Field-based method for evaluating the annual maximum specific conductivity tolerated by freshwater invertebrates

Most water quality criteria are based on laboratory toxicity tests and usually include chronic and acute magnitudes. Field-based criteria are typically based on long-term or continuous exposures, so they are chronic. Biological responses of quantified, short-term aqueous exposures are seldom documented in the field. However, acute values may be derived by estimating an upper limit using temporal variance and chronic values. This method estimates an upper limit from the variance of pollutant measurements from stream locations that attain the chronic criterion. The formula for deriving a 90th centile of a standard normal distribution is used to identify the upper limit, a criterion maximum exposure concentration (CMEC). The calculated CMEC is interpreted as a maximum exposure that 95% of organisms may tolerate if the chronic exposure is not exceeded. The methods of deriving chronic and acute criteria are illustrated with specific conductivity in a mountainous area in the eastern United States. The biological relevance of the CMEC was assessed using the maximum annual exposure during the life cycle of the most salt-intolerant genera. The method using the chronic criterion and the variance of water chemistry data is practical, whereas frequently collecting and analyzing paired biological and chemical samples at numerous sites is impractical and may give misleading results due to lags in biological response. This method can be used anywhere with sufficient data to estimate the temporal variability and may be applicable for field-based criteria other than the specific conductivity criteria illustrated here.