Stormwater TMDLs

Stormwater as a Pollutant of Concern

Total Maximum Daily Load (TMDL) calculations are commonly used in situations where a water is impaired by a specific pollutant (e.g. phosphorus, E. coli, etc.). The goal of the TMDL is to determine how much that pollutant needs to be reduced in order for the water to attain water quality standards.  For aquatic life impairments, there often is not one specific pollutant of concern; instead, the impairment may be caused by a mix of pollutants and physical alterations to the stream system.  Therefore, Vermont’s TMDLs use stormwater as it represents a combination of stressors. The use of this surrogate has the primary benefit of addressing the physical impacts to the stream channel caused by stormwater runoff such as sediment release from channel erosion and scour from increased flows. These physical alterations to the stream are substantial contributors to the aquatic life impairment. Also, reductions in stormwater runoff volume will help restore diminished base flow (increased groundwater recharge), another aquatic life stressor.

TMDL Development

In order to set a hydrologic target for the TMDLs, the impaired watersheds were compared with “attainment watersheds” that have similar physical characteristics, but still meet water quality standards. The watersheds were matched using statistical methods by UVM.  For the full report see below.

Statistical Analysis of Watershed Variables (pdf, 470 KB)

To set the targets based on flow, long term flow data for both the impaired and attainment streams was needed. That data not being available, the Stormwater Section used the P8 Urban Catchment Model to develop a time series of flow for 1990-1999 based on precipitation data and physical information about the watersheds. The model was calibrated using stream flow data from USGS and UVM gauges.

Stormwater Modeling for Flow Duration Curve Development in Vermont (pdf, 5.47 MB)

Since 2004, the Program has been undertaking various projects to collect information that has been used in the development of stormwater TMDLs and the subsequent implementation, and to form the basis of a monitoring program for the stormwater-impaired streams. These efforts have included:

• Stream Geomorphic Assessment to help focus remediation efforts where the stream condition is poor, and to provide a baseline to measure progress against for future monitoring. You can view the data collected for the impaired streams in an online SGA Data Management System, maintained by the River Management Section.
• Subwatershed Mapping to identify discharge points and their associated drainage areas. This data will be used in the modeling for the development of both the TMDL and watershed permits, and to identify higher risk areas within the overall watershed. This data can be found on the VCGI website.
• Flow Gaging and Precipitation Monitoring stations were established in each of the stormwater impaired watersheds.  Since altered hydrology is the dominant factor causing the impairments, this monitoring will help the Program measure the stream response to remediation efforts.  The data is availab at the UVM Flow Monitoring website for download.
• Impervious Surface Mapping to identify the impervious surfaces (roads, parking lots, roofs) that are a driving cause of increased stormwater flows.
• Engineering Feasibility Assessment to help develop the implementation plans the program has been collecting technical data on existing stormwater infrastructure and conducting limited engineering feasibility assessments.

Updated: March 2010