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Impervious Cover TMDL Methodology for Connecticut. Chris Bellucci Connecticut Department of Environmental Protection Biological Condition. % Impervious Cover. Impervious Cover …. Why now ?

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Impervious Cover TMDL Methodology for Connecticut

Chris Bellucci

Connecticut Department of

Environmental Protection

Biological Condition

% Impervious Cover


Impervious Cover …. Why now ?

  • Need for a methodology to address stormwater impacts to aquatic life
  • Literature supports relationship between IC and aquatic life
  • Surrogate measures make sense for stormwater TMDLs
  • Momentum for using IC (NEMO/Towns and MS4)
  • IC is a good way to show people the problem




Strength of Evidence – Scientific Literature

Adapted from 2003 Center for Watershed Protection Research Monograph Number 1


Support for Surrogate Approach

  • 40 CFR 130.2 (i) states that "TMDLs can be expressed in terms of either mass per time, toxicity, or other appropriate measure."
  • 40 CFR 130.7 (c)(1)(i) "states that TMDLs may be established using a pollutant-by-pollutant or biomonitoring approach."

“CTDEP developed a TMDL support document that provides a scientific basis that IC is an appropriate surrogate measure of impacts caused by stormwater (i.e. "other appropriate measure") and aquatic life use assessments using macroinvertebrates (i.e. "biomonitoring approach") provide an appropriate endpoint to measure progress of implementation.” From Draft Eagleville Bk TMDL


Applicability in Connecticut

The Streams

  • < 50 square miles drainage upstream
  • No sites within a mile downstream of POTWs
  • No streams with portion of watershed in another state
  • Rapid Bioassessment Protocol 3 level of effort
  • 12 riffle kicks, 200 organism sub-sample, ID to lowest taxon possible (generally species level)

Impervious Cover Data

  • ISAT tool developed by NOAA Coastal Services Center and UCONN
  • ArcView extension that takes land cover and estimates IC using land cover specific impervious cover coefficients
  • ISAT can be downloaded free at
  • Needs ArcView 3.x and ESRI’s Spatial Analyst to run

12% IC Threshold

% Impervious Cover and Macroinvertebrates

% of Reference Community compares 7 metrics- Taxa Richness, Modified HBI , Scraper/Filterer, EPT/Chironomidae, % Dominant Taxa, EPT Index, Community Loss



% Impervious Cover

N =86

N =39

Percent Impervious Cover and

Aquatic Life Use Support (ALUS)


Low Impervious Cover and Poor Bugs

Flow Alteration, Agriculture,

Nutrients, Spills


IC and Setting the TMDL Goal

  • Support Document established 12% IC threshold
  • TMDL Target is the 90th percentile of sites that Meet ALUS = 11% IC
  • TMDL Target = 11% IC
  • WLA and LA = Percent reduction to meet 11%

e.g. IC upstream of Impaired ALUS segment is 20%

Assume not enough info to separate WLA and LA

TMDL Target = 11% IC

WLA = (20% - 11% / 20%) x 100 = 45% Reduction in IC

LA = 45% Reduction in IC

MOS = 1% (12% Threshold - 11% Target)


TMDL Goal -What Does It Mean?

The IC TMDL target is not intended to be used as the basis for permit limits, or a "not to be exceeded" limit on development, or a measurement of compliance with Water Quality Standards.

  • Stormwater management
    • BMPs to reduce effects of IC
    • Disconnect IC where practical
    • Remove IC where practical
  • Meeting the TMDL goal will be assessed by measuring the aquatic life use directly and not by assessing the amount of IC reduction.

When to Use IC Method for TMDLs?

Biological assessment determines ALUS not met

303 (d) Listed ALUS-Cause Unknown

303 (d) Listed ALUS -

Cause = Stormwater

SI Investigation determines stormwater is most probable cause

IC TMDL Methodology



Where to next?

  • Mini-study designed to collect macroinvertebrate data from streams 7-15% IC
  • Collect ancillary data that will help describe these sites e.g. Habitat, Landuse, Town Zoning, Stream buffer widths, other?
  • Goal: Gain a better understanding of the important variables that influence attainment of ALUS

Ancillary GIS Data?

  • Obtain GIS data with Riparian Cookie Cutter
  • Landuse (e.g. % Forest, % Protected Land)
  • Forest continuity
  • Watershed shape (long and narrow vs short wide)
  • Storm Drains/sq mile
  • others ?

Will Mitigation Matter?

Can we offset Impacts of IC by BMPs and “Smart” Land Use Planning

(i.e. Will the bugs get better?)



Describe the variables that make some streams with high IC good and some streams with high IC bad

Need to reassess our aquatic life goals for streams with high IC



The Storm Water Management Model is a comprehensive computer model for analysis of quantity and quality problems associated with urban runoff. Both single-event and continuous simulation can be performed on catchments having storm sewers, or combined sewers and natural drainage, for prediction of flows, stages and pollutant concentrations. Extran Block solves complete dynamic flow routing equations (St. Venant equations) for accurate simulation of backwater, looped connections, surcharging, and pressure flow. Modeler can simulate all aspects of the urban hydrologic and quality cycles, including rainfall, snow melt, surface and subsurface runoff, flow routing through drainage network, storage and treatment. Statistical analyses can be performed on long-term precipitation data and on output from continuous simulation. The model can be used for planning and design. Planning mode is used for an overall assessment of urban runoff problem or proposed abatement options.

Recommendation - Stormwater Management


Connecticut Impervious Cover Model

When it rains, stormwater runoff carries pollutants to streams, alters natural streamflow, and causes negative impacts to fish and bugs that live in the stream. The greater the IC, the greater the chance of killing or reducing numbers of sensitive fish and bugs.

Recommendation - Stormwater Management


Occam's Razor

“Pluralitas non est ponenda sine necessitas”

Plurality should not be posited without necessity

"when you have two competing theories which make exactly the same predictions, the one that is simpler is the better."

William of Ockham (c.1285–1349)


Wireless Rechargeable Razor-

NEAEB 2006

“For CT, IC model is mo’ better than competing stormwater models for TMDL Development”