IMPACT OF DISSOLVED ORGANIC MATTER ON INORGANIC ARSENIC SPECIATION: EVIDENCE FROM A BLACKWATER SWAMP

Arsenic concentrations and speciation were determined by selective hydride generation, gas chromatography with photoionization detection in waters of the Great Dismal Swamp, southeastern Virginia, USA. Great Dismal Swamp waters have high dissolved organic carbon concentrations (445 - 9600 µmol/kg, mean = 3538 µmol/kg) and low pH (3.6 ≤ pH ≤ 6.4). Total dissolved As concentrations [i.e., As(III) + As(V)] range from 2.2 nmol/kg to 21.4 nmol/kg. Arsenite, As(III), concentrations range from ~ 1 nmol/kg to 17.7 nmol/kg, and arsenate, As(V), ranges from ~ 1 nmol/kg to 14.1 nmol/kg. Arsenate is the predominant form observed in the inflow waters to the Great Dismal Swamp, whereas As(III) is more abundant within the Swamp proper. Arsenite accounts for 54% to 81% of total dissolved As in Lake Drummond and other waters sampled from Great Dismal Swamp proper, and between 8% and 37% of dissolved arsenic in the Swamp's inflow waters. Arsenite is strongly correlated to DOC (r = 0.94) and inversely related to pH (r = -0.9), both at greater than the 99% confidence level. Arsenate is weakly related to pH and DOC (r = 0.4 and -0.37, respectively), and neither relationships is statistically significant. No statistical relationships exist between As(V) or As(III) and PO4 concentrations in Great Dismal Swamp waters. The predominance of As(III) in Great Dismal Swamp waters reflects the high DOC concentrations, which promotes As(III) desorption from minerals and/or particulate organic matter and inhibits As(III) adsorption to these same substrates. In addition, stable aqueous organic matter complexes may form with As(III). Lake Drummond and the Great Dismal Swamp drain via the Feeder Ditch and Great Dismal Swamp Canal to the Northwest River, a major drinking water source for Chesapeake, Virginia. High DOC concentrations in Northwest River waters (1830 - 1940 µmol C/kg) are used to predict As(III) concentrations in this drinking water source [5.4 - 5.6 nmol As(III)/kg] based on the strong correlation between DOC and As(III) in the Swamp.