Determination of site-specific acceptable concentrations of copper using fixed monitoring benchmarks in water: Soil porewater and stream water

Abstract

Biotic ligand model (BLM) has been used widely to predict Cu concentration in water that affects 50% of species (50% effective concentration, EC50) with site-specific water characteristics to assess its toxicity ecotoxicologically. It, however, has a limitation that its predicted Cu concentrations change over time because the characteristics of water varies. To determine a site-specific acceptable Cu concentration from varying BLM predictions, the fixed monitoring benchmarks which is a probability based method was introduced. It assesses the compliance of dissolved Cu concentrations in the site water with the BLM-predicted instantaneous water quality criterion(IWQC) and determines the highest acceptable concentration of dissolved Cu that the site water may reach potentially by probability. Soil porewater samples collected from Janghang city, Republic of Korea, during the 5 months of sampling period showed their characteristics including pH, DOC, temperature and cation concentrations changed each time when sampled. That resulted in different BLM-predictions over the period and based on the probability, the Cu concentrations in this site are expected to exceed the BLM-predicted safe concentrations more frequent than once in three years. In order to make its concentrations to be in compliance with the BLM predictions, the maximum dissolved Cu concentration it should reach is determined to be 0.17 mg/L to protect barley and local plants in the region. To expand the concept of FMB to spatially different locations, it was applied to stream waters within Han River basin with 12 different monitoring stations in Korea. Each station had different water characteristics from June 2014 to October 2016 which resulted in different BLM-predicted IWQC regionally. When the dissolved Cu2+ concentration and the Ca2+ concentration were assumed to be identical in each station, the predicted copper FMB values differed up to 95% in 12 stations from 2.63 to 7.41 μg/L. Current water quality monitoring system in Korea does not measure cation concentrations and measure Cu concentrations only at selective sites. Ca2+ itself is not a hazardous contaminant in water but it affects the toxicity of copper when dissolved in water. It changes the copper FMB from 3.999 to 28.87 μg/L when the Ca2+ concentration changes from 0 to 2 mM. The distribution of dissolved Cu2+ concentrations also affect the copper FMB values from 4.30 to 23.7 μg/L when the standard deviation of the distribution differs 1, 1.1, 1.2, 1.3, 1.4 and 1.5. The predicted copper FMB values are very sensitive to the dissolved Ca2+ concentrations and Cu2+ concentrations in water and to make the predictions more accurately, it is important to measure Ca2+ and Cu2+ concentrations in monitoring system in Korea.