Browsing by Author "Walraevens, Kristine"

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Groundwater exploitation and hydraulic parameter estimation for a Quaternary aquifer in Dar-es-Salaam, Tanzania.
(Elsevier Ltd, 2009-07-03) Mjemah, Ibrahimu Chikira; Van Camp, Marc; Walraevens, Kristine
The fact that groundwater exploitation has largely increased since 1997 in the Dar-es-Salaam aquifer, calls for a directed attention towards possible problems of aquifer overexploitation that may arise in the near future. Hydraulic parameters are important for developing local and regional water plans as well as developing numerical groundwater flow models to predict the future availability of the water resource. The determination of aquifer parameters through pumping tests has become a standard step in the evaluation of groundwater resource potential. The pumping tests in the study area were con- ducted in August 2004 and August 2005, where 39 boreholes were tested out of 400 visited. In the study area there are over 1300 recorded boreholes drilled by Drilling and Dam Construction Agency (DDCA) by the year 2005. Total groundwater exploitation in the study area was estimated at 8.59 10 6 m 3 /year, based on yield data collected during the 2004–2005 field campaigns. The pumping tests included single- well tests and tests with measurements on the pumping well and at least one observation well. The tests were conducted for 6 h and 30 min. The pump was shut down after 6 h of pumping and the remaining 30 min were used for recovery measurements. The pumping test analysis methods used include: Neuman type curve matching and Walton type curve matching, checked by specific well capac- ity assessment and Thiem–Dupuit/Thiem’s method. The curve-matching results from the aquifer tests show the following parameters: an average transmissivity and hydraulic conductivity of 34 m 2 /d and 1.58 m/d, respectively for the unconfined aquifer; the semi-confined aquifer has an average value of 63 m 2 /d and 2.14 m/d for transmissivity and hydraulic conductivity, respectively. For the case of the storativity, the unconfined aquifer has an average elastic early-time storativity of 0.01, while the lower aquifer has an average storativity of 3 10 4 . Specific well capacity method and Thiem–Dupuit/Thiem’s method confirm results for transmissivity and hydraulic conductivity of the semi-confined aquifer, while values for the unconfined aquifer are somewhat larger (by a factor of 2–3). The hydraulic param- eters calculated appear to reasonably agree with the geological formation of the aquifers, as deduced from borehole descriptions.
Groundwater exploitation and recharge rate estimation of a quaternary sand aquifer in Dar-es-Salaam area, Tanzania
(Springer-Verlag, 2010-09-10) Mjemah, Ibrahimu Chikira; Van Camp, Marc; Martenes, Kristine; Walraevens, Kristine
Dar-es-Salaam City gets water supply from surface water and groundwater. The groundwater is used to supplement surface water supply and has increasingly become a major source of water supply in the city. The study area comprises three major parts: the central coastal plain with quaternary fluvial–deltaic sediments, the deltaic Mio-Pliocene clay-bound sands and gravels in the north- west and southeast and the Lower Miocene fluviatile sandstones of Pugu Hills in the west of the study area. The main objective of this study was to quantify the integrated water balance. The major source of renewable groundwater in the aquifer is rainfall. Hence, the average recharge of 256.2 mm/year (for the year 2006) to the aquifer was estimated using the balance method of Thornthwaite and Mather, which is equal to 99.4 hm 3 /year for the whole alluvial aquifer. This value was balanced with total groundwater abstraction of 8.59 hm 3 /year, baseflow to rivers of 75.7 hm 3 /year and discharge into the sea (15.11 hm 3 /year).
Hydrogeological mapping and estimation of potential evapotranspiration and recharge rate of Quaternary sand aquifers in Dar-es-Salaam, Tanzania
(International Journal of Geomatics and Geosciences, 2015-11) Mjemah, Ibrahimu Chikira; Walraevens, Kristine
Dar-es-Salaam City is the largest urban centre in Tanzania, with a population of over 4 million. It gets water for domestic use from surface water (Ruvu and Kizinga rivers) and groundwater. The groundwater was used to supplement surface water supply and has by now become the major source of water supply in the city. However, despite the importance of groundwater in the city, limited researches on water quantity and quality have been conducted and generally the groundwater database is inadequate. The main objective of this research is to investigate the availability of groundwater by developing a hydrogeological map of the area and estimate the groundwater recharge rate of the Quaternary sand aquifer in the plain. The deposits in the study area cover two major periods, Quaternary and Neogene. They are made up of sedimentary rocks, which were proposed to be deposited in a fluviatile/deltaic environment with marine intercalations along the seaward margin. The Uluguru Mountains were probably the source of much of the materials deposited in the area. Nearly 150 m of Quaternary deposits consist of basically three geological layers: alluvial, coastal plain and coral reef limestone deposits. These deposits are mostly of Pleistocene to recent age and are found mainly moving from the coast towards the mainland within the river valleys. The Neogene deposits are of Miocene and Pliocene period. Two types of formations characterize these deposits: undifferentiated deposits (Mio-Pliocene clay-bound sands and gravels) and the fluviatile kaolinitic Pugu Sandstone (Lower Miocene). Hydrogeologically, the study area has two types of aquifers in the Quaternary deposits: an upper unconfined sand aquifer (5-50 m thick) and the lower semi-confined sand aquifer (the most productive zone 10-100 m thick). The groundwater recharge is estimated by using monthly precipitation data for 36 years, runoff and potential evapotranspiration. The average groundwater recharge rate in the area is 121.7 mm per year. This delivers substantial amounts of groundwater stored in the sand aquifers of the area.
Investigating seawater intrusion due to groundwater pumping with schematic model simulations: The example of the Dar es Salaam coastal aquifer in Tanzania
(Elsevier, 2014) Camp, Marc Van; Mtoni, Yohana; Mjemah, Ibrahimu C.; Bakundukize, Charles; Walraevens, Kristine
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
Modeling approaches and strategies for data-scarce aquifers: example of the Dar es Salaam aquifer in Tanzania
(Springer, 2013) Camp, Marc Van; Mjemah, Ibrahimu Chikira; Farrah, Nawal Al; Walraevens, Kristine
Management of groundwater resources can be improved by using groundwater models to perform risk analyses and to improve development strategies, but a lack of extensive basic data often limits the implementation of sophisticated models. Dar es Salaam in Tanzania is an example of a city where increasing groundwater use in a Pleistocene aquifer is causing groundwater-related problems such as saline intrusion along the coastline, lowering of water-table levels, and contamination of pumping wells. The lack of a water-level monitoring network introduces a problem for basic data collection and model calibration and validation. As a replacement, local watersupply wells were used for measuring groundwater depth, and well-top heights were estimated from a regional digital elevation model to recalculate water depths to hydraulic heads. These were used to draw a regional piezometric map. Hydraulic parameters were estimated from short-time pumping tests in the local wells, but variation in hydraulic conductivity was attributed to uncertainty in well characteristics (information often unavailable) and not to aquifer heterogeneity. A MODFLOW model was calibrated with a homogeneous hydraulic conductivity field and a sensitivity analysis between the conductivity and aquifer recharge showed that average annual recharge will likely be in the range 80–100mm/year.
The origin of high sulfate concentrations and hydrochemistry of the Upper Miocene–Pliocene–Quaternary aquifer complex of Jifarah Plain, NW Libya
(Springer-Verlag Berlin Heidelberg, 2016-10) Alfarrah, Nawal; Berhane, Gebremedhin; Mjemah, Ibrahim Chikira; Van Camp, Marc; Walraevens, Kristine
he high uncontrolled groundwater extraction in Jifarah Plain, NW Libya, causes a modification of natural flow systems, inducing seawater intrusion and causing groundwater quality deterioration. The principal aim of this study is to identify the hydrogeochemical processes in this coastal aquifer in order to verify the main sources of sulfate concentration increase that occurs in the system. In order to achieve this aim, water samples were collected from 134 sampling wells in the study area and analyzed for the major cations and anions; physical and chemical parameters were measured, such as water level, electrical conductivity, pH and temperature. The analytical results obtained in the hydro- chemical study were interpreted using Piper diagram, ion correlations with Na ? /Cl - , SO 42- , Cl - and TDS, in con- junction with calculation of the ionic deviations of the con- servative freshwater/seawater mixture and saturation indices using the PHREEQC 2.16 software. The large SO 42- anomaly observed in groundwater near the coast was explained by the presence of seawater intrusion and upconing of deep saline water in these areas. This conclusion is based on high chloride concentrations, the inverse cation exchange reactions and the lower piezometric level compared to sea level. Inland, in Sabratah, the high SO 42- values are related to gypsum dissolution from the Upper Miocene Formation in the lower part of the upper aquifer. These locally high SO 42- concentrations in the south of the study area show overall increase in the upstream direction, which also suggests the dissolution of evaporites from the mountain aquifers in the south. High SO 42- concentration is also related to the effect of the scattered sebkha deposits in some areas along the coast.
Quantification of groundwater exploitation and assessment of water quality risk perception in the Dar es salaam quaternary aquifer, Tanzania
(MDPI, 2019) Mussa, Kassim Ramadhani; Mjemah, Ibrahimu Chikira; Walraevens, Kristine
This study quantified groundwater exploitation and assessed water quality risk perception in the Dar es Salaam quaternary aquifer through a socio-economic survey. Annual total groundwater exploitation was estimated, using the daily per capita consumption of groundwater derived from the household survey. A logistic regression analysis was performed to ascertain the influence of sex, marital status, education level, employment, income, and cost of water on groundwater quality risk perception. It was revealed that most residents of Dar es Salaam use groundwater as a main source of water supply. The results of this study further showed that 78% consider groundwater as a reliable source. Averting strategies for insu cient quantity of groundwater consist of minimizing less necessary water uses, while for poor quality, buying bottled water and water treatment by boiling and using chemicals. The chance for water quality risk perception is 0.205 times greater for married than unmarried household heads, and it is 623 times higher for employed versus unemployed household heads. To get an overall view of the importance of groundwater for domestic needs in Dar es Salaam it is imperative to combine a time series data of groundwater and surface water exploitation.
Saltwater intrusion and nitrate pollution in the coastal aquifer of Dar es Salaam, Tanzania.
(Springer, Environimental Earth Science., 2013-11-03) Mtoni, Yohana; Mjemah, Ibrahimu Chikira; Bakundukize, Charles; Van Camp, Marc; Martenes, Kristine; Walraevens, Kristine
Dar es Salaam Quaternary coastal aquifer is a major source of water supply in Dar es Salaam City used for domestic, agricultural, and industrial uses. However, groundwater overdraft and contamination are the major problems affecting the aquifer system. This study aims to define the principal hydrogeochemical processes control- ling groundwater quality in the coastal strip of Dar es Salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater qual- ity. Major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors: dissolution of calcite and dolomite, weathering of silicate minerals, seawater intrusion due to aquifer over- exploitation, and nitrate pollution mainly caused by the use of pit latrines and septic tanks. High enrichment of Na ? and Cl - near the coast gives an indication of seawater intrusion into the aquifer as also supported from the Na–Cl signature on the Piper diagram. The boreholes close to the coast have much higher Na/Cl molar ratios than the boreholes located further inland. The dissolution of calcite and dolomite in recharge areas results in Ca–HCO 3 and Ca–Mg–HCO 3 groundwater types. Further along flow paths, Ca 2? and Na ? ion exchange causes groundwater evolution to Na–HCO 3 type. From the PHREEQC simu- lation model, it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals. The results of this study provide important information required for the protection of the aquifer system.
Saltwater intrusion in the quaternary aquifer of the Dar es Salaam region, Tanzania
(GEOLOGICA BELGICA, 2012) Mtoni, Yohana; Mjemah, Ibrahimu C.; Msindai, Kristoko; Van Camp, Marc; Walraevens, Kristine
A description of the occurrence of the salt water intrusion from seawater close to the coastline of Dar es Salaam Region is presented. The present status of intrusion in two pilot areas located between Mlalakuwa and Msimbazi Rivers (locality I) and Msimbazi and Kizinga Rivers (locality II) is evaluated. The causes, effects, remedial measures and future possibility of intrusion are discussed. An investigation on the hydrodynamics of the phenomenon and analysis of the formations as well as the interfacial transition zone is currently being performed. Water samples from Mikocheni, Oysterbay, Msasani, Masaki and City Centre areas show elevated chloride, sulphate and sodium concentrations. Using Stuyfzand (1986) water classification method, about 60 percent of the samples are classified as fresh-brackish to salt and 30 percent as extremely hard. Groundwater was classified into several categories by subtype, i.e., Sodium-Chloride, Sodium-Bicarbonate, Calcium-Bicarbonate, Magnesium-Bicarbonate, Sodium-Mix and Calcium-Mix. 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.
Sources of salinity and urban pollution in the Quaternary sand aquifers of Dar es Salaam, Tanzania
(Elsevier, 2014) Walraevens, Kristine; Mjemah, Ibrahimu Chikira; Mtoni, Yohana; Camp, Marc Van
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 pHP7, and boreholes located near the Indian Ocean, where coral reef limestone deposits are located and the watertype evolves towards CaHCO3. In the lower aquifer, Cl is 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