An Updated Unified Zinc Biotic Ligand Model for Protection of Freshwater Aquatic Life and its Application for Site-Specific Water Quality Objectives

A unified biotic ligand model (BLM) was developed to predict acute Zn toxicity to four invertebrates and two fish, and chronic toxicity to three invertebrates and a fish. Developed using a comprehensive ecotoxicity database, this unified BLM represents the first update to the unified Zn BLM in nearly 14 years. For comparative purposes, a unified Zn multiple linear regression (MLR) model was also developed. Both models are unified because they each use a single set of parameters to characterize the effects of toxicity modifying factors (e.g., pH, dissolved organic carbon [DOC], and major ions) on acute and chronic Zn toxicity. While both models were capable of accurately predicting Zn toxicity, the unified BLM performed marginally better based on quantitative model performance scores (MPS) and qualitative single-variable pH and DOC evaluations. The unified Zn BLM, which also performed better than the U.S. Environmental Protection Agency's hardness equation, was then used to normalize separate acute and chronic species sensitivity distributions (SSDs) to develop acute and chronic 5th percentile hazardous concentrations (HC5s) analogous to USEPA's aquatic life ambient water quality criteria (WQC). The unified Zn BLM-based WQC were shown to be protective of threatened and endangered species in California and appear to be protective of chemosensory endpoints for salmonids. Using monitoring data for California as a test case, chronic unified BLM-based WQC were lower than hardness-based WQC in 77% of samples, yet fewer WQC exceedances were observed. Implementation of the Zn BLM for site-specific water quality objectives (SSWQOs) using the fixed monitoring benchmark (FMB) approach indicates that for the California dataset, dry weather and wet weather samples should be considered separately to develop SSWQOs for each condition.