Integrated approach to choosing suitable areas for the realization ofproductive wells in rural areas of sub-Saharan Africa

Giorgio Ghiglieri1 & Alberto Carletti2

1Department of Territorial Engineering, Desertification Research Group (NRD), University of Sassari, Viale Italia, I-07100 Sassari, Italyghiglieri@uniss.it2Desertification Research Group (NRD), University of Sassari, Viale Italia, I-07100 Sassari, Italy

Received 26 August 2009; accepted 6 August 2010; open for discussion until 1 June 2011

Citation Ghiglieri, G. & Carletti, A. (2010) Integrated approach to choosing suitable areas for the realization of productive wells in ruralareas of sub-Saharan Africa. Hydrol. Sci. J. 55(8), 13571370.

Abstract Comprehensive geological, hydrogeological and hydrogeochemical investigations were carried out in thesouth of Hodh El Chargui (southeast Mauritania). Obtaining a hydrogeological conceptual model is crucial forgroundwater resources development and management. This is especially true in developing countries and in therural areas of sub-Saharan Africa. The chosen areas are represented by lithologies referring to a long geological timeframe, dating from the Upper Neoproterozoic to the Quaternary age. We developed a methodology in order toidentify sites that were suitable for the realization of productive, protected and correct wells to supply safe water tothe rural community. A multicriteria approach to studying hydrogeology was used in the project area. In order toidentify some main areas in which to carry out pilot interventions, criteria relating to water accessibility andavailability, and to hydrogeological and water quality, were considered. Moreover, during the project, it waspossible to transfer know-how and hand over responsibilities to the local population and bodies.

Key words groundwater; access to water; developing countries; Mauritania

Approche intgre pour le choix de zones pertinentes pour la ralisation de puits de production dans lesrgions rurales de lAfrique sub-SaharienneRsumDes tudes gologiques, hydrogologiques et hydrogochimiques compltes ont t menes dans le sud duHodh El Chargui (sud-est de la Mauritanie). Lobtention dun modle hydrogologique conceptuel est cruciale pourle dveloppement et la gestion des ressources en eaux souterraines. Cela est particulirement vrai dans les pays endveloppement et dans les rgions rurales de lAfrique sub-Saharienne. Les rgions choisies sont caractrises pardes lithologies qui renvoient une longue priode gologique, du Noprotrozoque Suprieur au Quaternaire.Nous avons dvelopp une mthodologie afin didentifier des sites pertinents pour la ralisation de puits productifs,protgs et corrects qui permettraient dalimenter les communauts rurales en eau de qualit. Une approchemulticritres a t utilise dans la rgion dtude pour tudier lhydrogologie. Afin didentifier quelques zonesprioritaires au sein desquelles conduire des interventions pilotes, des critres lis laccessibilit et la disponibilitde leau, ainsi quaux qualits hydrogologiques et de leau, ont t considrs. En outre, durant le projet, il a tpossible de transfrer le savoir-faire et les responsabilits aux populations et institutions locales.

Mots clefs eaux souterraines, accs leau; lenvironnement aride; Mauritanie

1 INTRODUCTION

As highlighted by MacDonald et al. (2008), At least44% of the population in sub-Saharan Africa (some320 million people) do not have access to clean, reli-able water supplies (JMP, 2004). The majority of thosewithout access (approx. 85%) live in rural areas wherethe consequent poverty and ill-health disproportio-nately affect women and children (JMP, 2004). Inresponse, the international community has set the

Millennium Development Goals (MDGs), which com-mit the UN membership to halve the proportion ofpeople who are unable to reach, or afford, safe drink-ing water by the year 2015 (United Nations, 2000).Another objective is to provide a quantity of at least 20litres per day per person to 60% of the population.Accessibility to drinking water is a fundamental rightthat is still lacking for a large part of the worldspopulation (UNEP, 1994). Further, MacDonald et al.(2008) stated: Across large swathes of Africa, South

Hydrological Sciences Journal Journal des Sciences Hydrologiques, 55(8) 2010 1357

ISSN 0262-6667 print/ISSN 2150-3435 online 2010 IAHS Pressdoi: 10.1080/02626667.2010.527845http://www.informaworld.com

America and Asia, groundwater provides the onlyrealistic water supply option for meeting dispersedrural demand. There are many reasons for this: it isgenerally found close to where it is needed, the naturalquality is usually good and it is resistant to evenprolonged droughts. Alternative water resources canbe unreliable and expensive to develop (Foster et al.,2000).

In an attempt to address this problem, the presentwork was carried out within the ACP-EU WaterFacility Programme, through the HEFEM Project(HEFEM: appui aux municipalits rurales pour lascurisation et la gestion de leau) (http://www.projet-hefem.org:8080/servlet/ae5Mau). The general objec-tive was to guarantee easier access to water for theinhabitants of 13 rural municipalities of the provincesof Nema and Timbedgha, in the south of Hodh ElChargui, southeast Mauritania.

The area had not previously been the object ofsystematic hydrogeological studies. A multicriteriaapproach to studying hydrogeology was used in theproject area. In order to identify the main areas inwhich to carry out pilot interventions, water accessi-bility and availability, as well as hydrogeological andwater quality criteria, were considered. Furthermore,during the project, it was possible to transfer know-how and hand over responsibilities to the local popu-lation and bodies.

The paper draws on an extensive literature reviewbased primarily on the published literature; however,in some cases it also draws on grey literature(papers, books, theses, conference proceedings, etc.)where the data are considered to be of good quality andwhere this provides clearer information. A representa-tive literature set was acquired through this process,although, as noted within the text, in some areas theavailable literature and evidence is sparse.

2 CONCEPTAND SCOPE OF THEMETHODOLOGICAL APPROACH

Lack of water is certainly one of the biggest problemsafflicting the population of Mauritania, as well as thecountries in sub-Saharan Africa, in general. Cobbing& Davies (2008) stated, It is now broadly acceptedthat if the MDG targets are to be met, groundwaterwill have a central role to play (see for instanceMacDonald et al., 2005; Pietersen, 2005). However,there are some risks inherent in proceeding with largeincreases in groundwater use without a scientificapproach to, and sustainable management of,

groundwater development. Frequently, many ruralwater supply projects suffer from very little hydrogeo-logical input. Instead, they are seen only in terms ofengineering problems: e.g. drilling, pump installation,tanks and taps.

Thompson et al. (2000) wrote: The benefits andcosts of providing a safe, convenient and reliablewater supply to people in the developing world hasbeen the subject of a vast and wide-ranging researcheffort over the last four decades. Research has focusedon various aspects: the relationship between waterand disease; the efficacy of water supply projects inimproving health; the causes and consequences ofdifferential access to, and control of, water resources(particularly with regard to gender and wealth); andthe financing of water supply infrastructures. Despitethis plethora of literature and research, relatively littleis known about a number of key aspects relating to asimple, useful, scientific-based procedure that couldbe applied in developing countries when planninginterventions aimed at improving access to safewater. Of all the regions of the world, the researchgap is most acute for sub-Saharan Africa, a regionwhose population has the least access to improvedwater supplies (Cosgrove & Rijsberman, 2000;MacDonald et al., 2005; Pietersen, 2005).

Concerning the methodology of work, Cobbing &Davies (2008) highlighted: Accurate and appropriatedata collection during project implementation, togetherwith data interpretation and knowledge disseminationcan prevent past mistakes being repeated and, more-over, reduce the ultimate cost of water supply schemesfrom both a human and a financial point of view, andFonjong et al. (2004) stated, Furthermore, projects ofthis type must take into consideration the sources andlevels of income and other socio-economic aspects ofthe community, its population and its system of man-agement, before proposing any technical solutions. Infact, in achieving clean water for all, indigenous knowl-edge and community participation have a determinantrole to play. This is because most topbottomapproaches to water schemes have shown limits inproviding safe water to communities. Such failures inwater provision and distribution should be consideredas a clarion call to encourage community involvementin all water supply schemes. It is also an indication ofthe need to promote community-based initiatives invol-ving the rural population in the planning, designing,implementation and management of their waterresources in accordance with local situations andneeds.

1358 G. Ghiglieri & A. Carletti

In order to identify suitable sites for the realiza-tion of productive, protected and correct wells to sup-ply safe water to the rural community in the study area,the authors developed a methodology by means ofintegrated criteria. In Fig. 1 the proposed methodologyis shown in a flowchart. Criteria have been groupedinto three categories:

Water access criteria (water availability); Hydrogeological criteria; and Water quality criteria.

The guidelines of the World Health Organization(WHO, 2006) define minimum standards for waterquality by establishing concentration level limits fora set of organic and inorganic chemical parameters.Moreover, the guidelines contain a classification basedon accessibility, expressed in terms of time and dis-tance to be covered to reach the nearest water supplypoint, and on the quantity of water being distributed or

used (Table 1). With regard to this last point, theminimum standard that has to be guaranteed in thedeveloping countries corresponds to the basic accesscategory (distance of less than 1 km and water avail-ability of 20 litres per day per person).

The hydrogeological criteria, aimed at identifyingpotentially exploitable aquifers, has also taken intoaccount some technical factors available in situ(Ghiglieri et al., 2010). Given that in these rural areasneither equipment for deep drilling nor electric distri-bution lines are available, the most strategically exploi-table aquifers are shallow ones. MacDonald et al.(2008) reports that in southern Africa sand rivers arean important source of water for rural areas. Researchinto the occurrence of groundwater in sand rivers hasbeen undertaken in Botswana (Wikner, 1980; Davieset al., 1998), Namibia (Jacobson et al., 1995) andZimbabwe (Owen, 1989). However, their exploitationis limited by the vulnerability of these aquifers to

Fig. 1 Integrated criteria flowchart.

Table 1 Requirements for water service levels and health implications (WHO, 2006).

Service level Access measure (distance or time) Needs met Level ofhealthconcern

No access Quantity collectedoften below 5 L per capitaper day

More than 1000 m or 30 min totalcollection time

Consumption cannot be assured.Hygiene not possible (unless practised at thesource)

Very high

Basic access Average quantityunlikely to exceed 20 L percapita per day

Between 100 and 1000 m or 530 mintotal collection time

Consumption should be assured.Handwashing and basic food hygiene possible;laundry and bathing difficult to assure unlesscarried out at source

High

Intermediate access Averagequantity about 50 L per capitaper day

Water delivered through one tap onplot or within 100 or 5 min totalcollection time

Consumption assured. All basic personal and foodhygiene assured; laundry and bathing should alsobe assured

Low

Optimum access Averagequantity 100 L per capitaper day

Water supplied through multipletaps continuously

Consumption: all needs met. Hygiene: all needsshould be met

Very low

Source: Howard & Bartram, 2003.

Groundwater research on Mauritania (sub-Saharan Africa) 1359

pollution: they are easily exposed to any possibleanthropogenic or animal contamination sources.

Furthermore, in rural areas there are widespreadphenomena of pollution due to bad management andlack of maintenance of water supply structures. Gyau-Boakye et al. (2008) states that, in rural areas of sub-Saharan Africa and particularly in Ghana, the waterin many hand-dug wells tends to be muddy and pol-luted due to a high nitrate content (3060 mg/L) andabundant coliform.

The authors used a classification in order to deter-mine the quality of the groundwater. The basic ground-water quality assessment was carried out through aclassification that was proposed in various scientificworks (Civita & De Maio, 2000; Barbieri et al., 2005;Ghiglieri et al., 2006, 2009; De Maio et al., 2007;Civita et al., 2009), and acknowledged by Europeanand Italian law (DPR 236/88, 1988; EC, 2000; DPR 31/01, 2001), as well as by the World Health Organization(WHO, 2006). This classification considers the destina-tion use of the water, distinguishing between waterintended for human consumption and that intendedfor irrigation purposes (Barbieri et al., 2005; Ghiglieriet al., 2006; Civita et al., 2007, 2009).

The classification takes into account a number ofchemical and physical parameters, and undesirablesubstances: depending on whether concentrations arehigher or lower than the guide values and than max-imum admissible concentrations (values that do notexceed the recommended limit), it assigns an assess-ment of usability (from optimum to very bad). Thus,for each intended use, these different parameters indi-cate respective values which identify various classesof water. Indeed, a classification based on parametersthat can vary over time, depending on natural or

anthropogenic factors, represents a snapshot of thequalitative state at the precise moment that samplingwas carried out. Therefore, in order to study the evolu-tion through time and space of the qualitative states ofthe groundwater, a programme of monitoring is ofprime importance.

The parameters taken into consideration, regard-ing the basic quality of waters destined for humanconsumption, are split into two groups:

Group 1 includes the chemical-physical para-meters: hardness, TH (French degree of hardness,10 mg CaCO3/L), electrical conductivity (EC) at20C (S/cm), chlorides, sulphates and nitrates(in mg/L).

Group 2 includes the undesirable substancesNH4

+, Fe2+ and Mn22+(in mg/L).

The values used to indicate the limits of the differentclasses were calculated starting from the Guideline Value(GV) and the Maximum Admissible Concentration(MAC) indicated by DPR 236/88 and EC (2000)(Ghiglieri et al., 2006; Civita et al., 2007, 2009).

Basic quality is identified by combining the twoclasses determined, in both groups, by the parameterincluded in the worst one. The possible combinationsof the six quality classes, three for each group (A1, B1,C1, A2, B2 and C2), give rise to nine final basicquality classes.

Table 2 shows the intervals of values which iden-tify the quality classes: optimum (A), intermediate (B)and poor (C), relative to each group. To each of these, aquality assessment is given, as shown in Table 3.

The hydrogeological and water quality criteriamust be used to identify suitable sites in which torealize productive, protected wells. Indeed, it is

Table 2 Classification of groundwater quality.

Group 1: Group 2: AssessmentChemical-physical parameters: Undesirable substances:

Class

TH EC (20C)(S/cm)

Cl-

(mg/L)SO4

2-

(mg/L)NO3

-

(mg/L)NH4

+

(mg/L)Fe2+

(mg/L)Mn2+

(mg/L)

A 15a30d 0.2c >0.05c Poor

TH: French degrees of hardness (10 mg CaCO3/L); EC: electrical conductivity.aMinimum recommended value (EEC, 2000; DPR 236/88; DPR 31/01).bGuideline value (GV) (EC, 2000; DPR 236/88; DPR 31/01).cMaximum Admissible Concentration (MAC) (EEC, 2000; DPR 236/88; DPR 31/01).dIntermediate indicative value between MAC and GV (EEC, 2000; DPR 236/88; DPR 31/01).eValue double that of GV.

1360 G. Ghiglieri & A. Carletti

normally recommended that new boreholes and pro-tected dug-wells are sited upstream, as opposed to inthe direction of the groundwater flow, and away frompit latrines and refuse dumps in order to avoidpollution.

Once the list of suitable areas has been defined, itmust then be submitted for approval to the TerritorialCommunities and to the variousMunicipal Authorities,in order to reach an agreement. In fact, in makingthe above-mentioned decisions and interventions,technical results are not sufficient, since social andeconomic factors play a relevant role and, thus, localauthorities and communities must first approve thedevelopment plans (Foster et al., 2000; MacDonaldet al., 2005; Cobbing & Davies, 2008).

3 STUDYAREA

3.1 Location, morphology, climate settings

The study area (Fig. 2(a)), corresponding to theMoughataa (Provinces) of Nema and Timbedgha, issituated in the southeast of Mauritania and lies entirelywithin theWilaya (Region) of Hodh El Chargui. In thisarea, the most populated apart from the capitalNouakchott, there are 281 600 inhabitants, 35 700 ofwhom are nomads (ONS, 2002). The two Moughataaare situated in the southern part of the Wilaya andoccupy a surface of 20 000 km2 extending as far asthe border with Mali. This area is divided into 13municipalities, nine of which belong to Nema, whilethe other four belong to Timbedgha. In these ruralareas, there are small villages inhabited by both noma-dic and settled people whose main activities are grow-ing cereals and rearing livestock, including sheep,goats, cattle and camels.

The region of south Hodh forms a large depres-sion delimited by the Afoll Mountains to the west andby the cliffs of Dhar to the east. Dhar is a plateau

extending eastwards as far as the border with Mali.The morphology of the region, excluding Dhar wherethe topographic heights reach 420 m a.m.s.l., is quiteflat, with heights varying between 250 and 180 m a.m.s.l. As a whole, the area is strongly sanded up. Whengoing from north to south, the aeolian semi-activedune fields, which are sometimes thicker than 100 m,diminish in thickness (less than 20 m). They resemblefixed dune fields extending in an ENEWSW direc-tion that progressively leave their place to the emer-ging rocky basement.

The zone is ploughed by many wadi which, dur-ing the dry season, resemble quite sanded-up and dryriverbeds called Batha. Very often, these are subject toan enlargement of the riverbed and to the erosion of thebanks caused by occasional intensive precipitation andby degraded vegetation cover.

The area shows a typical Sahelian climate: duringthe year, a long dry season (OctoberJune) follows avery short humid season (winter) lasting from Julyuntil September. The influence of monsoons duringthe winter is significant, because, although the rainsthey bring are very irregular, these are often violentand catastrophic. Precipitation is generally scarce: thisis especially noticeable when moving northwards.Temperatures are high on average (over 30C) andthe frequency of dry and warm winds, from the northand northeast, favour evapotranspiration. The meanannual rainfall, as reported by the Nema station andcalculated over 56 years (19502006), averages264 mm. The mean annual maximum and minimumtemperature is respectively 36.8 and 24.6C. Theseclimatic factors contribute to an extremely limitedgroundwater recharge.

3.2 Geological setting

The general geological descriptions (Fig. 2(c)) arebased on the official cartography of the Governmentof Mauritania (Lahondere et al., 2005), while thehydrogeological setting results from the work carriedout in the context of this research. Lithologies refer to along geological time frame, dating from the UpperNeoproterozoic to the Quaternary age.

The study area is within the southwestern edge ofthe Taoudeni basin, a wide geological entity (about1 500 000 km2), corresponding to the sedimentaryNeoproterozoic and Paleozoic, and covering a largepart of the West African Craton. The rocks that makeup the ancient basement in the area are part of theGroupe de Tniagouri (Upper Neoproterozoic). This

Table 3 Assessment of basic groundwater quality forhuman consumption.

Class Assessment of use

A1A2 OptimumB1A2C1A2 GoodA1B2 AcceptableB1B2C1B2A1C2 MediocreB1C2 LowC1C2

Groundwater research on Mauritania (sub-Saharan Africa) 1361

is divided into three different units: the FormationdOurkem, at the base, constituted by dolostoneswith barytine, which are generally between 2 and15 m thick; the Formation de Bouly made of chertand thin bedded pelites, with thicknesses reaching100 m; the Formation dOuld Yenje, at the top, com-posed of purple pelites and shales (with thicknesses ofbetween 100 and 500 m). In the south and southeast ofNema, important massive bodies outcrop: they consti-tute large laccoliths, formed by gabbro and doloriterocks that were injected between the pelitic formationsduring the Lower Jurassic.

From these, a number of veins, mainly aligned inan ENEWSW direction, cross the Hodh Depression;these are often hidden by aeolian sandy deposits. In theeast of Nema, Cretaceous sandstones rest, in angulardiscordance, on the pelitic basement and on dolorites;these constitute the plateau of Dhar. These formationsare covered, in slight angular discordance, by theFerricrete: a Tertiary ferruginous duricrust (a fewmetres thick).

Quaternary superficial formations are mainlyrepresented by: the aeolian sandy deposits thatcover a large portion of the study area and appearboth as ancient steady dune cordons and still activedunes; the khabha and sebkha deposits formed by thebuild-up in depressed zones of thin materials trans-ported by streaming water and to which evaporationdeposits can be associated; the old palaeo-valley

alluvial sediments; and the actual wadi alluvialsediments.

3.3 Hydrogeological setting

At present, due to the complex hydrogeological frameand to the broad expanse of the study area, it is difficultto accurately define all of the different hydrogeologi-cal parameters like aquifer geometry, storage capacity,permeability, recharge areas, etc.

The hydrogeological setting has been describedand reclassified by combining an existing terrainarrangement (Lahondere et al., 2005), and an existinghydrogeological large-scale study of Mauritania(Friedel, 2008) with the new hydrogeological andhydrogeochemical surveys carried out within this work.

A conceptual hydrogeological model, on a regio-nal scale, was reconstructed. The lithostratigraphicformations can be grouped into three main hydrogeo-logical units (HU):

Upper Neoproterozoic Hydrogeologic UnitThis includes in a single complex the formations ofthe Groupe de Tniagouri (Hods pelites aquifer). Inthe study area it can reach up to a hundred metres inthickness. The aquifer has a fractured permeabilitydepending on the degree of the fracture system (jointsand faults) that serve as good conduits for groundwatercirculation on a regional scale.

Fig. 2 (a) Study area; (b) well location; and (c) geological (source: Lahondere et al., 2005) and hydrogeological map.

1362 G. Ghiglieri & A. Carletti

Mesozoic Hydrogeologic Unit This is dividedinto the Dolorites complex (Jurassic) and theSandstones complex of the Dhar of Nema(Cretaceous). The aquifer hosted in the dolorites canreach considerable thicknesses connected with themain laccoliths, while dolorites in the shape of dykeshave lower thicknesses. Permeability, when present, isof fractured media type and the network of fracturefrequency is tied to the fissured zone and faultingstates. The Cretaceous formations which constitutethe Dhar Plateau can be grouped together in a singlehydrogeological complex characterized by secondarypermeability. The aquifer has a lateral boundary con-dition in the East of the study area, due to the thicknessreduction (up to a few metres) of sandstones and to thepresence of the sub-emerging pelitic basement.

Quaternary Hydrogeologic Unit This formationcontains aeolian sandy deposits, outcropping mainlyin the western area, and ancient and recent alluvialdeposits, emerging in the wadis. The average porosityand permeability (hydraulic conductivity) is good.Alluvial deposits constitute some aquifers with limitedextension and they are recharged both by surface infil-tration (which is extremely sensitive to climatic fluc-tuations), and laterally from groundwater circulatingin neighbouring aquifers. Some aquifers (often perched)are hosted in the aeolian deposits. In the more northernregions (Aouker), these formations have remarkablethicknesses, reaching up to 100 metres. By contrast,in the study area thicknesses are considerablyreduced (less than 20 m) and disappear progressivelytowards the southeast, where the impermeable base-ment crops out.

4 METHODS OF INVESTIGATION

With the aim of obtaining hydrogeological data, twofield surveys were carried out in the area. The first,which also included surveys outside the limits of thetwo Moughataa, was particularly time-consuming(about 6 months) and difficult in terms of accessibilityand logistics: in all, 533 water points were surveyed(Fig. 2(b)). Every water point was identified with analphanumeric code and the following general datawere collected for each one: elevation, geographiccoordinates, piezometric level and hydrogeologicalcharacteristics; constructive and technical data of thewells were obtained, a global positioning system(GPS) being used to locate each feature. Analyses ofgroundwater samples, including pH, Eh, dissolved

oxygen, electric conductivity, and temperature werecarried out in situ. For each water point a monographicdata sheet was edited; afterwards data were organizedin a digital database and in a GIS support, usingArcView 9.2 and the open source software gvSIG.

Once the water point census was completed, thehydrogeological information was pre-processed interms of well depth, hydraulic head (deeper pelitesaquifers always have a greater hydraulic head), theconditions of the wells, piezometric contour lines andEC values. This, in combination with the hydrogeolo-gical information (i.e. extension and thickness of thesequences), allowed us to associate each well with itsaquifer and define the most convenient and efficientmonitoring network. This was also selected taking intoaccount a representative geographic spatial distribu-tion, the water access criteria (see Section 5.3) and theaccessibility to water points.

The second field survey, lasting about 2 months,was conducted on the monitoring network by the selec-tion of 99 representative wells (Fig. 2(c)). In all of thesethe following characteristics were collected: piezometriclevel and in situ analyses of a groundwater sample (pH,Eh, dissolved oxygen, electric conductivity, tempera-tures). Moreover, 40 samples were collected for labora-tory analysis: the sampling locations were chosen insuch a way as to represent the different hydrogeologicalsettings encountered in the study area (Fig. 2(c)).

The hydraulic head values were computed bysubtracting the observed water levels from co-locatedelevations and spatialized using geostatistical interpo-lation techniques.

For chemical determination, samples were col-lected in 100-mL bottles and filtered through 1.20and 0.45 m membrane filters. The samples werecollected directly into plastic bottles at the wellhead,with care taken to prevent air entrapment. Sampleswere preserved under cool conditions prior to labora-tory analysis. The collected samples were analysed, totest for major elements, in the head office of the projectin Mauritania in a purposely equipped laboratory. Thechemical determination, following a standard proce-dure, was carried out as follows:

Temperature, pH and electrical conductivity at25C (S/cm) were measured in situ with a porta-ble pH-conductivity metre (mod. HI 98130HANNA Instruments).

Bicarbonates through basic acid titration, with HCl0.1Nwith indicator mixedmethyl red bromecresolgreen.

Groundwater research on Mauritania (sub-Saharan Africa) 1363

Total hardness, calcium hardness, chlorides, ammo-nia, nitrites, nitrates, fluorides, phosphates, manga-nese, zinc and boron using spectrophotometer(multiparametric photometer Aqualytic PC Multi)through colorimetric reaction techniques.

Sulphates using spectrophotometer (multipara-metric photometer Aqualytic PC Multi) using tur-bidity techniques.

Potassium with kit Visocolor-Eco using turbiditytechniques.

Magnesium through the difference between totalhardness and calcium hardness (APHA et al., 1992).

Sodium using flame photometry (CIBACORNING, Mod. 410).

The precision of chemical analyses ranges from0.6 to 8% and was better than 5% on average. Thephysical characteristics and the chemical analyses ofall groundwater samples, corresponding to the April2007 campaign, are presented in Table 4 where con-centrations are expressed in mg/L.

Moreover, water samples were collected to deter-mine microbiologic analyses. The evaluation of totalcoliform, Escherichia coli and Enterococcus waseffected in terms of their presence or absence usingthe DST (Defined Substrate Technology): for totalcoliform and Escherichia coli, with Colilert kit andincubation at 35C 0.5C for 24 h and forEnterococcus, with Enterolert kit and incubation at41C 0.5C for 24 h. For both Escherichia coliand Enterococcus, the results were read under ultra-violet light (366 nm).

5 RESULTS AND DISCUSSION

In the context of this work, the elaborations concernexclusively the pelites aquifer (Upper NeoproterozoicHydrogeologic Unit) and dolorites aquifer (MesozoicHydrogeologic Unit). At present, no significant dataare available for the other two HUs.

5.1 Regional groundwater circulation andflowpaths

In order to reconstruct the regional groundwater flowsthe piezometric data corresponding to the April 2007survey were elaborated.

Figure 2(c) presents the piezometric contour linesand the main groundwater flow directions. The piezo-metric contour lines concerning the southeast of the

study area are related to the dolorites aquifer, whereas,due to its extension, the pelites aquifer has beendivided into four representative geographic areas.

The groundwater flow direction lines in bothaquifers, generally, converge into the valleys wherethe main wadis have settled: this fact gives rise to animportant lateral recharge process for the aquifershosted in the alluvial and sandy deposits. North ofthe city of Timbedgha, groundwater flow related tothe pelites aquifer is in an ENEWSW direction.

5.2 Groundwater geochemistry and waterquality indicators

Plots of major ion concentrations in groundwater sam-ples from the aquifers are presented in a Piper diagram(Fig. 3(a)). Most of the samples are included in thecentral sector of the diagram: the distribution showsnon-dominant hydrochemical water types. The princi-pal feature emerging from the whole data set regardingthe pelites aquifer (Upper Neoproterozoic) is that wecan identify mineralization processes in the ground-water. The end members (fresh and mineralizedgroundwater) for the pelites aquifer (Fig. 3(a)and (b)) are identified considering the TDI (total dis-solved ions) values (Asnachinda, 1997; Mayer, 1999),which range from 292.77 to 3167.52 mg/L (Table 4).As shown in Fig. 3(b), the fresh groundwater samples(403018p1 and 603035p1) are located in the east andsouthwest of the study area respectively, correspond-ing to the recharge areas of the pelites aquifer. Incontrast, the mineralized groundwater samples(410025p1, 602006p1 and 602034p2) are detecteddownstream with respect to groundwater flow direc-tion. This underlines the fact that the chemical compo-sition of waters is derived from geochemical evolutionprocesses related to waterrock interactions.

In Fig. 4, assessment classes according to basicquality classes for human consumption related to 40groundwater samples are represented and expressed inpercentage.

Only certain chemical parameters have influencedthe classification: subsequently these were expressedthrough the degradation index (DI) (Ghiglieri et al.,2006). This index, which is calculated for the para-meters nitrates (NO3), nitrites (NO2) and fluorides (F

-),is obtained from the ratio between the measured con-centration and the threshold concentration foreseen byWHO guidelines (WHO, 2006). The strength of thisindex is that it clearly shows the degree of water

1364 G. Ghiglieri & A. Carletti

Table4Hydrochem

icalparametersforthegroundwatersampled

inthesecond

survey.

IDUTM

EUTM

NHU

TpH

EC

DO

Ca

Mg

Na

KHCO3

Cl

SO4

NO3

NO2

NH3

PO4

FMn

Zn

TDI

Facies

403008P1

700551

1808876

UN

31.6

7.5

830

3.44

67.00

39.50

64.32