Potential arsenic contamination in drinking water sources of Tanzania and its link with local geology

Recent studies on arsenic (As) occurrence particularly in African waters show that several sources of drinking water have elevated concentrations above national and international guidelines. In Tanzania, elevated concentrations of As above WHO guideline (10 μg/L) in Lake Victoria Gold fields is emerging as a threat to public health depending on groundwater and surface water as drinking water sources. In this study, spatial statistics and GIS tools have been used to delineate the relationship between As occurrence and local geological settings. Among the 12 mapped local geological units, the most targeted aquifers for potable water are characterized by granitoids, migmatite, mafic and ultramafic meta-sediments (~50% of water points). The probability of having As levels above WHO guideline was 0.71 and 0.33 for surface water and groundwater systems respectively. 2.3 Water sampling and laboratory analysis Water sampling was carried out at the end of dry season during October 2016. A total of 29 water samples were collected, of which 18 samples were taken from groundwater sources and 11 samples from surface water sources were collected (Fig. 1). The physio-chemical parameters such as, pH, temperature (T), electrical conductivity (EC), redox potential (Eh) and elevation (H) were measured in the field. Major anions were analyzed by ion chromatography (IC Dionex DX-120) in the Land and Water Resources Engineering laboratory at KTH Royal Institute of Technology. Major cations were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) at Linköping University in the Department of Thematic Studies. 2.4 Creation of spatial database and data analysis ArcGIS software was used to create spatial database comprising the location and description of each water sample, physio-chemical parameters and major ions and As. The data analysis part involved calculating and mapping of summary statistics, i.e. minimum, maximum, average and standard deviation. 3 RESULTS AND DISCUSSION 3.1 Spatial exploration of water quality parameters with local geological settings The collected water samples were from abstraction points targeting aquifers with following sediment types: i) predominantly alluvial and eluvial sediments (aQ) with slightly alkaline pH (7.4) and high Eh (mean +416 mV); ii) migmatite-granitoid-metasediment complex (miNA) with neutral pH (7.0) and higher Eh (356.4 mV mean); and iii) volcanosedimentary complex-Greenstone Belt with banded iron formation (BIF) with approximately neutral pH (6.9). The higher EC values between 715 and 843 μS/cm indicate that aquifer sediments originate from the parent rocks in Tanzanian Craton. Similarly, the higher mean Eh values between 356-416 mV suggest an oxidizing environment in all geologic units. 3.2 Probability of occurrence of arsenic contamination in groundwater The probability of having contaminated aquifers was calculated based on number of samples with arsenic concentrations exceeding WHO guideline value constrained by local geologic units. Figure 2 shows a probability map of potential arsenic contaminated aquifers. The most probable aquifers with As levels exceeding 10 μg/L are found in the lithologic groups aQ and gsNA rocks/sediments (50-90%). Aquifers in the migmatite-granitoid-meta-sediment complex (miNA) indicate comparatively less likelihood of elevated levels of As in well water. However, this is just a preliminary observation based on the small sample size, and work is currently in progress to link the overall hydrogeochemical characteristics, such as major ions, As and other trace elements with the mapped geological units. Figure 2: Probability map of arsenic contaminated drinking water sources in Lake Victoria Gold Fields in Mara region. 4 CONCLUSIONS AND RECOMMENDATIONS Arsenic contamination in Lake Victoria Basin is a really problem in drinking water sources. The most targeted aquifers composed of migmatite-granitoidmetasediment complex and metasediments seem to have high levels of arsenic exceeding WHO guideline. The drilling practice during potable water supply should consider the type of geological units and sediments to avoid continual exposure to arsenic toxicity among Lake Victoria Basin communities. The behaviour of excess arsenic needs to be investigated with respect to seasonal variations and depth.