Browsing by Author "Nkotagu, Hudson H."
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Item Application of Environmental Isotopes to Groundwater Recharge Studies in a Semi-Arid Fractured Crystalline Basement Area of Dodoma Tanzania(Elsevier, 1996) Nkotagu, Hudson H.The distribution of 18O and 2H in various water sources indicates that groundwater recharge is due to local rainfall occurring within the basins. Groundwater recharge takes place under a bypass flow mechanism and matrix diffuse flow and is 3% and 2% of the long-term mean annual rainfall of 550 mm for the Makutapora and Hombolo basins, respectively. Chloride mass balance indicates that 60% and 40% of the total groundwater recharge takes place through macropores and matrix flow, respectively. Sporadic variations in 18O, 2H and chloride among adjacent boreholes suggest existence of a discrete fractured aquifer and/or dominance of local recharge. The relationship between δ2H and chloride indicates that groundwater salinization is due to the leaching of surficial and soil salts during high intensity rainfall, which causes high surface runoff and flash floods. It has been concluded that the isotopic and chemical character of groundwater in fractured semi-arid areas may provide the most effective complementary means of groundwater recharge assessment and therefore is very useful in the management of the water resources.Item Application of Environmental Isotopes to Groundwater Recharge Studies in a Semi-arid Fractured Crystalline Basement Area of Dodoma, Tanzania(1999-02) Nkotagu, Hudson H.The distribution of 18O and 2H in various water sources indicates that groundwater recharge is due to local rainfall occurring within the basins. Groundwater recharge takes place under a bypass flow mechanism and matrix diffuse flow and is 3% and 2% of the long-term mean annual rainfall of 550 mm for the Makutapora and Hombolo basins, respectively. Chloride mass balance indicates that 60% and 40% of the total groundwater recharge takes place through macropores and matrix flow, respectively. Sporadic variations in 18O, 2H and chloride among adjacent boreholes suggest existence of a discrete fractured aquifer and/or dominance of local recharge. The relationship between δ2H and chloride indicates that groundwater salinization is due to the leaching of surficial and soil salts during high intensity rainfall, which causes high surface runoff and flash floods. It has been concluded that the isotopic and chemical character of groundwater in fractured semi-arid areas may provide the most effective complementary means of groundwater recharge assessment and therefore is very useful in the management of the water resources.Item Carbonate Isotope Geochemistry as a Record of Riverine Runoff(2005-01) Dettman, David L.; Fest, Manuel R. P.; Nkotagu, Hudson H.; Cohen, Andrew S.Evaporation dominates the removal of water from Lake Tanganyika, and therefore the oxygen isotope composition of lake water has become very positive in comparison to the waters entering the lake. The surface water in Lake Tanganyika has remained relatively unchanged over the last 30 years with a seasonal range of +3.2 to +3.5‰ VSMOW. Water from small rivers entering the lake seems to have a δ18O value between −3.5 and −4.0‰, based on scattered measurements. The two largest catchments emptying into the lake deliver water that has a δ18O value between these two extremes. This large contrast is the basis of a model presented here that attempts to reconstruct the history of runoff intensity based on the δ18O of carbonate shells from Lake Tanganyika cores. In order to use biogenic carbonates to monitor changes in the δ18O of mixing-zone water, however, the oxygen isotope fractionation between water and shell carbonate must be well understood. The relatively invariant environmental conditions of the lake allow us to constrain the fractionation of both oxygen and carbon isotope ratios. Although molluskan aragonitic shell δ18O values are in agreement with published mineral-water fractionations, ostracode calcite is ∼1.2‰ more positive than that of inorganic calcite precipitated under similar conditions. Ostracode shell δ18O data from two cores from central Lake Tanganyika suggest that runoff decreased in the first half of this millennium and has increased in the last century. This conclusion is poorly constrained, however, and much more work needs to be done on stable isotope variation in both the waters and carbonates of Lake Tanganyika. We also compared the δ13C of shells against predicted values based solely on the δ13C of lake water dissolved inorganic carbon (DIC). The ostracode Mecynocypria opaca is the only ostracode or mollusk that falls within the predicted range. This suggests that M. opaca has potential for reconstructing the carbon isotope ratio of DIC in Lake Tanganyika, and may be a useful tool in the study of the history of the lake’s productivity and carbon cycle.Item Environmental Controls on Shell-Rich Facies in Tropical Lacustrine Rifts: A View From Lake Tanganyika's Littoral(2010-07) Mcglue, Michael M.; Soreghan, Michael J.; Michel, Ellinor; Todd, Jonathan A.; Cohen, Andrew S.; Mischler, John; O'connell, Christine S.; Castañeda, Oceana S.; Hartwell, Richard J.; Lezzar, Kiram E.; Nkotagu, Hudson H.Lake Tanganyika, the world's largest tropical rift lake, is unique among its counterparts in East Africa for the remarkable diversity of mollusk-rich sediments in its littoral zone. Molluscan shell beds are, however, a common feature of ancient lacustrine rift deposits and thus a better understanding of their spatial and temporal development is important. Targeted surveys across the littoral region of the Kigoma Basin reveal three surficial shell-rich facies that differ widely in depositional style and geometry. A unifying characteristic of these deposits is the volume of shells of Neothauma tanganyicense, a large, viviparous gastropod endemic to the lake. Reservoir-corrected radiocarbon dating indicates that Neothauma deposits in these surficial sediments are time averaged over at least the last ∼1600 calendar years BP. Preservation of fossil Neothauma shells in the littoral zone depends on both environmental conditions and on post-mortem shell modifications. Interaction between shells and mobile siliciclastic grains, facilitated by wave action and storms, represents a particularly destructive taphonomic process in the study area. Rank scoring of damage to Neothauma suggests that stromatolitic encrustations or early calcite coatings may help mitigate shell destruction caused by hydraulic fragmentation and abrasion. Persistence of Neothauma in littoral beds has important implications for the structuring of specialized communities of shallow-water benthos, as well as for improving analog models for hydrocarbon reservoirs in lacustrine carbonates.Item Environmental Controls on Shell-Rich Facies in Tropical Lacustrine Rifts: A View from Lake Tanganyika's Littoral(Society for Sedimentary Geology, 2010) McGlue, Michael M.; Soreghan, Michael J.; Michel, Ellinor; Todd, Jonathan A.; Cohen, Andrew S.; Mischler, John; O'connell, Christine S.; Castañeda, Oceana S.; Hartwell, Richard J.; Lezzar, Kiram E.; Nkotagu, Hudson H.Lake Tanganyika, the world's largest tropical rift lake, is unique among its counterparts in East Africa for the remarkable diversity of mollusk-rich sediments in its littoral zone. Molluscan shell beds are, however, a common feature of ancient lacustrine rift deposits and thus a better understanding of their spatial and temporal development is important. Targeted surveys across the littoral region of the Kigoma Basin reveal three surficial shell-rich facies that differ widely in depositional style and geometry. A unifying characteristic of these deposits is the volume of shells of Neothauma tanganyicense, a large, viviparous gastropod endemic to the lake. Reservoir-corrected radiocarbon dating indicates that Neothauma deposits in these surficial sediments are time averaged over at least the last ∼1600 calendar years BP. Preservation of fossil Neothauma shells in the littoral zone depends on both environmental conditions and on post-mortem shell modifications. Interaction between shells and mobile siliciclastic grains, facilitated by wave action and storms, represents a particularly destructive taphonomic process in the study area. Rank scoring of damage to Neothauma suggests that stromatolitic encrustations or early calcite coatings may help mitigate shell destruction caused by hydraulic fragmentation and abrasion. Persistence of Neothauma in littoral beds has important implications for the structuring of specialized communities of shallow-water benthos, as well as for improving analog models for hydrocarbon reservoirs in lacustrine carbonates.Item Environmental Controls on Sshell-Rich Facies in Tropical Lacustrine Rifts: A View from Lake Tanganyika's Littoral(2010-07) McGlue, Michael M.; Soreghan, Michael J.; Michel, Ellinor; Todd, Jonathan A.; Cohen, Andrew S.; Mischler, John H.; O'connell, Christine S.; Castañeda, Oceana S.; Hartwell, Richard. J; Lezzar, Kiram E.; Nkotagu, Hudson H.Lake Tanganyika, the world's largest tropical rift lake, is unique among its counterparts in East Africa for the remarkable diversity of mollusk-rich sediments in its littoral zone. Molluscan shell beds are, however, a common feature of ancient lacustrine rift deposits and thus a better understanding of their spatial and temporal development is important. Targeted surveys across the littoral region of the Kigoma Basin reveal three surficial shell-rich facies that differ widely in depositional style and geometry. A unifying characteristic of these deposits is the volume of shells of Neothauma tanganyicense, a large, viviparous gastropod endemic to the lake. Reservoir-corrected radiocarbon dating indicates that Neothauma deposits in these surficial sediments are time averaged over at least the last ∼1600 calendar years BP. Preservation of fossil Neothauma shells in the littoral zone depends on both environmental conditions and on post-mortem shell modifications. Interaction between shells and mobile siliciclastic grains, facilitated by wave action and storms, represents a particularly destructive taphonomic process in the study area. Rank scoring of damage to Neothauma suggests that stromatolitic encrustations or early calcite coatings may help mitigate shell destruction caused by hydraulic fragmentation and abrasion. Persistence of Neothauma in littoral beds has important implications for the structuring of specialized communities of shallow-water benthos, as well as for improving analog models for hydrocarbon reservoirs in lacustrine carbonates.Item Geochemistry of Shallow Groundwater at Kigamboni Peninsula along Dar es Salaam Coastal Strip Tanzania(Elsevier, 1989) Nkotagu, Hudson H.The paper presents types of groundwater, hydrochemical facies, and chemical genesis of shallow groundwater at Kigamboni peninsula. Five types of groundwater have been identified as sodium bicarbonate, sodium sulphate, sodium chloride, calcium bicarbonate and magnesium bicarbonate type of waters. Hydrochemical facies change from calcium-sodium facies near the shore to sodium-calcium facies landward for cations, and from chloride-sulphate-bicarbonate to bicarbonate-chloride-sulphate facies respectively for anions. Calcium and Magnesium in high concentrations near the shore are attributed to dissolution of dolomitic limestone, being indicated by higher ratio of Ca2+/Mg2+ than one for samples K6, K7 and K9. The rest of the samples show low ratios indicating dolomite dissolution with probable calcite precipitation. Whereas the higher alkali metal concentrations than alkaline earths, observed landward, may probably be due to cation exchange process, similar pattern seen for chloride and sulphate concentrations also suggest mixing of fresh recharge water with interstitial marine water and or dissolution of airbone salts by recharging rainwater resulting into oceanic salt recycling. However, the water is observed to be suitable for general domestic and irrigation purposes. It has been concluded that the chemical character of shallow groundwater in the studied area, is a result of the interaction between recharge water with lithology and mixing with probable trapped marine water.Item The Groundwater Geochemistry in a Semi Arid Fractured Crystalline Basement Area of Dodoma(Elsevier, 1996) Nkotagu, Hudson H.The chemical character of groundwater is generated through processes including the leaching of surficial and near-surface soil salts, cation exchange and mineral dissolution of, in this case, mainly amphiboles and sodic plagioclase. Molar ratios thermodynamic analysis using a NETPATH geochemical model and standard Piper trilinear and Gibbs diagrams have been used to establish these geochemical processes. Though the groundwater is dominantly of NaCl type, it is apparently suitable for general irrigation purposes.Item The Groundwater Geochemistry in a Semi-Arid, Fractured Crystalline Basement Area of Dodoma, Tanzania(1999-02) Nkotagu, Hudson H.The chemical character of groundwater is generated through processes including the leaching of surficial and near-surface soil salts, cation exchange and mineral dissolution of, in this case, mainly amphiboles and sodic plagioclase. Molar ratios thermodynamic analysis using a NETPATH geochemical model and standard Piper trilinear and Gibbs diagrams have been used to establish these geochemical processes. Though the groundwater is dominantly of NaCl type, it is apparently suitable for general irrigation purposesItem Hydrogeochemistry of Fluoride and Salinization Mechanism of Groundwater in the Singida Region, Central Tanzania(2009) Nkotagu, Hudson H.This study focuses on the determination of factors that control high fluoride groundwater and salinization in basement aquifers of a semi-arid area in central Tanzania. Water samples were collected from productive dugouts, shallow wells and boreholes. The presence of 18O, 2 H, and major cation and anion chemistry, including fluoride, was determined with the use of standard analytical techniques. A sodium-bicarbonate type of water dominates the study area. The 18O - 2 H relationships suggest that the infiltration of water and groundwater has undergone evaporation while in some cases preferential flow mechanisms are suggested. Shallow wells and dugouts were found to have higher concentrations of both fluoride and chloride and this fact is partly attributed to the effect of evapotranspiration. The main mechanism for fluoride input into groundwater as well as salinization is attributed to the leaching of surface and near-surface soil salts, especially trona. No clear relationship was observed between the depth of wells/boreholes and fluoride concentration. Similarly, there is hardly any clear relationship between the fluoride concentration and the geomorphology of the study area. A more detailed investigation is needed to determine the relationship between sampling depth, fluoride concentration and salinization.Item Hydrogeochemistry of Fluoride and Salinization Mechanism of Groundwaters in the Singida Region, Central Tanzania(2009) Nkotagu, Hudson H.This study focuses on the determination of factors that control high fluoride groundwater and salinization in basement aquifers of a semi-arid area in central Tanzania. Water samples were collected from productive dugouts, shallow wells and boreholes. The presence of 18O, 2 H, and major cation and anion chemistry, including fluoride, was determined with the use of standard analytical techniques. A sodium-bicarbonate type of water dominates the study area. The 18O - 2 H relationships suggest that the infiltration of water and groundwater has undergone evaporation while in some cases preferential flow mechanisms are suggested. Shallow wells and dugouts were found to have higher concentrations of both fluoride and chloride and this fact is partly attributed to the effect of evapotranspiration. The main mechanism for fluoride input into groundwater as well as salinization is attributed to the leaching of surface and near-surface soil salts, especially trona. No clear relationship was observed between the depth of wells/boreholes and fluoride concentration. Similarly, there is hardly any clear relationship between the fluoride concentration and the geomorphology of the study area. A more detailed investigation is needed to determine the relationship between sampling depth, fluoride concentration and salinization.Item Hydrological evaluation of Two Contrasting Watersheds of the Lake Tanganyika Catchment(2005-01) Nkotagu, Hudson H.This study was conducted to delineate the impact of human activities on stream flow and water chemistry as well as other factors that influence the chemical character of both surface and groundwater in two contrasting watersheds of the Lake Tanganyika catchment. The study sites the Mwamgongo and Mitumba streams along the northern Tanzanian coastline of the lake are representative of disturbed and undisturbed watersheds, respectively, but are quite similar in other characteristics of slope, bedrock geology and size. Separation of stream flow components was undertaken using classical hydrograph analysis along with chemical methods using both Cl and 18O data. All the data show that groundwater accounts for the predominant source of total stream flow in both the Mwamgongo and Mitumba watersheds (65 and 70% respectively). The streams have an average δ18O of about -3.0% and less than 10 mg/l for Cl. The basin recession constants of 9.4×10-3-d-1 and 9.6×10-3-d-1 for Mwamgongo and Mitumba, respectively, indicate existence of both fissured and fractured aquifer systems. The chemical data exhibit low values of all determined ions. This supported the hypothesis that natural processes influence the water chemical character of the study area. An Mg–HCO3 type of water dominates in the two watersheds. Despite their similar size and bedrock character the Mwamgongo watershed has an order of magnitude in sediment transport than the Mitumba one. The data show that the disturbed watershed discharges less groundwater and more sediments, and has a poorer water quality than the forested Mitumba watershed, which lies within the Gombe National Park. The data show that soil erosion processes are more active at Mwamgongo, and that both the surface runoff component of the total stream flow and increased dissolved salt load is greater in the deforested Mwamgongo watershed than in the Mitumba watershed. The chloride and δ18O data complemented each other in delineating the amounts of groundwater in the total stream flow as the results using both data differed insignificantly. It may be concluded that the undisturbed watershed has a higher retention of good quality water and traps more sediments than the disturbed one. In addition, the groundwater component plays a dominant role in the total annual stream flow at each watershed.Item Hydrological Evaluation of Two Contrasting Watersheds of the Lake Tanganyika Catchment(2005) Nkotagu, Hudson H.Item Hydrology of Selected Watersheds along the Lake Tanganyika Shoreline. Lake Tanganyika Biodiversity Project(2000) Nkotagu, Hudson H.; Mwambo, K.Item Lake Tanganyika Ecosystem Management Strategies(Taylor & Francis, 2008) Nkotagu, Hudson H.Lake Tanganyika is a large East African rift valley system holding about 1/6 of the world's liquid freshwater with about 2000 species of organisms (fauna and flora), of which about 700 are endemic. The lake faces a number of threats including excess sedimentation, overfishing, pollution, habitat destruction, along with climate change. Efforts to better understand these involved an assessment of the magnitude of the threats, through the Lake Tanganyika Biodiversity project (LTBP) in which a number of outputs such as Draft conventions, special study reports and the Strategic Action Programme were achieved. The preparation of detailed projects to address the threats through the Lake Tanganyika Management Planning Projects (LTMPP) was another strategy, as well as projects prepared for management of catchment and pollution control, along with fishing management. It can be concluded that Lake Tanganyika faces essentially man-induced threats compounded by climate change, probably resulting in declining productivity of the lake and declining water levels. It is concluded that in order to maintain sustainability of the lake, both regional and global joint efforts are required.Item Lake Tanganyika Ecosystem Management Strategies(2008-03) Nkotagu, Hudson H.Lake Tanganyika is a large East African rift valley system holding about 1/6 of the world's liquid freshwater with about 2000 species of organisms (fauna and flora), of which about 700 are endemic. The lake faces a number of threats including excess sedimentation, overfishing, pollution, habitat destruction, along with climate change. Efforts to better understand these involved an assessment of the magnitude of the threats, through the Lake Tanganyika Biodiversity project (LTBP) in which a number of outputs such as Draft conventions, special study reports and the Strategic Action Programme were achieved. The preparation of detailed projects to address the threats through the Lake Tanganyika Management Planning Projects (LTMPP) was another strategy, as well as projects prepared for management of catchment and pollution control, along with fishing management. It can be concluded that Lake Tanganyika faces essentially man-induced threats compounded by climate change, probably resulting in declining productivity of the lake and declining water levels. It is concluded that in order to maintain sustainability of the lake, both regional and global joint efforts are required.Item The Lake Tanganyika Environmental Management strategies(Taylor & Francis, 2008) Nkotagu, Hudson H.Item Limnogeology of the Lakes Sagara And Nyamagoma(2008-03) Nkotagu, Hudson H.; Athuman, Charles B.The present study focuses on the limnogeological aspects of the lakes Sagara and Nyamagoma within the Malagarasi Wetland Ecosystem. Abiotic parameters from water and sediment samples were determined during the dry and wet seasons using appropriate gears and methods. The data show variation of the abiotic parameters in both lakes from the water surface to the bottom. Lake Nyamagoma is less turbid (0.7-24 NTU) than Lake Sagara (25 to 65 NTU). The water transparency was also high at Lake Nyamagoma (0.75-1.5 m) thus supporting the observed turbidity trend. The physical chemical changes may be attributed to processes such as cooling; dissociation; dissolution; decomposition; adsorption; precipitation; cation exchange and photosynthesis. However; higher nutrient concentration was observed in the wet season than dry season with the exception of SiO2 consequent to dilution effect by surface runoff. The mineralogical content (Kaolinite; Smectite; lllite and Quartz) of the lake sediments indicate existence of good drainage conditions at the lakes as supported by the geology of the area. It is recommended that a long-term limnogeological monitoring and evaluation be conducted so as to understand future nutrient hydrodynamics and hydrological functioning of the wetland ecosystem.Item Limnogeology of the Lakes Sagara And Nyamagoma(2008-03) Nkotagu, Hudson H.; Athuman, Charles B.The present study focuses on the limnogeological aspects of the lakes Sagara and Nyamagoma within the Malagarasi Wetland Ecosystem. Abiotic parameters from water and sediment samples were determined during the dry and wet seasons using appropriate gears and methods. The data show variation of the abiotic parameters in both lakes from the water surface to the bottom. Lake Nyamagoma is less turbid (0.7-24 NTU) than Lake Sagara (25 to 65 NTU). The water transparency was also high at Lake Nyamagoma (0.75-1.5 m) thus supporting the observed turbidity trend. The physical chemical changes may be attributed to processes such as cooling; dissociation; dissolution; decomposition; adsorption; precipitation; cation exchange and photosynthesis. However; higher nutrient concentration was observed in the wet season than dry season with the exception of SiO2 consequent to dilution effect by surface runoff. The mineralogical content (Kaolinite; Smectite; lllite and Quartz) of the lake sediments indicate existence of good drainage conditions at the lakes as supported by the geology of the area. It is recommended that a long-term limnogeological monitoring and evaluation be conducted so as to understand future nutrient hydrodynamics and hydrological functioning of the wetland ecosystem.