville collectively. In the study area, sustainable withdrawals are estimated to range from 2 to 7 million gallons per day (mgd), while recent sandstone demands range from 35 to 38 mgd. As a result, sandstone water levels have been declining for over a century. In some areas, the uppermost St. Peter Sandstone is currently dewatered, while the lowermost freshwater aquifer, the Ironton-Galesville Sandstone, is at-risk of dewatering in localized areas. Further declines will increase the risk to both aquifers. With Joliet planning to switch off the sandstone aquifers by 2030 and communities in Kendall County considering leaving the aquifer shortly after, updated groundwater flow models have assessed the regional impact of this reduction on sandstone water use. This report focuses on a single model scenario, referred to as the Current Trend, and outlines its underlying assumptions. A sensitivity analysis was conducted to demonstrate the impact of pumping assumptions in different portions of the region.Joliet and Kendall County communities switching from the sandstone aquifer will not alleviate the regional risk to the sandstone supply. The impact of alternate demands from new and existing industries, which were assumed to remain unchanged in the future, may still pose a risk to the regional sandstone supply. Regional action is needed to ensure a sustainable future water supply and to preserve the sandstone aquifer as an option for future back-up supply needs.