Application of a fixed monitoring benchmark approach to evaluate attainment of time-variable water quality criteria: Copper biotic ligand model as a case study.

In 2007, the Biotic Ligand Model (BLM) became the basis for the US Environmental Protection Agency (USEPA) freshwater water quality criteria (WQC) for Cu. Applying the BLM typically results in time-variable WQC, which are not unique to the BLM; they result from any criteria approach that depends on water chemistry (e.g., ammonia criteria or hardness-based equations for metals). However, widespread use of the BLM has renewed interest in developing an approach that considers variability when setting permit limits or benchmarks. To aid in establishing these benchmarks, we developed a fixed monitoring benchmark (FMB) approach: a probability-based method that incorporates time variability in BLM-predicted instantaneous water quality criteria (IWQC) and instream Cu concentrations. The FMB approach provides benchmarks that can be used to simplify implementation of time-variable WQC. Although it appears reasonable to apply this approach to derive a site-specific regulatory limit, the FMB does not technically represent a limit above which aquatic effects are expected. Rather, it represents a fixed concentration intended to yield the same level of protection as time-variable IWQC, which rely upon toxic unit (TU) distribution; each TU is calculated for a single sample using the Cu concentration and IWQC for this sample. The distribution of TUs for a particular site is used to estimate the probability that instream Cu concentrations are below associated IWQC. Our results suggest that Cu variability and corresponding IWQC, and their degree of correlation, indicate the magnitude of the FMB relative to the IWQC distribution. The FMB approach determines a maximum Cu distribution such that the resulting WQC exceedance frequency is consistent with the level of protection that is intended for the applicable water quality standard (WQS). This approach makes use of time-variable BLM-based WQC in regulatory contexts wherein a single benchmark is consistent with past practices and established implementation methods. Integr Environ Assess Manag 2018;00:000-000. © 2018 SETAC.