Water Quality Modeling in New York

LINAP’s Role in Developing Modeling Tools

The Long Island Nitrogen Action Plan (LINAP) is a multiyear initiative with a similar goal of reducing nitrogen in Long Island’s surface, coastal, and groundwaters.  This partnership includes NYSDEC, the Long Island Regional Planning Council (LIRPC), and Suffolk and Nassau counties, with input from numerous local and regional interested partners and stakeholders. The partnership’s work includes developing modeling tools that can assist with the understanding of nitrogen loading and transport on a subwatershed basis and its resulting discharge to receiving coastal waters and embayments along Long Island’s North Shore.  LINAP’s efforts will be integrated into LISS’s efforts.

A flowchart diagram of the Long Island Sound Systemwide Modeling Program. Cells # 1-3 for New York are described below. For Connecticut see the Connecticut Modeling Page, and the Systemwide Modeling Page for information on Cells # 4-6.

Watershed, Groundwater, and Embayment Circulation Modeling (diagram, # 1-3)

LINAP partners are developing watershed nitrogen loading models to estimate the contribution from wastewater, fertilizer, stormwater, and atmospheric deposition to groundwater and receiving waters. These watershed plans are considered early actions of the LINAP program and are in the process of being finalized. Once these early action plans are developed and implemented, they will provide a base to start nitrogen reduction strategies. A critical part of these initial plans is the inclusion of adaptive management, to allow for redirecting resources when necessary. Additionally, while these plans are being developed with the best available information, it is known that as additional data is collected or models developed, these plans will be refined to better estimate the nitrogen reduction strategy.

Suffolk County’s Subwatersheds Wastewater Management Plan (SWP) has evaluated parcel-specific land uses of nearly 200 subwatersheds. The SWP  developed initial nitrogen load reduction goals, established ecological sensitivity priority ranks for each surface waterbody, and provided implementation recommendations for a phased county-wide wastewater upgrade program.

The Suffolk County SWP models include:

  • Nitrogen Loading Model (diagram, #1)): The NLM model is a landside model that takes nitrogen inputs on a parcel scale to model the amount of nitrogen entering the groundwater. This is a quasi-steady-state model that shows a snapshot in time, but is ran over a 200 year period to estimate the nitrogen loading over the past 200 years.
  • Groundwater Model (diagram, #2): The SC SWP developed an in-house groundwater model to estimate travel times of the groundwater through the aquifer system to the receiving waterbody. The travel times range from 0 to 2 years to over 200 years. This model allows for the parcel specific loads (from NLM) to be simulated through the aquifer system to the ultimate discharge point of the embayments.
  • Hydrodynamic Models (diagram, #3): EFDC and FVCOM are hydrodynamic models that were used to develop residence times for each of the embayments studied. Understanding the residence time of a waterbody is crucial in the evaluation of the ability of that waterbody to accept an amount of loading from the land without degrading its quality. (FVCOM was used for the Great South Bay on the south shore of Long Island, while EFDC was used almost everywhere else.)

Nassau County’s Subwatershed Plan (SWP) is underway and is following a similar template as the Suffolk County SWP.  When complete, it will help determine how much nitrogen is entering the groundwater from different sources (e.g. wastewater, fertilizer, stormwater, atmospheric deposition). This effort is being led by Stony Brook University’s School of Marine and Atmospheric Sciences in partnership with the County, the Department of Environmental Conservation, and the Long Island Regional Planning Council. The SWP is expected to be completed in 2020. Contact  NYSDEC to learn more.

The Nassau County SWP models include:

  • Nitrogen Loading Model (diagram, #1): The NLM model is a landside model that takes nitrogen inputs on a parcel scale to model the amount of nitrogen entering the groundwater.
  • Hydrodynamic Models (diagram, #3): EFDC and FVCOM are hydrodynamic models that were used to develop residence times for each of the embayments studied. Understanding the residence time of a waterbody is crucial in the evaluation of the ability of that waterbody to accept an amount of loading from the land without degrading its quality. (FVCOM was used on the south shore of Long Island, while EFDC was used on the north shore.)

2. USGS Groundwater Modeling (diagram, #2)

The U.S. Geological Survey (USGS), working with NYSDEC and the Peconic Estuary Program, is developing a solute-transport model for the eastern portion of Long Island which flows into the Peconic Bay. The modeling looks at water table fluctuation over time, water use, and nitrogen loading—as a function of changing land use and atmospheric deposition rates—from predevelopment (e.g., 1900) through the present. In 2021, the USGS will extend the solute-transport model(s) to the central and western portions of Long Island, in order to conduct a similar analysis for the entire LIS watershed on Long Island.

The subwatershed nitrogen-load modelling conducted in Suffolk and Nassau counties (described above) assumes that the nitrogen was derived from current sources but does not fully account for the cumulative loading that has taken place between predevelopment through to current conditions. As such, the County subwatershed models have some inherent simplifying assumptions that limit the ability to predict future changes in nitrogen concentrations in the aquifer, as well as, nitrogen loads to wells and waterbodies. The solute-transport approach provides a more realistic depiction of nitrogen loading through time and, ultimately, more accurate predictions of concentrations of nitrogen in the aquifer and nitrogen loads to receiving waters.

Work in 2021 includes:

  • Develop data sets needed to represent current and historic land uses relevant to nitrogen loading in coastal watersheds;
  • Incorporate data sets as nitrogen source terms in numerical models capable of simulating groundwater-transport processes to estimate current loading rates and nutrient concentrations in the aquifer;
  • Use current-condition solute-transport models to simulate changes in nitrogen-loading rates in response to proposed wastewater-management actions; and
  • Prepare reports and data releases that document nitrogen sources, numerical models, and their application to nitrogen-load estimates.

The groundwater modeling work and supporting data will be publicly available on the project web page.

Upon project completion, the entire Long Island portion of the LIS watershed will be represented in a solute-transport modeling analysis. LISS partners will be able to use the results for additional LIS-specific scenarios such as nitrogen-removal progress on Long Island. Furthermore, this modeling effort, combined with the companion groundwater modeling effort in Connecticut being conducted by USGS, in cooperation with CTDEEP, will provide complete coverage of the groundwater contributing area to the entire LIS watershed. Coordination between these modeling efforts will eventually allow for a comprehensive analysis of time-varying nitrogen loading and the simulation of the effects of various nitrogen-management scenarios at the regional watershed scale for LIS.

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