Modelling of the Hydrogeological Behaviour of the Tassette Aquifer: Study of the Possibilities of Exploiting This Aquifer as an Alternative against the Limestone Problem in the Commune of Thies ()
1. Introduction
Senegal is a country facing several problems in terms of its groundwater resources according to the different zones and regions. Indeed, Senegal’s groundwater suffers from ills such as overexploitation, salinization, excess fluoride, limestone or iron, or anthropogenic pollution. Good management of these resources would therefore be necessary to ensure the improvement of the living conditions of populations and sustainable development [1] [2] . Thies, a city near Senegal’s capital Dakar, whose drinking water is supplied mainly from the Maestrichtian aquifer, is particularly affected by the significant presence of limestone. When the water is thus rich in limestone, it is called hard water. The advantage of hard water is that it covers part of the Ca2+ and Mg2+ needs of man and it is more pleasant to drink, however, limescale deposits in pipes imply the reduction of diameters which can be the cause of system malfunction, a decrease in working pressure, energy and additional expenses due to the alteration of water transfer pipes [3] [4] [5] .
In order to deal with this problem of hard water, transferring water from Tassette would be a good alternative. At this level, a battery of boreholes has been set up by the National Water Company of Senegal (SONES) and hydrogeological and geophysical studies as well as qualitative and quantitative monitoring are conducted by the Directorate of Water Resources Management and Planning (DGPRE). A more in-depth study aimed at feeding the city of Thies could be envisaged.
Tassette is part of the department of Thies. The distance separating it from the city is equal to about 20 km [6] . The water of the Tassette aquifer being of better quality than that present in Thies, this transfer could be done either completely, that is to say with cessation of the use of the Thies aquifer, or by a possible dilution of the water of the Thies aquifer by that coming from Tassette to lower the concentration of minerals such as limestone [7] .
2. Methods and Materials
2.1. Geographical Location of the Study Area
The region of Thies, located in the west of Senegal at 14˚46' North - 16˚54' West, is limited to the east by Fatick and Diourbel, to the west by Dakar and the Atlantic Ocean, to the north by Louga and to the south by the Fatick region. It thus covers nearly 3.4% of the country’s surface [8] [9] . Figure 1 shows the location of the Tassette area.
2.2. Visual Modflow Flex Software Overview
Visual Modflow Flex is a software created by SCHLUMBERGER Water Services;
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Figure 1. Location of the Tassette area.
It has an intuitive and easy-to-use interface that simulates groundwater flows, their quality, quantity, etc., thanks to a set of tools [10] [11] [12] .
With this software, it is possible to choose between a conceptual model approach and a classical numerical model approach [13] [14] [15] .
2.3. Estimated Water Needs of the City of Thies in 2037
For the estimation of the population Thiessoises by 2037 we start from the geometric method given by the following relation:
Pn = Po × (1 + T)n (1)
With
Pn = Future population
Po = Current population
n = horizon
T = rate of increase
We will consider an increase rate of 2.5% [16] .
Daily consumption is equivalent to:
Cj = Pn × Cu (2)
Cu = Unit consumption
2.4. Elaborating Model Input Parameters
1) Geologic of aquifere
At the study area level, three (3) electrical sounding sections were developed. Two (2) oriented from southwest to northeast (SW-NE) and one (1) approximately west to east (W-E). From these profiles, isobath maps of the roof and wall of the aquifer and isopaque maps of thicknesses were prepared.
2) Sampling
v The Paleocene globally is exploited by about 300 boreholes. In the area more specifically, we mainly note:
v The drilling of the Notto-Diosmone-Palmarin system which takes a total flow of 5500 m3/d;
v Four (4) functional rural boreholes operating between 46 and 178 m3/d, an average flow of 112 m3/d;
v Seven (7) SONES boreholes used mainly for the supply of Dakar and taking overall a flow of nearly 15,000 m3/d.
2.5. Model Calibration
For steady state, we determine the hydraulic conductivity at saturation of the zone. The calibration will thus be carried out from data such as operating rates, hydraulic conductivity, boundary conditions.
There are three conductivity zones to arrive at an acceptable calibration of the model: a conductivity zone equal to 7 × 10−1 m/s located in the northwest part, a second in the central part, around 4 × 10−4 m/s and in the southeast, the conductivity would be 3 × 10−5 m/s.
For each simulation scenario, we have entered in a table the flow rates used.
• Scenario 1
For this situation, it is considered the current AEP with the drilling already present in the area for a horizon of 20 years.
Seven (7) boreholes set up by SONES provide nearly 150 m3/h each, or 3000 m3/d; four (4) rural boreholes with flows between 46 and 178 m3/d, or an average of about 112 m3/d; NDP system drilling that exploits 5500 m3/d.
Drilling information for this scenario is reported in Table 1.
• Scenario 2
In addition to the data from scenario 1, we consider in it an increase in the number of collection structures for the supply of the city of Thies by 2037.
Assuming that this increase in exploitation is carried out from a battery of five (5) boreholes dug in the central part of the model in order to avoid overexploitation in the southeastern and western parts and at the same time limit brackish water inflows.
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Table 1. Location and operating rates of scenario 1 structures.
The water needs of the city of Thies for 20 years having been estimated above at 37,000 m3/d, providing this quantity of water would imply an operating rate of 370 m3/h by boreholes, considering a pumping time of 20 hours, i.e. a daily volume of 7400 m3 produced by each borehole (Table 2).
• Scenario 3
We start here from a contribution of part of the water needs of the city of Thies, still for the horizon of 20 years, for a possible dilution with the water currently used to reduce limestone concentrations. The captor field added to the previous scenario is retained. However, Thies’ water requirements for scenario 3 being halved 18,500 m3/d, operating flows would therefore return to 185 m3/d for each structure. The pumping time considered to be 20 hours, the volume pumped at each borehole is therefore 3700 m3/d (Table 3).
• Scenario 4
This scenario consists of covering nearly a third of the water needs of the inhabitants of the city of Thies. The Tassette aquifer does not have a very large capacity, and considering in this case that scenarios 2 and 3 do not provide good results and are therefore not able to meet the expectations of this modelling, this option would therefore, if the results are conclusive, be a plus both for the population concerned and for the quantitative and qualitative conservation of that groundwater.
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Table 2. Location and operating flows of scenario 2 structures.
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Table 3. Location and operating rates of scenario 3 structures.
Indeed, in this scheme, we seek to provide nearly a third of the estimated needs for this population and for 20 years. We are therefore starting from a flow of 12,000 m3 withdrawn per day, in addition to the current situation, that is to say scenario 1. This quantity divided between boreholes F8, F9, F10, F11 and F12 is equivalent to a flow rate of 2400 m3/d per drilling, or 120 m3/h per borehole for a daily pumping of 20 hours. Table 4 shows the flow rates sampled for this scenario.
3. Results and Discussions
This part is particularly devoted to the different results obtained for this modeling, we will first estimate the drinking water needs of the inhabitants of the city of Thies and this result will make it possible to evaluate the behavior of the aquifer in cases where these quantities are taken.
The principle of the model and calibration will help to better understand the processes by which the results obtained are generated in Visual Modflow Flex.
To meet the demand of the city of Thies in a horizon of twenty (20) years, the necessary water production should be about 37,000 m3 per day for a population of 552,709 inhabitants.
At this level, the simulation results indicate an evolution of the piezometric surface almost identical to steady-state piezometrics.
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Table 4. Location and operating rates of the works in the scenario.
The loads in the different structures retain substantially the same values in the first periods. But a rather small discount would be observed at the end of the 20 years of project. This variation is mainly due to the pumping of large flows around the battery of the 7 boreholes of the SONES. The balance of inputs and outputs during the simulation is estimated according to 10 Time Step steps of 20 stress periods each. Figure 2 shows the balance of model inputs and outputs for scenario 1 to 2037.
The simulation results for this situation show a very significant variation in piezometrics even altering the saturation of the water table in places with loads of about −24 m. Figure 3 shows the balance of water inflows and outflows in the area.
For this scenario, we find that the decrease in the potential withdrawal to supply the city of Thies gives less extreme results compared to the second situation. Indeed, the simulation with the same process and the same calibration data as the previous one reveals a piezometric surface remaining above the level of the roof of the aquifer. Figure 4 shows Balance of model inputs and outputs for scenario 3 to 2037.
Regarding this option, the charges are negative in places and mainly towards the center where the catchment field for Thies is located. They remain positive towards the East up to nearly 4 meters. Figure 5 shows Balance of model inputs and outputs for scenario 4 to 2037.
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Figure 2. Assessment of model inputs and outputs for scenario 1 to 2037.
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Figure 3. Assessment of model inputs and outputs for scenario 2 to 2037.
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Figure 4. Assessment of model inputs and outputs for scenario 3 to 2037.
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Figure 5. Assessment of model inputs and outputs for scenario 4 to 2037.
4. Conclusions
Groundwater quality is the most cost-effective way to supply households that are not served by public or private distribution systems, such as rural villages, and could serve as a catalyst for economic growth by increasing irrigated land and improving crop yields and crop diversity.
In short, the collection and exploitation of the various data related to the Paleocene aquifer, and more particularly in the Tassette area and its surroundings, made it possible to propose a general presentation of the area and its characteristics. As the Tassette area is indicated by several studies as part of the favourable potential for water transfers, it is therefore necessary to establish a model for the efficient and effective exploitation of this resource. In this sense, different scenarios were proposed and simulated during this work in order to deduce their effect on the water table, its capacity and its piezometric load. The results of this step were rather conclusive in terms of the almost unchanged piezometric surface.
It will especially be necessary to carry out regular maintenance of the supply networks and avoid high temperatures at the level of the appliances such as hot water ducts, water heaters, washing machines, etc., leading to the precipitation of limescale and the formation of scale. In addition, including the Maastrichtian in this same Tassette area would perhaps make it possible to have a modeling with better results because it has greater potential.