EBTAG Annual Workshop and Field Trip
May 13, 2014

¹Environmental Programs, Los Alamos National Laboratory, groffman@lanl.gov

²Adalante Consultants Inc.

³Winward Environmental, LLC

The Biotic Ligand Model (BLM) is based on a gill site interaction conceptual model and predicts metal bioavailability and toxicity by incorporating metal speciation and competing cations in solution. The Environmental Protection Agency (EPA) released nationally recommended copper water quality criteria (WQC) for aquatic life based on the BLM. Although the New Mexico water quality standards allow the use of the BLM on a site specific basis, the hardness-based method to calculate copper WQC is the default statewide approach. The application of the BLM for zinc, cadmium, and aluminum WQC estimations is under investigation as well with promising results. Understanding copper bioavailability is critical for a better understanding of immediate complex environmental conditions. Using the BLM for copper is the current EPA policy and represents the current science.

Water quality samples and BLM parameters were collected during 2013 from 19 ephemeral, intermittent, and perennial stream segments on the Pajarito Plateau, north central New Mexico. Chronic and acute copper and zinc instantaneous water quality criteria (IWQC) were estimated and compared using the BLM and hardness based WQC. In most cases the copper and zinc IWQC estimated with the BLM was higher than the hardness -based IWQC. The BLM-based acute copper WQC ranged from 3.4 to 361 µg/L while the hardness based IWQC ranged from 0.5 to 12.4 µg/L. Meanwhile, acute zinc BLM-based WQC ranged from 167 to 1,138 µg/L, while hardness based IWQC for zinc ranged from 6 to 149 µg/L. In all cases the toxicity units (Cu observed/BLM IWQC) values were less than one, indicating observed copper and zinc concentrations did not exceed respective criteria levels. These findings indicate that bioavailable copper and zinc concentrations estimated by the BLM are much lower and more representative than what is estimated using the hardness-based WQC . In addition, these results suggest that in many cases the hardness-based WQC are over protective of the environment, lacking the resolution inherent to the BLM to represent complex aquatic environmental conditions. In an aqueous environment, copper and zinc complex with inorganic and organic ligands, sorption sites on suspended particulates, common ion effect, and are particularly sensitive to pH. These geochemical processes are considered by the BLM. The hardness based WQC consider only competition from magnesium and calcium, a far simpler and less accurate basis. NPDES permits and state water quality assessments would be more accurate based on the BLM for copper and zinc.