Water Quality Modeling in Connecticut

As part of Connecticut’s Second Generation Nitrogen Strategy, the Connecticut Department of Energy and Environmental Protection (CTDEEP) will be modeling watershed surface and groundwater flows and nutrient loading, along with estuarine modeling of the Pawcatuck River, Niantic River Estuary, and other selected embayments.

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

1. Watershed Modeling (diagram, #1)

A watershed model simulates  the movement of water, sediment, and pollutants over the landscape and draining to a waterbody. The model first represents the movement of water, starting with precipitation and snow melt. In pervious areas (with natural vegetation) water may evaporate, run off on the surface, or infiltrate and move through the soil or ground water. In impervious areas (roads, roofs) water does not infiltrate, so only evaporation and surface runoff are considered. The model next simulates the movement of particles, which often carry pollutants. This is done through erosion in the pervious area, and washoff of particulate matter in the impervious area. Finally, the model represents movement of pollutants, in association with either water or particulate matter.

In 2002, the Connecticut Department of Energy and Environmental Protection (CTDEEP) developed the Connecticut Watershed Model (CTWM). This statewide watershed model was developed as a tool to evaluate nutrient sources and loadings, including nitrogen, phosphorus, and organic carbon, within each of six nutrient management zones and assess their delivery efficiency to LIS. The model supported initial efforts to manage watershed nutrient sources, but the final report identified deficiencies that if addressed would improve the accuracy and utility of the model moving forward. Work is now underway to address deficiencies in the 2002 effort by updating the CTWM with the following improvements:

Accuracy & Utility Design2002 CTWM
Specifications
2020 CTWM Specifications
Geospatial
Resolution
Aggregation of 34 subregional watersheds ranging in size from 11- 386 square miles.Aggregation of HUC-12 scale watersheds
(~332) plus any additional watersheds that
contribute directly to LIS embayments.
Calibration
Sites
6 sites20 sites with USGS gages; 6 sites will have continuous quality monitoring.
Land Use/Cover Categories6 modeled land uses: forest, agriculture/other, wetland, urban impervious, & roads.Higher resolution data will be used to improve description of land use/cover delineation.
ParametersNutrientsNutrients (nitrogen and phosphorus), Streamflow,
& Suspended Sediments; will track sediment delivery and
loading to improve selection of BMP.
Nutrient Source InformationLimited water quality data associated with point and nonpoint sources at 15 sites using data from 1991-1995.Water quality data connected to NPDES permits from
sewerage treatment, industry, & stormwater from 2002-2012
through 2022. Model will also include a minimum of 4 simulations with actual precipitation, predicted future precipitation, and two management scenarios.

2. Groundwater Modeling (diagram, #2)

Groundwater discharge to Long Island Sound along the north shore coast of Long Island Sound is small relative to discharge from the major rivers but is locally an important nutrient source to coastal embayments. The US Geological Survey (USGS), in cooperation with CTDEEP, is developing a regional groundwater flow model of coastal Connecticut as well as adjacent areas of Rhode Island and New York (excluding Long Island). The model will simulate groundwater budgets, groundwater travel time distributions, and will estimate groundwater loading to receiving waters (rivers or directly to Long Island Sound).

USGS is working to calibrate the groundwater model over two years. In 2020 USGS is refining Niantic River watershed modelling to simulate groundwater nitrogen loading to freshwater receptors (mainly rivers) and the coastal embayment. In 2021, USGS will extend the nitrogen model to the entire model domain. CTDEEP will use the calibrated regional model to better understand groundwater flow systems in coastal areas on the north shore of Long Island Sound, including quantitative information such as groundwater budget components at the watershed- and model-domain scales and travel times to freshwater and coastal receptors.

The work will include:

Model Work ComponentComponent Tasks
Data Collection & CompilationGenerate detailed grids to capture finer-scale N inputs and transport patterns.

Examine seasonal discharge patterns in study area due to short groundwater residence times.

Perform sensitivity analyses for the Niantic River model in order to inform model refinement selection.
Model Refinement for Nitrogen Loading SimulationsPre-process and select nitrogen loading and attenuation datasets.

Compile existing base flow data for Coastal CT to compare with simulated loads.

Leverage existing water quality sites with compiled data and evaluate/refine nitrogen model.
Nitrogen Loading SimulationsUse MODPATH software to simulate nitrogen loads using a mass-loaded particle tracking approach.

Account for nitrogen loading from atmospheric deposition, fertilizers (agriculture & grass/turf), and septic systems during simulation.

Calculate total loads, loading curves, and attenuation rates for each HUC12 basin and priority embayment within the model area.
Effects of Nitrogen Management Scenario SimulationsNitrogen management scenarios will likely focus on developed areas with medium to high groundwater nitrogen loads and be selected in consultation with CTDEEP and LISS.

Likely scenarios include:
1. Septic system conversion to public sewer.
2. Reduction in lawn fertilizer application rates.
3. Green infrastructure installation.

USGS and CTDEEP expect that the outputs from the project will also provide an estimated time context for management scenarios that have an impact on nitrogen in LIS. USGS could assign travel times from recharge to discharge for different land covers in the present, past, or future to understand the nitrogen discharge input function with time or under future management scenarios. These outputs combined will help CTDEEP prioritize nitrogen reduction actions where groundwater nitrogen discharges are greatest.

The groundwater flow model and supporting data will be publicly available and study results will also be presented on the project web page.

3. Embayment Modeling (diagram, #3)

Connecticut continues progress on its Second-Generation Nitrogen Strategy, which prioritizes embayments for further study and the preparation of protection or restoration plans.

In 2017, CTDEEP prioritized the following eight embayment complexes (some complexes include multiple embayments):

  • Pawcatuck River
  • Stony Brook Frontal
  • Mystic River
  • Niantic River
  • Farm River
  • Sasco Brook
  • Saugatuck River
  • Norwalk Harbor

CTDEEP has initiated monitoring and modeling efforts in the Pawcatuck River Estuary and Little Narragansett Bay; CTDEEP is also conducting a data synthesis and modeling effort on the Niantic River Estuary. These efforts will serve as a framework for CT DEEP to apply in other priority embayments. In 2020 CTDEEP is initiating monitoring and modeling of Mystic River and Norwalk Harbor.  In general, embayment modelling will include:

Model Work ComponentComponent Tasks
Data Collection & CompilationDevelop and implement monitoring plans specific to each embayment, include temporal and spatial considerations.

Collect water quality and flow data to parameterize the models, based on model needs and identified by a gap analysis.

Consider and data collected through the Unified Water Study and other efforts under an EPA-approved quality assurance project plan and compile as appropriate.
Embayment Model DevelopmentDevelop estuarine process models with input from the Connecticut Watershed Model described above:
1. Incorporate hydrodynamic input from existing models or simple dilution calculations.
2. Potentially use EcoGem box, Aquatox, and WASP models, as appropriate.
3. Explore simplified models to represent estuarine processes that evaluate nutrient loading impacts on eutrophication.
4. Run nutrient management scenarios to determine protection and restoration needs.

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