4b: Contaminated site assessment and management: Lessons for technical assistance under the Minamata Convention

Under the Minamata Convention (Articles 9 and 12) countries are encouraged to identify, assess, and remediate sited contaminated with mercury (Hg) in order to reduce the risks to humans and biota. However, there is an increasing concern over the potential for full recovery of lake and reservoir ecosystems, many of which have experienced a legacy of Hg contamination. While the cessation of the direct source may result in decreases of Hg concentrations in water and biota, the long-term recovery of the ecosystems is not always achieved.

Using three case studies of Hg-contaminated lakes (Calero Reservoir in California, USA, Lake Pena Blanca in Arizona, USA, and Onondaga Lake in New York, USA), this work illustrates current practices for mitigating Hg contamination in lake sediments. The three lakes experience seasonal anoxia in the hypolimnion and substantial build-up of methylmercury (MeHg). Calero Reservoir is located in the New Almaden mining district, one of the largest sources of mercury in North America. Elevated total Hg and MeHg concentration were observed in sediment, water, and biota. Engineered oxygen addition to bottom waters, using pure oxygen gas, was chosen as remedial alternative. Lake Pena Blanca receives runoff from Hg tailings of abandoned gold mines located within its watershed. Removal of the Hg source and complete excavation of the lake sediments was undertaken. New, clean sediments were placed, the lake was naturally refilled with water, and fish was restocked in the lake. Onondaga Lake received ionic Hg inputs from the waste stream of a chlor-alkali plant that operated on the shores of the lake. Elevated Hg concentrations were reported in the water column and pelagic fish. Whole-lake nitrate addition to the hypolimnion of the lake was the preferred remedial alternative.

The author compares and contrast the effectiveness of the three remediation techniques and provides recommendations for the selection of proper, cost-effective, and environmentally-sound remediation based on site-specific conditions and regulatory goals.

The Minamata Convention on Mercury was created to protect human health and the environment from the anthropogenic emissions and releases of mercury compounds by curbing and/or controlling the different life-cycle stages of mercury, including the management of contaminated sites. Phytoremediation, a group of technologies that reduce, remove, degrade or immobilize environmental toxics that are primarily of anthropogenic origin, is considered to be a cost-effective, safe and environment-friendly technology to clean-up contaminated sites. While phytoremediation is a promising technology, its application in mercury-contaminated sites is poorly understood, thus uncertainties remain as to its application. Here, we develop a framework for evaluating phytoremediation technologies for mercury-contaminated sites using the source-to-outcome based approach. A review of published works on mercury phytoremediation was done and the identification of the key environmental decision-making criteria is guided by the core/non-core model for sustainable remediation of contaminated sites. The existing phytoremediation technologies for mercury revealed that potential wider environmental impacts may result from the various inputs of substances and organisms by the remediation work. These include movement of mercury to groundwater resources through chelating agents, threats to biodiversity and ecosystem structure and function of non-native and transgenic species, and production of contaminated plant materials that need environmentally-sound disposal, among others. The source-to-outcome framework developed specifically for mercury-contaminated sites is important in understanding and visualizing the specificity of both the problem caused by mercury and the phytoremediation process and their relationship in the feedback mechanism. We propose that to further expound on the work, one should include the identification of core/non-core socio-economic factors that can be undertaken to provide a more holistic view of the decision-making process by ensuring that the analyses are grounded on the principles of sustainable development.

In China, the majority of mercury mines locate at Karst region in the southwest of China, especially in Guizhou province. The Karst region is characterized as the vulnerable and scarcity of farmland, the farmer depend on farmland as the main revenue resource. Soil surrounding Hg mining areas is heavily contaminated with mercury and thus causing the contamination of crops with mercury. The inhabitants living at Hg mining areas were exposed to mercury by consumption of Hg-contaminated crops. Therefore, the establishment of theme for safety crop production at mercury mining area is necessary for maintaining the eco-safety at mercury mining area.

Although many remediation methods have been developed, no one can be used in such a large scale. The three mercury mines including YunChangPing mercury mine, Wanshan mercury mine and Wuchuan mercury mine, which are located in Guizhou province, were chosen as our study areas. In order to screen the crops which are characterized as low accumulation of Hg, the distribution of Hg in air, calcine, soil and crops at mercury mining area were investigated. We proposed a safety agricultural regulation scheme which can largely ensure the original agricultural use of farmland and the original benefits of farmers based on growing the crops which are characterized as low accumulation of Hg and the database of local land use. By using the protocol of environmental economy, we calculated the potential benefits of agricultural products raised by the use of proposed agricultural regulation scheme and quantify the extent of decrease of human exposure to Hg.

In comparison to previous studies, the concentration of Hg in atmosphere and surface water showed a trend of decrease, but the values were still greatly exceeded those at background sites. No obvious change of mercury concentration in soil was observed. The total mercury concentration in the edible parts of different crops is significantly different from each other. The Hg concentration in the edible part of turnip (corresponding rhizosphere soil mercury content <10 mgkg-1), watermelon (corresponding rhizosphere soil mercury content <50 mgkg-1), strawberry and corn (corresponding rhizosphere soil mercury content <100 mgkg-1), and potato (corresponding rhizosphere soil mercury content <250 mgkg-1) are lower than the national food quality standard (GB2762-2012), and thus belonging to low Hg accumulation crops. By changing agriculture practices, we can significantly reduce mercury exposure in local inhabitants in mercury mining areas in Guizhou, China.

Savannah River Site (SRS) is a nuclear reservation owned by the U.S. Department of Energy that was used during the Cold War to refine nuclear materials for deployment in nuclear weapons. In fulfilment of the mission and vision of SRS, the legacy waste generated on-site is treated in a way that protects public health and the environment. Treatment of legacy waste at SRS includes vitrification into borosilicate glass and storage in stainless steel canisters. In addition to its environmental and public health impact, mercury, which is present at high concentrations due to its use as a catalyst in the dissolution of uranium-aluminum alloy fuel rods, can be corrosive to the off-gas equipment used in the vitrification process. Therefore, a method has been developed to speciate organic mercury in high-activity samples of radioactive tank waste to assure compliance with SRS Waste Acceptance Criteria. This quantitative method was developed and optimized for various derivatizing agents and automated purge and trap (PT), gas chromatography (GC), and atomic fluorescence spectroscopy (AFS) to be performed without sample distillation. Two derivatizing agents were evaluated, tetraethylborate and tetrapropylborate, to allow separate work to be performed on the samples for ethylmercury speciation. Total method runtime was significantly reduced, from 5.33 workhours per sample to 0.60 workhours per sample, by the removal of distillation. Due to the unique characteristics of this work, analyst-time, consumables, and waste generation were also optimized to prevent excessive radiation exposure. This method has produced quantitative results from simulated tank samples with a mean accuracy of 89.6% and mean precision of 4.61% RSD. Derivatization by propylation did not produce significantly different quantitative results for methylmercury (p>0.05), while permitting the quantification of ethylmercury. Direct analysis of samples, without distillation, improved mean accuracy by 11.4% (p=0.00172). This method was compared with other industry accepted methods of methylmercury and ethylmercury analysis, including inductively coupled plasma mass spectrometry, and atomic absorption spectroscopy, and was found to produce statistically similar quantitative results (p>0.05). Due to the highly accurate and precise quantification, fast method runtime, compact nature of the instrumentation, and the absence of excessive sample preparation, this method was determined to be ideal for the highly unique and hazardous nuclear waste tank samples analyzed at Savannah River Site and Savannah River National Laboratory. Future research will explore assimilating this method with inorganic mercury speciation using thermal desorption/AFS.