Should the sediment quality triad become a tetrad, a pentad, or possibly even a hexad?

Goal, Scope, Background. The Sediment Quality Triad (SQT) as originally developed 20 years ago involved three lines of evidence (LOE): sediment chemistry, sediment toxicity, and benthic community structure. The first two LOE have been consistently included in all SQTs conducted to date, however the last LOE has been changed to 'alteration of resident communities' an in some cases has been excluded or replaced. In other cases additional LOE have been added to the SQT. Main Features. This paper examines the history of different LOE used as part of the SQT, and provides recommendations regarding future use of various LOE in SQT studies. Results and Discussion. A total of 14 LOE replacing or additional to the alterations to resident communities LOE have been proposed and applied. These LOE fall into four categories: direct replacements for or additions to existing LOE; variations on existing LOE; additional LOE; and LOE to determine causation. An additional possible LOE not previously proposed or applied is determination of genetic diversity. Conclusion and Perspectives. Future applications of the SQT should ensure that LOE appropriately address all components of a risk assessment (Problem Formulation, Exposure and Effects Assessment), address causation, and also consider other (non-chemical) stressors. Choice of specific LOE will depend on the stressors of potential concern (SCPCs) and the receptors of potential concern (ROPCs), focusing on ecological relevance.