Ecological impacts observed in contaminated urban river systems often can be attributed to the presence of chemical stressors (e.g., chemicals in sediments) as well as non-chemical stressors (e.g., "flashiness" of flow, high rates of sedimentation, low DO, etc.) typically found in urban environments.  This issue presents a unique and very difficult challenge when attempting to characterize sources of ecological risk within urban river systems.

I am curious how others have tackled the problem of quantifying 1) ecological risk resulting directly or indirectly from exposure to chemical stressors; and 2) ecological risk attributed directly or indirectly to the presence of non-chemical stressors.  Although this is an important academic question, it is also an extremely important and potentially costly problem faced by those conducting ecological risk assessments that may be used to inform the remedial decision process mandated under various regulatory programs.