Environmental Risk Assessment of Fluctuating Diazinon Concentrations in an Urban and Agricultural Catchment Using Toxicokinetic–Toxicodynamic Modeling

Temporally resolved environmental risk assessment of fluctuating concentrations of micropollutants is presented. We separated the prediction of toxicity over time from the extrapolation from one to many species and from acute to sublethal effects. A toxicokinetic–toxicodynamic (TKTD) model predicted toxicity caused by fluctuating concentrations of diazinon, measured by time-resolved sampling over 108 days from three locations in a stream network, representing urban, agricultural and mixed land use. We calculated extrapolation factors to quantify variation in toxicity among species and effect types based on available toxicity data, while correcting for different test durations with the TKTD model. Sampling from the distribution of extrapolation factors and prediction of time-resolved toxicity with the TKTD model facilitated subsequent calculation of the risk of undesired toxic events. Approximately one-fifth of aquatic organisms were at risk and fluctuating concentrations were more toxic than their averages. Contribution of urban and agricultural sources of diazinon to the overall risk varied. Thus using fixed concentrations as water quality criteria appears overly simplistic because it ignores the temporal dimension of toxicity. However, the improved prediction of toxicity for fluctuating concentrations may be small compared to uncertainty due to limited diversity of toxicity data to base the extrapolation factors on

Assessing Site-Specific Risk to Aquatic Biota from Multiple Exposure Pathways

Many risk assessments rely on ambient water quality criteria (AWQC) to evaluate potential risks to aquatic biota from contaminant exposure. The AWQC are not intended to consider risk from sediment or dietary exposure pathways and few assessments account for site-specific bioavailability of metals. Risks to a variety of aquatic biota from metals in the Upper Clark Fork River (UCFR), Montana were evaluated using assessment tools specific to each exposure medium- water, sediment, and diet. Each medium was evaluated using extensive site-specific exposure data and, when possible, effects criteria that reflect site-specific metals bioavailability. Risk from exposures to water column metals (As, Cd, Cu, Pb and Zn) was evaluated by comparing observed dissolved metals concentrations (an average of long term monitoring) to chronic AWQC and a site-specific rainbow trout chronic toxicity reference value (TRV) developed by EPA based on testing UCFR water. (When environmental concentrations are below TRV’s, risks are usually assumed nominal.) Sediment metals exposure was evaluated using EPA draft Equilibrium Sediment Guidelines (ESGs--i.e., simultaneously extractable metals - acid-volatile sulfide and sediment porewater summed as toxic units) and using bulk Sediment Effects Concentrations (SEC) derived from toxicity tests with UCFR bed sediments. Trout dietary exposures were addressed by comparing extensive benthic macroinvertebrate tissue data with dietary TRVs derived from selected literature. Dissolved metals concentrations were generally lower than the water column TRVs for each metal. Metals in pore water and bulk sediments were below ESGs and SECs. Furthermore, benthic macroinvertebrate tissue concentrations did not exceed dietary TRVs. These results predict nominal risk to most UCFR aquatic biota under observed conditions for each of the exposure pathways evaluated

Evaluation of Exposure-Effects Relationships of Metals in the Benthic Macroinvertebrate Community in the Upper Clark Fork River, Montana

Previously published studies conducted on the Upper Clark Fork River (UCFR) suggest adverse effects due to metals-enriched sediments found in the depositional areas that comprise approximately 4% of the riverbed. While these studies measure exposure concentrations from depositional areas, effects measurements (e.g., benthic invertebrate abundance and diversity measures and tissue residues) have been predominantly obtained from coarse substrate riffle areas. Comparing exposure data from one habitat to effects data from another is problematic. An integrated approach was conducted to assess effects data by sampling benthic macroinvertebrates for community composition measurements, tissue residues, and sediment contaminant concentrations. Thirteen co-located sampling sites along the UCFR were sampled in both depositional and riffle habitats in 1996. Metals concentrations in bulk sediments and benthic invertebrate tissues decreased with distance from metals sources in the headwaters, but sediment porewater concentrations did not. Although there were some significant relationships between bulk sediment metals concentrations and tissue metals residues, BMI community metrics did not appear to vary along this gradient, or with exposure concentrations. This is consistent with an evaluation of sediment toxicity tests performed using sediments from Warm Springs Ponds, as well as the UCFR, which do not suggest that metals are of potential concern to benthic macroinvertebrates

Evaluating the Bioavailability of Metals in Sediments from the Upper Clark Fork River Basin, Montana

A sediment quality assessment was developed to evaluate the potential bioavailability of metals from riverbed sediments. The proposed approach consisted of multiple assessment methods using bulk sediment metals concentrations, equilibrium partitioning measures, and bulk sediment and pore water toxicity tests. Metals bioavailability was evaluated using: 1) EPA’s theoretical guidelines which compare acid-volatile sulfide concentrations to simultaneously extractable metals concentrations, and compare sediment pore water metals concentrations to ambient water quality criteria; 2) correlative guidelines, which compare bulk sediment metals concentrations against sediment no-effect concentrations (NECs), and 3) sediment toxicity tests. Sediment toxicity studies were also used to derive site-specific NEC values. The study site chosen for this assessment was the metals-enriched upper Clark Fork River (UCFR) located in southwestern Montana, USA. Results of this assessment indicate only nominal risk to most aquatic organisms posed by sediment metals concentrations in depositional areas in the UCFR. The uncertainty associated with using only one of the approaches for evaluating sediment contamination should be reduced by using the combined approach outlined here

The biotic ligand model: a historical overview

During recent years, the biotic ligand model (BLM) has been proposed as a tool to evaluate quantitatively the manner in which water chemistry affects the speciation and biological availability of metals in aquatic systems. This is an important consideration because it is the bioavailability and bioreactivity of metals that control their potential to cause adverse effects. The BLM approach has gained widespread interest amongst the scientific, regulated and regulatory communities because of its potential for use in developing water quality criteria (WQC) and in performing aquatic risk assessments for metals. Specifically, the BLM does this in a way that considers the important influences of site-specific water quality. This journal issue includes papers that describe recent advances with regard to the development of the BLM approach. Here, the current status of the BLM development effort is described in the context of the longer-term history of advances in the understanding of metal interactions in the environment upon which the BLM is based. Early developments in the aquatic chemistry of metals, the physiology of aquatic organisms and aquatic toxicology are reviewed first, and the degree to which each of these disciplines influenced the development of water quality regulations is discussed. The early scientific advances that took place in each of these fields were not well coordinated, making it difficult for regulatory authorities to take full advantage of the potential utility of what had been learned. However, this has now changed, with the BLM serving as a useful interface amongst these scientific disciplines, and within the regulatory arena as well. The more recent events that have led to the present situation are reviewed, and consideration is given to some of the future needs and developments related to the BLM that are envisioned. The research results that are described in the papers found in this journal issue represent a distinct milestone in the ongoing evolution of the BLM approach and, more generally, of approaches to performing ecological assessments for metals in aquatic systems. These papers also establish a benchmark to which future scientific and regulatory developments can be compared. Finally, they demonstrate the importance and usefulness of the concept of bioavailability and of evaluative tools such as the BLM. © 2002 Elsevier Science Inc. All rights reserved