Geochemical assessment,mixing behavior and environmental impact of thermal waters in the Guelma geothermal system,Algeria

Foued Bouaicha·He′nia Dib·Oualid Bouteraa·Nabil Manchar·Kamel Boufaa·Nabil Chabour·Abdeslam Demdoum

Abstract A study of thirteen geothermal springs located in the geothermal field of Guelma,northeastern Algeria,was conducted. Samples were collected during the period between January 2014 and February 2016.Geochemical processes responsible for the chemical composition of thermal and mineralized water were evaluated. The hydrochemical analysis shows that the thermal waters are characterized by the presence of two different chemical facies,the first type SO4—Ca in the east,west and south of Guelma,the second type HCO3—Ca in the south.This analysis also attributed to sodium,chlorides,and sulfates to an evaporitic terrigenous origin by the molar ratio Sr2+/Ca2+.The thermal spring waters from Guelma geothermal system have a meteoric origin,and all samples are immature with strong mixing between hot and shallow waters with 19—38.5%rate of mixing.The silica geothermometer shows that these thermal waters have a temperature varying from 84 to 122°C and that the water came from a depth of 2100—3000 m through a fault system that limits the pullapart basin of Guelma.Potential environmental effluent from thermal spas could pollute in both the irrigation and drinking waters,and which imposes danger on the health of the inhabitants of the region.

Keywords Geochemistry·Geothermometry·Mixing·Thermal effluents·Guelma·Algeria

1 Introduction

The history of hydrotherapy in Algeria goes back beyond Neolithic.The discovery in Hammam Debagh of three stelae shows that there was an establishment at this place since the Punic age.The Romans,when they occupied the Numidian kingdom,had to build thermal spas on the main springs.Agglomerations developed at Aquae Tibilitanae(Hammam Debagh),Aquae Calidae(Hammam Righa),Aquae Sireness (Hammam Bouhnaifia), Thanera Masa Castra(Hammam Berrouaghia)(Dib 2008).

Algeria has a large number of hot springs(＞240 thermal springs).They are spread over all country and in particular in its northeastern part(with 70 principal thermal spring).The source of the geothermal events could be related to the magmatic events of Mio-Plio-Quaternary(Verdeil 1982),and a relatively high geothermal gradient that occurs in northeastern Algeria (about 45°C/km,(Haouchine-Bouchareb 2012).The region of Guelma constitutes a wide geothermal field characterized by a high geothermal gradient.Indeed,various thermal and thermomineral springs emerge in the region,some of these at relatively high temperatures,such as the case of springs from Hammam Debagh(H.Chellal,Geyser,H.Benadji),which shows a temperature at discharge between 30 and 94.7°C.The thermal activity of the Hammam Debagh area is characterized by artesian thermal springs,carbonic and sulfur gas seepage,travertine deposition,and hydrothermal alteration zones.The thermal waters of Guelma geothermal field have been used for balneology and bathing and are used in some of the most famous Algerian hot spas(Dib 1985,2008).

Numerous research has studied the processes and origins of hydrogeochemistry of thermal waters in Algeria(Bails 1888；Belhai et al.2016,2017；Benamara et al.2017；Cormy and Demians d'Archimbaud 1970； Dib 2008；Djemmal et al.2017；ENEL 1982；EURAFREP 1966；Foued et al.2017；Guigue 1940,1947；Issaa?di.A 1992；Laissoub.B 1974；Ouali.S 2015；Pouget and Chouchak 1923,1926；Rezig 1991；Saibi 2009；Verdeil 1974,1982；Ville 1852).Major ion ratios used as hydrogeochemical indicators can be used to study the origin,water—rock interactions and mixing behavior of thermal-groundwater(Awaleh et al.2017；Besser et al.2018；Han et al.2010；Mutlu 1998). Physico-chemical information in the geothermal reservoir can also designate structural elements of geothermal systems(Favara et al.2001；Thomas 1988；Wakita et al.1985).Some geological and hydrogeological studies have revealed a high structural complexity controlled by the tectonics activities(Maouche et al.2013).The intense faulting controls thermal springs and heat emergence,in the Guelma area(highest temperatures in the Mediterranean basin).

Some chemical geothermometers were applied to evaluate the geothermal reservoir temperatures for the Guelma geothermal system.A silicate and chloride mixing models are discussed in detail；it was used to assess the proportion of mixing with cold groundwater and the effects of the conductive cooling process as waters ascend,and also to check the estimated reservoir temperature obtained by chemical geothermometers.

The physiochemical characterization is very important to investigate the suitability of thermal waters for drinking and irrigation purposes.At spas,thousands of visitors are bathing.Some people,during the therapeutic period,drink without pre-set restrictions. Regarding drinkability and water suitability for physiochemical quality,thermal waters from Guelma according to WHO guidelines are not safe for consumption because samples are hard and cations and anions exceed WHO standards.The waste waters diffused from thermal stations on the environment could pollute the surface and groundwater(e.g.pollution of Zit-Emba dam by the effluents of Hammam Ouled Ali 1,2 and 3).In this case,the health of local inhabitants could be at risk.

The potential academic contributions of this paper are as follows.First,this article will be a transposable prototype towards other similar areas,because it deals with a very complex hydrogeological system of thermal waters in East Algeria.It helps to understand the hydrodynamics and hydrochemistry of thermal groundwater and especially the management of water resources in a semi-arid climate.We focus on the water chemistry in order to explore the compositional of hydrothermal and the origin of water mineralization,spatial distribution,formation processes,and evolution mechanisms of the thermal water, and understand mixing processes between hot and cold waters,using mathematical and geochemical methods, in the Guelma geothermal reservoir system,east of Algeria.This study can be used to assess the possibility of the geothermal resources,and take an important position in the sustainable exploitation and protection of thermal and nonthermal groundwater in the Guelma area.

2 Geological and hydrogeological settings

The northern part of Algeria presents a complex geological setting and is belonging to the North of the Maghrebide-Alpine belt.This chain extends from Gibraltar to Sicily-Calabria(Aubouin and Durand-Delga 1971).It is positioned between the paleo-margin of the African plate(in the South)and the European plate(North).Inside this domain,there is a consideration between internal areas,within the belt and represented by different massive,distributed along the Mediterranean shoreline and,the external areas located at its periphery.

The study area is located in the external zones within the Tellian zones(Wildi 1983).The investigated area has various geological units which are presented by sedimentary formations of secondary and tertiary ages.It has experienced Miocene folds,and the nappes thrust over the Atlasic foreland to the south.The Guelma region is presented by units which are underlying from ancient to younger by:an autochthonous unit of Lower Cretaceous age:(limestone,dolomite and clay),the Tellian nappe of Cretaceous to Eocene age(limestone,marls,calcareous marls and clay),Oligocene to early Burdigalian age presented by the Numidian flysch nappe(lower unit:clay,upper unit:sandstone)(Chouabi 1987；Darest 1909,1910；Lahonde`re 1987；Raoult 1974；Vila 1980)(Fig.1)and a post nappe autochthonous formation consisting of the Guelma molasses (sandstone, marls and yellow conglomerates)and the continental Miocene(conglomerate,marls,limestone,clay with gypsum).

Fig.1 Geological map

The valley of Guelma makes,along the Seybouse River,a Miocene fill accumulation.It represents a pull-apart basin(Yelles-Chaouche et al.2006)which is delimited by marginal faults.

The determination and the identification of the aquifers and the determination of their hydrogeological characteristics labor it possible to recognize the groundwater potentialities of the Guelma province.The hydrogeological study of the Guelma basin is necessary for the determination of the different aquifers existing and for a good understanding of the different formations that constitute them.

The study area it is a collapse zone filled with deposits of Miocene(clay and marls with gypsum)and quaternary(heterogeneous alluvium in the form of terrace).These alluvial deposits,which are often very permeable and especially very thick,constitute an important water source fed by the infiltration of rainwater and by the lateral contributions of the Seybouse catchment.The infiltrations lead the surface water to the deeper levels which form the seat of a very important thermal reservoir in the study area and the neighboring areas.

On the basis of the description of the different geological units and the interpretation of the geoelectric and gravimetric sections collected in the various studies elaborated by(CRAAG 2004),we were able to identify and locate the following three aquifers:(i)the alluvial aquifer of Mio-plio-quaternary age；located at the center of the study area,(ii)the Neritic and Senonian limestone beds in the Heliopolis region,(iii)the Eocene limestone beds of Ras El Agba-Sellaoua Announa and the carbonate formations of the Ain Larbi-Bou Hachana region of Ypre′sian age,it is located to the southeast of the study area.

3 Materials and methods

3.1 Sampling and analytical methods

Thirteen geothermal water samples were gathered from the Guelma basin in January 2014 and 2016(Fig.1).All water samples were filtered through 0.45 μm membranes on site.Samples were stowed in 500 ml polyethylene bottles which had been rinsed with deionized water three times before sampling. At each sampling point, 3 bottles of water samples were collected, and reagent-grade HNO3was added to one of them to get the pH below 1 for cation analysis.All samples were taken from springs for analysis stored in an icebox at a temperature ＜4°C,after imparted to the laboratory and stored in the refrigerator(＜4°C)until analyzed (APHA 2005). Unstable hydrochemical parameters,including temperature(T),pH,and electrical conductivity(EC),were measured in situ using a HANNA Hi-9813-6 Multi-parameter. The analyzed samples for major and minor dissolved chemical elements such as calcium(Ca),magnesium(Mg),chloride(Cl),bicarbonate(HCO3),were determined by the titration method Table 1.Sulfate (SO4) was measured by a spectrophotometric method,sodium(Na)and potassium(K)were determined using flame photometer.Trace metals Zn,Fe,Cu,SiO2and Sr were determined by Graphite Furnace Atomic Absorption Spectrophotometer (Perkin-Elmer AAnalyst 700)using multi-element Perkin-Elmer standard solutions.The precision of the chemical analysis was confirmed by calculating ion-balance errors where the errors were generally around 10%.

Table 1 Descriptive statistics for the thermal water parameters

3.2 Statistical analysis

3.2.1 Cluster analysis

Hierarchical Cluster analysis(HCA)Q-mode is a method for assemblage individuals or objects into unknown groups.This method can be used to group the frequently collected water quality data,each group indicates the water of a particular quality.HCA is widely used by a number of studies to successfully classify water samples(Alther 1979；Ayadi et al.2018；Belkhiri et al.2010；Farnham et al.2000；Foued et al.2017；Khelif and Boudoukha 2018).Comparisons based on multi-parameters from different samples were made and the samples were grouped according to their‘similarity'to each other(known as Q-mode classifications).In this paper,Q-mode cluster analysis was used on the water chemistry data(10 hydrochemical variables measured:electrical conductivity(EC),pH,Ca,Mg,Na,K,Cl,SO4,HCO3,and NO3)to group the samples as water quality clusters(Grande et al.2013).In the present work,Q-mode HCA was used to categorize the samples into separate hydrochemical groups and Ward's linkage method(Ward 1963)was used in this analysis.A classification(dendrogram of Q-mode HCA)by means of Euclidean distance for resemblance measurement, together with Ward's method for linkage,produces the most characteristic groups where each member inside the group is more similar to its fellow members than to any member external the group(Gu¨ler et al.2002).

For statistical analysis,all the variables were log-transformed and more closely match to normally distributed data.Afterward,they were standardized to their standard scores(z-scores)as defined by(Gu¨ler et al.2002).

3.2.2 The Fuzzy c-means(FCM)clustering

Fuzzy c-means(FCM)is a clustering method which varies from normal clustering by calculating memberships per sample to each group.This clustering is a strong tool for unraveling datasets in similar groups(water facies),without any a priori knowledge.A great benefit of a statistical technique like FCM clustering is its possibility to separate a data set in same groups at the appropriate level(Frapporti et al.1993；Gu¨ler and Thyne 2004).The technique allows some vagueness in the description of the cluster model.Resemblance or similarity of a sample with a group is indicated by a membership(Mohammadrezapour et al.2018)between zero(completely different)and one(exactly the same).All fuzzy clustering algorithms trust on elements of the fuzzy-set theory(Zadeh 1965).In fuzzy clustering(also mentioned as soft clustering),data elements can fit into more than one cluster,and associated with each variable is a group of membership levels.These indicate the power of the association between that data element and a given group.Fuzzy clustering is a process of assigning these connection levels,and then using them to assign data elements to one or more groups；many of them are founded on the fuzzy c-means(FCM)clustering algorithm proposed by(Dunn 1973)and used by(Bezdek et al.1984).The FCM algorithm attempts to divider a finite combination of n elements X=｛x1，...，xn｝into a collection of c fuzzy clusters with according to some particular criterion.Given a limited set of variables,the algorithm returns a list of c cluster centers C=｛c1，...，cc｝and a separate matrix W=wi，j∈[0，1 ]，i=1，...，n； j=1，...，c where each element tells the degree to which element xibelongs to the cluster cj. The FCM was achieved by minimizing objective function:

3.2.3 Principal component analysis/factor analysis

PCA/FA are usually applied to determine the relations existing between measured properties that were originally considered to be independent sources of (groundwater hydrochemical data)to summary the most important factors and to decrease the data with minimal loss of information,and play the important role of confirming temporal and spatial variations caused by natural and anthropogenic issues (Mustapha and Aris 2012； Schaefer and Einax 2010).In addition,they have been efficiently and extensively applied to the assessment of surface water quality(Belkhiri et al.2010).Furthermore,numerous multivariate statistical analyses such as principal component analysis(PCA),are applied for explanation of reaction process and complex water quality data in the study area.

In this work,PCA/FA are applied to hydrochemical data from the Guelma geothermal field to extract the principal factors corresponding to the different sources of variation in the data and to identifying the spatial interrelationships within a set of variables in the study area(Awaleh et al.2015；Hamad et al.2018).PCs with eigenvalue ＞1 are only taken into consideration(Kaiser 1960).A Varimax rotation was executed on this PC to help the interpretation of factors based on hydrochemical or anthropogenic processes controlling groundwater quality. Hydrochemical statistical analysis of all samples was performed with Xlstat software(2016)and R studio software v1.0.143(2017).

4 Results and discussion

4.1 Water facies

The hydrochemical water type identification is a very useful tool in the assessment of water chemistry,hydrochemical spatial and evolution distribution of water composition. For Guelma thermal water samples, Piper diagram(Piper 1944)and Cl—SO4—HCO3ternary diagram(Fig.2),illustrates that the thermal springs of the region of Guelma are characterized by a predominance of sulfate compared to the carbonate,cations are mainly represented by calcium,the main groundwater types are SO4—Ca and HCO3—Ca,which may be explained by the leaching of Triassic formations in study area.To identify possible groups of waters based on physicochemical compositions,ion species Ca2+,Mg2+,Na+,K+,Cl-,SO4,HCO3,Fe,Cu,Zn,SiO2and Sr were considered for application of a clustering Q-mode(HCA)and the fuzzy c-means(FCM),in which resemblance relationships among water samples were examined. The HCA process was performed by Ward's linkage method with the Euclidean distance as a degree of similarity of samples.Two methods show the same results and the data are divided into three clusters(Fig.3)(Table 2).

Group 1 is formed by nine spring(Geyser,H.Chefa,H.Ouled Ali 1,H.Ouled Ali 2,H.Ouled Ali 3,H.Belhachani,H.Guerfa,H.Benadji and H.Chellal).The order of abundance of the major ions is Ca ＞Na ＞Mg ＞K and SO4＞HCO3＞Cl,and the hydrochemical type is characterized by Ca-SO4facies (Fig.3). This water type characterizes the north,south,and west of the Guelma.This group is characterized by a high salinity(1090 ＜EC ＜1960 μS/cm, mean=1482,22 μS/cm),however；calcium(min=1563 mg/L,max=368,74 mg/L,and mean=232.01 mg/L)and sulfate(min=365 mg/L,max=800 mg/L,and mean=494.22 mg/L)are also exceeded.All thermal waters exceeded the required calcium(75 mg/L)(WHO 2011)and bicarbonates(120 mg/l)for drinking water standards.

The second group type of thermal water,has low salinity(mean of EC=545 μs/cm) and abundance in orders Ca ＞Na ＞Mg ＞K and HCO3＞SO4＞Cl.dissolution of carbonate releases Ca and HCO3when waters enriched in CO2after being in contact with the atmosphere(Foued et al.2017).The dominance of Ca and HCO3,are related to the geology of the reservoir(limestone neritic facies).This cluster situated in the south of the Guelma hydrothermal field and represented by H.Assassla and H.Romia.

Fig.2 Piper and Cl—SO4—HCO3 ternary diagram

Fig.3 Cluster dondrogram

Group 3 consists of H.N'bails and H.El Mina are mainly localized in the eastern part of the study area that is mostly covered by evaporite diapir. This group is dominated by sulfate(319.5 ＜SO4＜337.25 mg/L),with a clear dominance of calcium,sulfates,bicarbonate,and chloride.The concentration of Ca and HCO3oscillate between 304.61 and 404,81 and from 384.3 to 402.6 mg/L respectively. Chloride values fluctuate from 319.5 to 337.25 mg/L.

The main thermal waters from Guelma exceeded the desirable limit of Ca and HCO3,at the same time,twelve well exceeds the standards required for consumption for sulfate and chloride.Water type is strongly influenced by the geology of the Guelma(water—rock interaction and leaching the Triassic formations).

Table 2 Fuzzy membership matrix

To differentiate the main hydrochemical facies and understand the different processes affecting the groundwater,we using the Piper diagram(Piper 1944)(Fig.2).The anionic diagram is dominated by the presence of SO4-Cl,while the area of Na dominance is situated in the cationic triangle.Therefore,most water samples can be classified into the SO4—Ca type(saline water),suggest that the geothermal reservoir has been influenced by evaporation and cation exchange,as well as leaching and dissolution of evaporite formations characterized the pull-apart basin of Guelma.

4.2 Hydrogeochemical process and origin of mineralization

4.2.1 Ionic relations and sources of major components

Hydrochemistry is more difficult to interpret in sedimentary environments such as the Tellian domains of the study area,as the mineralogical and chemical properties of the reservoirs are very heterogeneous.The heat flow,the tectonic regime and the hydrogeological conditions(the depth of water infiltration and the residence time in the reservoir)are specific to each zone.But we will try to interpret the essential factors that control the chemical evolution of the thermal waters in the Guelma basin.

To define the characteristics of thermal waters in study area we use the following ratios:Na/Ca,Na/Mg,Na/K,Ca/Mg,Cl/SO4,(Na+K)/Cl,(Cl+SO4)/HCO3,Cl/HCO3and (Na+K)/(Ca+Mg) (Rimi et al. 1998) and the Cation Exchange Value (CEV=[Cl-(Na+K))/Cl])(Brown 1998).All values of CEV and ratios are recapitulated in(Table 3).

The CEV values for seawater ranged from+1.2 to+1.3 (Custadio 1983). However, low continental salt water gives values close to zero(negative or positive).The CEV values ranging from-1.53 to 0.8 were positive for 61.53%and negative for 38.47%.

The CEV positive values indicate a greater exchange of Na cation in the water and the Ca cation in the rock and to a minor extent for the Mg cation,which is taken more strongly than Ca cation(Hem 1985).Ca and Mg cations are the main source of hardness in water,this phenomenon present in 61.53%of springs.When CEV close to zero(case of Geyser),that means the exchange between Na and Ca is weak.Negative values of CEV shows the thermal waters tended to fix the sodium,potassium and in the same time the solubility of calcium and magnesium；it is a case of 38.47%of the samples(H.Ouled Ali 1,2,3,H.Assassla and H.Romia).

The Ca/Mg ratios show the dissolution of Gypsum increases calcium concentration and consequently the ratio of Ca/Mg.When this ratio is higher than 0.5,it induces dedolomitization.As shown by Table 3,Dedolomitization also appears to characterize the hydrothermal chemistry of the Guelma area.This process would be produced by irreversible sulfate dissolution accompanied by dolomite dissolution and the precipitation of calcite,according to the next reaction:

Na/Mg,Na/Ca and(Na+K)/(Ca+Mg)ratios for all thermal water samples are high；this reveals an important process of ion exchange. The (Cl+SO4)/HCO3ratio values were high,except for Assasla and Romia spring,with about 1.0 and 1.10 respectively, that means that thermal waters of the Guelma arise from a mixture of shallow and the warm waters resulting from interaction with reservoir rocks.

The(Cl+SO4)/HCO3were high values that mean the majority of thermal waters of the Guelma geothermalrequire a long time at the depth to react with the reservoir rocks.

The Ca—SO4scatter plot is displayed in Fig.4,a typical SO4-type composition can be identified for the geothermal waters of the geothermal system of Guelma(Belhai et al.2016；Dib 1985,2008；Foued et al.2017).Therefore,all hydrothermal waters from the Guelma are plotted along the gypsum dissolution line. The correlation coefficient is R2=0.813,on the other hand in order to identify the origin of sulfate in groundwater,special attention was a focus to strontium(Sr),which is a constitutive element of some minerals combined with evaporites.It is found associated with gypsum in the form of Celestite(SrSO4).

The molar ratio(Sr2+/Ca2+)provides information on the origin of salinity and supports this hypothesis.According to(Meybek 1984),it is greater than 5%in the evaporites of the Alpine Triassic,whereas,in Morocco,it is greater to 7%in the waters of the Turonian aquifer of the Tadla basin(Hsissou et al.1996).In Algeria,it is superior to 3%in the evaporites of Djurdjura(Abdesselam et al.2000)and it is greater than 5%in the groundwater of the superficial aquifer of Loutaya(Brinis et al.2009),and between 19%and 63%in the thermal waters of the southern Se′tifien(Boudoukha and Athamena 2012),Sr/Ca ratios for thermal waters of Guelma oscillates between 9.14%and 52.52%which is characteristic of water that has circulated in reservoir rocks that is in contact with evaporites formations(Brady and Walther 1992)have shown that increasing the temperature of water accelerates chemical reactions,which could explain the high values of the Sr2+/Ca2+ratios.Hsissou et al.(1996)and Gouaidia et al.(2012)also showed that the salinity of water is controlled by chlorides,sodium,and sulfates.

Fig.4 Ca versus SO4 diagram

According to the correlation matrix results(Table 4),the R-value indicates that EC is strongly correlated with Ca(R ＞0.7),HCO3,Cl and Sr,and moderately correlated with K (0.5 ＜R＜0.7)； which indicate that these ions control largely the mineralization of springs water. A similar source of Cl,Na,and K is designated by the strong and positive correlation between them.Ca,Mg,HCO3,and SO4are strongly correlated,whereas Fe,Zn,Cu and SiO2are moderately correlated.Sr has a robust positive correlation with(Ca,K)and medium positive correlation with(HCO3,Cl).

The diagonalization of the correlation matrix provided a new data set independent of each other and classed in decreasing order of significance called principal components(PCs)(Helena 2000).PCs were then rotated using varimax rotation to find a new set of factors with a max of variance to facilitate their interpretation in terms of hydrogeochemical process.Results show four Varifactors(VFs) (Table 5) with eigenvalues ＞1 (Kaiser 1960)explaining 87.82%of the total variance in the water quality data set.To interpret factor loading,we took on consideration the classification defined before by(Liu et al.2003).

Among fours VFs,VF1 explaining 27.27%of the total variance,had strong positive loadings(＞0.75)on EC,K,Cl and Sr,which confirms the influence of evaporite mineral associated to the Triassic formation.

VF2 representing 26.69%of the variance,is dominated by a strong positive loading on temperature,Fe,Cu,Zn and SiO2.This heavy metal factor shows that the dissolution of the minerals associated with these metals is very associated with the hydrothermal activity in the Guelma geothermal field.VF3 accounted for 24.04%of the total variance and had robust positive loading on Ca,Mg,SO4and HCO3,which indicate the signs of water—rock interaction such as carbonates and gypsum rocks.VF4(9.4%of the total variance)had a moderately positive loading on Na and pH.VF4 confirms the excess in Na of water samples which can probably be related to the dissolving of the Triassic formations but also to the phenomena of ions exchange.

4.2.2 Relationship mineralization-hydrothermal activity in Guelma area

The polymetallic mineralization(Sb,Pb,Zn)appear to be very comparable to the type of mineral deposits linked to hot springs(Hot Springs deposits),this mineralization in the geothermal system of Guelma is considered to be the youngest in northeastern Algeria,it is of age Pliocene to Plio-quaternary.Several epithermal deposits of antimony throughout the world are auriferous,but the problem that has always arisen is to locate this gold which is in most cases ultramicroscopic.The discovery of gold at Hammam N'Bails opens up new prospects for the search for precious metals in regions where epithermal deposits are located at(Sb,As)(Merdas 2006).In the Pliocene basin of Hammam N'bails,near the thermal springs,a very particular type of mineralization of antimony,zinc,and lead was deposited.A certain number of mineral deposits(Alunite-Kaolinite,Scorodite,Dussertite,Nadorite,Stibine,antimony oxides and possibly gold and native sulfur)is linked to the activity of thermal springs distributed according to deep accidents of large extensions(about one hundred of Km)of EW direction.

Table 5 Varimax rotation PCA loading matrix

Along these areas of weakness and near these deposits,there are thermal springs.The typical Hammam N'bails mineral concentrations,the Miocene post-nappe Beroughia mineralization and the mercuriferous of Azzaba region(north of study area)are good examples of mineralization related to the deep fractures still active today and which are characterized by intense hydrothermal and remobilization phenomena. The circulation of the mineralizing fluids would have borrowed the deep fractures(Boutaleb et al.2000).An examination of the distribution map of the Sb and As deposits in northeastern Algeria shows evidence of the association of these deposits with thermal springs.The principal deposits and indices of antimony are divided according to two privileged directions E—W and NE—SW Fig.5.

For some authors(Verdeil 1982),if the hydrothermal springs and mineral deposits are on the same accidents,this does not explain a genetic link between them.Although,the results of analyses on the thermal waters of Hammam N'bails spring and the associated travertines,show abnormal values in(Pb,Sb,Zn,As)and many different metals,which explain the genetic connection between these waters and the mineralization.

4.2.3 Prediction of the scaling

Hydrothermal solutions contain an extensive variety and a high concentration of dissolved elements.Important problems in hydrothermal systems are scaling and corrosion,where chemical elements dissolved in hydrothermal waters may have a trend to form deposits(such as aragonite,calcite,hematite,etc.)(Figure 6),or to cause corrosion of the operating equipment of the spas.

The prediction of scaling trends of geothermal waters is important in assessing the production characteristics of a geothermal reservoir and for taking essential protections to stop or control scale formation(Tarcan 2005).Some carbonate,silica and sulfate minerals saturation indices(SI)can support one to an evaluation which ones of these minerals can precipitate during the utilization of the thermal waters.

The Langelier saturation index(LSI)is used to calculate the stability prediction of calcium carbonate in water.

where the pH is the pH value measured；pHs is the saturation pH.

Fig.6 DRX for scaling of Hammam Guerfa

The LSI values obtained are between 0.181 and 1.133,which means that most springs have a tendency to scale.In the geothermal system of Guelma,the pH,the temperature,the pressure of carbon dioxide CO2are the main factors causing the precipitation of calcite.The appearance of moderate to high-pressure carbon dioxide in a geothermal reservoir increases its solubility.CO2gas is the most common in Guelma hot springs H.Chellal,Benadji and the Geyser spring,it is maintained supersaturation under the effect of the pressure at depth,especially in waters rich in bicarbonates.Because of this pressure,the CO2is dissolved,sometimes in large quantities in the water of the reservoir.The resulting pH is low,and the solution can dissolve large amounts of calcite.The total dissolved CO2is proportional to the partial pressure of this gas in the gaseous phase in contact with water (Haouchine-Bouchareb 2012).

During the ascending thermal waters rich with CO2,significant physicochemical changes occur.However,as soon as the total pressure becomes less than the partial pressure of CO2,it begins to change phase and gas bubbles appear(Fig.7a,b).

Carbonate minerals saturation increases when we have a loss of dissolved CO2in the thermal water.As a result,these minerals precipitate around the hot springs and form the many deposits of travertines observed(their thickness reaches about 15 m in H.Chellal)(Fig.7c,f).The PCO2values estimated are significantly greater than 1—2 orders of magnitude at atmospheric pressure 10-3.6atm (Dever 1985)highlighting the underground nature of the flow of these waters, according to the calculations made with PHREEQC from the pH and alkalinity values of thermal waters studied at discharge temperature.Calcite tends todeposit at the surface from a reservoir water discharge with high partial pressure(Fig.7a,b).

Fig.7 a El Chellal spring,b silica scaling,c El Chellal cascade,d thermal water basin,e natural drains of thermal waters and(f)

The thermal springs studied all contain a notable amount of silica whose maximum value is 72.8 mg/L for Hammam Debagh(Fig.7a,b).These waters that come out boiling are charged with silica dissolved at high temperature.While cooling,the silica becomes less soluble and is deposited forming concretions,low walls and basins(Fig.7d),which are not without reminding the carbonated concretions,but which are here siliceous concretions. They also form imposing cascades like those of Hammam Debagh(Fig.7c)where they are active(circulations of water could be observed in these travertines(Fig.7e).

4.3 Chemical geothermometers

Geothermometry is a tool for assessing the temperature and the depth of geothermal reservoirs of the latter chemical or isotopic equilibrium before rising.However,during its ascent,the hot water may be mixed with the superficial cold waters that may complicate the use of chemical geothermometers and conduct to overestimation or underestimation of temperatures.Note that the rise of thermal waters since their original reservoir usually comes with a decrease in temperature with a change of initial mineralization. According to (Bouri et al. 2007) the use of geothermometers assumes that there is no important chemical change in the water during its residence time and rising to the surface,despite the different possible cooling and often remarkable.

To estimate the temperature of the last thermodynamic equilibrium,several geothermometers have been applied,Table 6 recapitulates the temperatures of the Guelma thermal springs which were calculated using some geothermometers such as:silica(Fournier 1977；Fournier 1992；Giggenbach and Soto 1992；Verma 2000a,b),and Na—K(Arno′rsson et al.1983；Can 2002；Fournier 1979；Giggenbach 1988；Nieva and Nieva 1987；Tonani 1980；Truesdell 1975).

The highest temperatures are those indicated by the Na—K geothermometer of Giggenbach (1988) of about 135—224°C,and CCG geothermometer(Table 6),at the same time the lowest are given by the silica geothermometers.

The Na—K geothermometer(Giggenbach 1988)is correlated to the variation of sodium and potassium in thermal waters due to leaching of the Triassic formation.Moreover,the Na—K geothermometers of(Nieva and Nieva 1987),Fournier 1979,Truesdell 1975,Tonani 1980,Arno′rsson et al.1983 and Can 2002,seem too high in some springs from Guelma geothermal reservoir,due to the leaching of the evaporitic formations characterizing the study area.An allowable average of the thermal waters given by the Na—K geothermometers is at nearly 140°C,but it seems that of silica geothermometer is best appropriate to the thermal waters of Guelma geothermal system.Those suggested by(Fournier 1977)and(Fournier and Potter 1982)for quartz and(Verma 2000a)for chalcedony were efficaciously used with samples from the study area,and much less for the Arno′rsson et al.(1983)chalcedony geothermometer.In the present work,the quartz geothermometers of Fournier and Potter (1982) estimate the temperature of the Guelma geothermal reservoir in the range of 62—122°C,about 11°C higher than Verma's mean temperature which ranged between 69 and 110°C(Fig.8).

Fig.8 Estimated reservoir temperatures in °C of the Guelma area based on different geothermometers

The water temperatures,estimated at the origin,oscillate between 84°C in the Ouest and 122°C in the East part of Guelma geothermal field.This last value,much higher at measured temperatures,thus shows a dissipation average of 28°C during the rising from the reservoir to the surface.This dissipation could be due either to a mixture with surface water and/or to diffusion thermal related to the long pathway(between 5 and 7 km)(Maouche et al.2013)by these waters to arrive surface.Placed in the triangular diagram K—Na—Mg1/2(Giggenbach 1988) (Fig.9), the thermal waters from Guelma area are influenced in part by a dilution infiltrated surface water,probably in the carbonate formations karstified and sandstone recovered by Mio-plio-quaternary formations.

This geothermal tool also made it possible to estimate the depth of the different reservoirs. According to(Haouchine-Bouchareb2012)the Guelma belongs to an area where the geothermal gradient is 40°C/km.This makes it possible to estimate the depth of the geothermal reservoir between 2100 m in the eastern of Guelma and 3000 m in the western part.

4.4 Mixing evaluation

Mixing evaluation in the Guelma geothermal system,it is important to calculate mixing ratios.This can be reached using hydrochemical components.When waters attainment the surface are mixed waters,acknowledgment of the different components(final components)can be difficult.This is particularly true if we have a re-equilibration of water—rock that happens after mixing(Han et al.2010).To simplify calculation of mixing quantities,it is expected that the hydrothermal water is a mixture of two end-members,one thermal and one non-thermal water.In this paper,chloride was used to calculate mixing ratios because at high temperatures it typically does not participate in chemical reactions.To estimate the mixing rates of thermal waters in the Guelma region using the following formula(Han et al.2010):

Fig.9 K—Na—Mg1/2 diagram

where M is the percentage of the mixing,Cltis the chloride content in the thermal water,Clmis the chloride content in the spring which has a probable water mixture,Clfis the chloride content in the cold water.The examination of some probable areas by the mixture between thermal waters and cold waters have values between 19%and 38.5%, Table 7 presents the mixing results for some springs in the Guelma geothermal field.

The mixing proportion depends on the distance between the center of hydrothermal spring and the location near the fault zone.In the Guelma geothermal system,samples are located along deep-seated faults,which can reason of a higher mixing ratio.

A simplified graphical method has proposed by(Truesdell and Fournier 1977)to estimate the temperature of the deep thermal waters,which is founded on dissolved silica concentration versus enthalpy of the spring water.This model depends on the supposition that no conductive cooling has happened after mixing.Figure 10a illustrates the Silica-enthalpy mixing diagram for the Guelma thermal waters,where all samples plotted are very close and above the chalcedony solubility line.If we accept that there is no loss of heat after mixing,the enthalpy assessed for the thermal waters is about 500 kj/kg which resemble the temperature of 120°C,which is considered identical with those evaluated by quartz and Na—K geothermometers.

The enthalpy-chloride mixing model is particularly useful to understand the hydrogeology of a geothermal system.It is widely used to deduce information on subsurface processes such as boiling,mixing and/or cooling by conductivity or dilution.This model proposed by Fournier(1979),who assumed that most chloride in warm water is released by water—rock contact at high temperatures in the geothermal reservoirs. Its application fundamentally includes relating analyzed chloride levels to water enthalpy,which can be resulting from measured dischargetemperature.Major results can be concluded from Fig.10b exhibits the enthalpy-chloride mixing model for the Guelma thermal waters tends to mix with the cold waters which are probably mixed with cooler groundwater from the recharge zone area(i.e.as represented by Ain Al Arbi and Ain Sebaa Ayoune which related with H.Assassla and H.Romia).

Table 7 Estimation of chlorine mixture in thermal waters.Based on Cl contents in thermal and cold waters

Fig.10 Silica and chloride mixing model

4.5 Thermal Water quality assessment

4.5.1 Potential environmental impact of hot spring wastes

According to the WHO(2011)guidelines,the thermal spring waters are not suitable to drink in terms of the physicochemical quality；because the magnitude of all minors and major ions was upper than the standard limits and concentrations(Cu,As,Pb,Ba,and F are unacceptable in all springs).

Thermal waters tend to have high values of total dissolved solids(TDS)and may hold poisonous components such as arsenic,lead and other trace elements especially Barium(Belhamra and Hani 2016；Boudoukha et al.2012；Smedley and Kinniburgh 2002).The discharge of these waters into the nearby environment may not only increase anxiety about the aquatic ecosystem(Cai et al.2019；Long et al.2018；Mroczek 2005)but might also pose a health hazard to resident peoples(Long et al.2017a,b；Robinson et al.2003；Webster 1999；Yazdi et al.2015).However,the presence of poisonous heavy metal has been significant in these waters from the thermal wastes to the surface waters.The levels of As,Pb and others found in these surface waters are very high and exceed the admissible standards,thus testifying to water contamination(Belhamra and Hani 2016；Boudoukha et al.2012).

Hot springs water from the Guelma geothermal system flows into the Bouhamdane,Cheniour,Charef,Boumia and El Mellah wadi(upstream springs sampling sites)(Fig.11).The consequence of the thermal springs on the chemistry of the surface water is clear.Downstream from the point where the thermal spring water flows into a small river,the water is used to irrigate harvests.The potential health hazard posed by the warm spring water would be linked to(i)field irrigation with Pb and As-contaminated water,driving to an accumulation of Pb and As in the harvests and/or(ii)the drinking of surface and ground waters that may have mixed with the thermal spring release.Consuming of the harvest by humans,poultry and cattle could be driving to critical health problems that merit additional investigation.Toxicity of heavy metals which is often the consequence of long-time,low-level exposition to contaminants has frequently been investigated in water,air,food and many consumer products(Falc et al.2006).

The attendance of lead and arsenic in groundwater in concentrations sufficient to disturb human health establishes a universal high-priority groundwater quality problem(Duker et al.2005a,b；Webster 1999).

Arsenic it is a known human carcinogen(Chen et al.1992),other health effects include a sore throat,irritated lungs(Smith et al.1996),nausea,vomiting,decreased production of red and white blood cells,abnormal heart rhythm,damage to blood vessels(Duker et al.2005a,b；MILTON et al.2001),and skin pigmentation abnormalities(Haut 2017；Tondel et al.1999).

Fig.11 Wadi map with Zit Anba and Koudiat Haricha dam

5 Conclusion

Hot springs from Guelma geothermal system were found to be aligned along the main faults(N—E and NE—EW),which facilitate the communication between different reservoirs and transport of deep thermal waters to the surface.This geothermal reservoir is fed by meteoric water,mainly from the regional aquifers. Rainfall in the area penetrates through fractured systems in the Jurassic and Cretaceous carbonate rocks between 2100 m in the east and 3000 m in the south and west,with deep circulating regional water and heat absorbed from the nearby rocks.Thermal waters obtain their original mineralization(carbonate)and mineralize more in chloride,sodium and sulfate in contact with the salt of Triassic formations and by ionic exchange with clays.Three groups of waters were defined using HCA and FCM.An excellent correlationship was found with their geological and geographical location.During the ascent,the thermal water gives in the North and West,the thermal zones of Ouled Ali and Debagh,in the East,those of H.N'bails and El Mina and in the south part H.Guerfa,H.Belhachani,H.Assassla and H.El Romia.

These thermal waters vary in facies,with Ca-SO4facies in the north,west and south parts,an HCO3—Ca facies situated in the south of the Guelma hydrothermal field and represented by H.Assassla and H.Romia.These waters also show significant levels of trace elements and metalliferous witnesses of a deep circulation.

The reservoir temperature in Guelma geothermal system was estimated by means of different geothermometric methods.Various chemical geothermometers(Na/K and SiO2)estimate temperature for the deep geothermal reservoir of 84—122°C.Hydrogeochemical studies and field observations propose that the most possible scales to be precipitated through the extraction of hot waters in the Guelma basin are carbonate minerals.

Finally,in the Guelma geothermal system,the diffusion of thermal waters into shallow aquifers could pollute the groundwater used for drinking purposes.Furthermore,the thermal spring effluents into the rivers could influence irrigated harvests in downstream fields.In both situations,the health of local inhabitants could be at risk.For future research work,gas geothermometry techniques,as well as several gas ratios,will be used to estimate the geothermal reservoir temperature and to examine the eventual magmatic intrusion at depth,which makes the first step towards developing a geothermal conceptual model for all northeastern Algeria.However,the limitations of this study are that it does not apply isotopic techniques to the geothermal discharges,which necessitates further research.