Risk-Screening Environmental Indicators (RSEI) Model

Modeling Water Releases in RSEI

EPA's Risk-Screening Environmental Indicators (RSEI) model calculates results for the following kinds of water releases reported to the Toxics Release Inventory (TRI):

  • Direct water releases from reporting facilities.
  • Transfers to wastewater treatment plants (Publicly Owned Treatment Works, or POTWs). Some portion of each transfer from a facility to a POTW remains in the water after treatment and is released as a water discharge.

Water releases account for a much smaller percentage of total pounds and total RSEI Score than air releases. RSEI models potential exposure to chemicals released into surface water in one of two ways: by drinking tap water from a public water system whose water intake was located in the stream path of a chemical release; or by eating contaminated fish caught in a water body in the stream path of a chemical release.

Illustration of how RSEI calculates downstream concentrations for a water discharge, showing stream path of a facility discharge and a downstream drinking water intake.RSEI uses the following data to model water releases:

  • Stream geography and hydrography (flow and volume estimates) from NHDPlus Exit (based on the National Hydrography Dataset (NHD)).
  • Stream segment that receives the facility’s or POTW’s chemical discharge.
  • Chemical-specific water decay rate.
  • Chemical-specific bioconcentration factor (BCF).
  • Chemical-specific overall removal rate for POTWs.
  • U.S. decennial census data.
  • Age- and sex-specific body weights, drinking water ingestion rates, and fishing ingestion rates.
  • EPA’s Maximum Contaminant Levels (MCLs) for chemical concentrations in drinking water systems.
  • Drinking water intake locations.
  • Number of people served by each drinking water system.
  • Number of people by county (or by state if county data are unavailable) with fishing licenses.

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RSEI does not include specific data on the following components related to water modeling:

  • How the release rate of the chemical varies over time (RSEI assumes a constant rate of release).
  • How other biological processes in streams (like sedimentation or biodegradation) affect the chemical concentration in the water (RSEI only models dilution and decay).
  • What chemicals are produced from decay or transformation of modeled chemicals (RSEI only models the chemical released).
  • How fish consumption rates vary geographically (RSEI uses nationwide values from an EPA survey, and assumes a fixed proportion of subsistence fishers with a higher ingestion rate than recreational fishers).
  • How many people fish in a given stream (RSEI assumes a portion of the population within 50 miles, based on available fishing license data).
  • What portion of the drinking water system is supplied from any given intake (RSEI assumes any intake supplies the whole system).
  • Where the people served by a drinking water system live (RSEI makes no assumption in this regard).
  • How a facility distributes its discharges across discharge pipes (RSEI assigns one discharge point per facility).
  • What specific technologies are used by POTWs (RSEI assumes only secondary treatment at all POTWs).

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For each stream segment downstream from a facility discharging to water, RSEI calculates the following metrics:

  • Population (number of people in grid cell).
  • Chemical concentration.
  • Toxicity-weighted concentration (toxicity weight x concentration), provided in the RSEI Geographic Microdata
  • Estimated dose. Human exposure assumptions (body weight, fish ingestion rate and drinking water ingestion rate) are used to transform the concentration into an estimated dose for each stream segment.
  • RSEI Score (dose x toxicity weight x population).

Section 5 of the RSEI methodology document provides detailed information on how RSEI models water releases.

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