Rare earth element concentrations and speciation in organic-rich blackwaters of the Great Dismal Swamp, Virginia, USA

Abstract

Concentrations of rare earth elements (REE), major inorganic solutes, and dissolved organic carbon (DOC) were measured in waters from the Great Dismal Swamp in southeastern Virginia, including Lake Drummond (pH=4.3), and its principal surface inflows and outflow. Concentrations of REE are high in the lake (average Nd=8 nmol/kg), and the acidic DOC-rich inflow and out flow waters (i.e., Nd=12.5 nmol/kg in Jericho–Washington Ditch and ∼8 nmol/kg in Feeder Ditch, respectively). The slightly acidic (pH 6–6.4) inflow waters from west of the Suffolk Scarp that comprise the chief source of surface water flow into Lake Drummond exhibit lower REE concentrations (Nd=3.4–7 nmol/kg). Great Dismal Swamp waters are all enriched in the middle REEs compared to the light and heavy REEs (LREE and HREE) when normalized to upper continental crustal (UCC) values. Rare earths are positively correlated to DOC concentrations in Great Dismal Swamp waters and inversely correlated with pH. Speciation calculations using a modified version of the well established Humic Ion-Binding Model V predict that organic matter complexes are the predominant form of dissolved REEs in Great Dismal Swamp waters despite significant competition from aqueous Fe and Al for organic matter binding sites. Adsorptive cathodic stripping voltammetry (ACSV) was employed, using a previously developed competitive ligand equilibration approach that involved adding the o-cresolphthalexon (OCP) ligand, to directly investigate REE complexation with natural organic ligands in Great Dismal Swamp waters. Although preliminary, the results of the ACS voltammetric titrations are consistent with dominance of organic matter complexes of REEs in Great Dismal Swamp waters. The results of the speciation modeling and the voltammetric titrations all point towards control of dissolved REEs in Great Dismal Swamp waters by complexation of these heavy metals with natural organic matter.