Water Well Investigation in Nampula Province - Slug Tests in Weathered Crystalline Rock

Abstract: In Nampula Province, Mozambique, locating productive wells is a problem due to the heterogeneity in local geology, which consists of mainly weathered crystalline rock. This study estimates hydraulic conductivity of 10 hand pumped water wells in rural areas of Nampula Province using slug tests, and is a follow up of Andersson and Björnström (2013) and of Enkel and Sjöstrand (2013). This thesis aims to increase the knowledge of these aquifers and similar aquifers. The thesis will help to describe the hydrogeological properties of the area in a bigger attempt to in the future find better methods of finding safe and easily accessible water in areas of similar geology. At each site 6-10 slug tests were conducted, resulting in 68 tests in total for the 10 sites covered. The slug test data was interpreted using Bouwer and Rice (1976) and Cooper et al. (1967) solutions, using the standard commercial software Aqtesolv. Moreover the hydraulic conductivity was estimated using the specific capacity, where the specific capacity was assessed from previous pumping test conducted during the drilling process. The hydraulic conductivities are estimated to be around 0.2-3.9 m/day using the Bouwer and Rice (1976) solution and 0.2-9.7 m/day using the Cooper et al. (1967) solution. The results using the specific capacity gave lower hydraulic conductivities ranging from 0.1-0.6 m/day. However, the results are all within the range of values from fractured igneous and metamorphic rock (Domenico and Schwartz, 1990). The three sites with highest hydraulic conductivity were Camaculo, Muriaze and Naholoco EP1-2, which were in different regions. It is found that water wells in this area that are geographically close may not at all be close in terms of hydraulic conductivity, most likely explained by the local and heterogeneous weathering process. We also show that similar rock types can yield a quite big difference in hydraulic conductivity. The study demonstrated that slug test may very well be a suitable method in similar geological environment, however with strong recommendations of high safety precautions, especially regarding contamination via equipment and risk to ruin pump parts. Moreover, the Cooper et al. (1967) and the Bouwer and Rice (1976) methods are both developed for porous aquifers, but it is found that the methods may be applied for weathered rocks as well.