The Establishment of Integrated Ecological Assessment Framework Using Toxicity Indicators of Heavy Metals in Korean Freshwater System

Abstract

In the environment, heavy metals naturally occur in background concentrations, but they have increased excessively due to anthropogenic activity. Heavy metals are non-biodegradable, semi-permanent, and bioaccumulative, with toxic effects on the ecosystem. Therefore, it is necessary to identify appropriate thresholds, such as a predicted no-effect concentration (PNEC), to protect aquatic organisms. In general, PNEC is calculated by plotting a cumulative distribution function using toxicity data from various organisms known as the species sensitivity distribution (SSD). SSD assumes that randomly sampled organisms can represent ecosystems and that a series of toxic responses to a chemical follow a specific probability distribution. A hazardous concentration for 5% of species (HC5) is estimated based on SSD and divided by an assessment factor (AF) to generate PNEC. To effectively protect the ecosystem from a chemical, it is necessary to derive PNEC and carry out an ecological risk assessment (ERA). The ERA is a process that assesses the possibility of an ecosystem being affected when exposed to a chemical. Performing the ERA on a chemical of interest requires data on various organisms’ toxic effects and environmental exposures. An exposure concentration distribution (ECD) is constructed in the same way as an SSD using the exposure concentration of a chemical in the environment. Ecological risk is then estimated using ECD and SSD. Such an ERA method has been used in many countries and jurisdictions because it is considered to offer useful and practical tools. However, some limitations still remain, and various attempts to improve the existing ERA methodology have been considered. As an alternative, ecological vulnerability focuses on individual organisms’ responses to a chemical and the resulting population or community changes. Vulnerability is estimated, taking into account chemical exposure, biological sensitivity, and population recovery. In order to account for vulnerability, the ecological and biological traits are regarded as vulnerability indicators. These indicators are aggregated into three categories and then further aggregated into the vulnerability, resulting in a trait-based vulnerability. However, toxicity is an important factor that cannot be overlooked when discussing heavy metal effects on organisms. Therefore, an improved ecological assessment has been performed in this study by adding a toxicity indicator to the trait-based vulnerability to investigate heavy metal ecological vulnerability. This study evaluates the ecological issues affecting Korean freshwater environments exposed to heavy metals by integrating heavy metal toxicity and the associated potential effects on organisms living in Korean freshwater systems. Four heavy metals were considered, including cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn). First, SSDs were developed using multiple models, and the optimal PNEC was derived from the model best suited for heavy metal toxicity data. The toxicity data used consisted of values for Korean freshwater organisms, and PNEC was determined using an AF reflecting statistical uncertainty. For the four heavy metals investigated, more than 25 sets of toxicity data for Korean-resident species were available, meeting the minimum amount of data required for SSD. However, since plant toxicity for Pb and Zn was not included, the species composition requirement was not satisfied. Of the multiple SSD models considered, the Normal distribution for Cd and the Gumbel distribution for the other three heavy metals were found to be optimal. In deriving PNEC, AF 2 and 3 sufficiently accounted for the statistical uncertainty of HC5, resulting in PNEC of 6.1, 3.4, 15.8, and 89.1 μg/L for Cd, Cu, Pb, and Zn, respectively. The uncertainty associated with PNEC derivation was moderated by elaborating resident organisms’ toxicity data while considering the most suitable SSD model and reasonable AF. However, heavy metal toxicity in specific taxonomic groups, mostly plants for which toxicity data is insufficient, requires further discussion. Next, the ecological risks posed by heavy metals in four major Korean rivers were quantified through statistical techniques. ERA was performed by relating pre-generated SSDs and PNECs to ECDs made for environmental concentrations of heavy metals accumulated through field sampling and literature investigation. The ecological risks of Cd and Pb in Korean rivers were negligible, and the derived PNECs were sufficient to assess conditions. However, Cu and Zn represented environmental exposure concentrations with suspected ecological risks. In particular, effective environmental measures are needed as a significant proportion of heavy metals exposure exceeds PNEC in industrial treated water (ITW). Lastly, to improve the existing ERA, the ecological vulnerability of living organisms in Korea to heavy metals was investigated. A trait database for vulnerability indicators was created by applying fuzzy coding. Each organism’s ecological vulnerability to heavy metals was evaluated in two stages: trait-based vulnerability and toxicity-based vulnerability. The constructed trait database has shown low resolution for some traits, but it is expected to play a role in the future by contributing information on Asian organisms lacking in existing trait studies. Trait-based vulnerability indicated apparent differences between invertebrates and fish. The significantly lower recovery and sensitivity scores for fish led to their designation as the least vulnerable species. On the other hand, annelids scored high in all categories and were identified as the most vulnerable species. Toxicity-based vulnerability, weighing in additional toxicity indicators, showed a significant change in vulnerability due to heavy metals for some organisms, including insects and bivalves. This suggests that toxicity should be considered when evaluating organisms’ vulnerability to chemicals. The vulnerability assessment performed also identified the potential importance of taking a comprehensive view of organisms’ intrinsic traits and toxicity to chemicals. However, the ecological vulnerability assessment, which is still in the development stage, requires improvement, and solutions must be found to fundamental controversies.