A method for deriving water‐quality benchmarks using field data

Abstract

Abstract-The authors describe a methodology that characterizes effects to individual genera observed in the field and estimate the concentration at which 5% of genera are adversely affected. Ionic strength, measured as specific conductance, is used to illustrate the methodology. Assuming some resilience in the population, 95% of the genera are afforded protection. The authors selected an unambiguous effect, the presence or absence of a genus from sampling locations. The absence of a genus, extirpation, is operationally defined as the point above which only 5% of the observations of a genus occurs. The concentrations that cause extirpation of each genus are rank-ordered from least to greatest, and the benchmark is estimated at the 5th percentile of the distribution using two-point interpolation. When a full range of exposures and many taxa are included in the model of taxonomic sensitivity, the model broadly characterizes how species in general respond to a concentration gradient of the causal agent. This recognized U.S. Environmental Protection Agency methodology has many advantages. Observations from field studies include the full range of conditions, effects, species, and interactions that occur in the environment and can be used to model some causal relationships that laboratory studies cannot. Environ. Toxicol. Chem. 2013;32:255-262. # 2012 SETAC

Derivation of a benchmark for freshwater ionic strength

“…The approach used to derive the benchmark [11] is based on an adaptation of the standard method for U.S. EPA's published Section 304(a) Ambient Water-Quality Criteria [12]. We used the statistical package R, Version 2.12.1 (December 2010), for all statistical analyses [29].…”

“…We excluded four sites that had an ionic mixture with more Cl À than SO 2À 4 þ HCO À 3 (conductivity > 1,000 mS/cm, SO 4 < 125 mg/L, and Cl À > 250 mg/L). This ionic mixture is expected to have a different toxicity [11,13]. Because Cl À was not measured at all sites, some sites with a different ionic composition may still occur in the data set.…”

“…That process was repeated 1,000 times to create a distribution of XC95 values for each genus. These distributions were used to calculate a two-tailed 95% confidence interval on the XC95 for each genus [11,27].…”

“…The estimated two-tailed 95% lower confidence bound of the HC05 point estimate is 228 mS/cm and the upper bound is 303 mS/cm. (See Figure 5 in [11] for a graphed illustration. )…”

“…The present study demonstrates the use of field data to develop a protective benchmark for ionic strength using a method developed by Cormier and Suter [11]. This method is adapted from the U.S. Environmental Protection Agency's (U.S. EPA's) standard method for deriving water-quality criteria [12].…”