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.
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.
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
- Precipitation
- NWIS - the USGS Data Archive
- Surface Water - Streamflow
- Groundwater Levels
- Water Table and Surface Maps
- Water Use
- Groundwater Budget
- Inflow to the Groundwater System
- Outflow from the Groundwater System
Below are other science projects associated with this project.
Long Island Water Availability
Long Island Precipitation
NWIS - the USGS Data Archive
Long Island Surface Water - Streamflow
Long Island Groundwater Levels
Long Island Water Table and Surface Maps
Long Island Water Use
Long Island Groundwater Budget
Long Island Inflow to the Groundwater System
Long Island Water Suitability
Long Island Water Suitability Case Studies
Long Island Groundwater System Potential Hazards
- Overview
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.
Sources/Usage: Public Domain.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.
Sources/Usage: Public Domain.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.
_______________________________
Table of Contents
State of the Aquifer, Long Island, New York - Introduction
- Precipitation
- NWIS - the USGS Data Archive
- Surface Water - Streamflow
- Groundwater Levels
- Water Table and Surface Maps
- Water Use
- Groundwater Budget
- Inflow to the Groundwater System
- Outflow from the Groundwater System
- Science
Below are other science projects associated with this project.
Long Island Water Availability
The foundation of any groundwater analysis, including those analyses whose objective is to propose and evaluate alternative management strategies, is the availability of high-quality data. Some, such as precipitation data, are generally available and relatively easy to obtain at the time of a hydrologic analysis. Other data and information, such as geologic and hydrogeologic maps, can require...Long Island Precipitation
The National Oceanic and Atmospheric Administration (NOAA) National Climatic Data Center (NCDC) offers several types of climate information generated from examination of the data in the archives. These types of information include record temperatures, record precipitation and snowfall, climate extreme statistics, and other derived climate products. A collection of statistical weather and climate...NWIS - the USGS Data Archive
As part of the U.S. Geological Survey's (USGS) program for disseminating water data within USGS, to USGS cooperators, and to the general public, the USGS maintains a distributed network of computers and fileservers for the acquisition, processing, review, and long-term storage of water data. This water data is collected at over 1.5 million sites around the country and at some border and...Long Island Surface Water - Streamflow
Surface water current conditions are based on the most recent data from on-site automated recording equipment. Measurements are commonly recorded at a fixed interval of 15 to 60 minutes and transmitted by satallite uplink or telephone telemetry to the USGS every hour. Values may include "Approved" (quality-assured data that may be published) and/or more recent "Provisional" data (of unverified...Long Island Groundwater Levels
Water-level measurements from observation wells are the principal source of information about the hydrologic stresses acting on aquifers and how these stresses affect groundwater recharge, storage, and discharge (Taylor and Alley, 2001). Water-level measurements are made by many Federal, State, and local agencies.Long Island Water Table and Surface Maps
The depth to the water table can be determined by installing wells that penetrate the top of the saturated zone just far enough to respond to water table fluctuations. Preparation of a water-table map requires that only wells that have their well screens installed near the water table be used. If the depth to water is measured at a number of such wells throughout an area of study, and if those...Long Island Water Use
The U.S. Geological Survey's National Water-Use Information Program (NWUIP) is responsible for compiling and disseminating the nation's water-use data. The USGS works in cooperation with local, State, and Federal environmental agencies to collect water-use information. USGS compiles these data to produce water-use information aggregated at the county, state, and national levels. Every five years...Long Island Groundwater Budget
A groundwater system consists of a mass of water flowing through the pores or cracks below the Earth's surface. This mass of water is in constant motion. Water is constantly added to the system by recharge from precipitation, and water is constantly leaving the system as discharge to surface water and as evapotranspiration. Each groundwater system is unique in that the source and amount of water...Long Island Inflow to the Groundwater System
Precipitation that infiltrates and percolates to the water table is Long Island's only natural source of freshwater because the groundwater system is bounded on the bottom by relatively impermeable bedrock and on the sides by saline ground water or saline bays and the ocean. About one-half the precipitation becomes recharge to the groundwater system; the rest flows as surface runoff to streams or...Long Island Water Suitability
Groundwater quality may be affected by natural and human factors (Johnston, 1988). Although the vulnerability of groundwater to contamination from the land surface is influenced by many factors, the degree of aquifer confinement, the depth of the well, and the surrounding land use are primary key factors that influence shallow groundwater quality.Long Island Water Suitability Case Studies
A collection of studies that focused on the quality of groundwater and surface water, are presented in this section. The reports associated with these areas of water quality concerns are linked as an online source for further reading.Long Island Groundwater System Potential Hazards
Hazards which may impact the ground water system adversely are presented in this web page. The impacts of these hazards are only shown here as a topic for further discussion and may need to be investigated with further details.