How the RiverCast is Created

RiverCast Water Quality Designations are based upon historical relationships between water quality, stream flow, and rainfall data, and on the United States Environmental Protection Agency's "Implementation Guidance for Ambient Water Quality Criteria for Bacteria" (USEPA, 2002). This document identifies the maximum level of bacteria measured in a single sample that would be within acceptable illness levels for recreational activities that may involve complete immersion in the water.

The historical data analysis compiled bacteria and turbidity (water clarity) data collected by the Philadelphia Water Department in the vicinity of the Fairmount Pool (Manayunk to Boathouse Row) over the period of 1998 to 2000, along with rainfall and streamflow data from the US Geological Survey for the same location and time period. This data was analyzed to identify flow, rainfall and turbidity conditions that correspond to bacteria levels defined by the federal regulations for recreational waters. These regulations define when it is or is not safe to have direct or indirect contact with the water.

The table below provides the acceptable illness levels, ranges of allowable bacteria concentrations, and the corresponding RiverCast color designations. RiverCast forecasts the expected range of bacteria levels based on current rain, flow, and turbidity measurements, compares it to the USEPA recommended limits, and posts the green-yellow-red designation.

The conditions and Water Quality Designations defined by the historical data were tested on a set of recent data (2004 to 2005) to assess the accuracy of the relationships and the RiverCast predictions. This testing showed that the RiverCast relationships are very conservative. 65% the time the RiverCast prediction was accurate. 35% of the time the prediction was conservative (higher bacteria levels were predicted than measured). There were no examples of predicted levels lower than the measured levels. This means that, based on the most recent year of data, the RiverCast Water Quality Designation will either accurately predict water quality or give a conservative recommendation to exercise more caution then might be required. If the RiverCast is Green, one can be very confident that bacteria levels do not pose significant risk to those in contact with the water.

REFERENCES

United States Environmental Protection Agency's - Implementation Guidance for Ambient Water Quality Criteria for Bacteria. May, 2002. U.S. Environmental Protection Agency, Office of Water (4305T), 1200 Pennsylvania Avenue, NW, Washington, DC 20460. EPA-823-B-02-003. http://www.epa.gov/waterscience/criteria/bacteria/