A cocktail of contaminants: why environmental toxicologists are moving away from single-chemical science

By: Dana Sackett

Cocktails, the good kind! Source: https://themorganbar.ie/parties/cocktail-menu/

For me, the word cocktail usually brings to mind delightful adult beverages or a foggy witch’s brew (still in Halloween mode).  However, more frequently, as an environmental scientist, this word is being used to describe the conditions of our aquatic environments.  Our waterways are often where the slurry of chemicals from agricultural run-off, human sewage treatment plants, industrial discharges, and stormwater runoff from roads and landfills end-up.  One of the great difficulties for environmental toxicologists is determining, and trying to predict, the impacts of every combination of chemicals when there are thousands released into the environment.

Cocktail, not the good kind. Whitewater River in MN. The area was the site of a large fish kill last year. Officials failed to identify a specific cause and instead suggested a mix of “biological, chemical and environmental conditions” could have been to blame. Alex Kolyer | File for MPR News. Source: https://www.mprnews.org/story/2016/07/13/minnesota-adds-pollution-sources-impaired-waters-list

These chemical mixtures can include pharmaceuticals, such as opioids and antidepressants, hydrocarbons, heavy metals, pesticides, personal care products, and hormones as well as many more.  A study by the U.S. Geological Survey in 1999 and 2000 found medications in 80% of water samples from 139 streams in 30 states, with 75% having more than one of these chemicals and 50% having at least 7. These medications included antibiotics, antidepressants, blood thinners, heart medications, hormones, and painkillers.

Point source of industrial pollution along the Calumet River. Source: https://oceanservice.noaa.gov/education/kits/pollution/03pointsource.html

Since its inception in the 1960’s, the discipline of toxicology has largely focused on characterizing and regulating individual chemicals.  Even today many environmental toxicology studies focus on the effects of individual chemicals or combinations of only two chemicals. One recent study examined the impact of low levels of two pollutants: 1) an antidepressant, which enters the aquatic environment after not being fully broken down in sewage treatment processes and has been seen to accumulate in the brains of fish, and 2) a fungicide, commonly used in agriculture and medicated shampoos and creams.  Individually these chemicals increased the swimming speed of small invertebrates and prevented their food source (leaf litter) from being degraded by fungus, which reduced their ability to eat the leaf litter.  Unexpectedly, when combined swimming speed was much slower than normal and though the invertebrates ate less, the reduced diet was unrelated to the state of their food source.  

Effects of Chemical Interactions
Different types of chemical interactions. Additive = two or more chemicals in combination have an overall effect that is equal to the sum of their individual effects. Synergistic = two or more chemicals in combination have an overall effect that is greater than the sum of their individual effects. Potentiating = a substance or chemical with no effect alone, causes a greater effect in another chemical when combined. Antagonistic= two or more chemicals in combination have an overall effect that is less than the sum of their individual effects.

While current legislation usually focuses on regulating single chemicals rather than chemical cocktails, there have been attempts to address the complexities of chemical combinations.  For instance, an amendment in 2003 to The Clean Water Act of 1972, through the Framework for Cumulative Risk Assessment moved risk assessment away from a single-chemical approach toward cumulative impacts of chemical mixtures with a focus on systems and communities near hazardous waste sites or heavily industrialized areas.  One difficultly is that many of these mixture assessments are still based on the effects of a single chemical and its known or assumed interaction with only one to two more chemicals.   

An example of a potentiating effect is grapefruit juice and a type of medicine known as statins Source: http://www.shieldhealthcare.com/community/health_care_professionals/2017/07/20/how-grapefruit-juice-affects-drugs/

Another challenge of examining these complex mixtures is knowing what to look for. As may be suggested by TV, there is no technique or instrument that can simply test for everything. Each chemical or class of chemicals being measured requires a specific collection technique, preparation process to isolate the chemical of interest, and testing process to measure the concentration present in the sample.  Each chemical you test for also comes with its own cost.  This means that a scientist must have some idea of what to test for before sampling the water and that anything the scientist does not test for will not be seen. 

USGS scientist in a stream with a water-quality sampling bottle on a pole
A USGS scientist collecting a water-quality sample from Boulder Creek, Colorado. The scientist is following a set of protocols to ensure the water sample is representative of all the water flowing down the creek at the sampling station. Photo credit: Jennifer Beck, USGS. Source: https://toxics.usgs.gov/highlights/mixtures/index.html

Despite these limitations, interest and research on chemical mixtures has been intensifying in the toxicology community over the past decade.  As a result, risk assessments are more commonly testing the toxicity of water systems as a whole (whole-effluent testing) and scientific organizations are coming up with lists of chemical mixtures commonly seen together with information on their combined impacts.  However, inventive new techniques will need to be used to understand the chemicals present, their interactions, and the impact of those interactions on our aquatic ecosystems.    

References and reading material:

De Castro-Catala N, Munoz I, Riera JL, Ford AT. 2017. Evidence of low dose effects of the antidepressant  fluoxetine and the fungicide prochloraz on the behavior of the keystone freshwater invertebrate Gammarus pulex. Environmental Pollution. 231: 406-414.

Fleeger JW, Carman KR, Nisbet RM. 2003. Indirect effects of contaminants in aquatic ecosystems. The Science of the Total Environment. 317: 207-233.

Hayes TB, Case P, Chui S, Chung D, Haeffele C, Haston K, Lee M, Mai VP, Marjuoa Y, Parker J, Tsui M. 2006. Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact? Environmental Health Perspectives. 114: 40-50.

Monosson E. 2005. Chemical mixtures: considering the evolution of toxicology and chemical assessment. Environmental Health Perspectives 113: 383-390.

Pal A, Yew-Hoong Gin K, Yu-Chen Lin A, Reinhard M. 2010. Impacts of emerging organic contaminants on freshwater resources: Review of recent occurrences, sources, fate and effects. Science of the Total Environment. 408: 6062-6069.

Relyea RA. 2009. A cocktail of contaminants: how mixtures of pesticides at low concentrations affect aquatic communities. Oecologia. 159: 363-376.






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