Hydrochemistry of Waters of Volcanic Rocks: the Case of the Volcanosedimentary Rocks of Thrace, Greece

HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS: THE CASE OF THE VOLCANOSEDIMENTARY ROCKS OF THRACE, GREECE

C. PETALAS1,∗,N.LAMBRAKIS2 and E. ZAGGANA3 1Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, Democritus University of Thrace, Vas. Soﬁas 12, 67100 Xanthi, Greece; 2University of Patras, Department of Geology, Laboratory of Hydrogeology, 26500 Patras, e-mail: [email protected]; 3Hellenic Centre for Marine Research (HCMR), Institute of Oceanography, Athens-Sounio Avenue, 19 013 Anavyssos, Greece, e-mail address: [email protected] (∗author for correspondence, e-mail: [email protected], Tel: +30-541-0-79385, Fax: +30-541-0-79385)

(Received 21 February 2005; accepted 2 September 2005)

Abstract. This work is referred to the characterization of the environmental hydrochemistry in the broader Sapes area Ð Thrace region, on the basis of physico-chemical properties of surface and groundwaters occurring in the volcanosedimentary formations of this area, where gold mining activities are planned to operate. Volcanic rocks are considerably altered where they are in contact with hydrothermal solutions. Aquifers are formed within these formations. Surface and ground waters are strongly metalliferous and their hydrochemical facies present similar but complex water types. Cer- tain characteristic chemical types are the following: Ca-Mg-HCO3-SO4, Ca-Mg-SO4-HCO3. Ca-SO4, Ca-Mg-SO4. Ca-Na-Cl-HCO3, Na-Cl. A small majority of the water samples present the following − 2− − order of anion dominance HCO3 > SO4 > Cl . Calcium is the dominant cation. Bicarbonates and sulfate ions are the dominant anions. The order of dominance for the heavy metals in surface and ground waters is as follows: Fe > Mn > Zn > Ni > Cu. The saturation index of waters regard- ing minerals is low. Computer simulation indicates that calcite and dolomite are common minerals in all water samples which are saturated in respect to quartz and argillaceous-siliceous minerals. The most pronounced property of waters is their acidic character. The high metal concentrations are related to water with low pH. Sulﬁde minerals control the low pH values of waters which is an important control factor for the evolution of the water chemical composition. The abundance of sulfates is attributed to the dissolution of the minerals pyrite (FeS2) and alunite (KAl3(SO4)2(OH)6. The waterÐmineral interactions are responsible for the chemical composition of waters. Water quality problems can be successfully handled by the use factor analysis. 17 chemical parameters can be substituted by ﬁve factors which successfully represent the hydrochemical processes as well as their geographic distribution. Volcanic rocks in the study area have the potential to produce acid drainage.

Keywords: Acid water, groundwater, hydrochemistry, saline water, volcanic rocks

1. Introduction

Recently, environmental problems caused by acid drainage, are evident in many parts of the world. In the last few years, these problems have been intensely studied

Water, Air, and Soil Pollution (2006) 169: 375Ð394 C Springer 2006 376 C. PETALAS ET AL. in order to minimize their adverse effect on the environment. The pollution of surface and ground waters by acid mine drainage cause serious damage to ecosys- tems. The occurrence of acid waters is related to the solution of sulfide compounds, such as sulfides, in water. Such compounds are very common in volcanic rocks and are originated as metallic deposits precipitated from hydrothermal fluids. Gold is usually found along with these minerals, in exploitable amounts, in relation to advanced argilic alteration in the andesitic rocks with silica and chalcedony bod- ies. Fluid inclusion data suggests an epithermal system for the ore forming-fluid (Lattanzi et al., 1991; Ruggieri, 1993). Usually, the utilization of the aforementioned compounds creates metal-reach metallurgical wastes, which causes serious environmental problems when they are leached by rain or surface waters. One of the most characteristic examples is the case of Sardinia, where after some decades of production of Pb, Zn, Ag, Cu, Sb, and Ba minerals, enormous volumes of metallurgical wastes caused serious problems of quality degradation on surface and ground waters (Cidu et al., 1997). Also, in Tuscany, the metallurgical wastes originated from the utilization of metal-reach sulfide compounds of Hg, Sb and Fe, created acid mine drainage that has caused serious environmental problems (Ben- venuti et al., 1997). Similar problems occur in South Korea, where the wastes of poly-metallic mines produce W, Mo, Fe, Sn, Cu, Mb, Zn and Au, Ag which are considered responsible for introducing heavy metals to the environment (Lee et al., 2001; Lee, 2003). In Greece, the intense utilization of silver-bearing galena during the period 3000 B.C.to1989 in the historical mining area of Lavrion, Attica, caused the accumulation of vast volumes of metallurgical wastes wich are considered to be responsible for the degradation of the environment (Marinos and Petrascheck, 1956; Konofagos, 1980; Kontopoulos et al., 1995). Stavrakis et al., (1994) and Kontopou- los et al., 1995 mention that the soils of the pit-area, but also in the surrounding area show high concentrations of Pb, Zn, Sb, Cu, Hg, Cd, As, Fe and Mn. Chronopoulos and Chronopoulou-Sereli, (1986) measured high concentrations of Pb, Zn, Cu, Fe and Mn in vegetable fibres. Alexakis and Kelepertsis (1998), and Stamatis et al. (2001) attribute the high metal concentrations in ground water to the geological environment. The presence of sulfide minerals has been postulated in the broader Sapes area (Thrace) Ð northeastern Greece. These minerals occur in volcanic rocks and hydrothermal fluids. This study was set up to characterize the environmental hydrochemistry and heavy metal contamination in the broader Sapes area, on the basis of physico- chemical properties of surface and groundwaters occurring in volcanosedimentary formations of this area. The results of the study will help identify rocks which have the potential to produce acid drainage. This work aims also to pre- vent the groundwaters and streams from further pollution in the future, taking into account that intense gold mining activities are planned to operate in the study area in the near future. Figure 1 shows the location map of the study area. HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 377

Figure 1. Principal geologic units and structural features in the study area. Water sampling points and stream systems are also shown. (Modiﬁed from the geological map 1:200000 of the Institute of Geological and Mineral Exploration, Greece). 378 C. PETALAS ET AL.

2. Geology of the Study Area

The study area belongs geotectonically to the Rhodope massif located at the southern boundary of Xanthi Ð Komotini basin which was formed during the tertiary upon the metamorphic rocks of Rhodope massif and the Perirhodopic unit. According to Jacobshagen (1978) the formation of the tertiary basins in the broader area was a result of the orogenetic phase of Eocene time. The broader Komotini-area, east of Komotini city, is made up by the following geotectonic units: a) The pre-alpine bedrock of the Rhodope massif, b) The perirhodopic belt (Makri and Drimou-Melia unit of Mesozoic era) and c) the tertiary basins related to taphrogenic tectonism of North Aegean (Middle to Upper Eocene Ð Pliocene), (Mposkos et al., 1988). Prin- cipal geologic units, a geologic cross section, structural features as well as water sampling points and streamline network in the study area are shown in Figure 1. Rocks of Tertiary age present in the study area are intensely altered and un- conformably overly the metamorphic bedrock (Makri unit). According to Mposkos et al. (1988) Makri unit is part or tectonic remnant of the Peri-Rhodopic belt which is geotectonically related to the Mesozoic geosyncline of Tethys. Makri unit is made up by the overlying metavolcanosedimentary series or greenschist series and the underlying metasedimentary series, which mainly is made up by marbles and calcitic phyllites. The geologic structure of the broader area is characterized by the presence of synclinal and antisynclinal structures of many kilometers long. The direction of their axes ranges from 40 to 50◦NE. Generally, folds are vertical-symmetrical without inclination of the axial surface (Mposkos et al., 1988). Subvolcanic and intrusive rocks display linear development following tertiary fault zones. Clastic sediments and volcanic rocks were deposited within the study area during a period characterized by intensive synsedimentary Ð intrusive activity. Pyroxene andesites and amphibole Ð biotite andesites, subvolcanic rocks (of rhyo-dakeitic composition) and plutonic rocks are dominant in the study area. According to Michael et al., (1989a, 1989b), Arikas (1981), Arikas et al. (1993) and Michael et al., (1995), pyroclastics and lavas in the Rhodope massif locally display paragenesis originated from hydrothermal alteration, usually related genetically to basic metal deposits. Neogene formations are mainly made up by argillaceous-siliceous minerals. In Thrace, lavas, pyroclastics and igneous intrusions are sufﬁciently altered by hydrothermal ﬂuids related to sulfer metalliferous deposits (Sideris, 1975; Arikas, 1981; Katirtzoglou, 1986). The petrochemical analysis of volcanic rocks indicates that they are mineralogically characterized as calc-alkali rocks, namely as basaltic andesites, andesites and dacites (Sideris et al., 1991). According to the same authors, the average chemical composition of these volcanics is characterized by SiO2 dominance. Basaltic andesites, andesites and dacites contain 55.17%, 60.14% and 65.29% of SiO2 respectively. These rocks contain also Al2O3 in a percentage of 18.28%, 17.91% and 16.24% respectively, minor amounts of Fe2O3 in a percentage HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 379 of 1.07%, 0.77%, and 0.55% respectively, FeO in a percentage of 7.13%, 5.16% and 3.71% respectively and smaller amounts of Mn, Mg, Ca, Na, K, Ti, and Cr oxides. The following distinct alteration zones are observed in Sapes area (Marantos et al., 1993): a) The siliceous zone which is composed primarily of quartz (95%) and minor amounts of oxides, e.g. iron oxides (1%) b) The zone of alunite having as main constituent alunite and minor constituents iron and manganese oxides, c) the sericitic Ð argillitic zone dominated argillaceous Ð siliceous minerals and pyrite. This zone contains trace amounts of iron oxides and hydroxides. d) The kaolinite zone which is mainly composed of kaolinite, coexisting with quartz, iron oxides and hydroxides constitute the minor minerals of this zone.

3. Hydrogeologic Environment

According to the climatic classification of K¬oppen the climate symbol of the study ◦ ◦ area is “Csa”, (2 < Tc < 18 C and Tw > 22 C), i.e “Mediterranean or Mesother- mal climate type with dry and warm summers and mild winters” (Lambrakis et al., 1999; Petalas et al., 1999), where Tc and Tw is the minimum and maximum annual temperature accordingly. According to the same authors the average annual precipitation of the study area is 700 mm and the real annual average evapotranspiration Etr is 300 mm. The runoff is restricted in the period DecemberÐMarch, while during the rest of the year it is very low to almost null. Floods are rarely produced by intense rainfall (when daily rainfall exceeds 50 mm). The average annual surplus of rain-water, expressing the difference between the relevant values of precipitation (P) and real evapotranspiration (Etr) is 390 mm (Lambrakis et al., 1999; Petalas et al., 1999). Aquifers are mainly found in the metamorphic formations (especially calc- schists) of Makri unit, in marbles and in volcanosedimentary formations in Maronia basin which are made up by pyroclastics, sediments, andesites and limestones. Phreatic or semiconfined aquifers are formed in different depths and extent. The hydrogeological behavior of several geological formations of the study area is as follows: The phyllitic system: Shallow aquifers of very low potential are formed within the metamorphic rocks phyllites and calc-schists of Makri unit. Additionally, a potential semiconfined aquifer is developed approximately at sea level within the carbonate rocks (marbles) of Makri unit. As water wells yields are appreciable (greater than 3600 m3/d) groundwater quality is subjected to excessive TDS concentrations. Sedimentary rocks: Southern of Perama, extensive areas are occupied by limestones of Eocene age containing a potential aquifer. In areas where the limestones are fractured and karstified the well yields range from 2400 to 3120 m3/d. The specific capacity of the limestone aquifers ranges from 1 m3/h/m to 3.5 m3/h/m. 380 C. PETALAS ET AL.

Volcanics (andesite, dacitic-andesite or tuffs): Their hydrologic properties have not been defined as extensively as those of other lithologies, probably because their hydrologic variability has discouraged their investigation. Their hydraulic properties differ greatly, and collectively they constitute a complex, heterogeneous, and anisotropic ground-water system. Their permeability is mainly secondary and is attributed to the tectonic activity, as indicated by their petrographic types in the area. The occurrence of the aquifer system results from the combined effects of fracture system, topography, and weathering. Weathering modifies both transmissivity and storage characteristics of these rocks. Transmissivity (T) of this aquifer system ranges from 57 to 130 m2/d. The storage coefficient ranges from 0.002 to 0.0009. The specific capacity ranges from 0.2 to 4.2 m3/h/m (Petalas and Diamantis, 1998). Based on aquifer tests, wells produce as much as 2160 m3/d. However, the exploitation potential of the volcanic aquifer system varies from place to place. Recently, wells completed in volcanics (andesites) in the area NW of Perama, with several meters of drawdown, yield 1920 to 2400 m3/d (Peta- las and Diamantis, 1998). These wells penetrated mainly andesites characterized by recent alteration which developed favourable conditions for the formation of a potential aquifer system. The natural recharge of the aquifers in all lithologic types derives entirely from precipitation (including snowmelt) and percolation from streams. More rarely recharge may also derive (in phyllite-aquifers) by lateral in- flow from marbles at the contact zones. In the broader area two distinct hydrogeological units are formed. Water-course aquifers of limited extent are also formed in the study area, within coarse-grained alluvial deposits traversed by perennial streams.

4. Methods

4.1. GROUNDWATER SAMPLING AND CHEMICAL ANALYSIS

A total of 38 representative samples from surface water and groundwater were collected for chemical analysis from the study area. The sampling took place during the autumn period of 2000, in a network from dug-wells (DW), deep wells (W) and surface waters (SW). The methods of sample collection and preservation used are specified in Title 40, Code of Federal Regulations, Part 136 (40 CFR 136) by the U.S. Environmental Protection Agency. Alkalinity, conductivity (EC), temperature, and pH, were measured in the field in order to acquire representative values of ambient aquifer conditions. The samples were collected and stored in each of two 500 ml plastic polyethylene bottles and submitted promptly upon return from the field to the Laboratory of Hydrogeology of the University of Patras. Chemical analysis was performed in one of the two sample bottles, by using a spectrophotometer (model 2− − 3− DR/4000 of the HACK), and the parameters SO4 ,NO3 ,NO2,NH4,PO4 , − SiO2 were determined. For the determination of Cl content, titration techniques HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 381 were applied by using AgNO3 0,1N (H¬oll, 1979). One of the two sample bottles was filtrated prior to the bottling. For sampling purposes, analytical ultra pure acid (dense HNO3)was added to the samples to fix the heavy metals in solution. Major cations as well as the heavy metals Fe, Mn, Cu, Ni and Zn were measured by using the atomic absorption technique (GBC Company Ð model 908AA). The chemical analyses were carried out in the laboratory and the analytical error does not exceed ± 5%. PHREEQC software (Parkhurst and Appelo, 1999), was used for calculating saturation indices.

4.2. STATISTICAL ANALYSIS

The R-mode multivariate technique is applied in this report on the data of chemical analyses to process hydrochemical data and to assist the understanding hydrochemical processes and relationships between the several variables (Hakli, 1970). The primary aim of factor analysis is the condensation of the maximum information of the hydrogeochemical parameters expressed by means of 17 chemical variables, (Table I) in a small number of factors. The aforementioned technique is routinely used (Voudouriset al, 1997; Stamatis, 1999) and it includes the following steps: The ﬁrst step consists of standardization (mean, xm = 0 and standard deviation σ = 1) of the raw data. This standardization is given by the formula: zi = (xi −xm)/σ , where zi is the ith value of the standardized variable z, xi is the concentration value of variable i. The second step involves the calculation of correlation coefﬁcients using the formula:

S(xi − xm)(yi − ym) = i r 1/2 2 2 S(xi − xm) S(yi − ym) i i

where x and y are the ith values of the standardised variables x and y; xm, ym are their respective means. The correlation coefﬁcients (r) are presented in a matrix form, referred to as the correlation coefﬁcient matrix. In a third step the determination of eigenvectors and eigenvalues follow by solving the correlation matrix. By this way, variables were converted to factors. The isolation of the factor number was based on the Kaiser criterion (Kaiser and Cemi, 1979) according to which, no-one of the factors (eigenvectors) has a value (eigenvalue) smaller than 1. In the next step the table with the eigenvalues was rotated according to the varimax rotation criterion (Davis, 1987) allowing thus better display of the results. Five factors resulted in this way. Finally, a weighting is assigned to each factor indicating its contribution to each sample (factor scores) necessary for its geographical distribution. We 382 C. PETALAS ET AL. ABLE I T Basic statistics of chemical parameters (mg/l) of water samples Dug wells (DW) Surface Water (SW) Deep wells (W) 70 10.664 19.871 1.3 18 5.719 4.007 1.5 63 21.827 22.069 570 265.7270.6 165.4220.15 0 0.152 0.039 388 0.217 0.043 195.094 0 146.488 0 34 1.8 0.18 361 0.269 0.034 198 0.421 0.056 0 111.615 0 0.37 0.01 0.133 0.002 0.118 0.004 5 520.5 9850 5820 341.091 147.364 302.883 198.951 23 0.3 1360 125 394.313 21.879 385.047 25 36.911 0 1324 448.727 35 491.484 6.409 10.148 3 3 4 2 4 ariable Min Max Average St. Dev. Min Max Average St. Dev. Min Max Average St. Dev. NO NH V PHCondNa 4.55 785K1 7.44Ca 5900 25Mg 5.577 2.460.909Cl 530 1.767.177 51HCO 20.5 5.072 430 156.545 420 27 190 3000 153.904 3.55 206.091 1362.5 82.209 1350 32 8.05 746.143 135.382 372.091 57.895 4.605 360 225 51 446.459 7410 4.142 15 74.5 38 445 2.359.818 5.9 96 2.581.372 167.688 48.832 572 110.311 8.55 40.906 19 123.281 7.5 6.476 128.596 25.177 1260 22.5 430 4 299.682 6.337 2180 167.136 476.397 466.136 83 155.555 846.157 44.273 34.708 SO NO FeMnNi 0.009Cu 0.005 0.06Zn 2.5 0.004 0.036 0.008 0.085 0.463 0.065 0.016 0.025 0.016 0.037 0.29 0.949 0.066 0.022 0.01 0.017 0.004 2.8 0.088 0.006 15 0.004 0.15 0.332 0.71 0.034 2.791 0.005 0.064 0.768 1.76 0.039 5.141 0.2 0.179 0.006 0.006 0.003 23.5 0.006 0.05 4.6 0.469 4.621 0.032 0.019 0.974 0.02 0.005 9.301 5.6 0.016 1.831 0.010 0.972 1.761 HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 383 are thus able to determine the most important elements which affect the observed variation.

5. Results and Discussion

5.1. GROUNDWATER QUALITY

In Table I, the concentrations of both elements and compounds are given for all of the three sample categories (DW, SW, and W). Apart form the nitrogen compounds, the rest of the elements and compounds do not present serious differentiations ranging within similar limits. Agricultural practices, including the use of fertilizers containing nitrogen, have been recognized in the study area as a key contributor of nitrate. For this reason, nitrate content in ground water showed a great range of variability. The most contaminated samples are those from dug wells (DW) and their average nitrate content much exceeded EEC/80/778 (1980) and WHO (1984) safe drinking water standard of 50 mg/L. Nitrate contamination of the ground water from deep wells (W) is lower because of the presence of confining units or a thicker vadose zone. Other elements and compounds also display high contents, so that the environmental and health impacts to be of considerable significance in most cases. High chloride contents (1000Ð2200 mg/l) were measured in samples D1, D7, W5 and W6. High chloride contents of samples D1 and D7 may be related to the presence of evaporitic salts (see also paragraph 5.5), while the high chloride contents of samples W5 and W7 may be attributed to entrapped relic of an old seawater intrusion in unflushed parts of aquifers composed of karstified limestones of Eocene age. Evaporated salts are associated in the study area with sedimentary deposits of Neogene age (Petalas, 1997). Fossil seawater could have originated from past invasions of the coastal aquifers accompanying rises in sea levels. The evidences for ancient seawater was primarily from tritium dating (Petalas, 1997). Many samples display low pH values. pH values ranges from 3.5 to 8.5. The higher pH values characterize samples from deep wells (W). The low pH increases the solubility of heavy metals in the samples D4, D5, D8, S4, S5, S6, S14, S15, W3, W4, W5, W6, W7, W8, W9 where their contents exceed the maximum allowable levels.

5.2. WATER CLASSIFICATION

As it appears in Piper diagram (Figure 2), the three sample categories (DW, SW, W) present similar but complex water types. The order of cation dominance is generally Ca2+ > Mg2+, although sodium (Na2+)isthe dominant cation in ﬁve water samples. A small majority of the water samples present the following order of − 2− − anion dominance is HCO3 > SO4 > Cl . Certain characteristic chemical types are the following: Ca-Mg-HCO3-SO4, Ca-Mg-SO4-HCO3. Ca-SO4, Ca-Mg-SO4. Ca-Na-Cl-HCO3, Na-Cl. 384 C. PETALAS ET AL.

Figure 2. Piper diagram showing the variation in groundwater and surface water chemistry of the study area.

5.3. AQUEOUS CHEMISTRY

As already mentioned, the most pronounced property of waters of the study area is their acidic character. 2− The binary diagram of Figure 3a illustrates the relation between pH and SO4 . Higher sulfate contents of waters are related to low pH. Figure 3b shows that TDS, expressed in the form of EC (Cond, µS/cm), is related also to pH. Moreover, it appears that this relation inﬂuences the salt content of surface waters more intensely than that of groundwaters. The relations between various elements and compounds reveal that water chemistry is the result of water-rock interaction, while processes as seawater dilution or evaporative concentration play a minor role. Figure 4a shows Cl− versus Ca2+ for waters on the study area. The dotted line represents the marine composition line relating two end member waters, fresh and sea water. A ﬁrst obser- vation is that most samples are plotted below freshwater/seawater mixing line, and calcium is dominant compared to chloride. Figure 4b shows that samples from all water categories tend to be scattered both above and below the freshwater/seawater mixing line particularly at high concentrations. In some samples sodium content appears to be in excess compared to chloride, while in some others the opposite is applied. Figure 4c shows that sulfate ions are dominant compared to chloride ions HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 385

2− Figure 3. Diagrams shown relationships of SO4 and specific electrical conductivity (Cond) versus pH. and most samples are plotted below the freshwater/seawater mixing line. As shown in Figure 4d, calcium ions are in excess compared to sulfate ions. Moreover this figure shows that the majority of water samples are ordered in the bargain of the second dashed line, which connects points with Ca/SO4 ratio, 1:1, implying that sulfate ions are not originated by gypsum dissolution. The abundance of sulfates (high values) could be attributed to the dissolution of the minerals pyrite (FeS2) and alunite (KAl3(SO4)2(OH)6. Alunite is a very abundant mineral in the study area. However, certain samples as W7 and W11 display Ca/SO4 ratio 1:1. In all cases what is observed is a scatter of samples in relation to fresh water/sea water mixing line. In that way, the aspect that the present chemical composition of waters results from water/rock interaction is confirmed. As it has already been mentioned, the higher metal concentrations on waters of the study area are related to samples with low pH. Acid waters originate by means of oxidation of sulfide minerals according to the following reaction for pyrite (Hem, 1985):

+ 1 + 1 → + FeS2 7 2 O2 3 2 H2O Fe(OH)3 2H2SO4 or in the general form by the reaction (Cidu et al., 1997):

+ 1 + → 2+, 2+ + 2− + +, (Fe, Me)S2 3 2 O2 H2O Feaq Meaq 2SO4 2H 2+ + 1 + + → 3+ + . 2Feaq 2 O2 2H 2Feaq H2O

2+ 3+ 3+ The oxidation of Feaq to Feaq , and the instability of Feaq species in solution when pH rises, leads to precipitation of iron oxyhydroxides, with the consequent 386 C. PETALAS ET AL.

Figure 4. Logarithmic plots of relationships between concentrations of chemical species: Ca2+,Na+, 2− − 2+ 2− and SO4 versus Cl , and Ca versus SO4 . The dotted line represents the marine composition line relating two end member waters (the concentration/dilution characteristic for seawater and freshwater). 2− 2+ The second dashed line of (d) relates points with a ratio of SO4 /Ca = 1:1. cooprecipitation and/or adsorption of other metals (Johnson, 1986; Kooner, 1993). 2− Additional amounts of SO4 in a non-acid environment, contributes to the increase of TDS by dissolution of soluble sulfates and this process does not affect pH. Chemical and Biological processes can contribute to the release of metals in water solutions (Stone and Morgan, 1987). Attenuation of high acid and metal loads may occur through buffering reactions with reactive solid phases in the rock. The principal acid-neutralization mechanisms are represented by dissolution of carbonates, hydroxides, and aluminosilicates.

H2SO4 + CaCO3 + 2H2O → CaSO4 · 2H2O + H2O + CO2 (Hem, 1985) ↔ 3+ + − Al(OH)3 gibbsite Al 3OH (Appelo and Postma, 1993) HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 387

The previous reaction results in increases in pH, however, it is possible to contribute to increased concentrations of calcium when Al displaces Ca on the exchange sites (exchanger X) according to the reaction:

1 3+ + 1 − ↔ 1 + 1 2+ 3 Al 2 Ca X2 3 Al-X3 2 Ca

Thus, the adsorption of aluminum facilitates the continuous dissolution of gibbsite and results in the creation of buffered conditions. Preceding reactions may occur in the study area. At present, the chemical evolution of acid waters is difficult to be es- tablished on a quantitative basis, because many of the interrelated factors involved (e.g., the mineralogy and relative abundance of the pH-buffering solids, the sequen- tial nature of pH-buffering reactions, the reaction rates under conditions of dynamic flow) have not been comprehensively evaluated yet (Blowes and Ptacek, 1994). Increases in alkalinity and pH are observed in certain water samples, as those from wells W1 and W11 that are characterized by the presence of low calcium concentrations. Two mechanisms may be responsible for the removal of Ca and the alkalinity increase; Dissolution of calcite accompanied by ion exchange processes, or/and hydrolysis of plagioclase (Albite, Anorthite and Analcite) which is controlled by the consumption of CO2. According to the first mechanism, ion exchange process is described by the following equations (Andrews et al., 1994):

2+ + Nα2X + Ca → CaX + 2Na + → 2+ + − + − CaCO3 H2O Ca HCO3 OH where X− indicates the exchanger The latter presupposes the presence of calcite and contributes to calcium release with sodium. Then, the released calcium in solution displaces Na on the exchange sites resulting in rich in sodium solutions unsaturated in respect to calcite. Ac- cording to the second mechanism, increased alkalinity may be caused by means of hydrolysis of plagioclase feldspars (Albite, Anorthite and Analcite). This mechanism is also controlled by the consumption of CO2, according to the following reactions (Andrews et al., 1994):

+ 1 + → + + − + 1 NaAl2Si3O8 5 2 H2O CO2 Na HCO3 2 Al2(Si2O5)(OH)4 + 2H4SiO4 + + → 2+ + − + CaAl2Si2O8 3H2O 2CO2 Ca 2HCO3 Al2(Si2O5)(OH)4 + / + → + + − + / NaAlSi2O6 31 2H2O CO2 Na HCO3 1 2Al2(Si2O5)(OH)4

+ H4SiO4

The occurrence of CO2 has been postulated in the basins of tertiary age in Macedonia-Thrace region and is related to sodium rich thermomineral waters in 388 C. PETALAS ET AL. this region (Lambrakis and Kallergis, 2005). Thus, it is likely that the previous processes take place locally in the study area (e.g. locations of wells W2, W11, and D4), and could be indicative of the occurrence of thermomineral waters. The prevailing of sodium ions in certain samples has also been mentioned in previous paragraphs. In saturated conditions with respect to calcite, the release of Ca from anorthite dissolution in the solution, as expected from the previous process for pH higher than 7, results in calcite precipitation and removal from the solution according to the following chemical equation:

2+ + Ca + HCO3 → CaCO3 + H

The formation of the previous minerals results also in the removal of sepiolite (Mg2Si3O75Ohx3H2O) by water.

5.4. WATERÐROCK INTERACTION AND SATURATION INDEX

Figure 5 shows that a considerable number of samples are saturated in respect to minerals calcite and dolomite. Because these samples are distributed in almost the total of the geological formations, it is evident that calcite and dolomite are

Figure 5. Histograms showing saturations indices of several minerals. HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 389 common minerals found in all of them. The higher values of saturation indices (SI > 1) are calculated in water samples from carbonate rocks. All the samples are unsaturated in respect to siderite, anhydrite and gypsum. The diagram of gypsum is not shown in Figure 5. Finally, all the samples are saturated in respect to minerals quartz and chalcedony. This is reasonable and expected because SiO2 predominates, particularly in pure volcanic rocks.

5.5. STATISTICAL ANALYSIS AND AREAL DISTRIBUTION OF THE CHEMICAL PARAMETERS

By using factor analysis a model of five factors is resulted. This model allows the reduction of 17 chemical parameters in five factors that express about 84% of the statistical information. The high communality values show that the model explains sufficiently the variance of almost all the statistical parameters (Table II). Factor 1, presents high loadings for the elements Na, K, Cl and for electrical conductivity (E.C.). These three elements are generally considered to be of marine origin, while the total salinity is expressed by the specific electrical conductivity (EC) of the water samples. This factor could be considered as the factor of salinity. The origin of salinity in places located far from the coast could be attributed to cosedimentary entrapment of salts. The second factor is a bipolar

TABLE II Varimax rotated factor loadings and communalities for Sapes aquifer dataset Variable Factor 1 Factor 2 Factor 3 Factor 4 Factor 5 Communality pH 0.039 0.883 0.128 −0.028 0.108 0.810 EC (µS/cm) 0.932 −0.255 −0.025 −0.105 0.198 0.985 Na (mg/L) 0.974 0.013 0.005 −0.01 −0.07 0.954 K (mg/L) 0.72 0.138 −0.37 0.009 0.16 0.701 Ca (mg/L) 0.313 −0.666 −0.378 −0.114 0.489 0.938 Mg (mg/L) 0.518 −0.306 −0.056 0.021 0.558 0.678 Cl (mg/L) 0.981 −0.014 0.063 −0.043 0.036 0.969

HCO3 (mg/L) 0.257 0.594 0.113 0.241 0.576 0.821 SO4 (mg/L) 0.132 −0.792 −0.51 −0.055 0.094 0.916 NO3 (mg/L) 0.004 0.111 0.128 −0.092 0.885 0.821 NH4 (mg/L) −0.079 −0.185 −0.122 0.92 −0.093 0.91 SIO2 (mg/L) 0.111 −0.19 −0.819 0.092 −0.166 0.754 Cu (mg/L) −0.035 −0.009 −0.005 0.954 0.06 0.915 Fe (mg/L) −0.075 −0.232 −0.829 −0.124 0.103 0.773 Mn (mg/L) −0.129 −0.734 −0.194 0.533 −0.045 0.88 Zn (mg/L) 0.09 0.019 −0.621 0.354 −0.114 0.532 Ni (mg/L) 0.239 −0.842 0.108 0.309 −0.032 0.875 %Var 22.7 22.2 14.1 14.0 10.7 83.7 390 C. PETALAS ET AL. factor and presents high positive loadings for pH and high negative loadings for sulfates and the elements Mn and Ni. This inverse relationship between pH and SO4-Mn-Ni is well correlated to the increased solubility of these elements in low pH values. The relationship between pH and the REDOX environments is also well known and accordingly factor 2 expresses the trace elements concentrations related to pH, but also the environmental dimension. However, this factor expresses also the narrow relations between environment, water, air and rocks. pH is accordingly an important control factor for the evolution of the water chemical composition. Factors 1 and 2 display similar variations related to the total variation (0.22 each of them) and accordingly they appear to have similar level of significance in the evolution of water quality. The next two factors display similar variations and accordingly have analogical level of significance. The rest of the statistical information on metals is attributed to these factors. Factor 3 displays variance 14%, presents high loadings for the elements Fe and Zn which relates with SiO2, while factor 4 presents high loadings for Cu and ammonia. Factor 5 with a variance of about 10% presents high loadings for nitrate ions. This factor is exclusively related to anthropogenic interventions (e.g. intense fertilizations) to the environment and accordingly to the configuration of water quality. From the precedents it becomes obvious that the presence of metallic minerals in the rocks of the study area is the decisive factor for the configuration of pH and chemical composition of waters. The geographical distribution of factors 1 and 2 is shown in Figure 6. The geographic distribution of the salinity factor indicates that higher concentrations of salts follow a main axis of prevalence with a NW-SE direction, south of Sapes municipality. Also, the increased values of salinity in a distance of a few kilometers from the coast could be attributed to sea water intrusion by means of natural mechanisms of salinization (e.g. mean sea level fluctuation in Pleistocene time). The geographic distribution of factor 2 is characterized by the presence of two axes. The first axis has a NW-SE direction following the direction of factor 1. The second axis, at first, is almost normal to the first axis, then, it gets a NS direction, south of Perama, where water samples display increased metal concentrations. Waters in this area present also significantly increased acidity due to the increased sulfides concentrations in this area.

6. Conclusions

Extended aquifers are formed in volcanosedimentary rocks of the study area. The transmissivity (T) of these aquifers is 57 to 130 m2/d, whereas their speciﬁc capacity ranges from 0.2 to 4.2 m3/h/m. Sulﬁde compounds of iron, manganese, zinc, nickel and cobalt are very common in the volcanosedimentary rocks. Surface and ground waters are strongly metalliferous and their hydrochemical facies present similar but complex water types as follows: Ca-Mg-HCO3-SO4, Ca-Mg-SO4-HCO3. Ca-SO4, HYDROCHEMISTRY OF WATERS OF VOLCANIC ROCKS 391

Figure 6. Areal distribution of the ﬁrst factor (salinization factor, a) and the second factor (b) in the study area. 392 C. PETALAS ET AL.

Ca-Mg-SO4. Ca-Na-Cl-HCO3, Na-Cl. Calcium and magnesium are the dominant cations. Bicarbonates and sulfate ions are the dominant anions. The general decreas- ing order of absolute abundances of major ions in the surface and groundwaters are − 2− − 2+ 2+ 2+ as follows: HCO3 > SO4 > Cl and Ca > Mg , although sodium (Na ) may be locally the dominant cation. High metal concentrations are related to water with low pH. The low pH increases the solubility of heavy metals and their contents very often exceed the maximum allowable levels. Sulfide minerals, during hydrolysis, offer great quantities of minerals in waters while they also lower the pH and therefore they cause acid drainage and consequently quality degradation in surface and groundwaters. The quality of the waters is mainly due to waterÐrock interactions. The presence of carbonates creates locally balance conditions in the acid character of the waters as they neutralize the low pH values. Anthropogenic activities also, e.g. intensive fertilizations, influence adversely water quality. Computer simulation indicates that a considerable number of samples are saturated in respect to minerals calcite and dolomite. All the samples are unsaturated in respect to siderite, anhydrite and gypsum and saturated in respect to minerals quartz and chalcedony. The increased values of salinity in groundwater a distance of a few kilometers from the coast could be attributed to the relics an old sea water intrusion. The water quality problems can be successfully handled by the use of factor analysis. It was concluded that 17 chemical parameters which were measured in the collected samples can be substituted by five factors which successfully represent (83.7% of the statistical information) the hydrochemical processes as well as their geographic distribution. Intense gold mining activities which are planned to operate in the study area in the near future must be appropriately managed as they may result in leaching of many toxic elements to the surrounding environment.

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