Near the end of 2014 a tsunami-like wave of concern swept over the Glenville community; mercury (Hg) sufficient to merit posting of human health consumption advisories had been measured in Lake Glenville walleye. It should not have been a surprise.
State-wide advisories for largemouth bass have been in effect on all NC lakes since 2006, and had been extended to walleye in several regional lakes as more were tested. Still, I was surprised at the very high Hg concentration that had been found in our walleye. The average Hg content of 20 walleye from Lake Glenville (1.67 parts per million) is 75% to 150% greater than that of other lakes in the area that have been tested, more than three and one-half times the threshold for ocean- caught market fish to be avoided, and in fact higher than the average for any market fish that has been reported by the FDA.
Although NCDHHS estimates that in NC over 13,000 newborn children per year have blood levels of Hg that put them at risk of neurological deficits such as learning disabilities, fine motor and attention deficits, and lowered IQ, some folks deny that there is a problem. Herein I will try to deepen member's understanding of our mercury situation. Perhaps such a review should be written by a medically trained person, but the issue also involves the physics, chemistry, and biology of the lake, so I will presume.
Hg is a naturally occurring metal in the earth's crust. More dense than lead, it is about as plentiful as silver, and about 20 times more plentiful than gold. Unlike our experience with other metals, it is a dense liquid at temperatures above -39° (either F or C) and, like other liquids, it vaporizes, to become airborne. Volcanoes, forest fires, coal burning and other industrial processes all release Hg in elemental, ionic, and particulate forms into the atmosphere where it slowly falls out over six months to a year, and hence can be carried and deposited as either dry deposition or washed out in rainfall (wet deposition) at great distances. It has been used in industrial processes and consumer products like dental fillings, mercurochrome, medical thermometers, electrical switches, florescent light bulbs, etc., for many years. Its occurrence in the surface environment has increased by a factor of 3 to 5 since preindustrial times. It has been much-used in mining operations, mostly in the west, but also in north Georgia, and if there were any of those in the Lake Glenville flooded area or watershed, there may still be some Hg leaching out from those sites.
I remember, as a young sprat growing up on a farm in the far northwest, coating seed potatoes with a Hg-based fungicide before planting. That procedure probably was used here in NC also, and there may still be some residues hanging around and leaching into our waterways. However, I do not believe that such legacy sources are of importance in Lake Glenville. Along with waste incineration, most uses have been phased out or banned in recent decades. Smokestack emissions from coal- burning electrical power plants are the major remaining cultural enhancement of environmental Hg. Coal contains only about 0.2 parts per million Hg, but a lot of coal is burned, and with Hg small concentrations matter.
All heavy metals are known for their toxicity, and Hg is the most dangerous of the lot. It has no known human metabolic function, but it is a potent neurotoxin in all its molecular forms. Most public notice has been in connection with its neurological effects, especially among developing children or fetuses, but Hg poisoning is also a causal or aggravating factor in adult maladies like hypertension, cardiovascular, and renal afflictions. Early neurological symptoms of Hg poisoning are loss of motor skills and dulled sense of touch, taste, and sight. Extreme cases can be crippling or fatal.
Brief contact with liquid elemental Hg is not notably dangerous. I played with some found in an unused high school chemistry lab. It would coat pennies or other metallic objects to bright silver color, and could be “cooked-off“ with a candle flame. The cooked-off vapor is very dangerous. Luckily, I lost interest in it early on; in later years two of my professional colleagues suffered debilitating effects from breathing Hg vapor in poorly ventilated laboratories. The principal remaining human exposures to Hg poisoning are respiration of elemental Hg vapor and, most commonly, consumption of Hg-contaminated seafood.
The greatest exposure to humans results from Hg that is deposited from the atmosphere and finds its way into water bodies, where microorganisms use it in their metabolism, converting it to a different form, methyl mercury (MeHg). Hg in this form is far more toxic, evidently because its molecule mimics a common amino acid, enabling it to cross the blood-brain barrier, where it resides for a long time with destructive effects, and the placental barrier to affect the fetal brain The biological half-life of MeHg is 2 to 3 months. Older and larger piscivorous fish typically are more contaminated than smaller and short-lived omnivores. Minute amounts of MeHg are usually tolerated by organisms, but its persistence means that as small organisms are eaten by progressively larger creatures their MeHg content is cumulative, or biomagnified, so the largest and oldest fish, like the humans who consume them, get the largest dose and also accumulate MeHg. Biomagnification factors exceeding one million are not uncommon.
Another name for acute MeHg poisoning is Minimata disease, named for a coastal region in Japan where over 2000 people were afflicted, of which more than a thousand died, from eating MeHg-contaminated fish. Similar but lesser disasters have occurred in Canada and Iraq. It is commonly said that terrestrial animals are not subject to MeHg contamination. That is mostly true; you will not experience Hg damage from eating pheasant or venison, as these animals are herbivores, but there are exceptions. Fish-eating animals, like seals, otters, or osprey, understandably suffer the effects of Hg poisoning. There are consumption advisories for some species of wild ducks harvested on peripheral waters of the Great Salt Lake, which has extremely high Hg concentration. Another interesting example is the Carolina wren. It accumulates MeHg by eating spiders, which in turn biomagnify MeHg by preying on insects. Early effects observed in such species often include reduced reproductive success.
I have found no reports of measurements of HG in water from Lake Glenville, or any other lakes of the Southern Appalachian region. Only about 10% of the Hg deposited on the surrounding forest lands finds its way into lakes. The major part is either stored in the trees and soils or re-emitted to the atmosphere. Using best guesses based on mercury deposition data from a site in the Great Smokey Mountains National Park, the area and volume of the lake, its watershed area, and several research papers, I estimated the mercury concentration that could be expected in Lake Glenville as about 2.2 nanograms per liter (ng/l). For those not conversant with nanograms or liters, that is the equivalent of dispersing about three- fourths cup of liquid mercury into the full pool volume of Lake Glenville. My estimate of 2.2 ng/l is not a particularly high value. It is 20% less than the average of values measured by the USGS and Canadian counterparts in 277 lakes of New England and eastern Canada. All of these states and provinces issue fish consumption advisories including total avoidance because they have some lakes with Hg concentrations approaching 20 ng/l. Even these levels pose no threat to human health from contact with the water. The EPA standard for drinking water is 2000 ng/l. The Hg content reported for our walleye amounts to a magnification factor of about 750,000 relative to my estimate for Hg in Lake Glenville water. Considering that about 10% of Hg in lake water is in the MeHg form, but 90% of that in fish is MeHg, that highly toxic form is magnified by nearly 7 million.
Several factors converge to make Lake Glenville and other regional reservoirs especially susceptible to occurrence of MeHg. First, Hg emissions are a global problem (yes, some of our Hg originated in China), but sites near a source experience more exposure than do more distant areas. The Tennessee Valley region has numerous coal-fired power plants and our heavy rainfall provides ample local washout of atmospheric Hg. Also, we have frequent thunderstorms which extend to greater height in the atmosphere than other rainstorms, and bring down more Hg from more distant sources. The bacterial conversion of Hg to MeHg is enhanced in oligotrophic lakes with low or no dissolved oxygen and low pH (i.e., acidic water). All these conditions are characteristic of Lake Glenville, and probably of all deep reservoirs in the Southern Appalachians. Our lake is manifestly oligotrophic; it is always acidic due to the acidity of our rain and the chemistry of the regional rock and soil. Notably, the same power plants that release Hg also spew carbon dioxide which increases the acidity of rain, and of our lake. At the end of every summer water at depths greater than about 30 feet contains little or no oxygen. The bottom sediments probably are perpetually anoxic, exactly the conditions the troublesome microorganisms need to convert Hg to MeHg. Lake Glenville may in fact have a more favorable outlook than some other regional reservoirs. Due to activation of the Hg stored therein by flooding of uplands, young reservoirs typically experience elevated Hg concentrations in water and fish for up to 30 years or more. At age 76 our reservoir should be well along with disposal of that initial charge.
Another factor, particular to Lake Glenville, is that due to years of spawning failures, all, or virtually all, of our walleye are 9 years or more of age. By having consumed more and larger prey, older and larger fish typically have accumulated a higher body burden of Hg than younger fish, so our walleye population has a higher average level of Hg than would a population with a more normal age structure. It is also conceivable that the spawning failures might in turn be a result of Hg contamination. As noted above, reproductive failures have been seen to result from Hg poisoning in other species. The recent advent of spawning failures suggests a recent increase of Hg concentration in the lake, perhaps due to change of lake management practice. Two possibilities come to mind. Prior to about year 2000 the lake had been lowered in winter to about the 80 foot datum, but under the recently negotiated agreement Duke Energy lowers it to only the 90-foot level. In consequence the nominal “flushing” of the lake has been reduced from about 40% to 20%. The total flushing is of course strictly determined by the amount of rainfall received, so what really has changed is the timing of the flushing process. Also, perhaps due to anomalous weather, early season water levels have been higher than historical, which increases erosion of the shoreline and falling of trees into the lake, releasing the Hg contained therein. Or, it may be that the Hg concentration in the region is in fact increasing, and began exceeding a critical threshold for walleye about a decade ago in Lake Glenville.
Mercury contamination is a global problem, and applies to market fish, mostly marine, as well. Over one-third of US exposure to Hg is believed to be due to consumption of tuna. Tuna are only moderately high in MeHg, but a lot of tuna is eaten. Over the last 50 years large reductions of mercury releases have been made in both North America and Europe, reducing Hg inputs to the North Atlantic Ocean by both atmospheric deposition and coastal and riverine waste discharges. In consequence, an 80% reduction of Hg has been measured in North Atlantic waters between 1990 and 2009. This is similar to the 85% reduction of lead between 1979 between 1998 and 2008 following the phaseout of leaded gasoline. In consequence, scientists from the Scripps Institution of Oceanography reported results from a review of 2662 analyses of Hg in fish done between 1969 and 2012. They found that the concentration of Hg in fish is decreasing, most notably in the North Atlantic, but that there are large regional differences. Part of the decrease of Hg concentration in the North Atlantic may be due to vertical mixing which is particularly vigorous there. Closer to home, it was reported in 2016 by staff of NOAA and Duke Univ. that bluefish, a popular coastal sport fish known for accumulating Hg and other pollutants, caught in North Carolina coastal waters in 2011 contained 43% less Hg than the same species collected in the same way during 1972. The Hg content is still too high, but 10% per decade reduction came as a pleasant surprise. On the other hand, it was recently found that Hg in tuna from the Pacific Ocean have been increasing at a rate of 3.8% per year since 1998, resulting in a near doubling of their Hg concentration. This increase of Hg in Pacific tuna is attributed to the increased Hg releases associated with rapid industrialization in east Asia.
A report on temporal trends of wet deposition of Hg using data from U.S. and Canadian mercury deposition networks published in October 2016 reveals that wet deposition of Hg had declined overall from 1997 to about 2008, then, up to 2013, it became more or less steady. Results are still uncertain because the data are highly variable, and the term short, but they support an hypothesis that post-2008, deposition continued to decline as a result of reduced emissions in industrial areas of the U.S., but began to rise in mountainous rural areas of the west due to global atmospheric transport from rapidly developing parts of the world. The nearest and most relevant measurement site to Lake Glenville is in the Great Smokey Mountains National Park, where deposition is found to be increasing at 1% to 2% a year recently.
The basis for fish consumption advisories for Lake Glenville merit special consideration. Such advisories have been vigorously criticized, mostly by the seafood industry, but also by power companies and their advocates due to the implied threat of restrictions on burning of coal. Because great individual variability exists of both the accumulation of MeHg from diet and the response to dosage, setting consumption limits is difficult. Toxicity experiments cannot be done on human populations. Data from extreme poisoning events like Minimata or Iraq identify the results of gross exposure, but the exposures resulting in gross effects were not well documented. Studies of populations with heavy dependence on seafood of the Faroe and Sechelles Islands and New Zealand have provided the basis for evaluating the effects of low-level chronic dietary exposure. The problem is complicated by the facts that there probably is no exposure to MeHg below which there is zero adverse effect, and that there are no human populations with zero exposure to Hg. It has been determined that the most sensitive susceptibility is to neurological damage to fetuses and children. Standards were identified at which no more than 5% of additional children above the most sensitive 5% should suffer adverse effects at the highest level of consumption. Then an extra factor of 10 was applied to allow for uncertainty of the determination. It has been necessary to assume that modest relaxation of advisories based on neurological development of children will provide an adequate safety margin for other, adult, effects.
There is some merit in the argument from the seafood lobby that seafood contains beneficial nutrient elements such as omega-3 that are essential to fetuses and children. Some of these elements have in fact been found to offset the detrimental effects of MeHg. This benefit is limited to low dosage however, while adverse effects of MeHg continue to increase with dosage. The beneficial effects of eating seafood have been evaluated by the FDA and are factored into the most recent advisories.
The question has been asked “What can we, the FLG or its members, do to help alleviate the problem?” We cannot change the physical and biochemical laws that move Hg through the ecosystem. An easy start would be to support or seek support for some actual measurements of Hg in lake water to help define the problem. Stocking to restore the walleye population to a more normal age structure might reduce the average Hg concentration to below the State action level (take away the signs at the boat ramps) as well as improve the walleye catch, but the body burden of the most-sought older and larger fish would likely be as great as at present.
I have seen virtual floating islands of fallen leaves that Lakeside property owners had blown off their property onto the lake. Bad practice! The largest component of mercury deposition into the surrounding forest is from litterfall (leaves, twigs, blossoms and nuts/fruits), but only about 10% of the Hg content of this material finds its way into the lake. If dumped directly into the lake 100% soon sinks and provides the resource for microorganisms to deplete the oxygen and convert the contained Hg to MeHg. Also of course, take care with disposal of fluorescent light bulbs and other Hg-containing refuse.
A geoengineering solution that has been applied in some lakes would be to eliminate the seasonal anoxia condition by pumping oxygen deep into the lake. This technology would benefit our cool and cold water fishes, walleye and trout, that are seasonally pinched between near-surface waters that are too warm, and deeper, cooler waters that are oxygen deficient, but is certainly beyond the resources of the FLG, and probably would be a hard sell to the County, State ,or Duke Energy.
Perhaps the most important action is to persuade our congressional and state representation that elimination or enervation of the EPA and other environmental agencies is not a good idea, or to replace these officials with more enlightened individuals. To expect that state-level government can or will prevail over the interests of big business, foreign governments, or even other states, is naive or disingenuous. The global community has been reasonably cooperative in regard to mercury and some other environmental pollutants, but, if the U.S. defects, probably all is lost.
As a boon to fishermen, every bass or walleye taken out of the lake carries its burden of MeHg with it, a singularly fun way to take mercury out of the lake. Catch all you can; just be careful of how many you eat, and don't forget about the tuna sandwiches and any other Hg-laden items tin your diet.
Donald Hansen, PhD
Dr. Hansen holdes a BS in physics and an MS and PhD in physical oceanography from the University of Washington in Seattle. Throughout the course of his career, he has served as a meterological officer in the US Army, an engineer with Boeing, a science teacher, a researcher with the National Oceanographic and Meteorologicial Administration (NOAA) and adjunct professor at the University of Miami. He began summering at Lake Glenville in 1993 where, in 1996 and to satisfy his own curiosity, he began making simple water quality measurements. Now in league with FLG, these measurementshave progressively improved in number, scope, and instrumentation. His findings can be found here on the FLG website and in the FLG Archives.