An official website of the United States government.

We've made some changes to EPA.gov. If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

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.

Top of Page

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).

Top of Page

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.

Top of Page