Land–ocean contributions of arsenic through a river–estuary–ria system (SW Europe) under the influence of arsenopyrite deposits in the fluvial basin

Water was sampled monthly from September 2005 to August 2006 at 14 stations distributed throughout the coastal system of Anllóns–Laxe, from where 30 surface sediment samples were also taken. After filtration through 0.22 μm polycarbonate filters, dissolved inorganic and total arsenic (UV oxidation) concentration was determined by HG-AFS. After microwave digestion, the arsenic in SPM and sediment was determined by AAS. Ultra-clean procedures were adopted during sampling, handling and analysis and the analytical accuracy was checked using certified reference material. Spatial distribution of As in water (0.2–4.0 μg L− 1), SPM (21–169 mg kg− 1) and sediment of the river reservoir was altered by the presence of arsenopyrite deposits in the middle fluvial basin that increases 2.1 ± 0.5 and 1.7 ± 0.5 times the concentrations of inorganic dissolved (DI-As) and particulate (P-As) arsenic, respectively. At the termination fluvial zone As fluxes can be calculated to be: [DI-As] = 7.09 · Q− 0.69. The Anllóns River exports to its estuary 460 kg a− 1 of dissolved (< 7% as organic) arsenic annually. It is higher (i.e. 0.83 kg s− 1 km− 2 of DI-As) than that of most of European rivers. In the estuary reservoir, the influence of arsenopyrite is also evident as the river concentration of DI-As, which was lower than in seawater during the wet season and higher during the dry season. Arsenic has non-conservative behaviour, as in other European estuaries, but the Anllóns shows an ambivalent pattern: as it usually gains DI-As during the wet season and loses it during the dry season, whilst P-As seems to behave contrary to the DI-As. When the fluvial arsenic reaches the ria its concentration varies due to the estuarine processes. In the wet season DI-As increases its concentration by one third whilst in the dry season it decreases by one fifth and the annual contribution to the ria is 10% higher than the fluvial output. In the case of P-As more data are necessary to quantify its behaviour; however, the estuarine sediments are contaminated by arsenic. In the ria reservoir DI-As levels were similar to those of the ocean, DO-As comprises 9–22% of the inorganic, P-As ranges from 3 to 40 mg kg− 1, and As-sedimentary can be classified as uncontaminated (4–18 mg kg− 1), except in the fishing ports. The ria circulation, reinforced by upwelling favours the exportation of arsenic to the ocean. In the Anllóns–Laxe system as a whole, the freshwater–saline interface processes do not lead to a decrease in the dissolved fluvial arsenic flux to the ocean.

► The arsenic transports through river, estuary and ria reservoirs are examined.
► River As concentration is doubled due to arsenopyrites in the fluvial basin.
► Estuarine mixing shows an As non-conservative ambivalent pattern: loss or gain.
► Land–ocean output of dissolved As was seasonally modified by the estuary.
► The arsenic exported to the ria has not accumulated there, but passes to the ocean.