Persistent Toxics Have Declined Over Past 30 Years
Uncertainties Remain about Impact of Novel Contaminants
WHY IT MATTERS
Our society releases thousands of biologically active chemicals into the environment. These compounds include medications, pesticides, industrial chemicals, and byproducts of combustion, corrosion and wear, among others. Such compounds are called toxic when they have negative impacts on human health or the environment, especially if they have negative effects at low environmental concentrations.
By definition, toxic chemicals can harm living organisms, including humans, animals, plants, and microorganisms. Contaminants can have both severe short-term and subtle long-term effects. Effects include mortality, neurological impairment, developmental effects, impacts to metabolic processes, increased risk of genetic mutations and cancer, disruption of hormonal systems, and changes in behavior. Toxic contaminants in the environment can affect success of individuals and species, thus altering composition of ecological communities and ecosystem processes.
Many organisms can concentrate persistent contaminants in their bodies, increasing contaminant exposure to organisms who eat them. In marine food webs, that process can occur repeatedly as predator becomes prey, in a process called “biomagnification”. Biomagnification means that even low environmental levels of persistent contaminants can pose health risks to organisms (from osprey and seals, to humans) who eat large, predatory fish.
Industrial uses of many persistent contaminants (like DDT and PCBs – See the sidebar for information on persistent toxic compounds) were phased out in the 1970s and 1980s, leading to reductions in environmental concentrations. We see the effects of those bans in long-term trends in levels of persistent contaminants in Casco Bay. Yet use of less-persistent toxic chemicals (including novel industrial chemicals, modern pesticides, pharmaceuticals and personal care products) has expanded. For many bioactive compounds, relatively little is known about their persistence or impacts in the marine environment.
Persistent Organic Contaminants
PAHs: Polycyclic aromatic hydrocarbons are principally derived from petroleum products or combustion of petroleum or other organic materials. Transportation and industrial activity are major sources.
PCBs: Polychlorinated biphenyls are chlorinated hydrocarbons that found a variety of industrial uses 50 years ago. Because of their toxicity, their use was phased out in 1979. Today, PCBs may be released when older products are improperly disposed of, or as a byproduct of poorly controlled burning of plastics.
Dioxins and furans: Dioxins and furans are not industrial products in their own right, but byproducts of incomplete combustion of organic matter and some industrial processes such as chemical manufacturing and chlorine bleaching of pulp and paper.
Butyltins: Tributyltin is a metal-organic molecule used in anti-fouling paint on large ships. Because of its toxicity and persistence in the marine environment, its use on smaller boats has been phased out. In the marine environment, it breaks down into related mono- and di-butyltins, so the group of related compounds are often studied together.
DDT Residues: DDT is well known as one of the more troublesome members of a class of pesticides known as organochlorines. Other members of this class include chlordane and lindane. Most organochlorine pesticides were banned decades ago because of their persistence in the environment. Like many organochlorines, DDT breaks down into closely related chemicals in the environment.
Benchmarks for risk assessment
A common way to evaluate risks associated with contaminants in sediments is to compare concentrations to published risk benchmarks, such as ERL and ERM:
- Effects Range Low (ERL) – A concentration below which negative effects on marine organisms are seldom observed.
- Effects Range Median (ERM) – A concentration at which negative effects are about as likely as not.
STATUS & TRENDS
Persistent Organic Contaminants on the Decline in Casco Bay’s Sediments
Sediment Contamination Tends to Be Highest Near Portland
Almost All Metals Decreasing in Sediments
Many metals are toxic and all are persistent in the environment. As each poses different risks, they need to be assessed separately. Almost all metals are decreasing in Casco Bay’s sediments. None were observed at levels above the ERM threshold, and thus levels are thought to pose limited risks to marine organisms. Persistent elevated levels of arsenic reflect presence of natural sources in the Casco Bay watershed. Elevated mercury bay-wide reflects atmospheric deposition.
Results from Casco Bay Sediment Sampling (1991-2011)
Deep Sediments of Portland Harbor Are More Contaminated
In 2018, the Portland Harbor Commission commissioned a study of contaminants in sediments of Portland Harbor. The study was undertaken as part of designing a “Confined Aquatic Disposal” facility to safely dispose of potentially contaminated dredged material from the Harbor. The results provide a window into historic levels of contaminants in the Harbor, because samples were collected from material buried under several feet of sediment, which has accumulated over many decades. The deep sediments of the Harbor act as a time capsule, revealing levels of contamination that were commonplace in the harbor 50 years ago, before passage of major environmental laws.
The highest levels of contaminants were generally found along the Portland waterfront. That partially reflects the waterfront’s industrial heritage but also decades of runoff from the City and uncontrolled wastes discharged directly to the Bay before construction of wastewater treatment plants.
Levels of organic contaminants (first graph) were substantially higher than elsewhere in Casco Bay, and also higher than in shallow sediments from Portland Harbor. (Note: The y-axis is on a log scale.) Over half the samples had concentrations of PAHs at or above levels of concern. Levels of PCBs are less extreme, although still higher than elsewhere in Casco Bay. DDT residues are consistently at levels of concern, an especially striking result, as the pesticide has been banned for decades, probably because of DDT’s long environmental persistence and low mobility in the Harbor’s sediments. Levels of metals (second graph) in Portland Harbor deep sediments regularly exceeded screening levels. Concentrations are often several times higher than seen in Casco Bay’s shallow sediments. Levels for mercury, copper, lead, and zinc, regularly exceeded levels of concern.
Urbanized Shores Have Higher Levels of the Pesticide Bifenthrin
Many persistent toxic chemicals, including DDT and most other organochlorine pesticides, were restricted or outlawed in the later part of the twentieth century. As a consequence, levels of such persistent toxics have declined in the environment. Novel compounds, such as less persistent pesticides, have come to replace outlawed or restricted chemicals in many uses. Replacement chemicals can pose lower environmental risk, but they are not risk-free, and many are less studied.
Because lobsters and other crustaceans may be vulnerable to certain pesticides, Maine’s Board of Pesticides Control conducted studies of fourteen “pyrethroid” pesticides, in Maine’s intertidal marine sediments in 2014 and 2015, including 18 locations around Casco Bay. Only bifenthrin, which is relatively easy to detect at low concentrations, was detected regularly. Bifenthrin (first approved for use in the U.S. in 1985) was detected in most samples, although at levels well below levels of concern.
As shown in this graph, the levels of bifenthrin are related to urbanization. Higher levels were observed when impervious surfaces like roads and parking lots were common within 500 meters of the sampling location. The relatively high sample from Yarmouth from 2014 is unexplained.
Many persistent toxic chemicals, including DDT and most other organochlorine pesticides, were restricted or outlawed in the later part of the twentieth century. As a consequence, levels of such persistent toxics have declined in the environment. Novel compounds, such as less persistent pesticides, have come to replace outlawed or restricted chemicals in many uses. Replacement chemicals can pose lower environmental risk, but they are not risk-free, and many are less studied.
Because lobsters and other crustaceans may be vulnerable to certain pesticides, Maine’s Board of Pesticides Control conducted studies of fourteen “pyrethroid” pesticides, in Maine’s intertidal marine sediments in 2014 and 2015, including 18 locations around Casco Bay. Only bifenthrin, which is relatively easy to detect at low concentrations, was detected regularly. Bifenthrin (first approved for use in the U.S. in 1985) was detected in most samples, although at levels well below levels of concern.
As shown in this graph, the levels of bifenthrin are related to urbanization. Higher levels were observed when impervious surfaces like roads and parking lots were common within 500 meters of the sampling location. The relatively high sample from Yarmouth from 2014 is unexplained.
Low and Decreasing Contaminant Levels in Casco Bay Blue Mussels
Contaminants in the ocean can pose human health risks if they find their way into seafood. Maine’s Surface Water Ambient Toxics (SWAT) program collects data on persistent contaminants in several marine species, including blue mussels. Mussels are filter feeders, and can concentrate contaminants from their surroundings. National and regional programs have tracked contaminants in mussels and oysters for many years. CBEP analyzed Casco Bay mussel data in relation to Gulf of Maine (Gulfwatch) and National (NOAA) benchmarks. Benchmarks used here (85th percentiles) show levels exceeded by 15% of observations from regional and national studies.
successes & challenges
- Concentrations of most toxic chemicals that have been studied in Casco Bay are below established levels of concern. Exceptions tend to occur principally, but not exclusively, in our more urban waters. Most persistent contaminants are less abundant today than in prior decades.
- Many persistent toxic chemicals, like PCBs and certain pesticides, were phased out in the late 20th century. Those policies led to reductions in concentrations in the Bay. Reductions of other toxic chemicals may reflect better stormwater management or reductions in tailpipe emissions from more fuel efficient, cleaner automobiles.
- Mercury still occurs at levels of potential concern for marine organisms in sediments across much of the Bay. A primary source of mercury entering the Bay is atmospheric deposition, associated with discharges from power plants to our west. Conditions may improve as coal-fired power plants are retired and replaced by natural gas and renewables.
- Historically, data collection on toxic contaminants in the marine environment has focused on persistent compounds which were identified as problematic in the 1960s and 1970s. Data on modern contaminants of concern, from flame retardants to microplastics, pharmaceuticals to personal care products, remains scarce.
View a PDF version of this page that can be downloaded and printed.
View references, further reading, and a summary of methods and data sources.
STATE OF CASCO BAY
If you would like to receive a printed State of Casco Bay report, send an email request to cbep@maine.edu.
This document has been funded by the U.S. Environmental Protection Agency under Cooperative Agreements #CE00A00348-0 and #CE00A00662-0 with the University of Southern Maine.
Suggested citation: Casco Bay Estuary Partnership. State of Casco Bay, 6th Edition (2021).