Robert P. Schreiber, Mark A. Maimone, Kristina K. Masterson, and William Fernandez, Camp Dresser & McKee; Deborah E. Malanchuk, New York City Department of Environmental Protection; and David Nickols, Hazen and Sawyer

Under the terms of Determination issued by the US EPA, The City of New York has developed a time-neutral, dual-track approach to meeting the Surface Water Treatment Rule of the Safe Drinking Water Act. While implementing various filtration avoidance measures, The City is proceeding with treatment plant design efforts that could be advanced in the event that filtration is someday deemed necessary. As part of these efforts, the City hired the consulting team of Hazen and Sawyer and Camp Dresser & McKee to prepare preliminary design plans for the treatment plant that could connect to two major aqueducts in south-central Westchester County. The current preliminary design plans for the new plant call for filter modules and interconnecting pipelines that extend significantly below the ground surface and existing water table. To predict dewatering flow rates and effects on nearby wetlands and structures, the project team developed a three-dimensional groundwater flow model while also collecting data on local hydrogeologic, meteorologic and land use conditions. Historical data archives maintained by the New York State Geological Survey provided key items revealing potential groundwater inflow from bedrock formations, as indicated by measured streamflows that the model could not initially match. Interconnection pipelines appear to cause the bulk of the dewatering flows and potential wetland / streamflow impacts. Further modeling during the next design phase will improve the accuracy of the predictions, leading to development of plans for mitigating the impacts and minimizing dewatering flow rates.