As part of the risk assessment framework, sediment constituent loads are often compared to common, but loosely relevant, benchmark levels as part of determining the potential for an impact to biological life. There are several technical and regulatory scenarios that benefit from the development of site-specific sediment criteria, including constituent- or condition- specific information gaps or the lack of observed biological response in previous monitoring.
Bioassays have the ability to serve as a powerful, integrated tool to support a decision maker’s weight-of-evidence approach. The participation of scientists with expertise in bioassay methods and their proper adaptation and interpretation are invaluable to a risk assessment team as they develop goals and plan sampling and analysis design. Site-specific criteria can be calculated using sediment amendment (i.e., spiking studies) and subsequent biological evaluation to more fully characterize local conditions as well as the viability of potential remediation alternatives. When undertaking these studies, multiple factors critical to understanding the design should be considered during planning stages, well before execution of any field work.
In partnership with an environmental consulting firm, Enthalpy provided testing for 25 sediment samples using the 20-day C. dilutus whole sediment bioassay. In addition to the samples that were screened for toxicity, a follow up study was undertaken to assess the risk associated with an exceedance in barium identified during bulk sediment chemical analyses. Barium was detected above the threshold effect limit (TEL), and this created the potential need for additional clean up and remediation actions which could have added significant cost responsibility for the client.
Our team designed a sediment spiking study to demonstrate that despite the elevated barium levels, there was no risk to the target species, Chironomus. A project-specific sediment amendment protocol was developed which included: assessment of the speciation of the barium present in the site sediment, selection of a suitable reference sample that accounted for key sediment and overlying water quality criteria (i.e. pH, TOC, alkalinity, etc.), and development of sediment amendment procedures and test method modifications that would ensure the success of the study.
The study met all acceptability criteria and was able to demonstrate that under the given site-specific conditions, barium resulted in no significant effects to the test organisms at five times the published effect threshold. The study was accepted without qualification by the regulatory body overseeing the project. This project highlights our understanding of the key biological and chemical influences which can affect study outcomes, our ability to modify and customize novel testing approaches to achieve project-specific goals, and our ability to produce data that leads to confident decision making and regulatory acceptance.