Investigation of overexploitation groundwater in arid areas: case of the lower Jurassic aquifer, Bechar province Southwest of Algeria

The conservation of natural non-renewable water resources is an important part of the expectations of the Algerian population, especially in the south of the country. Groundwater is provided in the southwest of Algeria, and the demand for water is high including agriculture. Studies have shown that the quality of these resources has changed after many years of continuous exploitation. To meet the satisfaction and the need for drinking water, for the city of Bechar, located in southwest Algeria, technicians and decision-makers have sought additional water resources, namely in the Jurassic groundwater located at the region of Mougheul (Sahara Algerian). The intense use of pumping at Mougheul has led to serious local problems (rapid drawdown and drying up of the source). The obtained results reveal that the piezometric charts, established in October 2001 and November 2009, show that the main flow of water is from north to south, exactly to the source of Mougheul. The flow rates of the drillings made are very varied, and between 6 and 30 l/s, this amounts to the heterogeneity of the medium. The pumping tests give low transmissivity values varying between 3.22 10–5 and 5.49 10−3 m2/s.


Introduction
Water scarcity is a major problem that permanently threatens human survival, especially in semiarid and arid regions, where increasing demand for water from local people and industries in the region is taking over confined and semiconfined aquifers. Therefore, ensuring the availability of sufficient water supplies of adequate quality for certain uses in dry areas is a major challenge, given the unpredictable climatic fluctuations in these areas.
Aquifers are very convenient water sources because they are natural underground reservoirs and can have a huge storage capacity. Since many aquifers contain water, its use for drinking water production and human consumption is obvious, and it is used worldwide as a source of irrigation water in agricultural areas. Because their storage capacity is important, they can provide a continuous source of water, even in dry seasons when precipitation is almost absent and surface water dries up quickly (Kendouci et al. 2016;Mebarki 2021).
In recent times, the concept of sustainability and sustainable development is very important, and it is defined as the level of groundwater development that meets the needs of the present generation without compromising the ability of future generations to meet their needs. But sustainability is also a multiple concept, when the total volume of withdrawals from the aquifer is close to or greater than the total recharge over several years, and it is often said that there is overexploitation. Basically, the aquifer is overexploited or runs the risk of being overexploited when the pumping rate is higher than the natural recharge rate (Kendouci et AL. 2019). However, in practice, an aquifer is often viewed as overexploited when some persistent negative consequence of an aquifer development is experienced or perceived, such as a continuous decline in the water level, a gradual deterioration in water quality, an increase in the cost of extraction, or environmental damage. The term "extensive use of groundwater" is applied when exploitation leads to significant changes in the dynamics of natural aquifers.
The objective of the hydrogeological study is based on the determination of the characteristics and properties of the aquifer. Our work is based on the observation and gathering of all possible information. This information was based mainly on measurements of water levels (static and dynamic) and gauges of the flows of works and sources (Osiensky et al 2006;Mebarki 2021).
In many semi-arid to arid regions, groundwater has been withdrawn at rates far in excess of recharge; therefore, careful management of the resources available in these areas is necessary to remove water stress due to the deterioration of water in terms of quality and quantity (Dawoud 2013). Indeed, in desert areas, the livelihoods of the population and economic development depend on the ability to maintain agricultural production and maintain the quality of crops, which in turn requires the systematic evaluation of many factors such as the quality of irrigation water, agricultural practices and sanitation systems (Kendouci et al. 2013).
Algeria, like most arid and semi-arid regions of the world, is facing a growing water crisis especially in the south, which is characterized by a dry desert climate and scarcity of surface water.
For our study, we will rely on the data for the year 2001 which were taken as reference data, because the water table was, during this period, very little used, not to say at rest, if we refer at the geological age of the aquifer formation (Lower Jurassic). The first dewatering of the aquifer only started, to our knowledge, during 2001. It was part of a longterm test pumping. At the level of five boreholes simultaneously, the average pumping time exceeds 72 h.

Material and methods
In the case of a search for underground water, one of the main objectives is to determine the quantity of water that can be exploited annually from the aquifers of the region studied. The factors determining these quantities of usable water are: 1. The transmissivity and the storage coefficient. 2. The extent of the aquifer and its boundaries. 3. The hydraulic characteristics of the roof and substratum of the aquifer. 4. The effects of the planned exploitation on the regional conditions of recharge and discharge of the aquifer.

Study zone
The area concerned by this study is represented by the village of Mougheul and its surroundings which correspond to a small portion of the northwest of the Algerian Sahara ( Fig. 1). The village of Mougheul is located about fifty km north of the capital of the wilaya of Béchar; it is connected to the latter by distinguished road under the name of CW3 ( Fig. 1). This zone is located on the Algerian-Moroccan borders and admits the limits ( Fig. 1) to the north Theniet Zerga, to the south the plain of Guelb El Harkat, to the east Oued El Abiod, and to the west the Menabha region. This region is distinguished by its arid climate with a long period of drought. The average annual height of precipitation is around 192 mm, with an average temperature of 17.5 °C. The monthly interannual average (2008-2018) of potential evapotranspiration is around 12,287.21 mm/year. The hydrological balance is in deficit according to the interannual step (2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). Recharging of groundwater takes place from intense infiltration by floods from outcrops ).

Hydrogeology of the region
The essential element of the entire hydrogeological unit of the Mougheul basin is the aquifer of dolomitic limestones of the Lower Jurassic whose formations are largely outcropping in the north at the Jebel Grouz level.
We are based on existing information to try to develop a draft on the hydrogeology of this geological formation. Jurassic dolomitic limestones (Fig. 2), the main aquifer of the region, largely outcrop to the north and northwest on the flanks of the Jebel Grouz and extend to the Ben Zireg region. It is the most important permeable base of the whole system of the study area and even of the northern part of Bechar.
These extensive outcrops give this aquifer system a large impluvium involving a certain recharge of the underground resource (Fig. 3). The direct feeding is done by its impluvium whose surface of outcrop is very extensive in the space. By their lithology, very favorable, these dolomitic limestones cracked are characterized by an important transmissivity. All the drillings realized in the region capture this formation, to a thickness greater than 80 m (Mebarki 2013).
The physicochemical values (Table 1) show that the waters of the Mougheul aquifer are excellent quality, and the groundwater is drinkable according to the drinking water standards of the WHO ).

The pumping test
The optimal length of a trial depends on several factors. If the pumping test is carried out for a water supply project, a steady flow regime is sought. This diet is obtained after a delay which varies from a few hours to a few days. The delay depends on the pumped flow, the type of aquifer, and the proximity of the recharge zones.
In general, the tests have a minimum duration of 72 h for the pumping phase and the same duration for the observation of the ascent, unless a return to the initial level in a shorter period of time. The water must also be sampled periodically for analysis, in order to ensure the consistency of the water quality. If the pumping test only serves to know the hydraulic characteristics of the aquifer, it can be made to last less than 72 h. However, it is recommended to make it last until a steady state is obtained, which can take several days in an unconfined aquifer. A test should not be carried out for less than 24 h in an unconfined aquifer.
On the other hand, a test, which gives the entire response in transient state before the steady state, provides information on impermeable boundaries and recharge zones. A shorter test, which gives only part of the transient response, can provide the T and S parameters of a confined aquifer without providing essential boundary information.

Piezometry
The piezometric map obtained (October 2001) shows that all the water flow lines converge toward an N-S direction drainage axis, with a hydraulic gradient of the order of 0.015 (Fig. 4). It is noted that the source of Mougheul is exactly downstream of this drainage axis; it presents the outlet of the sheet ( Table 2).
The piezometric map (December 2009) shows that the main flow of water is from north to south, with a hydraulic gradient of about 0.010 (Fig. 4). This piezometry showed a depression greater than 3 m. According to December 2009, when the drying of the source of Mougheul begins to be observed, it can attribute the drying of the source to the depression mentioned previously (Fig. 5).

Hydrodynamic parameters
The long-term pumping test of the drillings makes it possible to draw the curves of the descent (Fig. 6) which offer the possibility of calculating the hydrodynamic parameters by the application of the expression of the logarithmic approximation of C. Jacob.
Cooper and Jacob (1946) time-drawdown method is commonly used to interpret aquifer-pumping tests data due to its simplicity (Renard et al 2008). When using the Cooper and Jacob (1946) method, only transmissivity (T) can be estimated from single-well aquifer-pumping tests, while both transmissivity and storativity (S) can be estimated from multi-well aquifer tests (Gomo 2019;Mebarki 2013).
Cooper and Jacob (1946): (1) T = 2.3Q 4πΔs    The study of the ascent shows that the curves of the residual drawdown time (Fig. 7) are very fast indicating the good recharge of the aquifer.
The short-term pumping test (Fig. 8) was used to calculate the critical flows, the critical drawdowns and to propose the admissible values of the operating flows (Table 3).

Fictive radius calculation
In the calculation of the fictitious radius that will be used in the delimitation of the zone of influence, we note that only the Drilling D3 offers the possibility of calculating this value which is equal to 2.445 km. We note that the zone of influence also includes the Drilling D2, D5, farm well and Mougheul source (Fig. 9). This zone is the seat of a simultaneous pumping of all boreholes that are found to cause undoubtedly intense interference resulting in a sharp increase in the drawdown in drilling; this interference is most likely the main cause of the drying of the source of Mougheul.

Discussions
The services responsible for managing groundwater resources in many countries around the world have the appropriate financial and human resources, and groundwater users are officially registered with these bodies, and  (Mebarki 2021) different approaches have been tested to deal with the overexploitation of groundwater resources. In these countries, the legal framework and institutional procedures have enabled the design and implementation of joint strategies to control agricultural practices to ensure the sustainability of these uses. These approaches often use traditional integrated water resource management tools, such as pricing and quotas (Benfetta and Ouadja 2020).
Interpreting the results of a pumping test amounts to adjusting a conceptual model whose hydraulic behavior is considered similar to the geological aquifer system in question, for the vicinity of the catchment structure tested. This Fig. 6 a Curves of the descent, long-term test pumping of the drilling M (own elaboration), b Curves of the descent, long-term test pumping of the drilling D2 (own elaboration), c Curves of the descent, long-term test pumping of the drilling D3 (own elaboration), d Curves of the descent, long-term test pumping of the drilling D5 (own elaboration), e Curves of the descent, long-term test pumping of the drilling D10 (own elaboration), f Curves of the descent, long-term test pumping of the drilling D11 (own elaboration) adjustment work makes it possible to obtain hydraulic characteristics which, if the choice of the conceptual model is judicious, should make it possible to appropriately reproduce the hydraulic behavior of the geological aquifer system requested, namely: • The transmissivity of the aquifer formation and, depending on the case, its spatial variability or its anisotropy (when several piezometers were monitored during the test); • The storage coefficient and its spatial variability; • Depending on the hydrogeological context, the vertical hydraulic conductivity of the aquitard which covers the geological aquifer formation tested; • The efficiency of the pumping installation; • The position of the boundaries of the aquifer geological formation which are close enough to influence the flow in the vicinity of the catchment structure, therefore which were detected during the pumping test; • The piezometry in the sector under study; To determine the quantity of water that can be drawn annually from the aquifer under study without overexploiting it, it is therefore not necessary to limit oneself to carrying out and interpreting a pumping test. The following information should preferably be obtained before the pumping test: • The state of the current exploitation of the aquifer (number of wells capturing water in the studied aquifer, position of these wells, flow rates withdrawn, water quality, known quality problems, etc.); • The seasonal variations of the current exploitation (flows, volumes), which can be significant, for example in the case of agricultural use of groundwater; • Seasonal variations in piezometric levels, water quality; • The annual volume of current withdrawals and future withdrawals compared to the average annual flow transiting through the aquifer, established by the hydrogeological balance; • It is also necessary to evaluate, after the pumping test, the various impacts that will be created by the addition of the proposed well. This will disrupt a previous operation that was more or less stable over time.

Conclusion
One of the main quantitative objectives of the hydrogeological study is to determine the quantity of water that can be drawn annually from the studied aquifer without overexploiting it. It is therefore important that the 72-h pumping test at constant flow rate leads to a steady-state situation. It will also be checked that the water levels in the well and the piezometers (or monitoring wells) have returned approximately to their initial positions after 72 h of observation of the rise following the pumping. According to the calculations, it has been found that the D3 drilling has a great influence on the aquifer of the study area, which allows us to say that drilling is done in close proximity to each other; this led to probably at the lowering of drawdowns and the drying of the source of Mougheul over time. The insufficiency of the piezometers in the study area (1 piezometer), in spite of the importance of this aquifer, reflects a weak coverage of the data. In the Fig. 8 a Characteristic curves, short-term test pumping of drilling D4 (own elaboration), b Characteristic curves, short-term test pumping of drilling D5 (own elaboration) 2.24 × 10 -4 * 1.76 × 10 -6 − sense of conducting an effective strategy in the management of the water resource, it is necessary: • To avoid exploiting more than one borehole in the study area, it is possible to choose drilling D2 which has a significant transmissivity value compared to other drillings, with a flow not exceeding the critical flow. • To monitor the variation of drawdown in the boreholes and to make piezometers in the study area.