HYDROGEOLOGICAL CONTROLS OF SPATIAL VARIABIITY OF ARSENIC IN THE VOLCANO-SEDIMENTARY COMPLEX OF GEITA DISTRICT, NORTHERN TANZANIA

Accurate and precise understanding of water quality variables distribution is a pre-requisite to proper design of safe water supply systems especially in countries endowed with gold mineralization like Tanzania. In this study, the effect of topography, pH and well depth on variability of arsenic (As) have been investigated using measured concentration levels in shallow wells, intermediate deep wells and boreholes. The modelling process involved testing spatial dependence in measured concentration levels using global Moran’s Index and spatial regression analysis. A moderate significant Moran’s index (I = 0.366, p-value = 0.001) was obtained indicating that As occurrence in this local geologic formation is clustering in space. During spatial regression analysis, significant (p-value=0.002) model fit of 20% was obtained. Arsenic occurrence in the volcano-sedimentary complex was significantly negative correlated with altitude from mean sea level (p-value=0.012). In contrast, As concentration was positively correlated with pH and well depth, though the correlation with the latter was not significant. This is because some wells had no reliable depth information. This approach is very useful in understanding the space dependent natural processes controlling the distribution and mobility of geogenic contaminants such as As in drinking water sources.