Mercury pollution has detrimental effects on ecosystems and on human health. This case study describes the efforts of the South River Science Team to characterize industrial mercury pollution in the South River near Waynesboro, VA. The team studied the movement of mercury through the river ecosystem for 6 years and used their findings to help design remedial projects to reduce mercury exposure to humans and wildlife. This case study can be used to introduce concepts of mercury pollution, fate and transport, and the decisions involved in designing environmental remediation projects. Each section of the case study ends with a series of discussion questions meant to lead into the next section.

Introduction

Mercury pollution—caused by industrial disposal, the burning of fossil fuels, or mining operations—is everywhere. Air, land, and water can be affected by this dangerous heavy metal, and it threatens the health of both people and wildlife. Mercury is the only metal that is liquid at room temperature, giving it a number of properties useful to industrial applications. Although mercury use has been phased out of most consumer electronics, including light bulbs and electrical switches, mercury still finds frequent use in the chemical manufacturing industry. Its ability to mediate certain chemical reactions makes it a useful catalyst.

Ingestion of mercury can lead to a number of debilitating health effects. In the 1950s, residents of the town of Minamata in southwest Japan became afflicted with a mysterious disease that caused birth defects, neurological degradation, paralysis, and death. In 1956, it was discovered that the cause of the disease was mercury pollution in Minamata Bay, which bordered the town [1]. The Chisso Corporation, a chemical manufacturer, used mercury as a catalyst in the production of acetaldehyde (a precursor to a number of other products, such as acetic acid) and dumped their spent mercury into the bay. Subsequently, mercury accumulated in the tissues of fish and shellfish, which were an important part of the diet for the Minamata Bay community. This ultimately caused mercury poisoning for many individuals of the community. This was the first known large-scale mercury pollution disaster, and those afflicted with severe mercury poisoning were diagnosed with what came to be known as “Minamata disease.” In 2001, the Japanese government officially recognized 2,265 Minamata disease victims (of whom 1,784 died) [2].

Another mercury poisoning event occurred in 1971, when wheat grain seeds coated with a fungicide containing mercury were shipped to Iraq from the United States and Mexico. The Iraqi government received and distributed these seeds to rural areas. The grain was not recommended for direct consumption—the seeds were meant to be planted—but some farmers, either choosing to ignore the warning or unaware of the risks, consumed the grain anyway, causing mercury poisoning symptoms similar to those suffered by the residents of Minamata, Japan. Over 6,000 cases of mercury poisoning and 459 deaths were reported, with over a third of the cases affecting children under 10 years of age [3].

Because exposure to mercury can cause a high risk to human and ecological health, areas that become contaminated with mercury can become serious problems. This case study describes mercury contamination caused by industrial pollution in the South River of Virginia, USA. Scientists and regulators carried out multiple studies to learn about the extent and the risks of the issue and used the data to make decisions about how to clean the pollution and protect the health of people and the environment.

Case Examination

Part 1: Historical Background

Waynesboro is a small town within the Shenandoah Valley of Virginia. It is located about 100 miles west of the state capital of Richmond and is close to a number of tourist destinations such as the Appalachian Trail, the Blue Ridge Parkway, and Shenandoah National Park. The South River runs through the town, sourced from the streams and springs in the surrounding Blue Ridge Mountains. The South River flows north to meet with the North and Middle rivers to form the South Fork of the Shenandoah River, which flows into the Potomac and ultimately into the Chesapeake Bay (figure 1). Waynesboro is the home of a synthetic fiber manufacturing facility, which was formerly owned by the DuPont Corporation, and is situated on the banks of the South River.

Figure 1.

Map of the South River and the South Fork of the Shenandoah River. Areas affected by a fish consumption ban (red) and fish consumption advisory (orange) are displayed.

Figure 1.

Map of the South River and the South Fork of the Shenandoah River. Areas affected by a fish consumption ban (red) and fish consumption advisory (orange) are displayed.

Between the years 1929 and 1950, industrial production at the DuPont plant used mercury compounds as a catalyst in the production of Orlon fiber. This operation generated mercury-containing sludge, which was inadvertently released into the South River [4]. The Orlon production process changed in 1950, and the company stopped using the mercury catalyst. DuPont later discovered mercury in the soil around the plant during construction activities in 1976. Mercury is regulated as a toxic waste by the U.S. Environmental Protection Agency (EPA), so DuPont initiated studies to determine the extent of mercury contamination. Chemical analysis of the soil around the site revealed that there were concentrations of mercury exceeding 7,000 mg/kg (7,000 parts per million (ppm)) [5]. Soil with concentrations this high is classified as a toxic waste.

There were few environmental regulations during the time period in which DuPont contaminated the South River. The Resource Conservation and Recovery Act (RCRA), which gave the EPA the authority to control hazardous waste from “cradle-to-grave” and force industries to clean up pollution, was not enacted by Congress until 1976 [6]. Also in 1976, the World Health Organization published its first program on mercury safety. The lack of regulations, as well as the lack of knowledge of the health effects of mercury, prevented DuPont from making appropriate disposal decisions. This jeopardized the health of the people and the wildlife surrounding the South River.

The Virginia Department of Health responded to the findings of the mercury pollution of the South River in 1977 by enacting a fish consumption ban along over 200 km (125 miles) of the riverbank, affecting a large portion of the Shenandoah Valley [7]. The ban extended from Waynesboro to Front Royal, affecting the South River and the South Fork of the Shenandoah River. In 1979, the fish consumption ban was reconsidered when it was found that mercury levels were not as high as first predicted, and the ban was reduced to a consumption advisory along the South Fork of the Shenandoah River. The ban in the South River was retained for native smallmouth bass, but people could safely eat the non-native trout that had been stocked in the river (figure 1).

Based on the findings from the studies conducted between 1977 and 1980, DuPont and the Commonwealth of Virginia agreed in June 1984 on a legal settlement to compensate the state for the damage to natural resources resulting from mercury contamination of the South River. The settlement specified that DuPont was required to establish a trust fund that would financially support a 100-year-long monitoring program to determine whether mercury levels in fish, water, and sediment were declining [8]. The fund would be managed by the Virginia Department of Environmental Quality (VADEQ).

At the beginning of 1998, DuPont began a Release Assessment and RCRA Facility Investigation at the Waynesboro facility as required by law [9]. Data on fish tissue that were collected in the summer and fall of 1999 were compared with the results from the 1980s. Unfortunately, the data revealed that mercury levels were not declining as predicted and had stayed constant and even increased over time.

In February 2001, DuPont, VADEQ, and the EPA agreed to voluntarily establish an interdisciplinary team of individuals from industry, the government, citizens’ groups, academic institutions, and private research organizations to revisit the issue of mercury contamination and its consequences in the South River [5]. The team was called the South River Science Team, and their key objectives were to identify possible routes of human and environmental exposure to mercury in the South River watershed, to define possible risks and uncertainties, and to communicate that information to the public.

On October 20, 2003, the Sierra Club and the Natural Resources Defense Council delivered to DuPont a Notice of Intent to Sue Under Section 7002(a)(1)(B) of the RCRA[8]. The Notice alleged that the mercury levels in the fish of the South River were too high and that mercury presents an “imminent and substantial endangerment” to human health and the environment of the South River and the South Fork of the Shenandoah River [8]. The purpose of this lawsuit was to force the VADEQ to put pressure on DuPont, further speeding up the remediation process. The community generally supported this process because of the legacy of pollution that DuPont left behind. In July 2005, a Consent Decree was issued between DuPont, the Natural Resources Defense Council, and the Virginia chapter of the Sierra Club to address the mercury contamination in the South River watershed [8]. This Consent Decree required that a 6-year Ecological Study be performed to compile results from a range of scientific disciplines and develop a coordinated, integrated, watershed-level approach to addressing the mercury problems in the South River, characterize the movement of mercury in the South River, and ultimately come up with remediation decisions. The Decree also required DuPont to perform a Fish Consumption Survey and Health Advisories.

Before proceeding to the next section, pause here and discuss these provided questions.

  1. If you were putting together the Science Team, what types of expertise and interests do you think the members should include and represent?

  2. What questions should the Science Team ask in regard to how mercury enters and moves in the environment?

Part 2: Studies of the Fate and Transport of Mercury in the South River

In 2005, the South River Science Team began a 6-year-long series of studies of the South River. This research was used to characterize the fate and transport of mercury in the river system, which later informed decisions on how to clean up the contamination. In this context, fate refers to where pollution ends up and describes the receptors (living organisms) that are affected by the mercury, and transport is the process by which pollutants move through the environment and the reactions that modify or degrade them. Thus, fate and transport is the process describing how a pollutant moves through the environment and affects receptors. Multiple studies were conducted to determine how mercury contamination from the DuPont site moved and continues to move throughout the South River ecosystem and how different end receptors are affected because of this contamination. Four of these studies are described below to illustrate the fate and transport of mercury in the South River.

Study 1: The Movement of Aquatic Mercury Through Terrestrial Food Webs

In 2008, ecotoxicologist Daniel Cristol initiated a study to determine whether mercury moved from aquatic habitats to terrestrial habitats and how mercury levels in terrestrial food systems compared to the levels in aquatic food systems [10] Cristol’s team analyzed mercury concentrations in the blood of 13 species of terrestrial-feeding adult birds that were found within 50 m of the river and compared these mercury levels to birds at uncontaminated reference sites. They also sampled feathers (which store mercury) to indicate long-term mercury exposure.

Two species of terrestrial-feeding songbirds had mercury levels in their blood that were greater than the birds that consumed fish directly from the South River. The average concentration for an aquatic-feeding bird in the South River ecosystem is about 3.5 ppm, and the two species of terrestrial-feeding birds had blood mercury levels of 5 ppm and 7 ppm. These surprising results warranted further study, so the researchers watched adult terrestrial-feeding birds deliver prey to their young to determine whether there was a link. The most prevalent items in their diets were spiders, caterpillars, and grasshoppers. Samples of all of the organisms the birds were feeding their young were collected and analyzed for mercury levels. Dr. Cristol’s team ultimately found that the birds were feeding their young about 30% spiders by mass. These spiders contained, on average, 1.24 ppm of mercury. The caterpillars and grasshoppers analyzed had much lower concentrations of mercury—about 0.35 ppm. The spiders’ mercury levels were even higher than the fish collected from the South River at the site (roughly 0.73 ppm).

This study illustrates the effect of biomagnification—the tendency for the concentration of toxic compounds to increase in organisms higher in the food chain. Caterpillars and grasshoppers are primary consumers that eat terrestrial vegetation. When birds eat these organisms, they are only second-level consumers. As consumption reaches higher trophic levels, the amount of mercury increases in each organism as it accumulates in their tissues. The spiders eaten by the songbirds are themselves second-order consumers, feeding on insects that hatch from the South River. Therefore, the birds became third-order consumers when they ate spiders, and the effects of biomagnification increased.

Study 2: Sexual and Seasonal Variations of Mercury Levels in Smallmouth Bass

The Department of Fisheries and Wildlife Sciences at Virginia Tech conducted a study at the South River in 2007 to determine whether there were variations in the mercury levels of male and female smallmouth bass and whether these levels changed throughout the year [11]. The researchers chose to assess mercury roughly 20 miles downstream of the DuPont site, at a previously established monitoring point. Adult smallmouth bass were collected in April, July, and October, separated by sex, and their muscle tissue was analyzed for mercury concentration.

Mercury concentrations were positively associated with the age, length, and weight of smallmouth bass. Females had a 10% higher average mercury concentration in their muscle tissue than males, which the researchers attributed to differences in reproductive demands. They hypothesized that female smallmouth bass ate more, compared to males, to meet the energy demands of egg production.

Mercury concentrations in the fish peaked in the spring when they were 14% higher than in the summer and 21% higher than in the fall. This seasonal pattern had been found previously in research at other sites, and the Virginia Tech team hypothesized that this pattern occurred as a result of mercury methylation by sulfate-reducing bacteria in sediments. Methylation is the addition of a methyl functional group onto a substrate, and it is most efficient during environmental conditions such as warm sediment temperatures, low-oxygen conditions, and input of new soil from riverbanks. In the spring, the South River meets all of these criteria. Spring rains flood the river, eroding the riverbanks and adding mercury-laced soil to the sediment. As temperatures increase, bacteria methylate mercury at an increased rate, and oxygen levels in the water and sediment decrease. All of these factors combine to explain the pattern observed in the study.

Study 3: Mercury Loads in the South River

In 2009, Jack Eggleston, a hydrologist with the U.S. Geological Survey (USGS), contributed research to help the VADEQ calculate a total maximum daily load for mercury in the South River to help regulators develop plans to protect water quality [12]. In this case, the VADEQ’s goal was to lower mercury pollution in the river to keep fish methylmercury concentrations below 0.3 mg/kg of fish tissue (0.3 ppm)—a level considered to be adequately protective of human and ecological health. Eggleston and other USGS personnel collected 2 years’ worth of data, including streamflow (the volume of water that passes a point in a river), mercury levels in the river system (riverbanks, fish tissue, surface water, and sediment), and climate data (including temperature and rainfall). Groundwater flow, tributary influx (mercury received from tributaries to the South River), atmospheric deposition (the settling of mercury in the atmosphere contributed from the burning of fossil fuels), and the attachment and detachment (sorption and desorption) of mercury to sediments in the river were also included in this study.

Eggleston used the information that their team collected to develop a conceptual model for how mercury was transported through the South River system (figure 2). Their data were input into modeling software known as “Hydrological Simulation Program—FORTRAN,” which is a mathematical model designed to simulate the hydrology and movement of contaminants in a watershed. Using the model, Eggleston calculated that 189 kg (417 pounds) of mercury was entering the South River yearly and that 96% of the mercury was associated with soil erosion [12]. To bring methylmercury concentrations in fish tissue down to the desired value of 0.3 mg/kg, the mercury loading in the South River would have to decrease to 2 kg (4.5 pounds) per year, a reduction of 98.4%.

Figure 2.

The conceptual model of mercury sources and transport in the South River. Source: Ref. [12].

Figure 2.

The conceptual model of mercury sources and transport in the South River. Source: Ref. [12].

Study 4: Distribution, Behavior, and Transport of Inorganic and Methylmercury in a High Gradient Stream

A study published by members of the South River Science Team in 2010 neatly summarizes many of the team’s discoveries over the previous few years. Beginning in March 2006, the team took monthly samples of surface water, sediment, and soil at various locations downstream of the DuPont site and analyzed them for mercury concentrations. The team sought to identify connections between riverbank mercury and methylmercury production [13].

The research team found that although mercury contamination had made its way far downstream of the original source of contamination, the vast majority of it was constrained to the first 20 km below the DuPont site. Sediment mercury contamination reached its maximum concentration roughly 5 km downstream of the DuPont site and stayed relatively high for another 10 km. There was a relationship between temperature and methylmercury production, and surface water mercury concentrations were at their highest between March and April, when water temperatures increased, on average, from 8.4°C to 15.5°C. The concentrations of methylmercury decreased when water temperatures were lower than 4°C or higher than 20°C. The team concluded that this temperature range was the most suitable for sulfate-reducing bacteria, which can methylate mercury under favorable conditions.

In conclusion, the South River Science Team summarized much of the research conducted over the previous years.

Mercury was stored in floodplain soils and river banks, which are now slowly eroding, resulting in an ongoing release of mercury back into the river, which may continue for many more decades. The results of this study support the concept that soil erosion, specifically in the upper 20 km of the river, is the key continuing source of mercury to the South River. Controlling erosion of these banks may not be the only remedy necessary to reduce mercury in fish and wildlife, but it may initiate and/or accelerate the natural recovery of this system. [13]

After 6 years of research, the South River Science Team was confident in their understanding of the mercury pollution problem. In 2012, a final Ecological Study report was prepared by DuPont, illustrating the geophysical, chemical, and biological features that facilitate the fate and transport of mercury. At that point, the science team had to decide how to begin reversing the damage caused by mercury pollution and begin designing the remediation of the South River.

Before proceeding to the next section, pause here and discuss these questions.

  1. Why has mercury remained higher than previously predicted in fish tissue in certain areas?

  2. How is bioavailable mercury getting into the river ecosystem in the study area?

  3. How is mercury getting into the tissue of fish and aquatic animals in the study area?

  4. Are there specific mercury pathways that significantly contribute to mercury levels in fish tissue in the study area?

  5. What kind of remedial plans could be done to address the findings from the South River Science Team?

  6. What should be the focus of the remediation plans?

Part 3: Remedial Decisions

During their 6 years of research, the South River Science Team characterized the fate and transport of mercury in the South River. The researchers were confident in their understanding of the mechanisms responsible for the continuous input of mercury into the river system: The primary input of mercury into the South River was from eroding stream banks, which had sequestered the pollutant dumped from the former DuPont plant over half a century prior. Armed with data, the team now had to decide how to remediate the area, taking into account their ecological findings, laws and regulations, and budget.

Four remedial plans were considered for the South River remediation project. Each plan’s benefits and drawbacks, ease of implementation, potential effectiveness, and cost were evaluated [14]. Each alternative was designed to be broadly applicable to multiple contaminated sites along the South River’s banks.

The first plan the South River Science Team considered was to implement institutional controls, which reduce human exposure to mercury by legal measures such as fish consumption advisories [9]. Monitoring of water and fish tissue mercury would continue to occur, with the expectation that mercury levels would diminish over a 30-year monitoring period. No construction would be required, but fish consumption advisories would continue, and river access would be limited to designated points. This first alternative would be the cheapest and easiest to implement, but it did not guarantee the protection of human or ecological health (table 1).

Table 1.

Summary of Remedial Alternative Benefits, Drawbacks, and Cost.

Summary of Remedial Alternative Benefits, Drawbacks, and Cost.
Summary of Remedial Alternative Benefits, Drawbacks, and Cost.

The second alternative proposed was to stabilize the banks by planting natural vegetation (figure 3). By strengthening the riparian area of the river using natural mechanisms, erosion of mercury-contaminated soil would be reduced [9]. This plan would provide for the support of plant and animal life along the riverbanks but did not remove any of the mercury already present. The South River Science Team estimated that it would take 2 years for the plants in a stream bank to mature and provide adequate protection (table 1).

Figure 3.

Native trees were planted in the riparian zone of the South River to stabilize the river bank. Source: South River Science Team.

Figure 3.

Native trees were planted in the riparian zone of the South River to stabilize the river bank. Source: South River Science Team.

The third alternative was to engineer structural support for riverbanks (figure 4). The team evaluated the effectiveness of reshaping steep riverbanks to reduce erosion, the use of large boulders to protect the riparian area, and stabilizing the slope of the bank with reinforcements [9]. This plan also included the planting of native vegetation to supplement the strength of the bank. However, the cost for this plan would be high, and the need to design the engineering of each stream bank would slow down the rate of remediation (table 1).

Figure 4.

Geocells and boulders are used to stabilize a section of the river bank in the Constitution Park area of the South River restoration. Source: South River Science Team.

Figure 4.

Geocells and boulders are used to stabilize a section of the river bank in the Constitution Park area of the South River restoration. Source: South River Science Team.

The fourth and final remedial plan considered was to excavate and remove as much mercury-polluted streambank soil as possible. The clear benefit of this proposal was that it would permanently solve the problem of mercury contamination, but it came at a large cost. The hazardous soil would need special considerations for disposal. Large equipment would need to be able to access the riverbank, which is sometimes not feasible. Ecosystem functions, such as nesting sites along the bank, would be temporarily inhibited (table 1).

Before proceeding to the next section, pause here and discuss these discussion questions.

  1. Can you come up with any other possible remediation alternatives?

  2. Do these remediation alternatives take into consideration the fate and transport of mercury in the South River?

Part 4: Current Status

Each of the four remediation alternatives was assessed for their ability to protect human health and the environment, compliance with regulatory requirements, short-term and long-term effectiveness, and their reduction of toxicity, mobility, or volume of mercury in the South River. The South River Science Team concluded that the second and third remedial alternatives best met their evaluation criteria and provided the most efficient and cost-effective remedies. Two field pilot studies were initiated to determine whether the alternative remedial options were effective.

A pilot study was conducted in 2009, focusing on bank stabilization of the South River. The main objective of this study was to evaluate the reduction in riverbank erosion rates using a combination of engineered controls as well as native plantings [9]. The bank stabilization pilot was designed to incorporate three main components—a rock toe at the base of the bank slope to allow for protection, soil lifts to create a more stable slope, and to plant native vegetation along the bank to provide further stability (figure 5). Follow-up monitoring revealed that inorganic mercury concentrations in the sediment, water, and aquatic invertebrates along the pilot bank have decreased by over 90% since its installation.

Figure 5.

Design of the pilot study riverbank. Source: South River Science Team.

Figure 5.

Design of the pilot study riverbank. Source: South River Science Team.

The second pilot study was conducted in 2011. This study did not take place in the South River, but in a pond within the floodplain of the South River. The purpose of this pilot study was to test the efficiency of biochar in reducing the bioavailability of mercury [9]. Biochar is a charcoal-like substance that is made by burning organic material from agricultural and forestry wastes. The resulting biochar was then used as a soil amendment for carbon sequestration and soil/sediment health. After the application of the biochar, significant reductions of biological uptake of mercury were observed.

Conclusion

The DuPont plant is now owned by Invista and is no longer using processes that pollute the South River with mercury. Invista continues to operate and is an important employer in the area. Although some users of the river felt that the restoration did not go far enough, the community and participants are generally satisfied with the process.

The completion of the pilot studies has helped the South River Science Team identify the banks that would provide the most benefit when restored. Several riverbanks have already been stabilized or reengineered to reduce erosion. However, as of 2019, this process is only a beginning. Four banks comprising a half a mile of restoration have been built, with two more projects currently underway.

Remediation will have long-reaching effects on the South River and the South Fork of the Shenandoah River. Controlling and eliminating mercury will restore over 100 miles of the river system, improving the ecosystem and making fish safe to consume again. The research conducted by the South River Science Team used the knowledge gained by scientific studies to guide their decision-making processes for remediation alternatives, which has made a significant impact on the health of the environment.

Author Contributions

David Wilcox, Madison Whitehurst, and Robert Atwood are the primary authors of this article.

Acknowledgments

We would like to thank Robert Brent, a member of the South River Science Team, who provided insights into their findings. We would also like to acknowledge Robert Alexander for his support and encouragement. Finally, we thank Claire Baldacci, Christina Gedz, and Cody Girone for their comments and suggestions.

Competing Interests

The authors have declared that no competing interests exist.

Funding

Support for this article was provided by the James Madison University Interdisciplinary Environmental Minors Program.

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