Hydrogeochemical evolution of subsurface water in an inland valley wetland, Uganda – the role of silicate weathering in saprolite and alluvium

Laura Heiß1, Sonja Beuel1, Barbara Reichert1
1 Arbeitsgruppe Hydrogeologie, Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Universität Bonn

O 15.2 in Forum Junge Hydrogeologen

13.04.2016, 13:00-13:15, Plank Hörsaal, Geb. 40.32

 

The presented master thesis is embedded in the BMBF-funded project “GlobE – Wetlands in East Africa”. With the guiding theme "Reconciling future food production with environment protection" an interdisciplinary and international consortium assesses the wetlands' contributions to food security along climatic and social gradients. Their year-round water availability gives wetlands the potential to become production hotspots concurring with its other valuable ecosystem services. Since agricultural use is highly dependent on water quality it is necessary to outline the hydrogeochemical evolution of the wetland’s water.

A sound hydrogeological mapping, accompanied by several drilling surveys was performed on the basis of an existing geological map. Single piezometer pumping tests as well as granulometric analyzes were used to determine the hydraulic parameters of the aquifer system. Chemical and mineralogical composition of representative sediment samples were analyzed using XRD and XRF, with special focus on clay minerals. An adequate spatial coverage of major, minor, and trace elements as well as in-situ parameters of wetland water is given by snap-shot samples taken from suction cups, piezometers and pumping wells.  

The inland valley of Namulonge comprises Quaternary alluvial sediments overlying a saprolite. The existing geological map had to be modified with new stratigraphic correlations. The lateral expansion of the alluvial sediments comprises the valley bottom including lower parts of the wetlands slope. The upper organic soil layers consisting mainly of clay together with lower layers of sand, gravel and resorted saprolite rock fragments build a confined heterogenic layered aquifer system. Pumping test data evaluated with the time drawdown approach revealed transmissivities ranging between 10⁻⁵ and 10⁻⁶ m²/s with hydraulic conductivities of 10⁻⁵ m/s. The granulometric analyzes of depth-specific samples proved the heterogeneity of the sediment column with hydraulic conductivities ranging between 10⁻⁴ and 10⁻⁷ m/s. The alluvial sediments reflect a mineralogical composition of mainly quartz, kaolinite and feldspar with minor phases including smectite and illite/muscovite as a result of in-situ silicate weathering of granite, orthogneiss and slate. The quartz content decreases and the feldspar content increases from residual soil with prolonged saprolite depth, but the kaolinite content remains nearly constant. Hydrochemical composition is related to the geochemical evolution. The decrease of the quartz content in soils and rocks with depth is reflected clearly in the dissolved silica content of water. The high disaggregation of albite, anorthite and microcline in the upper layers is indicated by high concentrations of calcium, sodium and potassium. The deeper the weathering profile, the lower the aluminum concentration due to the formation of the secondary clay mineral kaolinite.



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