Science Inventory

Using mercury stable isotope fractionation to identify the contribution of historical mercury mining sources present in downstream water, sediment and fish

Citation:

Eckley, Chris S., C. Eagles-Smith, Todd P. Luxton, J. Hoffman, AND S. Janssen. Using mercury stable isotope fractionation to identify the contribution of historical mercury mining sources present in downstream water, sediment and fish. Frontiers in Environmental Chemistry. Frontiers, Lausanne, Switzerland, 4:1096199, (2023). https://doi.org/10.3389/fenvc.2023.1096199

Impact/Purpose:

Mercury emitted to the air can have a relatively long atmospheric residence time (0.5 to 2 years) allowing for its widespread distribution and contamination of remote ecosystems1. As a result of anthropogenic activities over the last century, the global Hg pool has increased 2-3 fold and fish consumption advisories for Hg are widespread across all states3. In addition to atmospherically deposited Hg, some watersheds also contain point sources of Hg pollution from Superfund Sites, mining related ativities, or other industrial activities. Distinguishing the relative contributions of Hg from different sources has long been a perplexing problem for Site Managers trying to reduce Hg levels in fish. Uncertainties regarding the sources of Hg in fish obfuscates setting meaningful remediation goals. Due to the unique biogeochemical conditions in a waterbody that govern how much methylmercury (MeHg) is produced, it is very difficult to identify what “background” levels would be in the absence of releases from a Superfund site. As a result, it has been challenging to determine how much Hg in fish at a Superfund site originates from site related releases versus other sources such as atmospheric deposition. This is further complicated by the geographic disconnect between releases from Superfund sites (i.e. the abandoned mine or industry) and the point of concern about elevated Hg in fish which can often occur many kilometers downstream (e.g. a reservoir or estuary). In addition, there remains large uncertainty regarding how far downstream from a Superfund site the fish Hg levels impacted from the site’s releases. GOAL AND OBJECTIVES: Until relatively recently, it has not been possible to “fingerprint” different Hg sources and the total amount of Hg in a fish (or other media) could not be attributed to any particular source. However, recent advancements using multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) has made possible to “fingerprint” Hg sources by utilizing differences in Hg stable isotope ratios2. This technique relies on a source/industry having a unique isotopic signature, which is not always identifiable if a pure sample of the industrial source is not available or the industrial sources relied on Hg that contained different isotopic signatures (i.e. Hg supplied from different mines/regions). The EPA has recently started to use this technique at a handful of sites, however its potential for widespread application is still far from realized and will benefit from additional studies at Superfund Sites. Existing studies using this technique have been complicated by multiple industrial sources with varying degrees of a unique fingerprint from different sources. The BBM Superfund Site provides an ideal situation for this technique to be applied because the Hg was mined and processed all on-site resulting in a single unique isotope signature that has already been measured in the site’s tailings. Concern about human exposure to Hg contaminated fish downstream of the BBM in the Cottage Grove Reservoir was a primary reason for its NPL listing. Because reservoirs in general tend to have elevated fish Hg levels compared to natural lakes and rivers, it remains unclear what portion of the Hg in Cottage Grove Reservoir fish is related to the sources from the BBM site, versus Hg from atmospheric deposition. There are two main objectives of this study: 1) determine the amount of Hg in different fish species in Cottage Grove Reservoir (OU3) that is attributable to the BBM; and 2) determine the extent to which fish downstream of the Superfund Site contain Hg attributable to the mine. This later objective has particular relevance to the Willamette River Hg TMDL being developed for this basin; but also answers important questions about the potential for environmental impacts downstream of a Superfund site.

Description:

Ecosystems downstream of mercury (Hg) contaminated sites can be impacted by both localized releases as well as Hg deposited to the watershed from atmospheric transport. Identifying the source of Hg in water, sediment, and fish downstream of contaminated sites is important for determining the effectiveness of source-control remediation actions. This study uses measurements of Hg stable isotopes in soil, sediment, water, and fish to differentiate between Hg from an abandoned Hg mine from non-mine-related sources. The study site is located within the Willamette River watershed (Oregon, United States), which includes free-flowing river segments and a reservoir downstream of the mine. The concentrations of total-Hg (THg) in the reservoir fish were 4-fold higher than those further downstream (>90 km) from the mine site in free-flowing sections of the river. Mercury stable isotope fractionation analysis showed that the mine tailings (δ202Hg: −0.36‰ ± 0.03‰) had a distinctive isotopic composition compared to background soils (δ202Hg: −2.30‰ ± 0.25‰). Similar differences in isotopic composition were observed between stream water that flowed through the tailings (particulate bound δ202Hg: −0.58‰; dissolved: −0.91‰) versus a background stream (particle-bound δ202Hg: −2.36‰; dissolved: −2.09‰). Within the reservoir sediment, the Hg isotopic composition indicated that the proportion of the Hg related to mine-release increased with THg concentrations. However, in the fish samples the opposite trend was observed—the degree of mine-related Hg was lower in fish with the higher THg concentrations. While sediment concentrations clearly show the influence of the mine, the relationship in fish is more complicated due to differences in methylmercury (MeHg) formation and the foraging behavior of different fish species. The fish tissue δ13C and Δ199Hg values indicate that there is a higher influence of mine-sourced Hg in fish feeding in a more sediment-based food web and less so in planktonic and littoral-based food webs. Identifying the relative proportion of Hg from local contaminated site can help inform remediation decisions, especially when the relationship between total Hg concentrations and sources do not show similar covariation between abiotic and biotic media.