Présentations Colloques

Oral Presentation
Session 2.02: Groundwater for sustainable water supplies in developing countries
Vassolo Sara
A case of natural contaminants in groundwater- High uranium concentrations in water supply wells in Burundi
Groundwater is often preferred over surface water as a source of potable water supply because of its favorable characteristics in terms of availability and bacteriological quality. However, the presence of natural contaminants is a risk that should be assessed. In areas with igneous or volcanic rocks, elements like fluorine and uranium often occur in groundwater at concentrations above drinking water standards. This contribution presents a case study from northern Burundi where dissolved uranium concentrations exceeding 700 Ig L have been encountered. The study area is located south of Lac Cohoha, on the border between Burundi and Rwanda. The geology is dominated by granitoids of the Kibaran belt system, which have a uranium content between 5 – 15 ppm. The original rocks have been highly weathered, and the weathered sequence forms unconsolidated aquifers that can reach a thickness of at least 91 m in the study area. The predominant land use is subsistence agriculture, with the main crops being beans, sweet potatoes, maize, cassava, banana, and sorghum. Groundwater recharge is estimated about one fifth of the mean annual rainfall, which is just over 1000 mm, and groundwater discharge occurs in valley bottoms and along the shores of Lac Cohoha. There, intense evapotranspiration occurs, and it is believed that this is the process that leads to the enrichment of dissolved uranium, which is supported by geochemical model calculations using PHREEQC. Based on a conceptual model of dissolution of feldspar minerals from the uranium-containing rock during groundwater recharge and flow, and evaporative concentration in the discharge area, the major trends of the groundwater chemistry, including uranium concentrations, can be reproduced. Whilst dissolved uranium concentrations may be influenced by other processes as well, the overriding influence of evapotranspiration highlights the need for caution when developing water supply wells near discharge zones in other regions where similar conditions exist.