Groundwater Sustainability of the Long Island Aquifer System

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Groundwater sustainability can be best defined as the development and use of groundwater in a manner that can be maintained for an indefinite time without causing unacceptable environmental or socioeconomic consequences. Informed management of the Long Island aquifer system can help ensure a regionally sustainable groundwater resource. This study will evaluate the sustainability of Long Island’s groundwater resource, now and for the future, by performing hydrogeologic mapping, monitoring of water quality and water levels, and developing a groundwater-flow model for this sole-source aquifer system.

Background

Most of Long Island, New York is entirely dependent on the underlying sole-source aquifer system, which currently supplies over 400 million gallons a day (MGD) of freshwater from more than 1,500 public-supply wells to over 2.8 million people in Nassau and Suffolk Counties. As the name implies, Long Island’s sole-source aquifer system is the only source of water available to meet the needs of the Island’s population.

Long Island’s aquifer system is comprised of several freshwater zones, or “aquifers”, generally ranging in increasing depth from the upper glacial, North Shore, Jameco, Magothy, and finally the Lloyd aquifer. Several major clay layers are also present including the Gardiners Clay and Raritan confining unit, which overlie most but not all of the Magothy and Lloyd aquifers, respectively. These clay units influence the aquifer system in several ways: 1) they act to confine and isolate the underlying freshwater zones, 2) they limit the rate of recharge to the units below, 3) they protect the underlying freshwater from surface contaminants, and 4) in coastal marine environments, they also influence the formation of seaward extended freshwater-aquifer wedges under natural-discharge conditions, and conversely, formation of inland saltwater-intrusion wedges under pumping conditions.

In some areas of Long Island, freshwater pumping has resulted in saltwater intrusion into the aquifer system and has also impacted streams, ponds, and coastal wetlands and estuaries that rely on groundwater discharge to sustain them. Additional human related activities, such as urban runoff and septic systems, have also affected the water quality of the aquifer system. Therefore, development and use of groundwater on Long Island is constrained by ecohydrological (i.e. the interactions between groundwater and surface-water ecosystems) and water-quality concerns.

Concerns

Groundwater is the primary source of freshwater in streams, lakes, and wetlands, and maintains the saline balance of estuaries. When large amounts of groundwater are withdrawn from the aquifer system, the water table is locally depressed, which in turn reduces the amount of groundwater available to discharge to streams, wetlands, and estuaries. Large-scale sewering practices have also reduced groundwater levels and discharge to surface-receiving waters.

Groundwater quality on Long Island has been impaired by saltwater intrusion and human activities. Increased saltwater intrusion from groundwater pumping has occurred in the Lloyd and Magothy aquifers on western Long Island since the 1940s (Cartwright, 2002), and in Suffolk County since the 1970s (Nemickas and Kozalka, 1982).

Contamination by human activities can be from point sources, such as industrial and commercial facilities, or from diffuse (nonpoint) sources such as domestic wastewater, road salt, fertilizers, pesticides, etc. Of particular concern, in Suffolk County, is the return of domestic wastewater to groundwater from septic systems.

Approach

The approach for this investigation consists of three main components: hydrogeologic-framework mappingsaltwater-interface mapping, and groundwater-flow modeling.  The hydrogeologic framework component will provide updated hydrostratigraphic surfaces and unit extents, building upon the last regional framework update of the Long Island aquifer system performed by the USGS in 1990 (Smolensky and others, 1990).  As part of this update, a network of new groundwater wells will be installed at about 25 locations in the Lloyd and Magothy aquifers throughout the Island to fill in substantial data gaps. The locations of the proposed groundwater wells will be selected by reviewing geologic, hydrologic, and water-quality information from the existing observation network. During and after completion of the newly drilled wells, lithologic core samples will be collected and analyzed to improve the understanding of the hydrogeologic framework. Borehole-geophysical logging techniques will also be used to provide additional information on the geology as well as aquifer salinity as part of the saltwater-interface mapping component of the investigation.

The saltwater-interface mapping component will use borehole-geophysical logs collected at existing and newly installed wells and surface-geophysical soundings using Time-Domain Electromagnetic (TDEM) technology (Sachin and others, 2007) at selected inland locations considered most susceptible to saltwater intrusion to delineate the seaward extent of freshwater in the Long Island aquifer system.  This effort will build upon earlier studies conducted at a much coarser scale throughout the Northern Atlantic Coastal Plain aquifer system. (Charles, 2016). 

A three-phased modeling approach will be used to simulate groundwater-flow conditions throughout the Long Island aquifer system that will include (1) an initial model based on existing information for current (2005–2015) average conditions, (2) the addition of time-varying stresses to simulate changes in hydrologic conditions from (1900–2015), and (3) a final model that incorporates the new interpretation of the hydrogeologic framework and salinity distribution into the model simulations. The final model will be used to simulate various scenarios, including changes in groundwater withdrawals, aquifer-recharge management, and climate change. These scenarios will be developed in collaboration with the New York State Department of Environmental Conservation and the project Steering Committee.