What can cause failure in a bioassay or a biomonitoring test?

Aquatic toxicity tests, often called bioassay or biomonitoring, were developed to estimate the effluent impact on receiving streams. Using living higher life-forms such as fat head minnow and daphnia (water flea), the test looks for both acute and chronic toxicity when subjected to various concentrations of wastewater effluent.

Often the chronic toxicity which is reproduction and weight gain are the harder part of the test to pass. But what causes failure?

Obvious causes of failure

• Ammonia (very toxic to aquatic organisms)
  
• Toxic compounds such as phenol, cyanides, surfactants, and pesticides/herbicides
  
  

But what happens when you are meeting all effluent permit limits on listed parameters, but are still failing parts of the bioassay test? Then it is time to looks at less obvious sources of toxicity and how multiple inhibitory compounds at low concentration can work concurrently to inhibit weight gain or reproduction.

Non-obvious causes of failure:

• Intermediate microbial transforms can actually be made more soluble than the influent compounds and therefore be more toxic to aquatic organisms
• Interaction by minor inhibiting compounds to create a larger amount of inhibition than any compound alone

How to handle test failure:

• Look at changes in influent
• Use upstream acute toxicity testing if influent makeup is unknown or questions exist
• GC/MS testing to look for most common toxic compounds
• Evaluate treating effluent with activated carbon, peroxide, or zeolite to improve toxicity profile