Long Island Inflow to the Groundwater System

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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 is lost through evapotranspiration (Cohen and others, 1968).

In determining the effects of pumping and the amount of water available for use, it is critical to recognize that not all the water pumped is necessarily consumed. For example, not all the water pumped for public supply is consumed. Some of the water returns to the groundwater system as infiltration through onsite-septic systems (recharge from wastewater return). Most other uses of groundwater are similar in that some of the water pumped is not consumed but is returned to the system. Thus, it is important to differentiate between the amount of water pumped and the amount of water consumed when estimating water availability (Alley and others, 1999).

The estimated total inflow to the groundwater system in 2005-2010, is an accumulation of contributions of several sources. These sources include the natural groundwater recharge from precipitation, infiltration from onsite septic systems, infiltration from leaking water mains, and storm water runoff diverted to recharge basins. The estimated average daily inflow of fresh water into the groundwater system was about 1,500 Mgal/d for the 2005-2010 index period. This includes the natural groundwater recharge from precipitation of around 1,180 Mgal/d, and the remaining fresh water inflow of approximately 300 Mgal/d from other sources enumerated above.

Recharge from Precipitation

Recharge from precipitation has been estimated to be about half of the total precipitation under natural pre-developed conditions. Impervious surfaces from urbanization across Long Island have altered the percentage of recharge from precipitation. However, new tools such as a Soil-Water-Balance (SWB) computer code have been used to calculate spatial and temporal variations in groundwater recharge. The SWB model calculates recharge by use of commonly available geographic information system (GIS) data layers in combination with tabular climatological data. The code is based on a modified Thornthwaite-Mather soil-water-balance approach, with components of the soilwater balance calculated at a daily time-step (Westonbroek and others, 2010). Recharge calculations are made on a rectangular grid of computational elements that may be easily imported into a regional groundwater-flow model. Recharge estimates calculated by the code may be output as daily, monthly, or annual values.

A recent application of the SWB code was documented in the appendix section of Masterson and others, 2013; the modeled output included Long Island. The SWB calculated groundwater recharge used a 1-mile grid spacing across Long Island. The results of the SWB code show the spatial variability of the calculated average recharge across Long Island for the period 2005-2009 (figure 26). Cohen and others (1968) estimated that annual recharge ranged from about 10 to 35 inches of water, and is comparable to the output of the SWB program, as can be seen here.


map of Simulated output of Soil-Water-Balance Code (SWB) of recharge across Long Island
Figure 26. Simulated output of Soil-Water-Balance Code (SWB) of recharge across Long Island, N.Y. from 2005-2009.(Public domain.)

Source: Masterson and others, 2013

The SWB calculated average groundwater recharge across Long Island is represented as 1 square mile gridded output. Cumulatively summing each grid value provided the estimated daily average groundwater recharge for 2005-2009 totaled around 1,180 Mgal/d.

Sutter (1937), one of the earliest regional water supply reports for Long Island, estimated 1,373 Mgal/d of precipitation recharged the groundwater system. Cohen and others (1968), evaluated a groundwater budget for the 1940-1965 period where they estimated about 820 Mgal/d entering as groundwater recharge; however the water budget area was smaller and excluded the areas of King and Queens Counties, and both the North and South Forks. Buxton and others (1999), used a numerical groundwater-flow model of Long Island’s groundwater system for the 1968-83 period where they estimated the average groundwater recharge was 1,126 Mgal/d. The area modeled included Kings and Queens Counties, however, excluded the north and south forks of Long Island. The SWB average daily groundwater recharge estimate therefore compares favorably with these previous published estimates of average daily recharge.

Recharge From Wastewater Return

Nassau County’s Sewer and Storm Water Authority collects and treats most of the sewage generated in the County. Two of Nassau County’s largest treatment plants process 85% of the sewage collected within the County. These two plants each treat approximately 58 million gallons per day (Mgal/d), Ten other independent treatment facilities operate within the County, these ten facilities process 15% of the County’s effluent. The effluent treated is discharged into nearby estuaries, and therefore leaves the groundwater flow system.

Source: Nassau Sewered Areas

Approximately thirty percent of Suffolk County's residents have their sewage collected and treated by over 190 sanitary sewer systems, over 150 are private. The treatment amounts permitted are approximately 59 Mgal/d; 43 Mgal/d discharge to surface waters and about 16 Mgal/d discharge to the ground. The remaining seventy percent of Suffolk County's residents have on-site sanitary wastewater treatment and disposal system. These on-site systems include a septic tank or cesspool for solids settling, connected to leaching fields to allow clarified water to seep into the ground.

Source: Suffolk Sewered Areas

In unsewered areas of Nassau and Suffolk Counties, 85 percent of the water pumped for public supply is estimated to infiltrate back to the groundwater system, whereas in sewered areas, only about 20 percent returns. The amount of public-supply water that returns to the groundwater system in Nassau and Suffolk Counties varies spatially (Buxton and others, 1999). Approximately 1,100,000 people on Long Island are estimated to live in an unsewered area and are using onsite-septic systems during the period 2005-2010. These onsite-septic systems return about 74 Mgal/d of water from these systems back into the ground

Recharge Basins

Recharge basins are unlined excavations in the glacial deposits; they range from about 10 to 20 feet in depth and from less than 1 to about 30 acres in area. In Nassau and Suffolk Counties there are more than 2,000 recharge basins (figure 27).

map of 2,000 recharge basins across Long Island
Figure 27. Generalized locations of 2,000 recharge basins across Long Island.(Public domain.)

Most of the runoff from highways in these counties is collected by storm sewers and routed to these recharge basins. A recharge basin is generally used only where the water table is sufficiently deep to remain below the floor of the basin at least most of the time. Therefore, only a few recharge basins are located in nearshore areas where the water table is within a few feet of the land surface. In addition, on Long Island many street storm water inlets are open bottomed (ie; not connected to a storm sewer) and therefore, function as small recharge basins (Franke and others, 1972).

Seaburn (1970) studied the inflow of two recharge basins in residential developments in Nassau County. From the rainfall-inflow relation for one of these basins, he estimated that, on average about 15 percent of the total precipitation falling on the drainage area of the basin discharged into the basin.

In 1968, Franke and McClymonds (1972) estimated that the total drainage area of all the recharge basins in Nassau and Suffolk Counties was probably on the order of 250 square miles. They assumed that 15 percent of total rainfall on this area enters recharge basins and that virtually all of this water recharges the groundwater reservoir, with the estimated daily average recharge to the groundwater system from these basins being on the order of 80 Mgal/d (Franke and others, 1972). The assumption that most of the water entering a recharge basin ultimately recharges the groundwater system is based on the observation that water entering most basins percolates into the ground fairly rapidly (commonly within a day or so).

It is likely that during the 2005-2010 index period, the total drainage area of recharge basins on Long Island has increased since 1968 because of development, largely in Suffolk County. Population growth in Suffolk County increased 25% since 1970. If we simply increase the total recharge basin drainage area by 25%, then the total drainage area would equal 313 square miles in 2010 and, by using the same approach described above, the average estimated daily average recharge to the groundwater system from these basins is on the order of 100 Mgal/d.

Infrastructure Leaks

    Every day in America, we lose nearly six billion gallons of treated water due to crumbling infrastructure. Throughout the nation, leaky aging pipes, and outdated systems are wasting 2.1 trillion gallons annually. That's roughly 16% of our nation's daily water use (Center for Neighborhood Technology web link). Leakage is usually the largest component of distribution loss. Leakage in public-water-supply systems results in loss of purified drinking water but also means wasting the energy and material resources used in abstraction, transportation and treatment. Estimates of the loss of water for the period 2000-2010 through major city public-supply distributions systems average about 12.8 percent (http://growingblue.com/wp-content/uploads/2012/07/Leakage-Rate-US-Cities.png) nationally and 14.2% for New York City.

    Accounting for this loss of supply from infrastructure leaks is needed in evaluating a present day groundwater budget. Conservatively using an estimated leakage rate of around 11% for publically supplied water throughout Nassau and Suffolk Counties is equivalent of about 42 Mgal/d returned to the groundwater system on average during 2005-2010. Virtually all of Kings and Queens County has combined sewers, and the major source of returned water is leakage from water-supply and sewer lines, which carry 700 Mgal/d from a reservoir system in upstate New York. About 58 Mgal/d is estimated to enter the groundwater system throughout Kings and Queens Counties from this leaky infrastructure (Buxton and others, 1999).



    Table of Contents

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

    Location and Physical Setting


    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