Long Island Outflow from the Groundwater System

Science Center Objects

The flow of water leaving, or discharging, the groundwater system of Long Island occurs naturally through streams, as base flow, at the coastline as shoreline discharge and sub-sea discharge, and through pumping wells as withdrawals. Estimates of each component of outflow from the groundwater system is presented and summarized in this section using streamflow measurements, and a compilation of reported or estimated withdrawals.

Streamflow

There are over 100 stream channels on Long Island, typically less than 5 miles long, that flow to the tidewater that surrounds Long Island. The channels were formed by glacial melt water and therefore are more abundant along the southern shore than along the northern shore. Groundwater discharge to streams has a major effect on flow patterns within the groundwater system. Under pre-development conditions, about 21 percent of precipitation, equivalent to more than 40 percent of the groundwater leaving the system, was discharged to streams (Buxton and others, 1999).

Continuous streamflow records, ranging in length from about 7 to 73 years, are available for the 22 streams shown below (Table 1). Unless otherwise stated, all values of streamflow are for total streamflow and therefore, include direct runoff. The average base flow of the streams (that is, seepage from the groundwater reservoir) is about 90-95 percent of total average streamflow (Spinello and Simmons, 1992; Reynolds, 1982). The average measured streamflow for the entire period of record at each stream gage station is presented below. On average about 188 Mgal/d of cumulative measured discharge occurred at these stations. Assuming that 90-95 percent of this streamflow is base flow, approximately 170 to 179 Mgal/d of groundwater leaves the Long Island aquifer system as stream base flow.

A large amount of additional water undoubtedly seeps from the groundwater system into the lower tidal reaches of the streams in the nearshore areas (estuarine seepage) particularly the southern nearshore area. Most of this water is derived from precipitation that recharges the groundwater system in the nearshore areas, that is, it is not part of the deep circulating groundwater system. The estimated average amount of this additional unmeasured streamflow is on the order of 40-80 Mgal/d for the southern nearshore area and 10-15 Mgal/d for the northern nearshore area (Franke and other,1968).

Therefore, the combined estimated groundwater discharge to streams and as nearshore estuarine seepage is on average around 220 to 280 Mgal/d for the period 2005-2010. This amount is about 60 Mgal/d less than the 26 year period (1940-65) described in Franke and others (1972). Buxton and others (1999), estimated the average groundwater discharge into the streams was 460 Mgal/d in a predevelopment period, and 325 Mgal/d from 1968-1983; in other words, a 135 Mgal/d reduction in base flow from the predevelopment period.

 

chart of USGS stream gage stations which had continuous record for a period of record
Table 1. USGS stream gage stations which had continuous record for a period of record, the average daily discharge was calculatedusing the entire period of record. (Gage locations are shown in figure 10 and 18)(Public domain.)

Groundwater Withdrawals

Groundwater pumpage on Long Island and water usage presented under the “Water Availability-Water Use” section is summarized here. The public water supply withdrawal data reported to the New York State Department of Environmental Conservation (NYSDEC) in 2010 and the USGS National Water Use Information Program (NWUIP) estimates of 2005 (Kenny and others, 2009) groundwater withdrawals for domestic-self supply, industrial and irrigation were used to calculate the average amount of water pumped from the groundwater reservoir. Even though these data have different time periods, it is an assumption that they are approximate amounts. The estimated average groundwater withdrawals throughout Long Island is over 500 Mgal/d, for 2005-2010 index period.

The following is a compilation of groundwater withdrawal data from several reports represented in an animation. A recharge amount of 22 inches per year (estimate of long term natural recharge) was used throughout the animation, as the natural source of fresh water entering the system. The animation shows the average annual withdrawals for each county from 1900-2007, and the percentage of the natural recharge going towards withdrawals, for each county. This approach provides a simple comparison of the natural groundwater recharge with the withdrawals for each county; and is not representing returned water through leaks, recharge basins or on-site septic systems.

Map of compilation of groundwater withdrawal dataon Long Island from several reports represented in an animation 1904-2007
Compilation of groundwater withdrawal data on Long Island from several reports represented in an animation 1904-2007. Click here to view animation. (Public domain.)

Coastline and Sub-Sea Discharge

Subsurface outflow (sub-sea) under natural (pre-development) conditions mainly included the subsurface movement of groundwater northward to Long Island Sound, and southward (a) to the swampy lowlands bordering the south-shore bays, (b) directly into the bays, and (c) directly into the Atlantic Ocean.

Sufficient data are not available to directly estimate subsurface outflow, which may be the largest element of natural groundwater discharge from the water-budget area. However, an indirect method based on a water-budget concept suggests that total subsurface outflow from the water-budget area under natural conditions was on the order of 470 Mgal/d (Cohen and others, 1968). Buxton and others in 1999 estimated 503 Mgal/d discharged to the shore and 58 Mgal/d as sub-sea discharge for the 1968-83 period.

Assuming storage removal is negligible, we estimate that 690 Mgal/d is discharged to the shoreline and as subsea discharge for the 2005-2010 index period. This was calculated using the indirect method based on the water-budget concept, as in previous investigations, to estimate coastline and sub-sea discharge.

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      Table of Contents

      State of the Aquifer, Long Island, New York - Introduction

      Location and Physical Setting

      Freshwater

      1. Hydrolgeologic Units
      2. Fresh and Saltwater Relations/Interactions

      State of the Aquifer System

      1. Precipitation
      2. NWIS - the USGS Data Archive 
      3. Surface Water - Streamflow
      4. Groundwater Levels
      5. Water Table and Surface Maps
      6. Water Use
      7. Groundwater Budget
      8. Inflow to the Groundwater System
      9. Outflow from the Groundwater System
      1. Case Studies

      Interactive Content