Assessment of the Sedimentary Bedrock Aquifer in Rockland County, New York

Problem - Ground water pumped from the sedimentary bedrock aquifer that underlies southeastern Rockland County is a major source of public water supply. Extensive suburban development has increased water-supply demands over the last 40 years to the point where the aquifer is considered fully developed in terms of wellfield spacing. Continued development in the County has led to progressive increases in withdrawals from existing wellfields. The County Health Department reported that 2000 and 2001 withdrawals approached or exceeded estimates of recharge that replenishes the aquifer. This raises serious concerns about the status of the aquifer. These concerns are magnified by: 1) the export of wastewater (originally ground water) in sanitary sewers from the aquifer area and 2) the increase in stormflow and decrease in aquifer recharge as continued suburban development increases the amount of impervious land surface.

The chemical quality of ground water is an additional concern, in that contamination decreases the water supply or requires treatment of the water supply. Bedrock aquifers are particularly susceptible to contamination from human activities at land surface, and several supply wells in this aquifer have been taken off line or have had treatment systems installed to remove contaminants. Definition of source waters contributing to supply wells improves the ability to protect and manage the aquifer.

The last study to focus on the overall hydrogeology of this aquifer was completed by the USGS in 1959. The present situation calls for further characterization of the aquifer framework, documentation of aquifer and stream responses to present-day ground-water withdrawals, and comparisons of current with historical data. Such information can be used by water managers to ensure the long-term sustainability of this critical resource.

Objectives - This study will address the following questions:

1. Are current ground-water withdrawal rates depleting the aquifer?
2. What are the sources of water to supply wells?
1. Does streamflow contribute water to supply wells
2. Does leakage from sanitary sewers contribute to streamflow or to supply wells?
3. What are the shape and size of land-surface areas that contribute water to supply wells?