What can we learn from 28 years of monitoring of fish tissue polychlorinated biphenyls in Michigan's rivers?

Polychlorinated biphenyls (PCBs) are an important part of chemical legacies in the Laurentian Great Lakes basin. Used in industrial products worldwide, PCBs are now extensively monitored because of their potential toxicity to humans. Fish consumption is a major pathway for exposure. Edible portion (i.e., fish fillet) data from Michigan's fish tissue PCB monitoring program were evaluated using regression statistics, principal component analysis, and t-tests to answer three questions: (1) How do fish tissue total PCB concentrations vary across Michigan's rivers? (2) Are the PCB congener patterns uniformly distributed among tested sites and species? (3) Do monitoring methods limit our ability to discern trends in fish tissue PCB concentrations? Our results indicate that although contaminated sites have been successfully identified, based on higher PCB concentrations in samples from Areas of Concern (AOCs) compared to non-AOC sites, 77% of fish samples from 2010 to 2015 exceeded the safe fish tissue PCB concentration for unrestricted consumption (97 g/day) by sensitive populations. The PCB congener profiles vary among species and locations. Results demonstrate that these data are not useful for supplementing ongoing spatial and temporal trend analysis. Only 15 of the 83 species + waterbody pairs had adequate data for evaluating temporal trends with more than three data points. In general, the trends at each location varied based on the analytical method. Conclusions from this work can inform revisions to existing monitoring programs and improve our ability to protect human health. Integr Environ Assess Manag 2023;19:152-162. © 2022 SETAC.

Global and Local Impacts of Delayed Mercury Mitigation Efforts.

Mercury (Hg) is emitted to air by natural and anthropogenic sources, transports and deposits globally, and bioaccumulates to toxic levels in food webs. It is addressed under the global 2017 Minamata Convention, for which periodic effectiveness evaluation is required. Previous analyses have estimated the impact of different regulatory strategies for future mercury deposition. However, analyses using atmospheric models traditionally hold legacy emissions (recycling of previously deposited Hg) constant, and do not account for their possible future growth. Here, using an integrated modeling approach, we investigate how delays in implementing emissions reductions and the associated growing legacy reservoir affect deposition fluxes to ecosystems in different global regions. Assuming nearly constant yearly emissions relative to 2010, each 5-year delay in peak emissions defers by additional extra ca. 4 years the return to year 2010 global deposition. On a global average, each 5-year delay leads to a 14% decrease in policy impacts on local-scale Hg deposition. We also investigate the response of fish contamination in remote lakes to delayed action. We quantify the consequences of delay for limiting the Hg burden of future generations and show that traditional analyses of policy impacts provide best-case estimates.

Mining legacy across a wetland landscape: high mercury in Upper Peninsula (Michigan) rivers, lakes, and fish.

A geographic enigma is that present-day atmospheric deposition of mercury in the Upper Peninsula of Michigan is low (48%) and that regional industrial emissions have declined substantially (ca. 81% reduction) relative to downstate. Mercury levels should be declining. However, state (MDEQ) surveys of rivers and lakes revealed elevated total mercury (THg) in Upper Peninsula waters and sediment relative to downstate. Moreover, Western Upper Peninsula (WUP) fish possess higher methyl mercury (MeHg) levels than Northern Lower Peninsula (NLP) fish. A contributing explanation for elevated THg loading is that a century ago the Upper Peninsula was a major industrial region, centered on mining. Many regional ores (silver, copper, zinc, massive sulfides) contain mercury in part per million concentrations. Copper smelters and iron furnace-taconite operations broadcast mercury almost continuously for 140 years, whereas mills discharged tailings and old mine shafts leaked contaminated water. We show that mercury emissions from copper and iron operations were substantial (60-650 kg per year) and dispersed over relatively large areas. Moreover, lake sediments in the vicinity of mining operations have higher THg concentrations. Sediment profiles from the Keweenaw Waterway show that THg accumulation increased 50- to 400-fold above modern-day atmospheric deposition levels during active mining and smelting operations, with lingering MeHg effects. High MeHg concentrations are geographically correlated with low pH and dissolved organic carbon (DOC), a consequence of biogeochemical cycling in wetlands, characteristic of the Upper Peninsula. DOC can mobilize metals and elevate MeHg concentrations. We argue that mercury loading from mining is historically superimposed upon strong regional wetland effects, producing a combined elevation of both THg and MeHg in the Western Upper Peninsula.