Arsenic in solution, colloidal and particulate phases of East-Hainan estuaries

During two nominal dry seasons (December 2006, March 2009) and two wet seasons (July 2007, July/August 2008) the estuarine distribution and size speciation of arsenic was studied in the Wanquan river and the Wenchang/Wenjiao river estuaries, located in tropical East-Hainan (China). Arsenic in solution and in colloidal fractions was analyzed by anodic stripping voltammetry after oxidative UV digestion thereby including inorganic As(III) and As(V) as well as organic As forms. Particulate As was determined by high-resolution ICP-MS after a microwave assisted total decomposition protocol. Taking the two relatively similar estuaries and the two expeditions for each season together, the average concentrations of total dissolved As (<0.4 μm; td-As) of the freshwater endmember were 4.4 nM and 7.6 nM during the dry and the wet seasons, respectively, which suggests a low – if ever – level of contamination. With one exception (uptake by phytoplankton in the lower estuary), during the dry season td-As increased essentially conservatively along the salinity gradient to 14.4 nM as the mean value for salinities of about S=30. During the wet seasons, however, td-As only showed a tendency for a conservative increase; scatter in the data was attributed primarily to heavy rain and side inputs of variable As concentrations from overflowing aquaculture ponds and piles of excavated soil. The separation of colloidal fractions revealed that on average 82% of td-As remains in solution (<5 kDa), followed by 13% in small colloids (5 kDa to 30 kDa) and 5% in large colloids (30 kDa–0.4 μm). Particulate As (0.24±0.06 μmol/g) showed no dependence on either salinity, suspended particles, estuary or season. Relatively high distribution coefficients (logKD=4.48–4.81 in the freshwater; logKD∼4.18 at high salinities) arose primarily from the very low td-As. Except for certain impacts from aquaculture ponds, the two estuaries might be considered uncontaminated with respect to As.