Bakundukize: their discussion and ideas were absolutely significant for producing good articles. ... more Bakundukize: their discussion and ideas were absolutely significant for producing good articles. I also take this moment to give my sincere thanks to the Geology Department, University of Dar es Salaam (UDSM) for the in-depth information about the study area that I received from Dr. Msindai (who passed away on 29 th July 2011). His support and many thoughts and ideas about the subject of research during my fieldwork were very great. It was indeed a sad moment to see a friend/supervisor and my BSc (Geology) teacher dying at the moment nobody expected. I will remember him always for his invaluable and tireless effort in advising and supporting me in my study. I would like to express my deepest and sincere appreciation to Marc Van Camp for his expertise and invaluable time spent in assisting to develop a conceptual model for understanding the saltwater transport processes occurring in the study area. I am grateful to Jill Van Reybrouck, the laboratory chemist of the Laboratory for Applied Geology and Hydrogeology (UGent) for her great effort spent in analyzing water samples. I wish to extend thanks to Dr. Kristine Martens (UGent) for her valuable comments to this study and for travelling from Belgium to Tanzania to give assistance in carrying out the geophysical survey. Dr. Flora Magige (UDSM) is thanked for encouragement and provision of materials and relevant books for my research topic. Thanks are due to the Drilling and Dam Construction Agency (DDCA) and Ministry of Water and Irrigation (MoWI) for giving access to borehole/well drilling reports and sharing their indepth experience and knowledge of the study area. Additional thanks go to Jovit Josephat and Arnold Kessy for their guidance during the field work mainly for identifying boreholes. Resident and boreholes owners are thanked for the cooperation given for accessing their boreholes. The fieldwork campaign for data collection would have not been successful without great help from Dar es Salaam Municipalities: Temeke, Kinondoni, and Ilala for the assistance provided during the field campaign while visiting the boreholes/wells and for providing data and useful suggestions. Many thanks go to Gymkhana Golf Club for giving the permission to conduct geophysical survey at their premises and providing boreholes which enabled groundwater monitoring. Likewise the management and staff of Coca-Cola Company, Zanaki Secondary school, Mnazi Mmoja Primary school, Bunge Dar es Salaam offices, and other institutions are all thanked for allowing access to their boreholes/wells during data collection.
Several studies conducted on occurrences of evaporitic salts in various parts of the world often ... more Several studies conducted on occurrences of evaporitic salts in various parts of the world often interpret the formation of such salts as the result of the evaporation of subsurface brines, saline lake waters and sea water-contaminated groundwater, on one hand, and the deposition of sea sprays and wind-transported salts, on the other hand. This study seeks to understand processes governing the formation of evaporitic salts in a humid tropical context and to establish a link between the occurrence of these salts and the different processes controlling the hydrogeochemical evolution of groundwater in Bugesera region, in northeastern Burundi. Unlike previous studies, salt occurrences in the study area occur in a continental and humid tropical environment where chemical analyses of water samples from several small lakes reveal rather fresh water. While salt incrustations reported in the western Burundi occur in a sedimentary basin which is made up of a mixture of lacustrine and fluviatile sediments, the evaporitic salts in the depression of Bugesera appear in the upper layers of the weathered overburden developed on Precambrian basement rocks. These facts suggest that the lithological nature and setting of the area may not be the key factor controlling the formation of these salt deposits. X-ray diffraction (XRD) analyses performed on saline soils from two quarries in Bugesera region confirm the presence of salt minerals including particularly gypsum (CaSO 4 •2H 2 O), halite (NaCl) and thenardite (Na 2 SO 4). Other minerals identified such as goethite, kaolinite, muscovite and quartz may be part of the soil matrix. Chemical analyses on leachates of soil samples from the 2 quarries in Bugesera region confirm their saline nature as evidenced by high TDS (1826-17,679 mg/l) and pH values (8.3-8.5). The predominance of Na + , Mg 2+ , Ca 2+ , K + , SO 4 2− , Cl − in the chemical composition of the dissolved salts clearly reflects their mineralogical composition. This study clearly demonstrates that these evaporitic salts originate from the combination of three processes, namely the dissolution of aluminosilicate minerals which enrich groundwater in different salt-forming ions, the capillary rise which lifts the shallow groundwater and the dissolved ions up to the reach of evaporation and ultimately the evapoconcentration which precipitates the salts following well-established pathways. These findings are well supported by the results of the simulation of the evapoconcentration process with the PHREEQC hydrochemical model.
This study investigates a Precambrian basement aquifer in Bugesera region, a typical African rura... more This study investigates a Precambrian basement aquifer in Bugesera region, a typical African rural area in northeastern Burundi. Domestic water supply relies on groundwater which is tapped through hand-dug wells. Despite several attempts to increase the number of water points in the area, the water demand is still far from being met as a result of the high rate of well failure. This paper seeks to understand whether the hydrogeological structure and the spatial distribution of hydraulic parameters can explain the low productivity and the high failure rate of hand-dug wells. The hydrogeological structure inferred from the interpretation of a large number of vertical electrical soundings (VES) reveals a typical sequence of geoelectrical layers, which is characterized by an overall upwards fining from the fresh basement, over the fractured/weathered basement, to the overburden or saprolite with a clay-rich layer on top. Whereas the overall aquifer potential mainly depends on the thickness of the weathered overburden, the aquifer potential for shallow hand-dug wells is determined by the hydraulic conductivity of the upper few meters of the saturated zone. This upper zone was investigated in the pumping tests. The spatial distribution of the specific capacity reveals a wide variation of hydraulic parameters, depending on the well's position in the depth profile of the aquifer's hydraulic conductivity. The thickness of the potential aquifer is highest in the central part of the study area (pegmatitic and granitic intrusions) which has the highest overall aquifer potential compared to the surrounding metasedimentary formations. However, a thick weathered overburden will increase the groundwater potential of an aquifer for deep boreholes, whereas for hand-dug wells, the productivity can only be high if the thickness of the weathered overburden is small enough, or the water table is deep enough, to allow to tap the coarse part at the base of the overburden and/or part of the weathered/fractured basement. This study reveals that the main cause of low productivity and failure of many hand-dug wells in Bugesera region is not to be linked to poor groundwater resource potential but rather stems from the inappropriate random siting of wells and from poor well construction expertise which does not take into account the hydrogeological conditions. The results of the investigation indicate that in the typical weathering profile of the Precambrian basement in Sub-saharan Africa, the coarse lower part of the weathered overburden and the fractured/weathered basement offer good prospects for water resource. Depending on the depth at which this layer is encountered, either shallow hand-dug wells or deep drilled wells are most appropriate.
Groundwater first started to be used as a source of water supply for the Dar-es- Salaam City in 1... more Groundwater first started to be used as a source of water supply for the Dar-es- Salaam City in 1943. However, many of the existing boreholes in Dar-es-Salaam were drilled in 1997 after Tanzania experienced a severe drought; since then groundwater use in the city shows an increasing trend and currently over 50 percent of residents in Dar es Salaam City relies on groundwater supply. Today, there are more than 10000 boreholes for domestic, water supply and industrial purposes. Demand and reliance on groundwater from a growing population along the coast raises the need to increase efforts to protect and manage the resource. In the coastal aquifer of Dar-es-Salaam, groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. The results show that the origin of salinity in the area is strongly influenced by: groundwater ascending from deep marine Miocene Spatangid Shales through faults, saltwater intrusion on the...
Advances in the Research of Aquatic Environment, 2011
ABSTRACT Water balance for Dar es Salaam Quaternary coastal aquifer (DQCA) was calculated accordi... more ABSTRACT Water balance for Dar es Salaam Quaternary coastal aquifer (DQCA) was calculated according to the method of Thornthwaite and Mather. Monthly potential evapotranspiration estimates were calculated from 39 years (1971-2009) of routine meteorological data using Penman-Monteith, Thornthwaite, Hargreaves and Hamon methods. Results were compared in order to show possible differences that could be attributed to the methods. Determination of groundwater recharge rates gave a mean value of 184 mm year-1 which is equivalent to 71.39 x 106 m3 year-1 indicating that 16.5% of the long term average annual precipitation of 1114 mm ends up as groundwater recharge. Groundwater abstraction from DQCA was estimated to be 69.3 x 106 m3 year-1. These results are alarming in two aspects: high abstraction rate and the increasing trend of borehole drilling. Groundwater abstraction which is nearly equal to the amount of the groundwater recharge is far greater than the least conservative sustainable yield calculated at 70% (equivalent to 49.97 x 106 m3 year-1).
Groundwater is globally important for human consumption, and changes in quality can have serious ... more Groundwater is globally important for human consumption, and changes in quality can have serious consequences. The study area is within a coastal aquifer where groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. Groundwater chemistry data from the aquifer have been acquired to determine the geochemical conditions and processes that occur in this area and assess their implications for aquifer susceptibility. Analysis of groundwater samples shows that the dominant watertype is mostly NaCl with pH < 7 in both aquifers (i.e. upper and lower) except for the shallow wells where CaHCO3 prevails with pH >= 7, and boreholes located near the Indian Ocean, where coral reef limestone deposits are located and the watertype evolves towards CaHCO3. In the lower aquifer, Clis higher than in the upper aquifer. The origin of salinity in the area is strongly influenced by groundwater ascending from deep marine Miocene Spatangid Shales through faults, seawater incursion on the border of the Indian Ocean, and throughout, there is some salinity within the Quaternary aquifer, especially in intercalated deltaic clays in the fluviatile deposits, showing some marine influences. The seawater intrusion is linked to the strongly increasing groundwater exploitation since 1997. Another process that plays a major role to the concentration of major ions in the groundwater is calcite dissolution. Next to geogenic salinity and seawater intrusion, anthropogenic pollution as well is affecting groundwater quality in the aquifer. An important result of this study is the observation of high nitrate concentrations, that call for improved sanitation in the area, where domestic sewage with on-site sanitation (mainly pit latrines) also threatens the groundwater resource.
Groundwater is a last-resort source of domestic water supply in Dar es Salaam City because of the... more Groundwater is a last-resort source of domestic water supply in Dar es Salaam City because of the scarcity of surface water sources. The Tanzania Government, Non Government Organizations (NGOs), Community Based Organizations (CBOs) and international aid organizations have promoted the drilling of boreholes. From 1997 until the present, boreholes drilling has increased tremendously and the trend is expected to increase even more in the future. Initial assessment of the current state of water quality indicates that the coastal aquifers are vulnerable to saline intrusion. In some areas, overpumping of the groundwater has resulted in encroachment of sea water into the coastal aquifers. Water samples from Mikocheni, Oysterbay, Msasani, Masaki and City Centre areas show elevated chloride, sulphate and sodium concentrations. Using Stuyfzand (1986) water classification method, over 50 percent of the samples showed the type of fresh-brackish to salt and extremely hard. Residents depend on the shallow aquifer at less than 15 m, because at this depth the water is relatively less saline. The saltwater interface is likely to be moving inland on seasonal basis and some boreholes become more saline in late summer. The growing human population in and around the investigated areas has had an adverse impact on the water in the shallow aquifer through, among other factors, indiscriminate abstraction of water without knowledge of the aquifer's capacity. In order to prevent further deterioration of the aquifer, there is a need to establish an inventory of the wells, conduct proper pump tests and geochemical characterization to ascertain the capacity of the shallow aquifer and understand the processes affecting the overall quality of the groundwater.
Water supply requirements in Dar es Salaam city (Tanzania) are rising rapidly by population growt... more Water supply requirements in Dar es Salaam city (Tanzania) are rising rapidly by population growth and groundwater is increasingly used to fullfill the needs. The groundwater is taken from the Dar es Salaam Quaternary coastal aquifer (DQCA), stretching inland from the coastline. As thousands of wells have been drilled in the coastal strip and pumping rates are uncontrolled, seawater intrusion is deteriorating the quality of fresh groundwater. To investigate the response of the fresh/salt water interface to coastal pumping, simulations with a schematic two-dimensional cross-sectional model have been done. Depending on the depth of the wells in the 150 m thick DQCA and their distance from the coastline, different pathways of seawater intrusion and shifts of the interface can be recognized. The local presence of a semi-pervious layer can have a significant impact on the fresh/salt water distribution. Although the modeled section is not related to a specific location along the coastline but rather represents an average aquifer buildup, the results of the simulations can be used to formulate recommendations when drilling new wells and for a better monitoring of the salinisation process along the coast. It proves that even simple schematic models can give meaningful contributions.
The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deform... more The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670-61 O°C and pressures of 6.5-6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X,l0=O.5 in calcsilicate rocks. These findings, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P-Tpath was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt; followed by their residence and cooling in the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed. o 1999 Elsevier Science Limited. All rights reserved. RESUME-Le socle de la zone de Merelani de la Ceinture du Mozambique dans le nord-est de la Tanzanie a ete def& et metamorphise de multiples fois dans les temps panafiicains (Nr%oproterozdique). Le premier evenement metamorphique a eu lieu dans le facies des granuliies et les roches ont subi plus tard une evolution retrograde par refroidissement isobare dans le facies des amphibolites. Les donnees thermobarom&riques des gneiss de Merelani indiquent des temperatures de I'ordre de 670-610°C et des pressions de 65(1600 MPa. Le refroidissement isobare est illustrt! par la croissance tardive posttectonique de porphyroblastes de disth&ne aux depens de la sillimanite dans les paragneiss graphiteux, ainsi que par la zonation du grenat dans les m8rnes roches. Une evolution retrograde supplementaire a ete due a I'hydratation le long de zones et de failles de cisaillement, conduisant a I'alteration de la forsteriie, I'enstatite et la pargasite en mineraux du groupe de la serpentine. Dans les roches calcosilicatees alt&es par hydrothermalisme, la cristallisation de zoisite vanadif&re ftanzanite) a eu lieu B ce stade pour X,,=O.5. Les resultats, combines aux donnees texturales et thermobarometriques des Monts Uluguru et de la zone de Furua, suggerent que le trajet pression-temperature de refroidissement isobare a 6te predominant dans la (les) evolution(s) metamorphiquefs) des roches de la Ceinture panafricaine
Ga onmiddellijk naar paginanavigatie. Groundwater pollution caused by seepage from Tailing Storag... more Ga onmiddellijk naar paginanavigatie. Groundwater pollution caused by seepage from Tailing Storage Facilities (TSF) : a case study of gypsum dumpsite in north of Ghent, Belgium. Please use this identifier to cite or link to this item: Record Details. Record ID, 999117. ...
The current groundwater replenishment under natural infiltration is about 184 mm (equivalent to 7... more The current groundwater replenishment under natural infiltration is about 184 mm (equivalent to 71.39 x 10 6 m 3 year-1) indicating that only 16.5% of the long term average annual precipitation of 1114 mm ends up as groundwater recharge (Mtoni et al., 2011). The water balance of the catchment suggests for the average sustainable yield (calculated as 40% of natural groundwater recharge) a value of 28.56 x 10 6 m 3 year-1. The current groundwater abstraction is approximately 69.3 x 10 6 m 3 year-1) and such overexploitation of groundwater usually results in a rise of the freshwater-saltwater interface and lateral seawater intrusion in a coastal aquifer, and thus degradation of groundwater quality.
Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting overexp... more Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting overexploitation is entailing the risk of seawater intrusion. This problem has been systematically investigated for the first time, and appears to be widespread. Determining the depth of the fresh-salt water interface was deemed crucial in the view of sustainable exploitation of the Dar es Salaam quaternary coastal aquifer system. Vertical electrical soundings (VES) and horizontal resistivity profiles have shown a decrease of resistivity with depth and toward the coastline. Cross sections have shown the presence of salt water in the aquifer near the coastline, and the boundary between fresh and saline waters is mapped. The surface prospection resistivity data showed comparable results as reflected in hydrochemical/lithological cross sections and resistivity well logs. Salt water was found mostly in the area within 2 km of the coastline, and the depth to the interface in this zone is ranging from 1.3 to 20 m. Away from the coastline, the interface (if present) is at greater depth. VES executed at Kurasini (4 km from the sea) showed the fresh-salt water interface at a greater depth (43 m).
Abstract This study investigates hydrogeochemical processes controlling groundwater evolution in ... more Abstract This study investigates hydrogeochemical processes controlling groundwater evolution in a Precambrian basement aquifer situated in northeastern Burundi, in Bugesera region. The hydrogeochemistry has been evaluated based on the analytical results of 143 water samples comprising 66 samples from shallow wells, 61 samples from springs, and 16 samples from the shallow lakes. While the stability diagrams, the evolutionary sequence of groundwater inferred from the Piper diagram and bivariate plots highlight the weathering of aluminosilicates as the primary process controlling groundwater chemistry in this aquifer, the increasing trend of chloride levels between the highlands (recharge area) and the depression of Bugesera (discharge area) and the random distribution of high nitrate content even in water samples from springs suggest that additional processes such as evaporative concentration and anthropogenic pollution play a secondary role in the hydrogeochemical evolution of groundwater. The spatial distribution of different hydrogeochemical parameters shows a general increasing trend from the highlands where water samples, mainly from springs, show low mineralisation (TDS = 26–152 mg/l) and low pH (field pH = 4.6–6.6), towards the depression of Bugesera where higher values of mineralisation (TDS = 44–3229 mg/l) and pH (field pH = 5.6–7.7) are observed. The overriding control of aluminosilicates weathering on the hydrogeochemical evolution of groundwater in Bugesera region is clearly demonstrated by the predominance of NaHCO3 and CaHCO3 water types and the cross-plots which show an increasing trend of alkaline and alkaline earth cations (Na+ + K+ + Ca2 + + Mg2 +) released from the weathering of aluminosilicates along the flow line. Moreover, the poor correlation between the concentrations of SiO2 and Na+, Ca2 +, Mg2 +, K+, HCO3– and TDS reflects the incongruent character of the weathering of aluminosilicates which causes part of the silica released from the weathering of silicates to be taken up into the neoformation of clay minerals. The anthropogenic pollution is reflected by the occurrence of NaNO3, CaNO3, KNO3 and NaSO4 types even in spring water.
Introduction 1.1 Background In the last two decades, the use of groundwater in Dar es Salaam City... more Introduction 1.1 Background In the last two decades, the use of groundwater in Dar es Salaam City has increased considerably because of the scarcity of surface water sources. With only 25 percent of the population receiving water supplied by Dar es Salaam Water and Sewerage Authority (DAWASA), much of the water needs must be met through private boreholes (UN HABITAT, 2009). Since 1997, the Tanzanian Government, Non Government Organizations (NGOs), Community Based Organizations (CBOs) and international aid organizations have promoted the drilling of boreholes to try to meet the demand of water supply in the city. Despite the significant importance of groundwater in Dar es Salaam City, threats exist, including overabstraction that can lead to the intrusion of salt water in fresh water aquifers. A study by Nkotagu (1989) reported marine intrusion in the coastal aquifer of the Kigamboni Peninsula with elevated chloride, sulphate and sodium concentrations. A study made by the Ministry of Water (URT, 2007) to develop future water sources for Dar es Salaam City, reported an increase of salinity in several boreholes in the coastal area of the city. The increase of salinity at some points of the coastal area in Dar es Salaam Region has been surveyed over the last 20 years. According to the previous work, the cause of salinization was thought to be influenced by various sources of salinity. These include: seawater intrusion, airborne salts, water ascending from deep marine sediments, and clay-bound sands contributing salinity themselves (Mnzava, 1986; Msindai, 1988; Mjemah, 2007). Groundwater resources in coastal areas are always in danger of contamination by sea water intrusion. The likelihood of this taking place is increased when rates of extraction of groundwater close to the sea are raised due to increased demand on fresh water resources. This paper presents preliminary results of an ongoing PhD research on "Regional Hydrogeochemical Characterization of Processes in Quaternary Sand Aquifers of the Dar es Salaam Region". It examines if saltwater intrusion is a pressing problem along the Dar es Salaam coastal area. 1.2 Population The last national census was in 2002, so all current figures are estimates. The city's present population is estimated to be over 3.5 million people (Fig. 1) with a growth rate of 4.3%. The high population has proven to be a serious challenge in the provision of housing and essential services such as water, sanitation and health care (URT, 2004). Population in the municipality varies from settlement to settlement. High population densities are found in unplanned settlements, while low population densities are found in peripheral localities. 1.3 Water supply and management problems The increased demand of water in Dar es Salaam City and uncontrolled groundwater abstraction poses a danger of seawater intrusion due to overpumping. From 1997 until the present, boreholes drilling has increased tremendously and the trend can still be expected to increase in the future. It is estimated that about 1000 boreholes are drilled annually (Baumann et al., 2005). Individuals, the Community, industries, institutions and tourist hotels have sunk a number of boreholes from which large quantities of water are withdrawn using electrical and/or diesel pumps. All these put a strain on the fresh groundwater resources and increase the possibility of sea water intrusion. 1.4 Objectives of the study The objectives for this study were: (1) To preliminarily assess the current state of water quality in selected areas near the coastline, especially in view of assessing sea water intrusion; (2) To attempt, with data available and based on the groundwater samples of a first field campaign, to give an early warning of the threat of sea water intrusion along the Dar es Salaam coast; and (3) To review available information in relation to this study and design a strategy for further field surveys.
Bakundukize: their discussion and ideas were absolutely significant for producing good articles. ... more Bakundukize: their discussion and ideas were absolutely significant for producing good articles. I also take this moment to give my sincere thanks to the Geology Department, University of Dar es Salaam (UDSM) for the in-depth information about the study area that I received from Dr. Msindai (who passed away on 29 th July 2011). His support and many thoughts and ideas about the subject of research during my fieldwork were very great. It was indeed a sad moment to see a friend/supervisor and my BSc (Geology) teacher dying at the moment nobody expected. I will remember him always for his invaluable and tireless effort in advising and supporting me in my study. I would like to express my deepest and sincere appreciation to Marc Van Camp for his expertise and invaluable time spent in assisting to develop a conceptual model for understanding the saltwater transport processes occurring in the study area. I am grateful to Jill Van Reybrouck, the laboratory chemist of the Laboratory for Applied Geology and Hydrogeology (UGent) for her great effort spent in analyzing water samples. I wish to extend thanks to Dr. Kristine Martens (UGent) for her valuable comments to this study and for travelling from Belgium to Tanzania to give assistance in carrying out the geophysical survey. Dr. Flora Magige (UDSM) is thanked for encouragement and provision of materials and relevant books for my research topic. Thanks are due to the Drilling and Dam Construction Agency (DDCA) and Ministry of Water and Irrigation (MoWI) for giving access to borehole/well drilling reports and sharing their indepth experience and knowledge of the study area. Additional thanks go to Jovit Josephat and Arnold Kessy for their guidance during the field work mainly for identifying boreholes. Resident and boreholes owners are thanked for the cooperation given for accessing their boreholes. The fieldwork campaign for data collection would have not been successful without great help from Dar es Salaam Municipalities: Temeke, Kinondoni, and Ilala for the assistance provided during the field campaign while visiting the boreholes/wells and for providing data and useful suggestions. Many thanks go to Gymkhana Golf Club for giving the permission to conduct geophysical survey at their premises and providing boreholes which enabled groundwater monitoring. Likewise the management and staff of Coca-Cola Company, Zanaki Secondary school, Mnazi Mmoja Primary school, Bunge Dar es Salaam offices, and other institutions are all thanked for allowing access to their boreholes/wells during data collection.
Several studies conducted on occurrences of evaporitic salts in various parts of the world often ... more Several studies conducted on occurrences of evaporitic salts in various parts of the world often interpret the formation of such salts as the result of the evaporation of subsurface brines, saline lake waters and sea water-contaminated groundwater, on one hand, and the deposition of sea sprays and wind-transported salts, on the other hand. This study seeks to understand processes governing the formation of evaporitic salts in a humid tropical context and to establish a link between the occurrence of these salts and the different processes controlling the hydrogeochemical evolution of groundwater in Bugesera region, in northeastern Burundi. Unlike previous studies, salt occurrences in the study area occur in a continental and humid tropical environment where chemical analyses of water samples from several small lakes reveal rather fresh water. While salt incrustations reported in the western Burundi occur in a sedimentary basin which is made up of a mixture of lacustrine and fluviatile sediments, the evaporitic salts in the depression of Bugesera appear in the upper layers of the weathered overburden developed on Precambrian basement rocks. These facts suggest that the lithological nature and setting of the area may not be the key factor controlling the formation of these salt deposits. X-ray diffraction (XRD) analyses performed on saline soils from two quarries in Bugesera region confirm the presence of salt minerals including particularly gypsum (CaSO 4 •2H 2 O), halite (NaCl) and thenardite (Na 2 SO 4). Other minerals identified such as goethite, kaolinite, muscovite and quartz may be part of the soil matrix. Chemical analyses on leachates of soil samples from the 2 quarries in Bugesera region confirm their saline nature as evidenced by high TDS (1826-17,679 mg/l) and pH values (8.3-8.5). The predominance of Na + , Mg 2+ , Ca 2+ , K + , SO 4 2− , Cl − in the chemical composition of the dissolved salts clearly reflects their mineralogical composition. This study clearly demonstrates that these evaporitic salts originate from the combination of three processes, namely the dissolution of aluminosilicate minerals which enrich groundwater in different salt-forming ions, the capillary rise which lifts the shallow groundwater and the dissolved ions up to the reach of evaporation and ultimately the evapoconcentration which precipitates the salts following well-established pathways. These findings are well supported by the results of the simulation of the evapoconcentration process with the PHREEQC hydrochemical model.
This study investigates a Precambrian basement aquifer in Bugesera region, a typical African rura... more This study investigates a Precambrian basement aquifer in Bugesera region, a typical African rural area in northeastern Burundi. Domestic water supply relies on groundwater which is tapped through hand-dug wells. Despite several attempts to increase the number of water points in the area, the water demand is still far from being met as a result of the high rate of well failure. This paper seeks to understand whether the hydrogeological structure and the spatial distribution of hydraulic parameters can explain the low productivity and the high failure rate of hand-dug wells. The hydrogeological structure inferred from the interpretation of a large number of vertical electrical soundings (VES) reveals a typical sequence of geoelectrical layers, which is characterized by an overall upwards fining from the fresh basement, over the fractured/weathered basement, to the overburden or saprolite with a clay-rich layer on top. Whereas the overall aquifer potential mainly depends on the thickness of the weathered overburden, the aquifer potential for shallow hand-dug wells is determined by the hydraulic conductivity of the upper few meters of the saturated zone. This upper zone was investigated in the pumping tests. The spatial distribution of the specific capacity reveals a wide variation of hydraulic parameters, depending on the well's position in the depth profile of the aquifer's hydraulic conductivity. The thickness of the potential aquifer is highest in the central part of the study area (pegmatitic and granitic intrusions) which has the highest overall aquifer potential compared to the surrounding metasedimentary formations. However, a thick weathered overburden will increase the groundwater potential of an aquifer for deep boreholes, whereas for hand-dug wells, the productivity can only be high if the thickness of the weathered overburden is small enough, or the water table is deep enough, to allow to tap the coarse part at the base of the overburden and/or part of the weathered/fractured basement. This study reveals that the main cause of low productivity and failure of many hand-dug wells in Bugesera region is not to be linked to poor groundwater resource potential but rather stems from the inappropriate random siting of wells and from poor well construction expertise which does not take into account the hydrogeological conditions. The results of the investigation indicate that in the typical weathering profile of the Precambrian basement in Sub-saharan Africa, the coarse lower part of the weathered overburden and the fractured/weathered basement offer good prospects for water resource. Depending on the depth at which this layer is encountered, either shallow hand-dug wells or deep drilled wells are most appropriate.
Groundwater first started to be used as a source of water supply for the Dar-es- Salaam City in 1... more Groundwater first started to be used as a source of water supply for the Dar-es- Salaam City in 1943. However, many of the existing boreholes in Dar-es-Salaam were drilled in 1997 after Tanzania experienced a severe drought; since then groundwater use in the city shows an increasing trend and currently over 50 percent of residents in Dar es Salaam City relies on groundwater supply. Today, there are more than 10000 boreholes for domestic, water supply and industrial purposes. Demand and reliance on groundwater from a growing population along the coast raises the need to increase efforts to protect and manage the resource. In the coastal aquifer of Dar-es-Salaam, groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. The results show that the origin of salinity in the area is strongly influenced by: groundwater ascending from deep marine Miocene Spatangid Shales through faults, saltwater intrusion on the...
Advances in the Research of Aquatic Environment, 2011
ABSTRACT Water balance for Dar es Salaam Quaternary coastal aquifer (DQCA) was calculated accordi... more ABSTRACT Water balance for Dar es Salaam Quaternary coastal aquifer (DQCA) was calculated according to the method of Thornthwaite and Mather. Monthly potential evapotranspiration estimates were calculated from 39 years (1971-2009) of routine meteorological data using Penman-Monteith, Thornthwaite, Hargreaves and Hamon methods. Results were compared in order to show possible differences that could be attributed to the methods. Determination of groundwater recharge rates gave a mean value of 184 mm year-1 which is equivalent to 71.39 x 106 m3 year-1 indicating that 16.5% of the long term average annual precipitation of 1114 mm ends up as groundwater recharge. Groundwater abstraction from DQCA was estimated to be 69.3 x 106 m3 year-1. These results are alarming in two aspects: high abstraction rate and the increasing trend of borehole drilling. Groundwater abstraction which is nearly equal to the amount of the groundwater recharge is far greater than the least conservative sustainable yield calculated at 70% (equivalent to 49.97 x 106 m3 year-1).
Groundwater is globally important for human consumption, and changes in quality can have serious ... more Groundwater is globally important for human consumption, and changes in quality can have serious consequences. The study area is within a coastal aquifer where groundwater quality is influenced by various potential sources of salinity that determine the composition of water extracted from wells. Groundwater chemistry data from the aquifer have been acquired to determine the geochemical conditions and processes that occur in this area and assess their implications for aquifer susceptibility. Analysis of groundwater samples shows that the dominant watertype is mostly NaCl with pH < 7 in both aquifers (i.e. upper and lower) except for the shallow wells where CaHCO3 prevails with pH >= 7, and boreholes located near the Indian Ocean, where coral reef limestone deposits are located and the watertype evolves towards CaHCO3. In the lower aquifer, Clis higher than in the upper aquifer. The origin of salinity in the area is strongly influenced by groundwater ascending from deep marine Miocene Spatangid Shales through faults, seawater incursion on the border of the Indian Ocean, and throughout, there is some salinity within the Quaternary aquifer, especially in intercalated deltaic clays in the fluviatile deposits, showing some marine influences. The seawater intrusion is linked to the strongly increasing groundwater exploitation since 1997. Another process that plays a major role to the concentration of major ions in the groundwater is calcite dissolution. Next to geogenic salinity and seawater intrusion, anthropogenic pollution as well is affecting groundwater quality in the aquifer. An important result of this study is the observation of high nitrate concentrations, that call for improved sanitation in the area, where domestic sewage with on-site sanitation (mainly pit latrines) also threatens the groundwater resource.
Groundwater is a last-resort source of domestic water supply in Dar es Salaam City because of the... more Groundwater is a last-resort source of domestic water supply in Dar es Salaam City because of the scarcity of surface water sources. The Tanzania Government, Non Government Organizations (NGOs), Community Based Organizations (CBOs) and international aid organizations have promoted the drilling of boreholes. From 1997 until the present, boreholes drilling has increased tremendously and the trend is expected to increase even more in the future. Initial assessment of the current state of water quality indicates that the coastal aquifers are vulnerable to saline intrusion. In some areas, overpumping of the groundwater has resulted in encroachment of sea water into the coastal aquifers. Water samples from Mikocheni, Oysterbay, Msasani, Masaki and City Centre areas show elevated chloride, sulphate and sodium concentrations. Using Stuyfzand (1986) water classification method, over 50 percent of the samples showed the type of fresh-brackish to salt and extremely hard. Residents depend on the shallow aquifer at less than 15 m, because at this depth the water is relatively less saline. The saltwater interface is likely to be moving inland on seasonal basis and some boreholes become more saline in late summer. The growing human population in and around the investigated areas has had an adverse impact on the water in the shallow aquifer through, among other factors, indiscriminate abstraction of water without knowledge of the aquifer's capacity. In order to prevent further deterioration of the aquifer, there is a need to establish an inventory of the wells, conduct proper pump tests and geochemical characterization to ascertain the capacity of the shallow aquifer and understand the processes affecting the overall quality of the groundwater.
Water supply requirements in Dar es Salaam city (Tanzania) are rising rapidly by population growt... more Water supply requirements in Dar es Salaam city (Tanzania) are rising rapidly by population growth and groundwater is increasingly used to fullfill the needs. The groundwater is taken from the Dar es Salaam Quaternary coastal aquifer (DQCA), stretching inland from the coastline. As thousands of wells have been drilled in the coastal strip and pumping rates are uncontrolled, seawater intrusion is deteriorating the quality of fresh groundwater. To investigate the response of the fresh/salt water interface to coastal pumping, simulations with a schematic two-dimensional cross-sectional model have been done. Depending on the depth of the wells in the 150 m thick DQCA and their distance from the coastline, different pathways of seawater intrusion and shifts of the interface can be recognized. The local presence of a semi-pervious layer can have a significant impact on the fresh/salt water distribution. Although the modeled section is not related to a specific location along the coastline but rather represents an average aquifer buildup, the results of the simulations can be used to formulate recommendations when drilling new wells and for a better monitoring of the salinisation process along the coast. It proves that even simple schematic models can give meaningful contributions.
The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deform... more The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670-61 O°C and pressures of 6.5-6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X,l0=O.5 in calcsilicate rocks. These findings, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P-Tpath was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt; followed by their residence and cooling in the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed. o 1999 Elsevier Science Limited. All rights reserved. RESUME-Le socle de la zone de Merelani de la Ceinture du Mozambique dans le nord-est de la Tanzanie a ete def& et metamorphise de multiples fois dans les temps panafiicains (Nr%oproterozdique). Le premier evenement metamorphique a eu lieu dans le facies des granuliies et les roches ont subi plus tard une evolution retrograde par refroidissement isobare dans le facies des amphibolites. Les donnees thermobarom&riques des gneiss de Merelani indiquent des temperatures de I'ordre de 670-610°C et des pressions de 65(1600 MPa. Le refroidissement isobare est illustrt! par la croissance tardive posttectonique de porphyroblastes de disth&ne aux depens de la sillimanite dans les paragneiss graphiteux, ainsi que par la zonation du grenat dans les m8rnes roches. Une evolution retrograde supplementaire a ete due a I'hydratation le long de zones et de failles de cisaillement, conduisant a I'alteration de la forsteriie, I'enstatite et la pargasite en mineraux du groupe de la serpentine. Dans les roches calcosilicatees alt&es par hydrothermalisme, la cristallisation de zoisite vanadif&re ftanzanite) a eu lieu B ce stade pour X,,=O.5. Les resultats, combines aux donnees texturales et thermobarometriques des Monts Uluguru et de la zone de Furua, suggerent que le trajet pression-temperature de refroidissement isobare a 6te predominant dans la (les) evolution(s) metamorphiquefs) des roches de la Ceinture panafricaine
Ga onmiddellijk naar paginanavigatie. Groundwater pollution caused by seepage from Tailing Storag... more Ga onmiddellijk naar paginanavigatie. Groundwater pollution caused by seepage from Tailing Storage Facilities (TSF) : a case study of gypsum dumpsite in north of Ghent, Belgium. Please use this identifier to cite or link to this item: Record Details. Record ID, 999117. ...
The current groundwater replenishment under natural infiltration is about 184 mm (equivalent to 7... more The current groundwater replenishment under natural infiltration is about 184 mm (equivalent to 71.39 x 10 6 m 3 year-1) indicating that only 16.5% of the long term average annual precipitation of 1114 mm ends up as groundwater recharge (Mtoni et al., 2011). The water balance of the catchment suggests for the average sustainable yield (calculated as 40% of natural groundwater recharge) a value of 28.56 x 10 6 m 3 year-1. The current groundwater abstraction is approximately 69.3 x 10 6 m 3 year-1) and such overexploitation of groundwater usually results in a rise of the freshwater-saltwater interface and lateral seawater intrusion in a coastal aquifer, and thus degradation of groundwater quality.
Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting overexp... more Dar es Salaam City is highly depending on groundwater resources since 1997. The resulting overexploitation is entailing the risk of seawater intrusion. This problem has been systematically investigated for the first time, and appears to be widespread. Determining the depth of the fresh-salt water interface was deemed crucial in the view of sustainable exploitation of the Dar es Salaam quaternary coastal aquifer system. Vertical electrical soundings (VES) and horizontal resistivity profiles have shown a decrease of resistivity with depth and toward the coastline. Cross sections have shown the presence of salt water in the aquifer near the coastline, and the boundary between fresh and saline waters is mapped. The surface prospection resistivity data showed comparable results as reflected in hydrochemical/lithological cross sections and resistivity well logs. Salt water was found mostly in the area within 2 km of the coastline, and the depth to the interface in this zone is ranging from 1.3 to 20 m. Away from the coastline, the interface (if present) is at greater depth. VES executed at Kurasini (4 km from the sea) showed the fresh-salt water interface at a greater depth (43 m).
Abstract This study investigates hydrogeochemical processes controlling groundwater evolution in ... more Abstract This study investigates hydrogeochemical processes controlling groundwater evolution in a Precambrian basement aquifer situated in northeastern Burundi, in Bugesera region. The hydrogeochemistry has been evaluated based on the analytical results of 143 water samples comprising 66 samples from shallow wells, 61 samples from springs, and 16 samples from the shallow lakes. While the stability diagrams, the evolutionary sequence of groundwater inferred from the Piper diagram and bivariate plots highlight the weathering of aluminosilicates as the primary process controlling groundwater chemistry in this aquifer, the increasing trend of chloride levels between the highlands (recharge area) and the depression of Bugesera (discharge area) and the random distribution of high nitrate content even in water samples from springs suggest that additional processes such as evaporative concentration and anthropogenic pollution play a secondary role in the hydrogeochemical evolution of groundwater. The spatial distribution of different hydrogeochemical parameters shows a general increasing trend from the highlands where water samples, mainly from springs, show low mineralisation (TDS = 26–152 mg/l) and low pH (field pH = 4.6–6.6), towards the depression of Bugesera where higher values of mineralisation (TDS = 44–3229 mg/l) and pH (field pH = 5.6–7.7) are observed. The overriding control of aluminosilicates weathering on the hydrogeochemical evolution of groundwater in Bugesera region is clearly demonstrated by the predominance of NaHCO3 and CaHCO3 water types and the cross-plots which show an increasing trend of alkaline and alkaline earth cations (Na+ + K+ + Ca2 + + Mg2 +) released from the weathering of aluminosilicates along the flow line. Moreover, the poor correlation between the concentrations of SiO2 and Na+, Ca2 +, Mg2 +, K+, HCO3– and TDS reflects the incongruent character of the weathering of aluminosilicates which causes part of the silica released from the weathering of silicates to be taken up into the neoformation of clay minerals. The anthropogenic pollution is reflected by the occurrence of NaNO3, CaNO3, KNO3 and NaSO4 types even in spring water.
Introduction 1.1 Background In the last two decades, the use of groundwater in Dar es Salaam City... more Introduction 1.1 Background In the last two decades, the use of groundwater in Dar es Salaam City has increased considerably because of the scarcity of surface water sources. With only 25 percent of the population receiving water supplied by Dar es Salaam Water and Sewerage Authority (DAWASA), much of the water needs must be met through private boreholes (UN HABITAT, 2009). Since 1997, the Tanzanian Government, Non Government Organizations (NGOs), Community Based Organizations (CBOs) and international aid organizations have promoted the drilling of boreholes to try to meet the demand of water supply in the city. Despite the significant importance of groundwater in Dar es Salaam City, threats exist, including overabstraction that can lead to the intrusion of salt water in fresh water aquifers. A study by Nkotagu (1989) reported marine intrusion in the coastal aquifer of the Kigamboni Peninsula with elevated chloride, sulphate and sodium concentrations. A study made by the Ministry of Water (URT, 2007) to develop future water sources for Dar es Salaam City, reported an increase of salinity in several boreholes in the coastal area of the city. The increase of salinity at some points of the coastal area in Dar es Salaam Region has been surveyed over the last 20 years. According to the previous work, the cause of salinization was thought to be influenced by various sources of salinity. These include: seawater intrusion, airborne salts, water ascending from deep marine sediments, and clay-bound sands contributing salinity themselves (Mnzava, 1986; Msindai, 1988; Mjemah, 2007). Groundwater resources in coastal areas are always in danger of contamination by sea water intrusion. The likelihood of this taking place is increased when rates of extraction of groundwater close to the sea are raised due to increased demand on fresh water resources. This paper presents preliminary results of an ongoing PhD research on "Regional Hydrogeochemical Characterization of Processes in Quaternary Sand Aquifers of the Dar es Salaam Region". It examines if saltwater intrusion is a pressing problem along the Dar es Salaam coastal area. 1.2 Population The last national census was in 2002, so all current figures are estimates. The city's present population is estimated to be over 3.5 million people (Fig. 1) with a growth rate of 4.3%. The high population has proven to be a serious challenge in the provision of housing and essential services such as water, sanitation and health care (URT, 2004). Population in the municipality varies from settlement to settlement. High population densities are found in unplanned settlements, while low population densities are found in peripheral localities. 1.3 Water supply and management problems The increased demand of water in Dar es Salaam City and uncontrolled groundwater abstraction poses a danger of seawater intrusion due to overpumping. From 1997 until the present, boreholes drilling has increased tremendously and the trend can still be expected to increase in the future. It is estimated that about 1000 boreholes are drilled annually (Baumann et al., 2005). Individuals, the Community, industries, institutions and tourist hotels have sunk a number of boreholes from which large quantities of water are withdrawn using electrical and/or diesel pumps. All these put a strain on the fresh groundwater resources and increase the possibility of sea water intrusion. 1.4 Objectives of the study The objectives for this study were: (1) To preliminarily assess the current state of water quality in selected areas near the coastline, especially in view of assessing sea water intrusion; (2) To attempt, with data available and based on the groundwater samples of a first field campaign, to give an early warning of the threat of sea water intrusion along the Dar es Salaam coast; and (3) To review available information in relation to this study and design a strategy for further field surveys.
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