In Nassau County, New York, it is estimated that 27 million gallons a day of fresh water is flowing out to waste to sea from the south watershed. A barrier injection line utilizing renovated water along the south shore of the County would prevent the escape of this 27 million gallons a day and also supply the alternate flow which would maintain the fresh-salt water interface and prevent salt water intrusion. By the creation of this hydrologic dam, the 27 million gallons a day of fresh water thus made available could be utilized for additional water supply. The report discusses laboratory bench studies and model unit studies to determine parameters for a larger 400 gpm tertiary treatment pilot plant.