EXECUTIVE SUMMARY
Pore (or interstitial) water toxicity and sediment chemistry are important components of assessments which determine the bioavailability of contaminants in waterways receiving mining discharges. The methods and approaches used in pore water toxicity and sediment chemistry evaluations vary widely in their scope, application, sensitivity, accuracy and precision, resource requirements, scientific credibility, and effectiveness for mining applications. This technical evaluation provides an extensive review of the literature with subsequent recommendations based on consensus opinions of scientific peer-reviewed studies, technical reports, and experts from academia, industry, consulting, and government.
Pore water has been shown to be a major route of exposure for many benthic organisms. Contaminants in pore waters can be transported into overlying waters through diffusion, bioturbation, and resuspension processes. Contamination of the sediments and their surrounding pore waters may, therefore, adversely impact the aquatic ecosystem and interlinking food webs. The usefulness of pore water sampling for determining chemical contamination and/or toxicity will depend on the study objectives and nature of the sediments at the study site. Depositional sediments usually are the primary sediments of concern as they are generally the sites of highest contamination.
The literature on contaminated sediments reveals some trends and generalities, as follows: 1) There has been a tremendous increase in peer-reviewed publications and funded projects dealing with contaminated sediments in the past few years; 2) The focus on determining the bioavailable fractions of contaminants is clearly established as superior to “total” chemical concentrations for assessing ecosystem harm; 3) The “weight-of-evidence” approach utilising a combination of physicochemical, habitat, indigenous biota, and toxicity characterisations is now widely accepted as the superior approach to contaminant assessments, reducing uncertainty of single “tool” approaches; 4) Detection of sediment contamination is much more sensitive with improved analytical methods and sublethal indicators of toxicity and stress; 5) Sediments are complex and variable in nature; 6) Sampling and laboratory testing of sediments can significantly alter contaminant bioavailability; 7) Contaminated sediments are degrading numerous aquatic ecosystems; and 8) Pore water toxicity testing is used to a lesser extent than whole sediment toxicity testing.
Predicting what fraction of the total metal concentration is bioavailable is a challenge, since metals complex to varying degrees with so many different entities and their toxicity rapidily changes with speciation and complexation. However, it is apparent that bioavailability can be determined and sound conclusions derived if proper methods and quality assurance protocols are used. This technical evaluation resulted in the following conclusions: 1) Pore water toxicity testing provides a useful supplement to whole sediment testing; 2) Pore water metal concentrations may be the best sediment chemistry indicator of bioavailability; 3) Metal concentrations which exceed water quality criteria should be considered harmful; 4) Pore water toxicity testing must be done using carefully controlled collection, extraction, and testing conditions to ensure reliable results; 5) Pore water toxicity testing should only be conducted if adequate QA/QC guidelines are followed; 6) In situ peeper collection of pore waters is the most accurate method, reducing sampling artifacts for chemical analysis and toxicity tests which can be conducted with small sample volumes; 7) In situ sample collection and/or testing is preferable to laboratory extraction and/or testing; 8) Efforts must be made to reduce sampling related artifacts, such as oxidation and mixing of vertical gradients; 9) Toxicity testing should commence as soon as possible following extraction; 10) Centrifugation (10,000 x g, 30 min., 4°C) without filtration is the preferred pore water extraction method if in situ collection methods are not feasible; 11) Few laboratories have experience with pore water toxicity testing; however, qualified laboratories do exist in Canada; 12) Total metal concentrations in sediments are most reliable in situations where gross contamination exists; 13) Easily extractable fractions may be useful on a site specific basis, but are still considered to be in the realm of research; 14) The AVS/SEM approach shows promise with some metals in anaerobic sediments, such as Cd and Ni; 15) Dissolved organic C is likely a primary control factor for Cu availability; and 16) An integrated assessment approach is most accurate, combining toxicity testing, biological community characterization, habitat characterization, and physicochemical characterization in a tiered testing approach.
Given the need to adopt a uniform national approach to environmental assessments, it would be useful to conduct a field demonstration project at geologically diverse mining sites to better evaluate utility of pore water toxicity testing. This evaluation program should include field validation of the popular sediment quality guidelines for routine application in mining monitoring situations. Following this demonstration project, appropriate guidelines should be developed for regulatory application of pore water toxicity testing.
AETE
