Investigation of legacy industrial mercury in floodplain soils: South River, Virginia, USA

Mercury (Hg) was used during 1929–1950 as a catalyst to produce rayon acetate at the former DuPont plant in Waynesboro, Virginia, and released into the South River. Though the use of Hg ceased in the 1970s, the affected ecosystem is still a matter of concern. Here, high total mercury (THg) and total organomercury, stated as methylmercury (MeHg) are reported in historically contaminated floodplain soils and shallow groundwater from five locations sited 5.6 km downstream from the plant of one river edge site. In soils, THg ranged from 0.1 to 1201.5 mg/kg exceeding the health-based screening Hg levels for residential and industrial soils while MeHg varied between 0.1 and 54 µg/kg. Concentrations decreased sharply with depth indicating the stratification of legacy industrial Hg-rich soils underlain by the pre-industrial soils with minor Hg. Strong linear correlation between THg and MeHg was observed. Highest soil MeHg was associated with total carbon, poorly crystalline and amorphous Fe and/or Mn oxyhydroxides. Sequential extraction analyses indicated that Hg was present mostly in relatively recalcitrant forms, as determined with procedures that targeted β-HgS, HgS, HgSe, HgAu, thiol-bound Hg, Hg0, and some organo-complexed Hg, Hg2Cl2 phases. High Cu (≤ 404.3 mg/kg), Zn (≤ 151.3 mg/kg), Cr (≤ 123.9 mg/kg) were also identified. In shallow groundwater, THg ranged from 28 to 538 ng/L and MeHg varied between 1.2 and 137 ng/L. During the intermittent precipitation, the highest MeHg was linked to the highest Fe2+, Mn, SO42−, total alkalinity, and conductivity, which could be due to either the potential leaching and dissolution of soil minerals and/or the saturation of the vadose zone. A sharp increase in the soil moisture at the top 40–70 cm of the Hg-rich soils after rainfall and overbank flooding was followed by redox gradients from oxidizing (≈ + 600 mV) to reducing (≈ − 300 mV) and a reverse response in a transmissive gravel zone at the base of the bank (≈ − 400 to + 200 mV) with a defined lag with depth. These dynamic seasonal fluctuations at the South River might be crucial for solubilization of Hg, Fe, and Mn redox-sensitive minerals triggering the anoxic response for new MeHg production.