Chemical Pollution in the North Branch?
by Tom Wessels
Editor’s Note: This article is based on an analysis of the USGS report on contamination from organic chemicals which was published earlier this year. (The study was conducted in 2018 but was not published until February 2020). The report is available for download at: https://doi.org/10.3133/sir20205002
The long-term gradual decline in trout populations in the North Branch is certainly no secret, and has been fairly well documented. But May, 2018 saw a dramatic reduction in trout numbers that was, and remains to some extent, a mystery. What caused this rapid decline? Was it a lack of food? Disease? Predators? Warmer temperatures (and reduced oxygen levels) the previous summer? Winter cold and ice? Chemical pollution?
These questions spurred a number of research projects. Michigan DNR “spot checked” six sites on the North Branch and verified that there were indeed “much lower densities of trout”, and Michigan DEQ (now EGLE) surveyed three sites and found them “excellent for both habitat and macroinvertebrates”. In addition, several brook trout were captured live and transported to MSU for disease testing. The upshot of the DNR’s report was that the 2018 decline in trout population was probably not caused by:
· Predators, since there were no reports of unusually high number of mergansers or otters;
· Chemical dumping or a spill, since there were no reports of a large die-off of fish; or
· Disease, since test results at MSU did not indicate pathogens as the cause.
The report concludes by saying, “High flows [due to heavy rains and snowfall] were likely the most important contributor to the trout population decline [in spring, 2018].”
But, bear in mind that scientific studies of unusual events can rarely determine a single cause, and never with absolute certainty. Scientists always talk in terms of probabilities or likelihoods. And there are still some factors that can’t be ruled out as possible causes for the decline – short-term or long-term. While the biodiversity of macroinvertebrates appears high, we don’t know whether there are enough total insects – called “biomass” – to sustain a healthy trout population.And while trout may be reproducing adequately, fewer of them seem to be growing into adulthood, a process called “recruitment”.These are two aspects of a study currently ongoing by Dr. Mark Luttenton, of Grand Valley State University.Glen Eberly has written extensively about this study previously (see “So Where Are the Trout?” in our Spring/Summer 2019 newsletter).
Another factor that may be playing a role in the trout population decline is the presence of organic chemical pollution in the North Branch. This was the focus of a study conducted in June, 2018 by the US Geological Survey in collaboration with Lovells Township and the Mason Griffith chapter of TU. Passive sampling devices were deployed in the river at four locations – Bent Tree Drive, Ford Road, Twin Bridges Road, and Kellogg Bridge – for a period of 28 days.
Cover of USGS Report showing sampling devices placed in the river
Keep in mind that testing for organic chemicals is a very complex process for a number of reasons.
· There are so many of them! The USGS study tested for 97 different compounds.
· They are often present in such small concentrations – down to parts per quadrillion, or pg/L – beyond the detection limits of our instruments.
· Some of these chemicals tend to degrade into other compounds over time. For example DDE is a common product of the breakdown of the banned pesticide DDT.
· Many of these chemicals tend to increase in concentration as you move up the food chain from plants à insects à fish. This process is called “bioaccumulation”.
· Acceptable concentrations, or “benchmarks” for many of these compounds have not been established (it’s costly). And, when they have been established, they can be different depending on whether exposure is due to a one-time event (an “acute value”, usually tested over a 96 hour time span) or accumulated over an organism’s lifetime (a “chronic value”). They can also be different for human consumption versus for fish/aquatic life. In general, benchmarks for acute exposure tend to be much greater than those for long-term (chronic) exposure. And benchmarks for human consumption tend to be the lowest.
· Finally, benchmark concentrations are calculated for individual chemicals. Their effects can be much greater when they are present in combination with other compounds. This is known as the synergistic, or “cocktail” effect. But it’s difficult and costly to actually test – think of how many combinations are possible between the 97 compounds tested in this study! To try and predict the cocktail effect of a mix of chemicals, scientists at EPA and USGS have complex mathematical models.
This study tested the North Branch for 97 organic compounds which are grouped into four categories:
· Pesticides, both “legacy” pesticides (banned in the US) and “current use”;
· Polychlorinated byphenyls (PCBs) which are used in electrical transformers and capacitors,
· Polybrominatied diphenyl ethers (PBDEs), which are still being produced and have been used in flame retardants and a variety of other products such as plastics and electrical devices, and
· Polycyclic aromatic hydrocarbons (PAHs) which are released from burning coal, oil, gasoline, diesel fuel, wood and garbage.
Potential sources for these compounds could be golf courses, private residences, gas and oil production wells, the atmosphere, or Camp Grayling.
According to the data tables in the study, of the 97 compounds tested for, only 21 were detected above “quantitation limit”, in other words, at levels that can be reliably and precisely measured (at least at one of the four sites).
· No PCBs were detected,
· One (1) PBDE was detected (no threshold value known),
· Nine (9) PAHs were detected, none above known threshold values,
· Three (3) current use pesticides were detected, none above known threshold values, and
· Eight (8) legacy pesticides were detected, one of which was present in concentrations above EPA threshold for human consumption.
In conclusion, the study states that although “low concentrations of organic chemicals may still pose a risk to aquatic organisms because of low-dose additive and synergistic effects … and a lack of established water-quality benchmarks, … the organic chemical concentrations detected on the North Branch Au Sable River were relatively low and below almost all water quality benchmarks. These data establish current conditions of organic chemicals … against which future data can be compared.”