Groundwater-Flow Modeling - Long Island, New York

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Numerical models provide a means to synthesize existing hydrogeologic information into an internally consistent mathematical representation of a real system or process, and thus are useful tools for testing and improving conceptual models or hypotheses of groundwater-flow systems.  The goal of this effort is to develop a regional model for the Long Island aquifer system to simulate changes in water levels, streamflows, the position and movement of the boundary between fresh and saline groundwater, assess groundwater-age distributions, and to determine regional hydrologic response to changes in anthropogenic and natural stresses in order to assist in the determination of the groundwater sustainability of the Long Island aquifer system.

Modeling Approach:

Pertinent hydrogeologic information is needed to develop a conceptual model of how water enters, moves through, and ultimately leaves the aquifer system. The conceptual model then is used as the foundation for the construction of a 3D-numerical model capable of simulating groundwater-flow conditions throughout the Long Island aquifer system. The collection of digital-hydrogeologic data in the study area are needed for proper model development, including elevations and extents of hydrostratigraphic units, groundwater-salinity distribution, groundwater withdrawals, temperature and precipitation distributions, recharge and impervious-surface distribution, and additional information necessary for accurate model calibration including groundwater levels and streamflow.

illustration of aquifer model

Figure 1Conceptual Model of Water Movement through the Aquifer System of Long Island, New York: The figure above shows a conceptual model of how water enters, moves through, and ultimately leaves the aquifer system on Long Island, NY. The main components of this conceptual model are groundwater recharge from precipitation, returnflow from septic systems, recharge basins, and leaky infrastructure, and groundwater discharge to streams, estuaries, or subsea boundaries. (Public domain)

The sources (or inflows) and sinks (or outflows) of water to the Long Island aquifer system can be illustrated schematically (fig. 1) to show how water enters, flows through, and exits this aquifer system). The sole source of water for predevelopment conditions is recharge from precipitation.  All of this water entering the aquifer system is balanced by water leaving the aquifer system. Water leaves the aquifer system as discharge to streams, discharge to shallow-coastal waters, and deep-subsea discharge farther offshore in the Atlantic Ocean. An additional loss of water from the aquifer system for post- development conditions is water removed by groundwater withdrawals. Depending on the type of water use, and whether (or how much) a community is sewered a substantial amount of the water withdrawn from the aquifer system may be returned in the form of wastewater-return flow through domestic septic systems.

Modeling Components:

The main components associated with the model development include (1) hydrogeologic framework, (2) aquifer recharge, (3) water use, (4) freshwater/saltwater boundary, and (5) model calibration.

Hydrogeologic Framework:

The hydrogeologic framework consists of the extents and thicknesses of the major hydrogeologic units and the hydraulic properties of the sediments that comprise these units. The initial model development was based in part on the previous USGS analysis that defined the hydrogeologic-unit surfaces (Smolensky and others, 1990).  Initial estimates of aquifer permeability were derived from an analysis of the existing deep lithologic boring data as shown in these cross sections of grain-size distribution.  This information will be updated as part of the ongoing hydrogeologic framework mapping.

Aquifer recharge:

Recharge is the sole source of water to the Long Island aquifer system and it consists of four main components: (1) natural recharge from precipitation, (2) redirected recharge from impervious surfaces, (3) returnflow from domestic-septic systems, and (4) returnflow from leaky infrastructure.

Groundwater withdrawals:

Approximately 460 million gallons per day of water is pumped from the Long Island aquifer system, and this pumping consists of four main components: (1) public supply, (2) remediation pumping from contaminated sites, (3) farm irrigation, and (4) industrial and commercial facilities. Public-supply withdrawals account for nearly (94%) of the groundwater pumped from the aquifer system for current (2015) conditions.

Boundary between fresh and saline groundwater:

The boundary between fresh and saline groundwater – otherwise referred to as the freshwater/saltwater interface, represents the lateral extent of the fresh-groundwater system in the first phase of model development.  In this first phase, the freshwater/saltwater interface assigned in the model is based on the current information obtained from previous sampling and geophysical logging (Charles, 2016). This information is being updated as part of this ongoing investigation (see “Saltwater-Interface Mapping”). Subsequent updates to the groundwater-flow model will include a numerical solver that allows for the simulation of the position and movement of the freshwater/saltwater interface in response to changes in hydrologic stresses over time.

Model calibration:

Once the groundwater model datasets are assembled, the model simulated water levels and streamflows are compared to observed data to determine how well the model matches real-world conditions.  Often times, model parameters such as recharge and hydraulic properties need to be adjusted to improve this match by a process referred to as “model calibration”.  

Planned Deliverables:

Phase I model: The first model developed for this investigation is a 25-layer model that simulates only the freshwater portion of the groundwater system for average, current (2005 – 2015) steady-state conditions.  This model is being used to delineate the sources of water (recharge areas) and time of travel distributions for the coastal-receiving waters throughout Long Island.  A USGS report that documents the development of the Phase I model is now in review and is planned for publication in the summer of 2020.

Phase II model: The second model developed for this investigation will consist of a refinement of the Phase I steady-state 25-layer model to simulate changes in groundwater pumping and aquifer recharge from 1900 – 2015.  This model will incorporate all of the information collected in western Long Island as part of the first phase of the hydrogeologic - and saltwater-mapping component of the investigation.  This model will include a numerical solver that will allow for the simulation of the effects of changes in pumping and aquifer recharge on the position and movement of the freshwater/saltwater interface and to assess changes in water levels, streamflows, and coastal discharge in response to changes in pumping, natural recharge on western Long Island. A draft report documenting this model is planned for publication in the fall of 2021.

Phase III model: The third model developed for this investigation will consist of a refinement of the Phase II model that was focused on western Long Island.  This model will incorporate all of the information collected in central and eastern Long Island as part of the second phase of the hydrogeologic - and saltwater-mapping component of the investigation.  This model will include a numerical solver that will allow for the simulation of the effects of changes in pumping and aquifer recharge on the position and movement of the freshwater/saltwater interface and to assess changes in water levels, streamflows, and coastal discharge in response to changes in pumping, natural recharge on central and eastern Long Island. A draft report documenting this model is planned for publication in the fall of 2023.