Long Island Groundwater System Potential Hazards

Science Center Objects

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.

Sea Level Rise

The Center for Operational Oceanographic Products and Services has been measuring sea level for over 150 years, with tide stations of the National Water Level Observation Network operating on all U.S. coasts. Changes in Mean Sea Level (MSL), either a sea level rise or sea level fall, have been computed at 128 long-term water level stations using a minimum span of 30 years of observations at each location. These measurements have been averaged by month to remove the effect of higher frequency phenomena (e.g. storm surge) in order to compute an accurate linear sea level trend. The trend analysis has also been extended to 240 global tide stations using data from the Permanent Service for Mean Sea Level (PSMSL). In the vicinity of Long Island, the rate of sea-level rise ranges from about 2.35 to 3.9 mm/year based on long-term trend data.

Changes in climate and sea level will drive changes to the coastal groundwater system that will impact both human populations and coastal ecosystems. Increases in sea-level will raise the fresh water table in many coastal regions (figure 31). Impacts to humans may include an increase in the potential for basement or septic system failure. Sea-level rise can also contaminate groundwater supplies due to landward and upward movement of sea-water in coastal aquifers. The intrusion of saltwater into groundwater systems will also impact coastal ecosystems such as marshes by changing the elevation of the freshwater-saltwater interface.

A major concern for water managers on Long Island is the potential adverse effect of sea-level rise on the depth to the freshwater-saltwater interface near public groundwater supply wells. Pumping from public-supply wells in coastal aquifers underlain by saltwater can lower the water table with respect to sea level, decreasing the depth to the freshwater-saltwater interface beneath the pumping well. This increases the potential for saltwater intrusion, and potentially limits the amount of potable water available from the well.

illustration of the effects a rise in sea-level will affect groundwater flow in coastal aquifers
Figure 31. A rise in sea-level will affect groundwater flow in coastal aquifers (1). An increase in the elevation of the water table (dashed blue line) may result in basement flooding and compromise septic systems (2). A rise in sea level may also result in an upward and landward shift in the position of the freshwater-saltwater interface (3). Where streams are present, an increase in the water-table elevation also may increase groundwater discharge to streams and result in local changes in the underlying freshwater-saltwater interface (4). (USGS)(Public domain.)

 The USGS Center of Excellence for Geospatial Information Science (CEGIS) provides a viewer animating a sea-level rise with hypothetical depths of rise from 0-30 meters. An example for New York can be found at this link. The animation demonstrates the population amounts that might be impacted by sea-levels rising to a certain height.

Groundwater Flooding

The groundwater table rises and falls because of increases and decreases in recharge or discharges (pumping wells). Average total precipitation (and temperature) has been above normal on Long Island from 2004-2010 based on data provided by National Oceanic and atmospheric administration (NOAA) National Climatic Data Center (NCDC). The above normal precipitation has brought the groundwater levels to near record highs in some parts of Long Island. The impacts of a rising water table may include an increase in the potential for subsurface structure flooding (subway tunnels, basements) or on-site septic system failure. In 2010, the depth to the water table was estimated to show areas that have the potential for groundwater flooding (figure 32). An interactive mapping tool allows the user to select a location and retrieve the estimated depth to water at that location.

Map of the estimated depth to water in 2010, Long Island, NY.
Figure 32. Map of estimated depth to water in 2010, Long Island, NY.(Public domain.)


A drought is a period of drier-than-normal conditions that results in water-related problems. When rainfall is less than normal for several weeks, months, or years, the flow of streams and rivers declines, water levels in lakes and reservoirs fall, and the depth to water in wells increases. If dry weather persists and water-supply problems develop, the dry period can become a drought.

The term "drought" can have different meanings to different people, depending on how a water deficiency affects them. Droughts have been classified into different types such as:

  • Meteorological Drought - Lack of precipitation
  • Agricultural Drought - Lack of soil moisture
  • Hydrologic Drought - Reduced streamflow or groundwater levels

It is not unusual for a given period of water deficiency to represent a more severe drought of one type than another type. For example, a prolonged dry period during the summer may substantially lower the yield of crops due to a shortage of soil moisture in the plant root zone but have little effect on groundwater storage replenished the previous spring.

A groundwater drought typically refers to a period of decreased groundwater levels that results in water-related problems. The amount of groundwater decline that would be considered a drought varies regionally and locally due to differences in groundwater conditions and groundwater needs for humans and the environment.

For additional USGS background on drought, see:

The National Drought Mitigation Center (NDMC), provides several drought monitoring tools and information to help people assess drought severity and impacts.


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