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Science Lessons from a Mercury Site

2022; Wiley; Volume: 41; Issue: 3 Linguagem: Inglês

10.1002/etc.5288

1552-8618

Ralph G. Stahl,

Heavy metals in environment

Mercury is a devilish substance, changing forms and sometimes causing harm to humans and wildlife by evading our mitigation efforts. I spent 20 years of my DuPont Company career focused on mercury contamination at a former manufacturing site in Waynesboro, Virginia, situated along the South River. Those years taught me lessons about mercury, which are applicable to similar sites. They include (1) monitor the monitoring program, (2) do not work alone, (3) seek advice from experts, (4) the unexpected can happen, (5) keep the conceptual model evergreen, and (6) communicate with the public. Mercury at this site was used from 1929 to 1950 and entered the nearby environment. After mercury was discovered on site in 1975, numerous environmental investigations and engineering studies from 1976 to 1982 and fish consumption advisories for mercury became the basis for a settlement between DuPont and Virginia (Bolgiano, 1980; Lawler, Matusky and Skelly Engineers, 1982; Stahl et al., 2014). A natural attenuation remedy was selected, combined with a 100-year monitoring plan starting in 1984 that focused on surface water, sediment, and fish of the South River and areas downstream. However, a summary report of the fish monitoring results through 1996 (Messing et al., 1997) showed continued elevated mercury burdens, in contrast to engineering predictions of an approximate 20% decline by the late 1990s. The question became—why? Were the predictions wrong, was the conceptual model inaccurate, or was it something else? At DuPont our global remediation efforts are team-based, composed of internal staff and external consultants. The summary fish report noted in the previous paragraph stimulated us to meet with state and federal regulatory personnel in 1999, after which we decided collectively to employ recognized mercury experts from academia and other affiliations. We then formed the South River Science Team (SRST; www.southriverscienceteam.org) in late 2000, which at any one time included 40–50 mercury experts, academics and their students, state and federal regulatory staff, and nongovernmental groups, each working on various aspects of the problem. We met frequently to share, analyze, and discuss data; plan how to fill data gaps; and plan how to best communicate with the public. This collaborative effort was a major reason we were able to elucidate why the mercury burdens in fish had not changed, and it became a model we would attempt to replicate, not always successfully, at several of our other sites. As our work progressed, we considered whether mercury burdens in fish would be reflected in piscivorous birds that fed on them. It occurs in other river systems, but what about the South River? D. Cristol, an avian biologist, and his students were tasked to conduct a field investigation of belted kingfishers. They found blood levels of mercury up to 11 mg/kg—this is high but in some ways expected. But they also found mercury in blood and feathers of a few songbird species, opportunistically sampled in the field. The Carolina wren, a passerine that consumes insects and seeds but typically not fish, was one of these. This was not expected—how and where did these wrens get their mercury burden? The answer was that aquatic insects obtained their mercury from the South River and, on emergence, are eaten by bats or insectivorous birds or trapped in webs of bank spiders. Spiders are a food source for insectivorous birds, including the Carolina wren (Cristol et al., 2008). This new finding meant we had not only an aquatic/fish issue but a floodplain/terrestrial issue too. Later work also showed mercury in bats sampled along the South River. The 1982 engineering studies predicted that new, clean soil and sediment from the upstream, uncontaminated South River would be carried downstream and attenuate the mercury and lead to reduced burdens in fish and other biota. That was the "conceptual model" in 1982, which proved inaccurate by the early 2000s. After engaging a geomorphologist, J. Pizzuto, and his team of graduate students, we soon understood why. Their work showed that the South River is flashy, does not transport much suspended sediment during flood events, and has many riverbanks that are constrained by bedrock. Much of the riverbed downstream from the Waynesboro site is primarily small rock and gravel, not sediment. This suggested that there were few mechanisms for or evidence of new, clean soil and sediment entering the South River from upstream sources. Further, radioactive dating showed that mercury released from 1929 to 1950 remained in the riverbank and floodplain soils (Pizzuto, 2014). Our revised conceptual model is as follows: (1) the slow erosion of the riverbank and floodplain soils during flooding and freezing–thawing provides a continued source of mercury to the South River; (2) once there, mercury is methylated and enters the food chain; (3) this slow but steady input of mercury could continue for decades or longer; (4) there would be little or no change in mercury burdens in biota without mitigation actions. Locally, the mercury contamination of and fish consumption advisory for the South River were well known, but our new work starting in 1999 was not. Public communication efforts started in 2002 and continue today. They include posting information on our SRST website, contact through email and direct mailings, and presentations at local community events and scientific symposia. We also sponsored the Promotores de Salud, a local lay health program for the Hispanic communities focused on children and expecting or new mothers, because of concerns about recognition of the fish consumption advisories on the South River. My account may be useful to those facing similar challenges of investigating and mitigating environmental mercury contamination. Our revised conceptual model is the basis for the first phase of remedial work now completed on the South River at and immediately downstream of the former facility. It also informed our current monitoring plan, the results of which I am hopeful will begin to show that remedial efforts have reduced or eliminated the mercury burden in fish. The author declares that there are no conflict of interests. The statements made herein are those of the author alone. All data referenced in this paper are available at www.southriverscienceteam.org.