EFFECTS OF GEOLOGY AND URBAN SPRAWL ON THE WATER RESOURCES OF THE NEW YORK-NEW JERSEY HIGHLANDS REGION

       The Highlands Region of New York and New Jersey, underlain predominantly by crystalline rock, is the source of water supply for more than 4.5 million people. Geology and the degree of urbanization affect the infiltration, storage, and availability of Highlands water. Highlands reservoir systems supply more than 450 Mgal/d (million gallons per day), 90 percent of which is transferred out of the Highlands to major metropolitan areas to the east. Ground water is the primary source of water used within the region, where bedrock and glacial valley-fill aquifers supply more than 145 Mgal/d.

       As part of a recent assessment of the potential effects of continued suburban development on water resources in the region, the U.S. Geological Survey, in cooperation with the U.S. Forest Service, used a watershed model (TOPMODEL) to simulate the effect of projected increases in impervious surface cover and population growth on resource sustainability and to develop water budgets for 182 watersheds. On average, ground-water discharge to streams (base flow) accounts for 73% of total streamflow over the region. Variations in the percentage of total streamflow that occurs as base flow depend mainly on the geology and degree of development in a watershed. Base flow accounts for more than 80% of streamflow in watersheds where the geology is characterized by a high percentage of carbonate-rock and glacial aquifers, which have relatively high recharge rates and water-storage capacities. In contrast, base flow accounts for less than 50% of streamflow in some of the most urbanized areas with large ground-water withdrawals such as Rockland County, New York, and Morris County, New Jersey. The predicted rate of change in stream runoff, base flow, and evapotranspiration increases substantially for watersheds with a projected increase of 15% or more in impervious surface cover with respect to 1995 conditions. Water-budget calculations indicate that runoff may increase 50% or more, and base flow may decrease 10% or more, in these watersheds. Watersheds with current and projected ground-water withdrawals greater than 20% of estimated ground-water recharge were identified as being most sensitive to increases in impervious cover and ground-water withdrawals and therefore are most at risk with respect to the sustainability of ground-water yields.