ABSTRACT

Concerns over the state of water resources in Rockland County, NY, prompted an assessment of current (2005–07) conditions. The investigation included a review of all water resources but centered on the Newark basin aquifer, a fractured-bedrock aquifer over which nearly 300,000 people reside. Most concern has been focused on this aquifer because of (1) high summer pumping rates, with occasional entrained-air problems and an unexplained water-level decline at a monitoring well, (2) annual withdrawals that have approached or even exceeded previous estimates of aquifer recharge, and (3) numerous contamination problems that have caused temporary or long-term shutdown of production wells. Public water supply in Rockland County uses three sources of water in roughly equal parts: (1) the Newark basin sedimentary bedrock aquifer, (2) alluvial aquifers along the Ramapo and Mahwah Rivers, and (3) surface waters from Lake DeForest Reservoir and a smaller, new reservoir supply in the Highlands part of the county. Water withdrawals from the alluvial aquifer in the Ramapo River valley and the Lake DeForest Reservoir are subject to water-supply application permits that stipulate minimum flows that must be maintained downstream into New Jersey. There is a need, therefore, at a minimum, to prevent any loss of the bedrock-aquifer resource—to maintain it in terms of both sustainable use and water-quality protection.

A regional conceptual model of the aquifer framework was needed upon which other regional and local hydrogeologic data could be overlaid to define the regional groundwater flow system. From that perspective, water-resource questions could be addressed from a regional context.

The framework of the Newark basin bedrock aquifer included characterization of (1) the structure and fracture occurrence associated with the Newark basin strata, (2) the texture and thickness of overlying glacial and alluvial deposits, (3) the presence of the Palisades sill and associated basaltic units on or within the Newark basin strata, and (4) the streams that drain the aquifer system. The structure of the aquifer was in part defined by previous geologic mapping, including strike and dip measurements of the sedimentary strata that fill the basin, and lithologic mapping that shows westward coarsening from mudstones and siltstones to conglomeratic sandstones. Borehole geophysical surveys were conducted at 24 wells and provided critical subsurface structural data. Other data that contributed to the conceptual model of the aquifer framework included groundwater-level responses to pumping at production wells and groundwater and surface-water chemistry (particularly chloride). The strike of the tilted bedding constrains groundwater flow because the most productive water-bearing fractures are subparallel to bedding. The general strike of bedding is north-northeast and the dip is about 10 degrees to the northwest. The regional groundwater flow system was delineated by overlaying aquifer-wide groundwater-level data on the bedrock framework (bedding strike lines). Groundwater divides were identified, including a major southeast to northwest regional divide that partitions groundwater flow northeastward to discharge at the Hackensack River and its tributaries and southwestward to discharge in the Mahwah River, Pascack Brook, and Saddle River drainages.

Review of pumping-rate and water-level data from the bedrock aquifer during 1989–2004 suggests that there is not a year-to-year, aquifer-wide downward trend in water levels. There have been periods of several years where water levels at individual wells show declines, and groundwater levels have declined in response to new stresses as production wells have come online, especially if the wells have been used continuously. Once pumping is initiated, water levels decline toward a new equilibrium, if possible. In fact, water levels in a large area of the most productive west-central part of the bedrock aquifer have declined because of withdrawals and depths to water in this part of the aquifer are the greatest (100–150 feet).

The greatest concern regarding sustainability of groundwater resources is the aquifer response to the seasonal increase in pumping rates from May through October (an average increase of 25 percent in 2005). Investigation of pumping rates and water levels during these periods indicates that water levels in most wells decline beyond what is expected under natural conditions and that the effective aquifer yield can decrease as water levels drop or as entrained air from stressed aquifer conditions creates problems in the distribution system. Increases in pumping rates at certain productive well fields during summer result in water-level decline rates that are not sustainable and that represent the greatest stresses on the aquifer. Extrapolation of water-level decline rates under conditions of continuous pumping (a worst-case scenario, although the assumption of no decrease in aquifer yield over the summer is a best case scenario) indicates that between 10 and 15 wells would not be able to pump through the entire high-water-use season (May 15 to October 1). In most cases, pump rates would have to be reduced as aquifer yield declines. This analysis underlines the fragility of the aquifer given the fact that recent years (2003–06) have been relatively wet. Large seasonal water-level fluctuations in the most productive part of the aquifer indicate that recharge during the non-growing season thus far has been enough to replenish the aquifer prior to the next growing season. Streams also are affected by seasonal increases in groundwater pumping rates; nearly all streams in the productive west-central area of the aquifer went dry during dry periods in late summer of 2005.

Impervious surfaces increase the amount of stormflow and decrease the amount of base flow in streams. Analysis of stormflows in watersheds with 11.9 and 17 percent impervious surface area increased the percentage of rainfall that becomes stormflow in streams by 7 to 8 percent and by 12.5 to 16.5 percent, respectively.

Recharge was estimated from streamflow data and from groundwater-level data. Estimates from across the county in 1961 ranged from 24.8 inches in the northwest (New York Highlands area) to 14.7 inches in the southeast. Recharge largely parallels the annual amount of precipitation. Recharge is probably highest in the Highlands because of high precipitation, despite crystalline bedrock that acts as a relatively poor aquifer. Across the county, the thickness of glacial deposits that mantle bedrock also appeared to be a major control on the amount of recharge. The distribution of monthly recharge was documented, including substantial recharge during the growing season in 2006.

Water budgets were generated for three basins with streamflow data. During1959–94 and in 2006, groundwater pumpage for public supply accounted for 12 to 24 percent of recharge within the Mahwah River near Suffern, NY, watershed. Public-supply pumpage as a percentage of recharge in 2006 at the two other currently gaged watersheds (Pascack Brook and Saddle River) was 18 and 21 percent, respectively.

About 12.9 billion gallons of water was used in Rockland County in 2005. The majority (63 percent) was for base-line domestic supply (non-growing season rates of use); of this amount, about 6 percent was from domestic wells and 94 percent was from production wells and reservoirs. Commercial, industrial, and institutional users made up 10 percent of total water use, and growing-season increases accounted for 18 percent.

Sanitary sewers serve much of Rockland County and the majority of treated wastewater is discharged to the Hudson River, which is an estuary with brackish water adjacent to Rockland County. Inflow of stormwater and infiltration of groundwater constitute a significant additional contribution of water to the sanitary sewer system.

First posted February 22, 2011