Groundwater exploitation and hydraulic parameter estimation for a Quaternary aquifer in Dar-es-Salaam Tanzania

doi:10.1016/j.jafrearsci.2009.03.009

Journal of African Earth Sciences

Volume 55, Issues 3–4, October–November 2009, Pages 134-146

https://doi.org/10.1016/j.jafrearsci.2009.03.009 Get rights and content

Abstract

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 conducted 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 × 10⁶ m³/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 capacity 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 m²/d and 1.58 m/d, respectively for the unconfined aquifer; the semi-confined aquifer has an average value of 63 m²/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 parameters calculated appear to reasonably agree with the geological formation of the aquifers, as deduced from borehole descriptions.

Introduction

Groundwater flow varies in space and time and is dependent on the hydraulic properties of the rocks and the boundary conditions imposed on the groundwater system (Stallman, 1971). Hydraulic parameters (transmissivity, hydraulic conductivity and storativity) are important for developing local and regional water plans and developing numerical groundwater flow models to predict the future availability of the water resource. A pumping test can also indicate disturbing factors such as lateral flow boundaries, hydraulic continuity, constraints of fracture flow, and recharge. All of these are important to understand in order to establish proper management of the groundwater resources. From the water exploitation perspective, the “test pumping” is the best method by which to size and establish the optimal capacity and depth setting of the pump, as well as to establish system storage and operational needs. Proper pump sizing and depth selection can provide considerable savings to a water system over the lifetime of the well, through reduced power consumption and maintenance costs. On the other hand, a “pumping test” is a scientific experiment aimed at deducing hydraulic characteristics.

The study covers part of Dar-es-Salaam and Kisarawe areas. It focuses on the hydrogeological characteristics of the unconsolidated sediments of Quaternary age, which form the major aquifer in the coastal area of Dar-es-Salaam region. Dar-es-Salaam is the largest urban centre in Tanzania, with a population of about 3 million. The study area is located at latitudes 6°44′S to 7°00′S and longitudes 39°00′E to 39°19′E, with an area of about 700 km² (Fig. 1). In this study, emphasis was given to hydrodynamic characteristics of the sand aquifers because such data are lacking in the study area. The pumping tests were conducted in August 2004 and August 2005, where 39 boreholes were tested out of 400 visited. In the area, there are over 1300 recorded boreholes drilled by Drilling and Dam Construction Agency (DDCA) by the year 2005. However, some of them are not operating due to either pump failure or high salinity. 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 purpose of the tests was to determine the hydraulic parameters, the productivity of each well and the interference each pumped well produced in surrounding wells. The computer program Infinite Extent, a Windows-based pump test analysis package, was used to analyze the pumping test data (Starpoint Software, 1996).

The pumping test analysis methods used include: Neuman type curve matching for constant pumping rate in the unconfined aquifer and Walton type curve matching for constant pumping rate in the semi-confined aquifer. The specific capacity method was also employed for calculating transmissivity from drawdown and pumping rate at steady state. Additionally, for the tests with measurements on both the pumping well and at least one observation well, Thiem–Dupuit’s interpretation method was applied for tests in the unconfined aquifer and Thiem’s method for tests in the semi-confined aquifer, both at steady-state flow. The resultant transmissivity estimates were then divided by the aquifer saturated thickness to obtain horizontal hydraulic conductivity values.

Section snippets

Hydrogeological settings of the study area

The study area comprises three major parts, distinguished by the geological formations outcropping: the central coastal plain with Quaternary fluviatile-deltaic sediments, the deltaic Mio-Pliocene clay-bound sands and gravels in the northwest and southeast and the Lower Miocene fluviatile sandstones of Pugu Hills in the west of the study area (see Fig. 1). The Quaternary deposits of Pleistocene to Recent periods have total thickness of approximately 150 m within Dar-es-Salaam City area. They

Groundwater exploitation

Groundwater is used as a source of water supply for the Dar-es-Salaam City since 1943, after the first borehole was drilled at Temeke Dairy Plant with a depth of 30 m and a yield of 8 m³/h (Service Plan, 1997). Some other industries started to use groundwater later on, to a limited extent. In 1949 the surface water started to be the main source of water supply after the Mtoni Water Treatment Plant on River Kizinga was constructed, and this was further increased after the construction of Upper and

Execution of pumping tests

A pumping test (or aquifer test) is used to estimate the hydraulic properties of an aquifer or confining bed by pumping water out of a well and observing the drawdown in the observation well(s). Pump tests can predict the ability of the formation to yield water and can allow to infer hydraulic boundary conditions. From a pumping test, we can determine transmissivity (T) and storativity (S). Assessment of the hydraulic properties of the different aquifer systems encountered in the study area has

Specific well capacity distribution

Fig. 5 shows the distribution of the values obtained for the specific well capacity in the study area. This map is produced based on calculated specific capacities resulting from pumping tests on 39 wells located in the unconfined and the semi-confined aquifers (Table 4). The values obtained were then brought into a Geographic Information System (ArcView-GIS v.3.1) and gridded, contoured, and color-coded into ranges to produce the map shown as Fig. 5.

The map can be used to provide some general

Conclusion

The structure of ground water in Dar-es-Salaam has revealed unconfined and semi-confined aquifers. These aquifers are consisting of mainly sandy Pleistocene to Recent fluviatile-deltaic deposits that are separated by a clayey aquitard. This Quaternary aquifer system is overlying the Mio-Pliocene clayey deltaic deposits, acting as the substratum.

Total groundwater exploitation in the area was estimated at 8.59 × 10⁶ m³/year, based on yield data collected during the 2004–2005 field campaigns. In

Acknowledgements

The authors wish to express their sincere thanks to the Flemish Interuniversity Council (VLIR) – Institutional Co-operation Programme and the VLIR – Interuniversity Collaboration Programme (IUC) with Sokoine University of Agriculture (SUA-VLIR Programme) for funding this study. The authors highly appreciate the help from all other persons that provided information and/or assistance during data collection.

References (19)

S.J. Kollet et al.
Influence of aquifer heterogeneity and return flow on pumping test data interpretation
Journal of Hydrology
(2005)
N.S. Boulton
Analysis of data from non-equilibrium pumping tests allowing for delayed yield from storage
Proceedings of the Institute of Civil Engineering
(1954)
R.H. Brown
Estimating the transmissivity of an artesian aquifer from the specific capacity of a well
US Geol Survey Water Supply Paper
(1963)
Castany, G., 1982. Principes et méthodes de l’hydrogéologie [Principles and methods of hydrogeology]. Ed Dunod, Paris,...
Dupuit, J., 1863. Etudes théoriques et pratiques sur le mouvement des eaux dans les canaux découverts et à travers les...
C.W. Fetter
Applied Hydrogeology
(1994)
R.A. Freeze et al.
Groundwater
(1979)
Kappa Engineering, 1999. Saphir Well Test Interpretation Software, Version 2.30 Update Notes. Dallas, TX: Kappa North...
Kent, P.E., Hunt, J.A., Johnstone, M.A., 1971. The geology and geophysics of coastal Tanzania. Geophysical Paper No. 6,...

There are more references available in the full text version of this article.

Cited by (41)

Hydrogeochemical and isotopic characterization of the Gioia Tauro coastal Plain (Calabria - southern Italy): A multidisciplinary approach for a focused management of vulnerable strategic systems
2023, Science of the Total Environment
Citation Excerpt :
In response to this, coastal plains have always been considered strategic sectors for human settlement hosting most of the world's megacities with the relative issues due to the intensive use of the territory (McGranahan et al., 2007). In these areas, the intensive exploitation of groundwater reduces freshwater outflow to the sea, forming a local water table depression and inland seawater migration (Masciopinto, 2006; Mjemah et al., 2009; Van Camp et al., 2014). This phenomenon, named seawater intrusion, is a global concern and represents one of the most widespread processes that degrade groundwater quality and limit its utilization (Nisi et al., 2013; Alfarrah and Walraevens, 2018; Vespasiano et al., 2019; Boumaiza et al., 2020; Han et al., 2021; Kammoun et al., 2021).
This work pursues the hydro-geochemical and isotopic characterization of the complex groundwater system of the Gioia Tauro Plain, one of the most important industrialized and agricultural coastal areas of southern Italy. The anthropic pressure exposes the water resources at risk of depletion and quality degradation making the plain groundwater a system of high scientific and social interest.
The plain is characterized by a shallow aquifer, mostly recharged by local rains and a deep aquifer apparently less influenced by local precipitation. Both aquifers are mainly Ca-HCO₃ waters except for localized sectors where Na-HCO₃, Na-Cl and Ca-SO₄ waters are present. In deep aquifer, both prolonged interaction with sedimentary rocks, mainly deriving from the erosion of crystalline rocks, and direct cation exchange represent the primary factors controlling the formation of Na-HCO₃ waters. Mixing processes between these waters and either connate brine and/or deep thermal waters contribute to the formation of isolated high salinity Na-Cl-rich waters. In shallow aquifer, inputs of N-rich sewage and agriculture-related contaminants, and SOx emissions in proximity of the harbor are responsible of the increasing nitrate and sulphate concentrations, respectively. The Cl/Br and NO₃/Cl ratios highlight contamination mainly linked to agricultural activities and contribution of wastewater.
Along the northern boundary, the warmest groundwater (Na-Cl[SO₄]) were found close to a bend of the main strike-slip fault system, locally favouring the rising of B- and Li-rich deep waters, testifying the influence of geological-structural features on deep water circulation.
Despite the high-water demand, a direct marine intrusion is localized in a very restricted area, where we observed an incipient groundwater-seawater mixing (seawater contribution ≤7 %). The qualitative and quantitative conditions of the shallow aquifer still have acceptable levels because of the relatively high recharge inflow. A reliable hydrogeochemical conceptual model, able to explain the compositional variability of the studied waters, is proposed.
Evolution of salinity in the Gaza Strip over the last five decades deduced from field data
2023, Journal of African Earth Sciences
Citation Excerpt :
Coastal areas are also the most densely populated areas in the world, a fact that makes the need for freshwater even more acute (Bear et al., 1999; Gaaloul et al., 2012). Heavy groundwater abstraction from coastal aquifers reduces natural freshwater outflow to the sea and creates water table depression, causing seawater to migrate inland and rising salinity in wells close to the coast (Bear et al., 1999; Masciopinto, 2006; Mjemah et al., 2009; Van Camp et al., 2014). Inappropriate management of a coastal aquifer may lead to its destruction of freshwater much earlier than other aquifers which are not connected to the sea as the result of the threat of seawater intrusion (Bear et al., 1999).
Gaza coastal aquifer has been exposed to severe overexploitation to meet the human needs for water supply. This exploitation has resulted in lowering of the groundwater level to by more than 15 m below mean sea level in the south and around 5 m in the north. Groundwater level drop has exposed the aquifer to seawater intrusion and inland saline upconing in the central south and south-east of the Gaza Strip.
We developed five cross sections based on wells drilled over the past five decades representing aquifer stratigraphy in the five governorates (North, Gaza, Deir El Balah, Khan Younis and Rafah) of the Gaza Strip. Those cross sections have been used to map the chloride concentration over the past five decades (years 1972, 1980, 1990, 2000, 2010, 2017 and 2021).
The results show three main sources of aquifer salinity: (1) seawater intrusion, (2) lateral inflow from Eocene rocks in the east and/or from agricultural return flow as a result of using treated wastewater for irrigation in Israel close to the eastern border and (3) upconing due to fossil marine salt water trapped at the bottom of the aquifer. The clay layers which are subdividing the aquifer have caused a difference in sub-aquifer level chloride difference. Before the year 2000, the public water supply wells were placed far away from the sea and no seawater intrusion was noticed especially in the south of Gaza Strip (Khan Younis and Rafah governorates), while afterwards public water supply wells appeared within 2 km from the Mediterranean Sea. From then on, seawater intrusion has been taking place due to heavy groundwater abstraction. Also, lateral inflow with chloride concentrations of more than 1,000 mg/l is well noticed during the different years, related to eastern inflow from Eocene rock formation where the coastal aquifer rests directly on it, 10–15 km from the coast. Inland areas with high chloride contents were observed as a result of upconing from deep saline groundwater. Cumulative Distribution Function (CDF) curves were created for all wells in each cross section and showed a remarkable shrinkage of freshwater percentage in the aquifer.
Spatial variability of the groundwater exploitation potential in an arid alluvial-diluvial plain using GIS-based Dempster-Shafer theory
2021, Quaternary International
The spatial distribution of groundwater exploitation potential is crucial to the sustainable management of groundwater resources in arid regions. In this study, an effective approach is developed using a geographical information system (GIS) associated with the Dempster-Shafer (D-S) theory to explore the groundwater exploitation potential based on eight factors (i.e., slope, topography, distance to a river, canal density, lithology, land use, soil type, and groundwater depth)based on pumping test data. Different groundwater exploitation potential zones from the output map are classified as follows: very high (0.67–0.96, 8% of the total area), high (0.45–0.67, 8%), moderate (0.27–0.45, 14%), low (0.13–0.27, 24%), and very low (0.02–0.13, 46%), in the Hotan River basin of the Kunlun Mountains, China. The groundwater exploitation potential is highest in a gravel plain in the piedmont and gradually decreases from the piedmont to the desert. The predicted spatial variability of groundwater exploitation potential can explain 71% of the measured pumping test data. The uncertainty of the groundwater exploitation potential map ranges from <1 to 25.5% across the basin. The Dempster-Shafer model can reflect the combined influence of various factors affecting the groundwater exploitation potential, and the system uncertainty is within the belief functions. Based on the developed method, decision-makers are provided with an effective and reliable tool for groundwater exploitation and sustainable management.
Impact of domestic wells and hydrogeologic setting on water quality in peri-urban Dar es Salaam, Tanzania
2019, Science of the Total Environment
In densely populated urban areas of many low-income countries, water scarcity, poor water quality, and inadequate wastewater management present complex challenges to ensuring health and wellbeing. This study was conducted in an impoverished peri-urban community in Dar es Salaam, Tanzania that experiences water scarcity and relies on domestic wells for drinking water. The objective of this study was to identify the sources of domestic well water contamination and assess the relationship and association of water contamination with three variables 1) the proximity of the well to a sanitation system, 2) well age, and 3) well depth. Out of the 71 wells tested, samples from >80% of wells contained Escherichia coli (E. coli) and 58% had nitrate levels above WHO guidelines. The average concentration of total dissolved solids (TDS) was 882 mg/L, which exceeded the WHO guideline of 600 mg/L. Bivariate correlation analysis showed a strong correlation between water contamination and proximity of the well to a sanitation system along with well depth. Univariate regression analysis confirmed the association of contaminants with distance of a well from a sanitation system and well depth (p < 0.05) but age of the well did not show any significant influence on water quality. Our findings indicate significant contamination of wells from nearby septic tanks and pit latrines. New regulatory mandates for the distance of domestic wells from sanitation systems are essential to prevent groundwater contamination and to protect human health.
Overexploitation hazards and salinization risks in crucial declining aquifers, chemo-isotopic approaches
2019, Journal of Hazardous Materials
Citation Excerpt :
Groundwater overdraft or overexploitation occurs when extraction exceeds both natural and induced aquifer recharge over a long period [2,5]. Adverse effects of groundwater overdraft include, uneconomic pumping conditions, water quality degradation through induced intrusion of saline or poor quality groundwater, land subsidence and gradual depletion of groundwater storage [5–7]. Therefore, the exploitation of groundwater resources is of high importance and has become very crucial in the last decades, especially in arid and semi-arid regions, which are particularly at risk due to intrusion of salty water from different sources.
Bastam region is overusing its groundwater resources stocks. The groundwater levels, and hydrogeochemical data during a 14-year period (2002–2016) and isotopic composition (2016) of the 22 groundwater samples were investigated to determine the hydrogeological regime of Bastam Plain and identify the geochemical processes that control the groundwater quality. This plain is situated in a semi-arid region of northern Iran, mainly recharging from the carbonate Mountains in the north and southwest. In general, overexploitation of the groundwater due to low precipitation and changing precipitation regime has destructive effects on the hydrogeological setting and flow regime of Bastam crucial aquifer, including the decline of the groundwater level with rate of 0.9 m/year, water storage deficit of 17.32 Mm³/year and consequently, the intrusion of saline water towards the aquifer from the western and eastern salty plains. Based on the EC values, the groundwater samples divided into three groups of fresh waters with EC < 1000 μS/cm and Ca-Mg−HCO₃ water type which are located in the recharge zones, semi-saline waters (1000 < EC < 2500 μS/cm) with Na–Mg–Cl–SO₄ type at the center and saline waters with EC values more than 2500 μS/cm and Na–Cl type in the eastern and western parts of the plain. A few groundwater samples (group 2) fall on the mixing line between fresh and saline water in Piper diagram during the saline water intrusion, but most of the samples depart from this mixing line, indicating water–rock interactions in this area, resulting in deﬁcit of Na⁺ and surplus of Ca²⁺ concentration. The fresh water samples cluster along the local meteoric water line in δ¹⁸O and δ²H relationships, suggesting an integrative and rapid recharge with meteoric water. Hydrochemical and isotopic characteristics indicate that invasion of the saline water resulted from halite dissolution with minor evaporation in the surrounding salty playas is the main origin for the increasing salinity in the Bastam aquifer. The salinization risks limit the groundwater withdrawals from Bastam aquifer in some areas, threatening the future sustainable development of the region.
Poor understanding of the hydrogeological structure is a main cause of hand-dug wells failure in developing countries: A case study of a Precambrian basement aquifer in Bugesera region (Burundi)
2016, Journal of African Earth Sciences
Citation Excerpt :
An and Gn in Fig. 1 correspond respectively to the 16 transects of the first batch and the 11 transects of the second batch. Pumping tests are the most reliable method to estimate an aquifer’s hydraulic characteristics (Van Camp and Walraevens, 2009; Mjemah et al., 2009). In the study area, hand-dug wells with large diameter are the only direct means available for hydrogeological investigations.
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.

View all citing articles on Scopus

View full text

Groundwater exploitation and hydraulic parameter estimation for a Quaternary aquifer in Dar-es-Salaam Tanzania

Abstract

Introduction

Section snippets

Hydrogeological settings of the study area

Groundwater exploitation

Execution of pumping tests

Specific well capacity distribution

Conclusion

Acknowledgements

Journal of Hydrology

Analysis of data from non-equilibrium pumping tests allowing for delayed yield from storage

Proceedings of the Institute of Civil Engineering

Estimating the transmissivity of an artesian aquifer from the specific capacity of a well

US Geol Survey Water Supply Paper

Applied Hydrogeology

Groundwater