Determination of nutrients and heavy metal species in samples from Lake Maracaibo.

In this study, a new analytical method for the simultaneous determination of total N, P and S using hydrogen peroxide oxidation has been developed for the analysis of water and sediment samples. The products of the oxidation reaction (nitrate, phosphate and sulphate) were determined by ion chromatography. A method for the simultaneous chemical speciation of arsenic, selenium and chromium was developed using ion chromatography coupled with ICP-MS. Reversed phase chromatography and ICP-MS was used for the simultaneous determination of mercury and selenium species. For the speciation of vanadium a new method using HPLC with reversed phase and ICP-MS detection was developed. The species arsenite, arsenate, selenite, selenate, chromate, methylmercury, inorganic mercury, selenocystine, selenomethionine, vanadium (IV) and vanadium (V) in samples of water, sediment, fish muscle tissue and mussel, were determined using the developed methods. Simultaneous determination of nutrients and metal species were applied to the study of pollution in Lake Maracaibo, Venezuela. The distribution of As, Se, Pb, Hg, Sn and V in sediment was studied using a sequential extraction scheme and related to the physicochemical parameters and nutrient content. The major concentrations of arsenic and lead found inside the lake were associated with the fraction associated with the Fe/Mn hydroxides phase, however, mercury and selenium were distributed at the main zone of the lake in the organic-sulphide fraction. In the strait and the gulf, metals were distributed mainly in the residual phase with the exception of tin. Conditions which favour mercury methylation in the lake are discussed. In the centre of the lake, with anaerobic conditions, methylmercury was the predominant species for mercury. The results found for vanadium and arsenic speciation showed that the predominant species in all the samples of Lake Maracaibo was vanadium (IV) and arsenite, respectively. Results were compared with those from lakes with similar pollution problems.