Assessing Variation in Concentrations of Dissolved Iron in Minimally Disturbed Streams to Improve Water Quality Criteria

USEPA is moving toward revised approaches for establishing water quality criteria for aquatic life that include improvements in the consideration of time-varying exposure through the use of kinetic toxicity and population models. Use of these models, and the criteria derived from them, will benefit from a thorough understanding of the time variable exposure regime experienced by biota in minimally-disturbed aquatic systems. In this paper, variations in dissolved iron (Fe) concentrations at several water quality stations from the U.S. Geological Survey (USGS) Hydrologic Benchmark Network (HBN) collected between 1984 and 2005 are characterized. Results provide information that can be used to set appropriate bounds for assumed time-variable chemical concentrations used as inputs to kinetic toxicity and population models for aquatic systems. They further indicate that care may be needed to validate assumptions about the chemical concentration distributions used in these models.