The biogeochemical control of arsenic in certain estuaries

Semi-continuous hydride generation atomic absorption spectrometry for the analysis of dissolved arsenic speciation in water, was optimised and used to detect sub-nonagram quantities of inorganic arsenic(III) and arsenic(V), monomethylarsenic and dimethylarsenic species in saline waters. High performance liquid chromatography-hydride generation AAS was also developed for the analysis of organoarsenicals in marine macro-algae. The biogeochemical control of arsenic was then studied in two estuarine ecosystems. The temporal speciation of arsenic in Southampton Water (Hampshire U.K.) was monitored and related to planktonic and bacterial activity. The spring and summer depletion of arsenic(V) was associated with the production of methylated and reduced arsenic(III) by marine diatoms and flagellates, but not bacteria. At times up to 50% of the total dissolved arsenic was present in these forms. A UV photodecomposition technique was developed that broke down organoarsenicals that do not form hydrides, such as arsenobetaine, arsenocholine and arsenosugars, to hydride reducible derivatives. This allowed their detection in algal extracts and sea water. Photodecomposition of arsenicals present in Solent water revealed increases of up to 30% in total dissolved arsenic, above that measured by conventional hydride generation; mainly as dimethyl- and inorganic arsenic. The `extra' arsenic showed seasonality and was associated with an abundance of phytoplankton. The other system studied was that of the Tamar, an estuary dominated by non-riverine inputs of arsenic from past mining activity. Surveys revealed two maxima in inorganic arsenic. One derived from mine leachates in the upper reaches, and a second from the release of arsenic-rich porewaters by tidal scouring, in the mudflat dominated mid-estuary. Estuarine particulate material was shown to partially regulate the dissolved level of inorganic arsenic. Methylated arsenic showed no affinity for the suspended sediments.