Environment, Federal Government
Mercury, a pollutant that negatively affects human and ecosystem health, is an age old problem that has been simmering for a while. In the 1990s, a few U.S. states developed consumer advisories to protect against mercury entering the human body through the consumption of high-end fish. Since then, scientists have discovered a growing number of water bodies with mercury-contaminated fish tissue exceeding consumption guidelines. Today, there are fish advisories in every state.
The biggest driver for controlling mercury, and mercury-contaminated sediment specifically, is to prevent accumulation in the food chain. Regularly consuming fish above the guidelines can have serious toxicological effects. Mercury concentrations in waterbodies that can lead to fish tissue above the consumption guideline are low—in the subparts per trillion. So low, in fact, that it is often overlooked or discarded as insignificant.
Fighting Old and New Sources
Much of the historic mercury contamination came from mining, where mercury-contaminated runoff from gold and mercury mining activities discharged into downstream water bodies. Many industries employed mercury as a component of their processes and typically had elevated mercury in their waste streams. While mercury-contaminated effluent is now heavily controlled, historical releases into the environment have been significant.
One of the biggest current anthropogenic contributors are coal-fired power plants. Volatile elementary mercury and inorganic mercury contaminated particles, released through burning coal, enter the water cycle and circulate the globe in the atmosphere. This mercury undergoes atmospheric chemical transformations that can convert it into particulates that drop in pristine landscapes like forests and mountain streams. Ubiquitous microorganisms, like sulfate reducing bacteria present in healthy aquatic ecosystems, convert mercury into methylmercury. As a result, mercury has been found above fish consumption advisories in watersheds that have no historic mercury point sources.
Characterizing a Mystery
The dynamics of mercury in the food web is still a mystery, with manifestation varying on a system-by-system basis. Many factors can contribute to a water system’s ability to foster both the formation of methylmercury and uptake into the food web. In some cases, you can have the exact same concentration of methylmercury in two different lakes, but one may have 5x more mercury in fish tissue than the other. The health of a particular system greatly affects its potential to make that initial conversion into methylmercury. Parameters such as nutrient balance and aromatic dissolved organic carbon, can influence—even prevent—food web uptake.
The development of analytical technology has transformed this field of science; however, solving mercury problems goes beyond quantifying mercury concentrations. Forensic approaches, such as isotope fractionation, can identify mercury sources through unique isotope ratios—think fingerprint—to determine what mine or other source is contributing the contamination somewhere downgradient. Real-time in situ measurement tools are being developed to streamline assessments, which will reduce costs and streamline the investigation processes. On the horizon is the ability to measure the potential for mercury methylation and its build up in fish in a holistic system diagnostic approach.
Integrating Technology and Awareness for Holistic Solutions
This field of science is wide open for new and innovative solutions. Take eutrophication, for example. In waterbodies that have excessive nutrient loading, there is a subsequent increase in growth of algae and zooplankton that spurs microbial activity, such as sulfate-reducing bacteria that converts organic mercury to the lethal methylmercury. Limnologists have been developing lake and reservoir management practices and technologies to address eutrophication over the last century. Solutions to a eutrophication problem may very well be at least part of the solution for a mercury problem.
Three big themes that are a part of eutrophication management should also be considered in mercury contaminated systems: internal and external nutrient controls, bottom water and surface sediment oxidant controls, and biomanipulation (i.e., the controlling the structure and/or quantity of the different trophic levels of the food web). Modeling the comprehensive mass loading of mercury throughout a watershed and into receiving water bodies might also provide insight into transport and smarter treatment decision. However, technology alone is not enough. Even with posted fish consumption advisories in every state, there needs to be a more concentrated and creative approach to public outreach. Educating the consumer on consumption limits of not only recreationally caught fish, but also seafood found in grocery stores, needs to be a part of this effort to avoid exposure to the general public.
The integration of technology and human expertise is necessary to understand, manage and mitigate mercury’s effects. Answers will come from a more holistic understanding of mercury in our ecosystem and creative and innovative approaches for dealing with this tricky element.
Stephen R. Dent, Ph.D., has more than 9 years of experience in researching, designing and managing environmental engineering research and monitoring programs involving contaminant issues. His experience has focused on lake management and historical hard rock mine site investigations. Dr. Dent has expertise in evaluating mercury fate and transport, including biotic and abiotic mercury transformation analysis and food-web uptake assessments.