A REVIEW OF THE SOURCE, BEHAVIOR AND DISTRIBUTION OF ARSENIC IN NATURAL WATERS OF SOUTHERN TOGO, WEST AFRICA

About 500,000 people in Lome, the capital of Togo, West Africa, are gripped by arsenic contamination from drinking surface water, taken from the southern Togo rivers watershed. In areas, where about 76% people of the population depend on surface and groundwater, the nature and extend of surface water contamination is severe. Typical concentrations of As in freshwater are less than 10 mg/l and frequently less than 1 mg/l. These large-scale natural and anthropogenic As tend to be found in about 45 locations of southern Togo rivers watershed. Both environments tend to contain geologically young sediments and are found in flat, low-lying areas where groundwater flow is sluggish. The As content of the sediments taken from 45 locations in southern Togo does not appear to be very high in sediments ranging from 253 to 545 µg/kg. However the As contents of river water samples from the Mono, Zio and Haho Rivers show the concentration from 3-4 mg/l. There appear to be three distinct triggers that can lead to the release of As on a large scale in southern Togo river waters. The first may results from the development of high pH (>8.5) conditions, in dry season, as a result of the combined effects of mineral weathering and high evaporation rates. This pH change leads either to the desorption of adsorbed As (especially As(V) species) and a range of other anion-forming elements (V, B, F, Mo, Se and U) from mineral oxides, especially Fe oxides. The Arsenic retention in the sediments is highly variable and controlled by local processes as a result of natural weathering process of metamorphic bedrock. The second results from the development of strongly reducing conditions at near-neutral pH values, leading to the desorption of As from mineral oxides and to the reductive dissolution of Fe and Mn oxides, also leading to As release. Positive correlation is observed between dissolved As and Mo, which is also believed to be derived from ore minerals and oxides in the mineralized zones. The third may results from agricultural as well as industrial activities in urbanized regions of Lome. The discovery of high As concentrations in some river waters of southern Togo reiterates the need for reconnaissance surveys in mineralized areas of crystalline basement.