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 New York State Department of Environmental Conservation (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. The LINAP includes the following components: 1) Suffolk and Nassau County’s Subwatersheds Wastewater Management Plans, which incorporate nitrogen loading, groundwater, and hydrodynamic models; 2) Solute-Transport Modeling, which analyzes the effects of water table fluctuation, water use, and nitrogen loading as a function of changing land; and 3) USGS Long Island Groundwater Delineation Project, which is a comprehensive model to delineate groundwater recharge areas, travel times, and outflows to streams and estuaries. 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.

1. Suffolk and Nassau County’s Subwatershed Wastewater Plans Modeling (diagram 1, # 1)

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 models are being developed in support of county-wide watershed plans, which are considered early actions of LINAP. These plans will provide a basis for targeting nitrogen reductions. A critical part of these plans is the inclusion of adaptive management, to allow for redirecting resources when necessary. Additionally, while these plans are being developed with the current best available information, it is known that as additional data is collected or models developed, these plans will be refined to improve nitrogen reduction strategies.

The completed Suffolk County’s Subwatersheds Wastewater Management Plan (SWP) evaluates 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 (NLM): 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 runs over a 200-year period to estimate the nitrogen loading over the past 200 years.
  • Groundwater model: The in-house groundwater model estimates travel times of the groundwater through the aquifer system to the receiving waterbody. The travel times range from 0 to 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: Hydrodynamic models were used to estimate water 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 (The FVCOM model was used for the Great South Bay on the south shore of Long Island, while the EFDC model was used almost everywhere else).

Nassau County’s Subwatershed Wastewater 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 2021. Contact  NYSDEC to learn more.

The Nassau County SWP uses two of the same models as Suffolk County SWP, including:

  • Nitrogen Loading Model (NLM): 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 : Hydrodynamic models are being used to estimate residence times for each of the embayments being 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 (The FVCOM model was used on the south shore of Long Island, while the EFDC model was used on the north shore).

Outcome: The Suffolk and Nassau County Subwatershed Wastewater Plans, are using a Nitrogen Loading Model, with data associated for each of the subgroundwatershed, to facilitate informed decision-making directed towards nitrogen reduction strategies. Integrating improved scientific understanding will support more effective management decisions targeting removal of nitrogen. Loading estimates from the model’s output could be used to refine the Systemwide Eutrophication Modeling effort.

Expected Availability Date: 2021

Information about the Suffolk and Nassau Counties’ Subwatershed Wastewater Management Plans can be found on NYSDEC’s web page.

Contact: Susan Van Patten, [email protected], NYSDEC

2. Solute-Transport Modeling (diagram 1, #2)

The US Geological Survey (USGS), working with NYSDEC and the Peconic Estuary Program, is developing a solute-transport model for the portion of eastern  Long Island, NY that flows into the Peconic Bay. In 2021, the USGS will extend the modeling to the central and western portions of Long Island, which will cover parts of Kings, Queens, Nassau, and western Suffolk Counties. 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. Once the model is complete for the entire Island, LISS will be able to use the model to predict how nitrogen reduction strategies will impact the Sound.  

The subwatershed nitrogen-load modeling developed for 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 for the period from 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 planned for 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.

Outcome: Upon project completion the entire Long Island portion of the Sound watershed will be represented in a solute-transport modeling analysis. LISS partners will be able to use the resulting models to run additional Long Island Sound-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 Long Island Sound 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 the Sound.

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

Expected Availability Date: 2021

Contact: Donald Walter, [email protected], USGS

3. USGS Long Island Groundwater Delineation Project (diagram 1, #2)

A groundwater recharge area is where surface waters enter the groundwater through filtration, and therefore replenish aquifers. The aquifer then outflows freshwater and associated loading of nutrients and other dissolved constituents to surrounding estuaries and their tributaries. This transport mechanism of groundwater, with its nutrients and potential contaminants, to the Sound is a critical factor of its overall health and future fate. Multiple organization have assessed the outflow and loading impacts to the Sound, but assessments are not as accurate as delineation of recharge areas are not appropriately incorporated. As a response to the need to scientifically identify and evaluate these factors, USGS developed a comprehensive model to delineate groundwater recharge areas, travel times, and outflows to Long Island streams and estuaries.

The modeling effort was part of a groundwater availability study of the North Atlantic Coastal Plain (NACP) aquifer system. From 2015-2016, USGS collected groundwater-quality data to support additional flow modeling on Long Island under the National Water Quality Assessment (NAWQA) Program. This data was used to create a regional model, but the focus on Long Island provided a better understanding of linkages between water availability and sustainability, and therefore laid a foundation for the delineation of groundwater recharge areas. The project delineated upwards of 1,000 groundwatersheds, identified travel times for groundwater that flows through major Long Island aquifers (e.g, upper glacial, Magothy, and Lloyd) before entering the Sound, and produced geospatial layers and metadata for public dissemination.

The project’s approach included several steps: 1) flow model development, 2) particle-tracking analysis, 3) identification of future applications, and 4) report production. The flow model development used the datasets collected by NACP and NAWQA and were inputted into the USGS MODFLOW, which included modules for freshwater-saltwater interface and particle-tracking analysis. During the particle-tracking analysis, receiving surface-waterbodies were identified to provide particle end locations, in which particles are tracked from entry points in the groundwater system to exit points at surface waters. This process delineates the groundwater recharge areas for the receiving waters. The project’s products include:

  • A model grid for Long Island groundwater-flow stimulation
  • Maps and sections presenting 3-D distribution of hydrogeologic units
  • The present steady-state distribution of groundwater recharge
  • Simulated potentiometric-surface and water-table maps of major aquifers
  • Flow model calibration, sensitivity analysis, and model uncertainties statistics
  • Identification of groundwater recharge areas, and their associated travel times and outflow rates to receiving surface waters

Outcome: The model is expected to facilitate future analyses and projects, including, evaluation of parcel-scale nitrogen loading rates by groundwater recharge areas, identification of priority wastewater-management-related areas, determination of atmospheric and agricultural sources of nutrients, and application to other future modeling efforts, such as, geochemical modeling of nitrogen attenuation, refinement of flow-model and particle tracking, and transient-state modeling of historical and future scenarios of sea level rise, sewering and other time-varying factors. Furthermore, the results of the model will be used for the development of the NYSDEC’s Solute-Transport Model to determine nitrogen loading rates for selected groundwater recharge area using parcel-scale data. The results of this project will also help develop informed strategies to address pollutant loading to the Sound.

The report documenting the methodology and results is available online to the public on USGS’s web page.

Expected Availability Date: 2019

Contact: Paul Misut, [email protected], USGS

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