Water Quality Modeling in Connecticut

As part of Connecticut’s Second Generation Nitrogen Strategy, the Connecticut Department of Energy and Environmental Protection (CT DEEP) will be modeling:

  • The watershed, analyzing the movement of water, sediment, and pollutants over the landscape to the waterbody;
  • groundwater, incorporating groundwater budgets, travel time distributions, and loadings to receiving waters; and
  • embayments, analyzing the movement of water and impacts of nutrients from surface and groundwater sources.  

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, #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 groundwater. 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, CT DEEP 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 the Sound. 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
2020 CTWM Specifications
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.
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.

Outcome: The watershed model will be able predict sediment supply to tidal marshes, point and nonpoint source loads, and streamflow and pollutant loads under current and possible future precipitation and land use scenarios. These results will facilitate future implementation of effective management plans. The improvement of the CTWM will also help develop water quality plans for coastal embayments, lakes, and reservoirs. Additionally, the outputs of the model will be used as inputs for other water quality models for the Long Island Sound. For example, the model’s output will be used as an input for Connecticut’s embayment model, which will simulate in-estuarine processes. Furthermore, the output will provide freshwater inputs and nutrient (i.e., nitrogen and phosphorus) and sediment loading information for the Long Island Sound wide eutrophication model.

Information about the watershed modeling work is publicly available on the project web page.

Expected Availability Date: 2022

2. Groundwater Modeling (diagram 1, #2)

Groundwater discharge to 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 CT DEEP, 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 modeling 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. CT DEEP 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 CT DEEP and LISS.

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

Outcome: USGS and CT DEEP expect that the outputs from the project will also provide an estimated time context for management scenarios that have an impact on nitrogen in the Sound. 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 CT DEEP prioritize nitrogen reduction actions where groundwater nitrogen discharges are greatest. Additionally, the outputs of this model will complement the current work being conducted for New York’s Solute-Transport Model as the efforts are similar and will cover the entirety of the Long Island Sound shoreline.

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

Expected Availability Date: 2022

Contact: Janet Barclay, [email protected], USGS

3. Embayment Modeling (diagram 1, #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, CT DEEP 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

CT DEEP has initiated monitoring and modeling efforts in the Pawcatuck River Estuary and Little Narragansett Bay; CT DEEP 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, CT DEEP is initiating monitoring and modeling of Mystic River and Norwalk Harbor.  In general, embayment modeling 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.

Outcome: The results of the embayment modeling project will be used to validate upland watershed models and develop and estuarine nutrient process model specific to each embayment. In coordination with CT DEEP’s watershed and groundwater model, the embayment modeling project will help develop embayment specific nutrient targets to manage water quality, specifically to combat eutrophication. The outputs of the three modeling efforts complement each other and will ultimately lead to better management, protection, and restoration efforts to the north shore of the Sound. Additionally, the embayment modeling output will help the development of the Long Island Sound Systemwide Eutrophication Model, in which CT DEEP plans to coordinate with NYCDEP to enterprise on any synergies and avoid redundancies between efforts.

Information about the embayment modeling work is publicly available on the project web page.

Expected Availability Date: 2022

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