The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity As a Driving Force

minerals

Review The Cr(VI) Stability in Contaminated Coastal Groundwater: Salinity as a Driving Force

Ioannis-Porfyrios D. Eliopoulos 1, George D. Eliopoulos 2 and Maria Economou-Eliopoulos 3,*

1 Department of Pharmacy, School of Health Sciences, National University of Athens, GR-15771 Zografou, Greece; [email protected] 2 Department of Chemistry, University of Crete, GR-70013 Heraklion, Greece; [email protected] 3 Department of Geology and Geoenvironment, National University of Athens, GR-15784 Athens, Greece * Correspondence: [email protected]

Abstract: Chromium concentrations in seawater are less than 0.5 µg/L, but the Cr(VI) in contaminated coastal groundwater affected by Cr-bearing rocks/ores and/or human activities, coupled with the intrusion of seawater may reach values of hundreds of µg/L. A potential explanation for the stability of the harmful Cr(VI) in contaminated coastal aquifers is still unexplored. The present study is an overview of new and literature data on the composition of coastal groundwater and seawater, aiming to provide potential relationships between Cr(VI) with major components in seawater and explain the elevated Cr(VI) concentrations. It is known that the oxidation of Cr(III) to Cr(VI) and the subsequent back-reduction of Cr(VI) processes, during the transport of the mobilized Cr(VI) in various aquifers, facilitate the natural attenuation process of Cr(VI). Moreover, the presented positive trend between B and Cr(VI) and negative trend between δ53Cr values and B concentration may suggest that seawater

components signiﬁcantly inhibit the Cr(VI) reduction into Cr(III), and provide insights on the role − of the borate, [B(OH)4] ions, a potential buffer, on the stability of Cr(VI) in coastal groundwater.

Citation: Eliopoulos, I.-P.D.; Therefore, efforts are needed toward the prevention and/or minimization of the contamination by Eliopoulos, G.D.; Cr(VI) of in coastal aquifers, which are inﬂuenced by the intrusion of seawater and are threatened by Economou-Eliopoulos, M. The Cr(VI) changes in sea level, due to climate change. The knowledge of the contamination sources, hotspots Stability in Contaminated Coastal and monitoring of water salinization processes (geochemical mapping) for every coastal country may Groundwater: Salinity as a Driving contribute to the optimization of agricultural management strategies. Force. Minerals 2021, 11, 160. https://doi.org/10.3390/min11020160 Keywords: groundwater; contamination; chromium(VI); salinization; borate; chromium isotopes

Academic Editor: María Ángeles Martín-Lara Received: 29 December 2020 1. Introduction Accepted: 29 January 2021 Published: 3 February 2021 The scientiﬁc interest on the sustainable management of water resources and the effect on groundwater aquifers by either nature processes and/or extensive application

Publisher’s Note: MDPI stays neutral in industry, is an attractive and fundamental subject to global food security. Among with regard to jurisdictional claims in heavy metals, chromium (Cr) has become widespread in the environment. It appears published maps and institutional affil- in several oxidation states, the trivalent Cr(III) and hexavalent forms [Cr(VI)] being the iations. most thermodynamically stable Cr forms in nature [1,2]. Cr(III) is necessary for lipid and sugar metabolism, and it is an essential trace element for human and animal health, whereas Cr(VI) in the food chain (groundwater, soil and plants), has created an alarming situation for human life and ecosystems [3–5]. Potential sources for the relatively high Cr contents in soil and groundwater may be the widespread Cr-bearing peridotites, which Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. are parts of ophiolite complexes, covering more than 1% of the earth, along orogenetic This article is an open access article zones [6], intense ore mining/smelting, industrial and agriculture activities (fertilizers and distributed under the terms and pesticides) [7,8]. The weathering processes of ultramafic rocks, depending mainly on the conditions of the Creative Commons climate conditions and morphology, may result in the formation of laterites, both releasing Attribution (CC BY) license (https:// significant amounts of Cr among other heavy metals (Ni, Co, Mn, Fe). Groundwater from creativecommons.org/licenses/by/ sites characterized by the extensive presence of ultramafic rocks contain more than 10 µg/L 4.0/). Cr(VI), reaching values up to hundreds of µg/L of Cr(VI) [6,9–16]. The contamination of

Minerals 2021, 11, 160. https://doi.org/10.3390/min11020160 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 16

Minerals 2021, 11, 160 sites characterized by the extensive presence of ultramafic rocks contain more than2 of 1510 μg/L Cr(VI), reaching values up to hundreds of μg/L of Cr(VI) [6,9–16]. The contamination of groundwater by Cr may be derived from industrial activities, such as in the Czech Re- groundwaterpublic (a highly by Cr industrialized may be derived country from industrialin Central activities,Europe) [17], such at as the in thearea Czech of Friuli Republic Vene- (azia highly Giulia industrialized (northern Italy) country [18], the in Assopos Central Basin Europe) (Oinofyta [17], at or the Inofyta, area of near Friuli the VeneziaAssopos Giuliariver) (northernin Greece, Italy)exhibiting [18], as the high Assopos as 8000 Basin μg/L (OinofytaCr(VI) in shallow or Inofyta, groundwater near the Assopos [19]. Ad- river)ditionally, in Greece, ferrochromium exhibiting (FeCr), as high which as 8000 is aµ g/Lcritical Cr(VI) alloy in in shallow the production groundwater of stainless [19]. Additionally,steel, is produced ferrochromium in several European (FeCr), which countries is a critical (Finland, alloy France, in the productionItaly, Norway, of stainless Sweden, steel,the nations is produced formerly in several comprising European Yugoslavia, countries Germany, (Finland, Italy, France, Switzerland, Italy, Norway, and Sweden,the U.K.) thegenerating nations formerlyCr-bearing comprising wastes [7,20]. Yugoslavia, Germany, Italy, Switzerland, and the U.K.) generatingIn coastal Cr-bearing groundwater wastes [that7,20 ].is contaminated by either Cr-bearing rocks/ores or in- dustrialIn coastal wastes, groundwater Cr(VI) often that exceeds is contaminated the maximum by acceptable either Cr-bearing level for rocks/ores Crtotal in drinking or in- dustrialwater (50 wastes, μg/L) Cr(VI) [21]. Such often groundwater, exceeds the maximum at the interface acceptable zone level between for Cr totallandin and drinking sea, is watercontinually (50 µg/L) influenced [21]. Such by both groundwater, marine and atterre thestrial interface processes, zone and between it often land contains and sea, ele- isvated continually concentrations inﬂuenced of Na, by bothCl, B, marine Li, Se, andAs, S, terrestrial Ca, and Mg, processes, which andis characteristic it often contains of the elevatedseawater concentrations composition [22,23]. of Na, Although Cl, B, Li, Se, about As, one S, Ca, quarter and Mg, of the which global is characteristicpopulation lives of thein the seawater vicinity composition of the world’s [22 ,coastlines23]. Although [24], the about potential one quarter role of of the the presence global population of seawater livescomponents, in the vicinity which ofmay the significantly world’s coastlines facilitate [24 the], theCr(VI) potential stability, role inhibiting of the presence the Cr(VI) of seawaterreduction components, to Cr(III), is unexplored which may in signiﬁcantly groundwater. facilitate Additionally, the Cr(VI) because stability, high inhibitingtechnology themetals Cr(VI) such reduction as the rare to Cr(III), earth elements is unexplored (REEs in) have groundwater. become contaminants Additionally, in because the environ- high technologyment [25,26], metals this study such aspresents the rare new earth data elements on the composition (REEs) have of become coastal contaminants water, including in theREEs, environment from the industrial [25,26], this zone study of presentsthe Assopos new and data other on the Neogene composition Basins. of They coastal are water, com- includingbined with REEs, available from data the industrial from previous zone ofstudies, the Assopos and the and delineated other Neogene relationships Basins. are They pre- aresented. combined A major with aim available was the data interpretation from previous of studies,the Cr(VI) and stability, the delineated as a function relationships of the aresalinity presented. in groundwater A major aim resulting was the from interpretation the intrusion of the of Cr(VI) seawater, stability, the asprevention a function of ofgroundwater the salinity infrom groundwater further degradation, resulting from and theprotection intrusion of the of seawater, valuable thewater prevention resources. of groundwater from further degradation, and protection of the valuable water resources. 2. Chromium Background 2. Chromium Background StructureStructure of of Cr(III) Cr(III) and and Cr(VI) Cr(VI) and and Soluble Soluble Products Products AssumingAssuming that that Cr(III) Cr(III) forms forms hexa-coordinate hexa-coordinate complexes complexes with with the the octahedral octahedral arrange- arrange- ment of ligands, [Cr(H2O)3+63+] is the main Cr species in solutions of inorganic Cr(III) salts ment of ligands, [Cr(H2O)6 ] is the main Cr species in solutions of inorganic Cr(III) salts under strongly acidic pH, while at pH ≥ 4, Cr(III)-bound H2O molecules undergo hydrol- under strongly acidic pH, while at pH ≥ 4, Cr(III)-bound H2O molecules undergo hydrolysis,ysis, resulting resulting in in the the formation formation of of soluble soluble oligomeric oligomeric products, products, whilst whilst polymeric polymeric products products areare insoluble insoluble (Figure (Figure1 )[1)27 [27].].

4+ 4+ 3+ 3+ FigureFigure 1. 1. SolubleSoluble oligomeric oligomeric product, product, Cr(OH) Cr(OH)2(H2(H2O)2O)8 by8 hydrolysisby hydrolysis and polymerization and polymerization of [Cr(H of2 [Cr(HO)6 ] 2(modifiedO)6 ] (modiﬁed after [27]. after [27]. Surface waters commonly contain a mixture of soluble monomeric and oligomeric Cr-productsSurface waters[28]. Additionally, commonly containin the presence a mixture of oforganic soluble matter, monomeric the formation and oligomeric of stable Cr-productsCr(III) complexes [28]. Additionally, with small organic in the presencemolecule ofs can organic increase matter, their the mobility formation at the of stablesource Cr(III)of contamination complexes with sites smalland maintain organic molecules the solubi canlity increaseof Cr(III) their even mobility at neutral at pH the source[27]. of contaminationChromate sites and and dichromate maintain thehave solubility tetrahedra of Cr(III)l arrangements even at neutral of coordinated pH [27]. oxygen (FigureChromate 2). The andchromate dichromate anion (CrO have42 tetrahedral−) is the predominant arrangements formof of coordinatedCr(VI) in dilute oxygen solu- 2− (Figuretions at2). neutral The chromate pH, co-existing anion (CrOin equilibrium4 ) is the with predominant its protonated form form of Cr(VI) [HCrO in4] dilute− in an − solutionsapproximately at neutral 3:1 ratio pH, co-existingat these conditions in equilibrium [29]. with its protonated form [HCrO4] in an approximately 3:1 ratio at these conditions [29].

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FigureFigure 2. 2. StructuresStructures of of tetrahedral tetrahedral chromate, chromate, and and dichromate dichromate (modified (modified from from ref ref [27]). [27]). 3. Geological and Hydrological Outline 3. Geological and Hydrological Outline Detailed studies in Greece have shown that the main aquifers in the Greece are de- Detailed studies in Greece have shown that the main aquifers in the Greece are developed in alluvial formations, such as Quaternary and Neogene unconsolidated deposits veloped in alluvial formations, such as Quaternary and Neogene unconsolidated deposits (porous aquifers) covering the lowlands and semi-mountainous area of Greece and in (porous aquifers) covering the lowlands and semi-mountainous area of Greece and in car- carbonate rocks (karstic aquifers) [30–38]. Specifically, the Neogene Assopos–Thiva Basin bonate rocks (karstic aquifers) [30–38]. Specifically, the Neogene Assopos–Thiva Basin (Figure3c) is mainly composed of tertiary and quaternary sediments of more than 400 m (Figure 3c) is mainly composed of tertiary and quaternary sediments of more than 400 m thickness, and expands over an area of approximately 700 km2. Alternations of marls thickness, and expands over an area of approximately 700 km2. Alternations of marls and and marly limestone occurs in the lowest parts of the basin sequences, and continental marlysediments limestone consisting occurs of in conglomerates the lowest parts with of small the intercalationsbasin sequences, of marls, and continental marly limestones, sedi- mentsschists, consisting sandstones, of conglomerates clays and flysch with are small dominant intercalations in the upper of marls, parts. marly A sharp limestones, tectonic schists,contact sandstones, between the clays sediment and flysch types, are due domi to thenant intense in the neotectonic upper parts. deformation, A sharp tectonic is a char- contactacteristic between feature the of sediment the entire types, area [due30]. to The the morpho-tectonic intense neotectonic structure deformation, and evolution is a char- of acteristicthis basin feature are the of result the entire of E–W area to WNW–ESE[30]. The morpho-tectonic trending fault systems, structure with and peridotites evolution andof thisa Ni-laterite basin are the over-thrusted result of E–W on theto WNW–ESE Triassic–Jurassic trending carbonates fault systems, [30]. Quaternary with peridotites sediments and a coverNi-laterite large over-thrusted parts of the Assopos on the Triassic–Jurassic valley and host two carbonates types of [30]. aquifers: Quaternary (i) aquifers sediments within coverNeogene large conglomerates,parts of the Assopos sandstones valley and host marly two limestone types of toaquifers: a depth (i) of aquifers approximately within Neogene150 m; and conglomerates, (ii) karst type sandstones aquifers within and marly the Triassic–Jurassic limestone to a depth limestones of approximately at deeper levels 150 m;of and the basin,(ii) karst such type as theaquifers Mavrosouvala within the aquifer Triassic–Jurassic [31–33]. The limestones Neogene Basinat deeper of the levels Central of theEvia basin, is characterized such as the byMavrosouvala strong geomorphological aquifer [31–33]. contrast The Neogene and is built Basin up mainlyof the Central of Pleis- Eviatocene is characterized to Holocene sedimentsby strong geomorphological hosting the most productive contrast and aquifers is built in up this mainly area. of Strongly Pleis- tocenetectonized to Holocene ultramafic sediments rocks (harzburgites hosting the andmost lherzolites), productive over-thrusted aquifers in this onto area. Upper Strongly Creta- tectonizedceous limestones ultramafic and rocks flysch (harzburgites sediments, are an widespreadd lherzolites), in C.over-thrusted Evia. Alluvial onto deposits Upper are Cre- the taceoushost rocks limestones to the aquifer, and flysch which sediments, is probed are by manywidespread shallow in wells C. Evia. (10–180 Alluvial m) or deposits agricultural are theactivities host rocks [11, 12to]. the aquifer, which is probed by many shallow wells (10–180 m) or agri- culturalThe activities most important [11,12]. aquifers in the Mesogeia Basin in Attica, which is part of the Attica–CycladesThe most important zone, are aquifers constituted in the by Mesogeia Triassic–Jurassic Basin in limestones, Attica, which Upper is part Cretaceous of the Attica–Cycladeslimestones, marly zone, limestones, are constituted and travertine by Triassic–Jurassic limestones (permeablelimestones, Upper rocks) [Cretaceous34–38]. The limestones,main part marly of the limestones, Mesogeia Basin, and travertine including limestones the Koropi (permeable area (Figure rocks)2), is [34–38]. occupied The by mainQuaternary part of the formations Mesogeia (gravels,Basin, including sand, and the clays)Koropi which area (Figure characterize 2), is occupied a low productive by Qua- ternaryphreatic formations aquifer [ 35(gravels,]. The sand, alluvial and deposits clays) which form characterize a phreatic aquifera low productive displaying phreatic limited aquiferpermeability [35]. The and alluvial poor hydraulic deposits form characteristics, a phreatic dueaquifer to fine displaying grain compositions; limited permeability they feed anda large poor number hydraulic of wellscharacteristics, and boreholes, due to with fine depthsgrain compositions; ranging from they 10 to feed 20 m a [large35–37 num-]. ber of wells and boreholes, with depths ranging from 10 to 20 m [35–37].

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FigureFigure 3.3. SketchSketch mapmap showingshowing locations of contamin contaminatedated groundwater groundwater by by Cr(VI) Cr(VI) coasts coasts (a (a) )exam- examples suchples assuch in as California, in California, Pakistan, Pakistan, India, India, Northern Northern China, China, the the Mediterranean Mediterranean Sea, Sea, etc. etc. [23 [23];]; (b ()b Karstic-) Karstic-type aquifer developed in Neogene lacustrine formations of the Mesogeia Basin in Attica type aquifer developed in Neogene lacustrine formations of the Mesogeia Basin in Attica [34,35,37]; [34,35,37]; (c) simplified geological map presenting the sites of the groundwater well and soil sam- (c) simpliﬁed geological map presenting the sites of the groundwater well and soil samples at the C. ples at the C. Evia and Assopos–Thiva Basins [11,36]. Evia and Assopos–Thiva Basins [11,36]. 4. Materials and Methods 4. Materials and Methods Although the details of the analytical methods applied for the determination of the Although the details of the analytical methods applied for the determination of the groundwater composition, including chromium stable isotopes, are provided in relative groundwater composition, including chromium stable isotopes, are provided in relative references, a brief outline is given here. More than two hundred coastal groundwater sam- references, a brief outline is given here. More than two hundred coastal groundwater ples from domestic and irrigation wells from the C. Evia, Assopos–Thiva, and Attica Ne- samples from domestic and irrigation wells from the C. Evia, Assopos–Thiva, and Attica ogene Basins, and seawater samples from the Mediterranean Sea (Evoic gulf), collected Neogene Basins, and seawater samples from the Mediterranean Sea (Evoic gulf), collected during the period from 2007 to 2017, have been analyzed for major and trace elements by during the period from 2007 to 2017, have been analyzed for major and trace elements inductively coupled plasma mass spectroscopy (ICP/MS) [11,12,31–38]. In addition, rep- by inductively coupled plasma mass spectroscopy (ICP/MS) [11,12,31–38]. In addition, resentatives of those groundwater samples were analyzed in the present study for major representatives of those groundwater samples were analyzed in the present study for major and trace elements, including rare earth elements (REEs) and platinum-group elements and(PGEs) trace by elements, inductively including coupled rareplasma earth mass elements spectroscopy (REEs) (ICP/MS) and platinum-group (Table 1). Detection elements (PGEs)limits, quality by inductively control samples coupled and plasma the precisio mass spectroscopyn of the analyses (ICP/MS) were in (Table agreement1). Detection with limits,international quality standards control samples (~10%). Physical and the precisionand chemical of the parameters analyses (pH were and in total agreement dissolved with internationalsolids) of the standardswater samples (~10%). were Physical measured and in chemical the field using parameters a portable (pH Consort and total 561 dissolved Mul- solids)tiparameter of the Analyzer water samples(Turnhout, were Belgium). measured The analyses in the ﬁeldof total using chromium a portable were performed Consort 561 Multiparameterby GFAAS (Perkin Analyzer Elmer 1100B (Turnhout, system, Belgium).Waltham, MA, The USA), analyses with ofan total estimated chromium detection were performedlimit of ~1 μ byg/L. GFAAS The chemical (Perkin analyses Elmer 1100Bfor Cr(VI) system, were Waltham,performed MA,by the USA), 1.5-diphenylcar- with an esti- matedbohydrazide detection colorimetric limit of ~1 method,µg/L. using The chemical an HACH analyses DR/4000 for spectrophotometer Cr(VI) were performed (Loveland, by the 1.5-diphenylcarbohydrazideCO, USA). The estimated detection colorimetric limit of method, the method using was an determined HACH DR/4000 at ~4 μg/L. spectrophotometer (Loveland, CO, USA). The estimated detection limit of the method was determined at ~4 µg/L.

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Table 1. Major and trace element concentrations in groundwater and water leachates for peridotites from C. Evia, Assopos–Thiva, and Attica Basins.

Attica Assopos C. Evia Water Leachates, Peridotites Detection µg/L U.C. K.2 V.3 V.4 KA.5 KP.6 Ass.A1 Ass.A2 Ass.AK3 EV.1 EV.2 EV.3 EV.4 EV.5 R2BA R2BB R3 Limit As 2.5 6.4 1.3 1.2 2.3 2.2 1.0 3.5 4.8 1.5 0.5 0.9 <0.5 0.7 1.7 0.5 0.8 0.5 B 62 31 105 89 17 18 35 37 130 54 35 88 57 136 124 98 78 5 Li 7.8 9.8 2.4 3.0 2.3 3.3 5.9 12 32 8.4 3.3 7.5 8.9 5.5 1.7 0.2 15 0.1 Se 1.8 1.4 1.3 0.7 0.9 1.2 0.9 2.2 3.5 1.7 0.6 1.8 2.2 0.6 <0.5 <0.5 <0.5 0.5 Ba 35 50 42 41 35 40 37 64 42 4 16 74 27 21 3.30 2.36 5.08 0.05 Br 230 210 190 200 130 210 150 296 1400 237 132 210 290 143 13 <5 <5 5 Cu 0.9 2.3 2.0 2.2 1.2 1.1 1.1 0.6 2.1 2.6 0.9 2.7 2.6 1.8 1.5 0.3 0.6 0.1 Cr 8.3 19.0 6.0 6.2 6.9 6.1 59 114 900 105 52 128 320 91 35 64 32 0.5 Mn 0.05 0.29 0.08 0.09 0.77 0.06 0.73 0.05 <0.05 0.26 <0.05 0.21 1.21 0.36 0.13 0.15 0.48 0.05 Co <0.02 0.34 0.05 <0.02 <0.02 <0.02 0.31 <0.02 <0.02 0.08 0.02 0.15 0.28 0.12 0.04 <0.02 <0.02 0.02 Ni <0.2 1.5 <0.2 <0.2 <0.2 <0.2 3.0 0.8 5.3 5.1 15 10 6.5 6.8 4.2 2.3 0.7 0.2 Fe <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 <10 4.2 2.3 0.7 10 Zn <0.5 1.8 1.4 0.7 79.7 0.5 1.5 <0.5 74 1.6 <0.5 1.3 1.2 <0.5 35 5.5 6.1 0.5 Sb 0.11 0.09 0.17 0.17 0.21 0.21 <0.05 <0.05 0.2 <0.05 <0.05 <0.05 <0.05 <0.05 0.19 0.11 0.36 0.05 V 1.1 2.4 1.5 1.3 1.8 0.6 2.8 9.6 9.8 3.7 1.2 2.1 2.1 2.0 3.2 4.2 2.1 0.2 La <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.02 0.02 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Ce <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Pr <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Nd <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 0.01 <0.01 <0.01 <0.01 0.02 <0.01 <0.01 <0.01 0.01 Sm <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 0.02 Eu <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Gd 0.02 <0.01 <0.01 <0.01 0.02 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 0.01 0.01 <0.01 0.01 Tb <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Dy <0.01 0.01 0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Ho <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Er <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Tm <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Yb <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Lu <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Sc 2.0 3.0 2.0 2.0 2.0 2.0 4.0 6.0 7.0 10.0 5.0 5.0 7.0 5.0 2.0 3.0 3.0 1.0 Ru 0.06 0.14 <0.05 0.12 <0.05 0.30 0.12 0.06 0.13 0.15 <0.05 0.19 <0.05 0.14 <0.05 0.18 <0.05 0.05 Rh <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 Pt <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.03 <0.01 0.01 Pd <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 <0.2 0.2 Rb 2.24 1.40 1.03 0.88 1.08 0.44 0.47 0.65 0.7 0.57 0.43 0.89 0.41 0.81 0.25 0.50 0.16 0.01 Re <0.01 <0.01 0.02 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 <0.01 0.01 <0.01 <0.01 <0.01 0.01 Zr <0.02 <0.02 <0.02 <0.02 0.04 <0.02 <0.02 <0.02 0.02 0.03 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 <0.02 0.02 Mo 0.7 0.2 0.2 0.3 0.2 <0.1 0.1 0.2 0.2 0.2 0.1 <0.1 0.4 0.1 0.5 1.7 0.3 0.1 Nb 0.02 <0.01 <0.01 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.01 P <10 <10 <10 <10 169 <10 <10 <10 30 10 <10 <10 <10 <10 11 <10 <10 10 Sr 330 224 349 352 156 200 151 200 150 61 160 332 260 174 20 19 15 0.01 Th <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 0.10 0.10 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 0.05 U 2.61 0.38 0.25 0.30 0.42 0.40 0.15 0.34 1.3 0.23 1.84 3.28 0.20 <0.02 0.03 <0.02 <0.02 0.02 mg/L Si 8.5 12.3 10.4 10.8 7.8 9.4 22.3 28.8 11.2 49.1 25.3 24.7 37.6 25.4 12 12 13 0.04 Ca 85 118 113 125 144 126 80 47 39 38 81 102 92 79 26 26 25 0.05 Mg 45 14 11 12 8.2 10 74 70 137 114 48 94 124 66 1.9 6.1 2.9 0.05 Na 42 29 37 39 25 33 26 30 410 26 23 40 32 40 3.4 2.8 2.4 0.05 K 1.89 1.54 3.22 2.53 1.30 0.75 0.87 0.79 2,1 1.94 1.74 5.38 1.89 2.25 0.7 0.6 0.6 0.05 S 9.0 6.0 16 17 17 14 8 3 60 15 11 43 30 15 <1 <1 <1 1.0 Cl 71 77 70 78 41 59 40 97 590 46 30 51 57 47 <1 <1 <1 1.0 Minerals 2021, 11, 160 6 of 15

A series of batch water-leaching experiments for serpentinized peridotites were carried out in order to study the long-term (one week) leaching responses of Cr under atmospheric conditions [38]. Water and water leachate samples in an amount which would yield about 1 µg of total chromium were used for the Cr isotope composition, and the solutions were prepared following the method from ref [39]. Both Cr concentrations and isotope ratios were analyzed using an IsotopX “Phoenix”multicollector thermal ionization mass spectrometer (TIMS) equipped with eight Faraday cups at the University of Copenhagen, Denmark. Four Cr beams (50Cr+, 52Cr+, 53Cr+, and 54Cr+) were analyzed simultaneously with 49Ti+, 51V+, and 56Fe+ beams, which were used to monitor interfering ions. The ﬁnal isotope composition of a sample was determined as the average of the repeated analyses and reported relative to the certiﬁed SRM 979 standard as:

53 53 52 53 52 δ Cr(‰) = [( Cr/ Crsample/ Cr/ CrSRM979) − 1] × 1000. (1)

The raw data were corrected for naturally- and instrumentally-induced isotope fractionation using the double spike routine. To assess the precision of the analyses, a double- spike treated, certiﬁed standard reference material (NIST SRM 979) was used.

5. Global Distribution of Coastal Groundwater Climate change, due to increasing concentrations of greenhouse gases, may affect the saltwater intrusion through changes in precipitation and temperature [40]. Coastal aquifers within the zones of inﬂuence are threatened by rising sea levels worldwide (Figure3). The phenomenon of seawater intrusion has been recorded in many coastal areas, such as in Texas, Florida, Indonesia, Syria, in South Asia (Bangkok) in Africa (Nigeria, Egypt and Tunisia), China, Australia, and Europe (Cyprus, Turkey, Estonia, Italy, and Greece) [10,12,23,25,31–38]. Global review publications provide an extensive array of ﬁeld, laboratory, and computer-based techniques for the investigation of the seawater intrusion, its composition, and prediction of freshwater–saltwater interfaces over regional scales [23,41–46].

6. Geochemical Characteristics and Salinization of Coastal Groundwater A common feature of the groundwater from the Neogene lacustrine formations of the Attica Basin, which has been affected by occasionally enclosed (tectonically) serpentinized peridotites, and the C. Evia, Assopos–Thiva Basins and the Loutraki area (Figure3b,c) is the effect of the seawater intrusion [11–14,31–38,41], although they have been classiﬁed in various water types: the water samples from Attica as a Ca–HCO3 water type [37], while those from C. Evia and Assopos–Thiva Basins and the Loutraki area are of Mg–HCO3 type, due to the CO2-driven dissolution of dominant minerals, such as serpentine and Mg-carbonates/hydroxides [11,14,31]. In order to better understanding the hydrochemistry of the above aquifers and the driving force of the Cr(VI) stability in contaminated coastal water, representative groundwater samples from the Attica, Assopos–Thiva, and C. Evia Neogene Basins were analyzed in the present study for major and trace elements, including rare earth elements (REEs) and platinum group elements (PGEs) (Table1), which were combined with the relative literature data. Major and trace elements (Table1) are in a good agreement with those in previous studies, often exhibiting Cr(VI) concentrations over the maximum acceptable level for Crtotal in drinking water (50 µg/L), although the water contaminated by industrial activities in the area of Oinoﬁta (or Inofyta), near the Assopos river (Assopos Basin) has shown concentrations higher than 8000 µg/L [19]. Besides the elevated B, Br, Se, Li, As, V, U, S, Na, K, Ca and Mg concentrations, the concentrations for the majority of REEs are lower than the detection limit of the method (<0.01 µg/L), except values of 0.02 µg/L La and 0.01 to 0.02 µg/L Nd measured in the Assopos and C. Evia aquifers, and 0.02 µg/L Gd and 0.02 µg/L Dy measured in the Attica aquifers. The measured range for Sc of lanthanides was 2–3 µg/L in the Attica aquifers and 4–14 µg/L Sc in the C. Evia and Assopos Basins (Table1). With respect to platinum-group elements, Pd was lower than the detection limit Minerals 2021, 11, 160 7 of 15

of the method (0.2 µg/L). The Pt was lower than detection limit (0.01 µg/L) in the Attica aquifers, while 0.01 values were measured in the C. Evia and Assopos aquifers. The Ru concentrations ranged from 0.06 to 0.3 µg/L in Attica, and from 0.06 to 0.19 µg/L in the C. Evia and Assopos Basins (Table1). Additionally, these water samples were analyzed for the elements Au, Ag, Al, Ti, W, Tl, Te, Ta, Sn, In, Hf, Ge, Pb, Cs, Hg, Be, Bi, Cd, Ga and Y, but their concentrations were lower than the detection limit of the method. The measured concentrations for REEs and PGEs in water leachates for altered peridotites were also too low (Table1). The available data on groundwater from previous studies exhibiting the lowest B and Cr(VI) concentrations have been recorded in the Mavrosouvala wells of karst type (Table2), which are used for the municipal water supply of Athens city.

Table 2. Average composition of groundwater from the Assopos, Thiva, Attica Basins and C. Evia and seawater. Data from refs [19,32–38,46–49].

µg/L mg/L g/L ‰ Location n Cr Cr(VI) B Li As Se Ca Mg S Na K pH TDS δ53Cr Groundwater Karst type Mavrosouvala 3 <1 <1 18 2.5 2.4 <0.5 90.3 17 4 14 0.6 7.4 0.31 Attica, Athens 19 9.7 9 43 6.4 3.5 1.9 120 20 15 44 2.5 7.3 0.43 Attica, Koropi 31 12 10 130 8 8.4 10 135 55 37 217 8.8 7.4 0.48 Assopos basin Avlida 10 70 64 440 16 4 9.3 36 99.8 37 310 11 7.4 1.13 Oropos 3 61 57 120 23 3 3 38.5 55.3 15 109 1.6 7.5 0.59 Oropos As.K.W.900 850 130 32 4.5 8.8 40.2 140 60 408 2.5 7.7 1.42 1.16 ± 0.09 Avlona 2 72 66 36 0.8 2.3 1.6 64 72 6 28 0.8 7.4 0.41 1.01 ± 0.02 C. Evia 7 63 62 70 5.3 0.6 1.2 78 73 20 33.4 1.9 7.4 0.4 1.42 ± 0.37 C.Evia E7 260 250 40 7.3 0.7 2.2 64 13 44 32 2.4 7.34 0.28 0.98 ± 0.08 Water leachates (altered peridotites) C. Evia R2BA 35 36 124 3.2 1.7 <0.5 26 1.9 <1 3.4 0.7 8.1 0.56 ± 0.04 R2BB 64 63 98 0.2 <0.5 <0.5 26 6.1 <1 2.8 0.6 8.1 0.86 ± 0.06 R3 30 29 78 15 0.8 <0.5 25 2.9 <1 2.4 0.6 8.3 0.96 ± 0.06 Seawater Mediterranean 4 0.55 <1 4200 160 84 360 400 1200 1200 6380 485 1.13 ± 0.09 Assopos Estuary Ass.c. 0.26 <1 4500 190 88 360 392 1250 1260 6400 500 0.79 ± 0.14 Paciﬁc Ocean 6 0.3 0.72 ± 0.14 Atlantic Ocean 15 0.22 0.76 ± 0.24 Baltic sea 12 0.16 0.42 ± 0.20 Antarctica 3 0.29 0.57 ± 0.03 Geothermal water Aedipsos, Evia 9 n.d. 9500 1400 68 380 1080 310 480 9800 300 6.6 27 K. Vourla, C. Greece 5 n.d. 3400 380 29 145 540 350 200 45,800 110 6.1 9.0 Vendenheim, France 2 2.7 37,000 155,000 960 3450 62 180 27,400 3700 7.5 96

Present analytical data (Table1) combining those from previous studies (Table2) indicated that the best pronounced positive trend between B and Cr(VI) was recorded in C. Evia and Attica (Figure4a). Limited data from the neighboring Mediterranean seawater (Assopos river estuary and Evoic Gulf) were plotted in a separate ﬁeld characterized by much higher B and lower Cr(VI), comparable to those for geothermal water (Figure4a ). In addition, a good positive correlation between total dissolved solids (TDS) and B concentrations is a common feature of those water types (Figure4b). A negative trend between the mass ratio Ca/Mg and the Cr(VI) concentrations recorded in groundwater from the studied Neogene Basins was followed by geothermal water (Figure4c). In addition, a negative trend was clear between total organic carbon and Cr(VI) concentrations, for both lower and higher Cr(VI)concentrations of 50 µg/L in groundwater from the Assopos Basin (Figure4d). Minerals 2021, 11, x FOR PEER REVIEW 9 of 16

Minerals 2021, 11, 160 8 of 15 Minerals 2021, 11, x FOR PEER REVIEW 9 of 16

Figure 4. Plots of the Cr(VI) and total dissolved solids (TDS) versus B concentrations (a,b); Ca/Mg Figure 4. Plotsmass of the ratio Cr(VI) and andtotal total organic dissolved carbon solids (TOC) (TDS) versusversus Cr(VI)B concentrations concentrations (c (a,d,b) );for Ca/Mg coastal mass groundwa- ratio and total organic carbon (TOC) versus Cr(VI) concentrations (c,d) for coastal groundwater from Attica, C. Evia), the Assopos–Thiva ter from Attica, C. Evia), the Assopos–Thiva Basins, sea and geothermal water. Data from Tables 1 Basins, sea andand geothermal 2 [19,31–38,48,49]. water. Data from Tables1 and2[19,31–38,48,49]. Figure 4. Plots of the Cr(VI) and total dissolved solids (TDS) versus B concentrations (a,b); Ca/Mg mass ratio and total organic carbon (TOC) versus Cr(VI) concentrations (c,d) for coastal groundwa- 53 The plots of Theavailable plots stable of available chromium stable isotopes chromium [38,46] isotopesexpressed [38 as,46 δ53]Cr expressed values versus as δ Cr values ter from Attica,versus C. Evia),B and the Cr(VI) Assopos–Thiva concentrations Basins, for sea coastal and geothermal groundwater water. from Data the from C. Tables Evia, Assopos1 Basin andB and 2 [19,31–38,48,49]. Cr(VI) concentrations for coastal groundwater from the C. Evia, Assopos Basin and water leachatesand for water peridotites leachates showed for peridotites a negative showed trend (Figure a negative 5a). trendMoreover, (Figure plots5a). of Moreover, δ53Cr plots of δ53Cr values versus Cr(VI) concentrations for seawater from the Paciﬁc and Atlantic oceans, valuesThe versus plots Cr(VI) of available concentrations stable chromium for seawater isotopes from [38,46] the Pacific expressed and as At δlantic53Cr values oceans, versus and and Mediterranean and Baltic seas [43], showed a negative trend as well (Figure5b). BMediterranean and Cr(VI) concentrations and Baltic seas for [43], coastal show groued andwater negative from trend the as C.well Evia, (Figure Assopos 5b). Basin and water leachates for peridotites showed a negative trend (Figure 5a). Moreover, plots of δ53Cr values versus Cr(VI) concentrations for seawater from the Pacific and Atlantic oceans, and Mediterranean and Baltic seas [43], showed a negative trend as well (Figure 5b).

53 Figure 5. Plot ofFigure the δ 535.Cr Plot values of theversus δ CrB( valuesa), and versus Cr(VI) B ( (ab),) concentrationsand Cr(VI) (b) concentrations for coastal groundwater for coastal (GW) groundwa- from the C. Evia, Assopos–Thivater Basins, (GW) waterfrom the leachates C. Evia, for Assopos–Thiva rocks (serpentinized Basins, peridotites)water leachates and for soils, rocks and (serpentinized seawater (Mediterranean perido- Sea, tites) and soils, and seawater (Mediterranean Sea, Pacific and Atlantic oceans, and Baltic Sea). Data Paciﬁc and Atlantic oceans, and Baltic Sea). Data from refs [38,46,47,50]. from refs [38,46,47,50]. Figure 5. Plot of the δ53Cr values versus B (a), and Cr(VI) (b) concentrations for coastal groundwater (GW) from the C. Evia, Assopos–Thiva Basins, water leachates for rocks (serpentinized perido- 7. Discussion tites) and soils,7.1. and REE seawater and PGE (Med asiterranean Potential Sea, Contaminants Pacific and inAtlantic Groundwater oceans, and Baltic Sea). Data from refs [38,46,47,50]. Given that REEs have become contaminants in the environment, due to industrial activities [25,26], and hundreds of industrial plants are still active at the Assopos Basin, analyses of groundwater including REE are presented (Table1) in order to evaluate the level of REEs and potential environmental impact. Although the REE concentrations were lower

Minerals 2021, 11, 160 9 of 15

that the detection limit, using inductively coupled plasma–mass spectrometry, even the very low measured concentrations of La, Nd, Gd and Dy suggest that analysis using a more sensitive analytical method [51] is required to provide evidence for the REE behavior in that type of groundwater. With respect to PGE concentrations in groundwater, only Ru values (ranging from 0.06 to 0.30 µg/L) are often higher than the detection limit of the method (0.05 µg/L) (Table1), suggesting that Ru is relatively mobile in the Neogene unconﬁned aquifers. It has been shown that humic and fulvic acids are capable of taking the PGE into solution from sulphides and Platinum Group Minerals (PGM), The complete sequence of alteration of the PGM from the fresh rocks to the weathered rocks has been proposed [52]. However, the available thermodynamic data for aqueous organic or hydroxide complexes involving the PGE are limited, and their impact on natural hydrological processes, water quality, and ecosystem processes are still unexploded [53].

7.2. Interaction between Cr(III) with Major Seawater Ions in Coastal Groundwater

Among potential redox couples in natural aquatic environments are H2O/O2, Mn(II)/Mn(IV), − − N2/NO3, NH4/NO3, Fe(II)/Fe(III), HS /SO4 and CH4/CO2 [54,55]. In particular, H2O2, high- − valent Mn-oxides, and MnO4 may act as oxidants of Cr(III) [6,56,57] because their higher standard potentials favor that reaction [58]. Experimental data have shown that chromate adsorption by goethite is thermodynamically favorable through both inner- and outer-sphere surface complexation reactions [2]. These authors indicated that chromate adsorption decreases with increasing pH towards the zero-point-of-surface-charge (pHPZC 9.1) of the adsorbent surface, and ionic strength and temperature both affected outer-sphere surface complex. Experimental results have indicated that H2O2 becomes a reductant at low pH [55,59]:

− + 3+ 2HCrO4 + 3H2O2 + 8H → 2Cr + 8H2O + 3O2

However, under alkaline conditions, hydrogen peroxide is an efﬁcient oxidizing agent which can oxidize Cr(III) [59,60]:

− 2− 2Cr(OH)3 + 3H2O2 + 4OH → 2CrO4 + 8H2O

7.3. Salinity as a Common Feature of Geothermal and Coastal Groundwater The salinity, which is expressed by the total dissolved solids (TDS) and deduced from measurements of the electrical conductivity of groundwater, is a characteristic feature of sea and the geothermal water as well (Table2[ 23,48,49]). Geothermal water has been described in Greece, along the Hellenic Volcanic Arc, the Chalkidiki Peninsula, in Serres, many places in Thrace [61], in hot springs in the NW part of Evia (or Euboea) island, and in Central Greece [48], as well as in Hungary, Germany, France and many other countries [49,62]. In faulted regions, meteoric water can be circulated underground where it is heated by magma or hot rocks, and geothermal water ascends back to the surface; Na–Cl brines, with TDS values ranging from 99 to 107 g/L, pH values close to 5 and a reservoir rocks temperature value close to 225 ± 25 ◦C are well known in Germany [49,62]. According to these authors, their high Na, B, Li, Cl, and As concentrations may be related to processes of mixing between primary brines formed by advanced evaporation of seawater and meteoric freshwaters. Relatively low Cr concentrations (4.5 µg/L) and/or no detected concentrations in geothermal water [48,49,62] seems to be consistent with the assumption that hot brines have penetrated the granite basement below the sedimentary cover and the low pH from about 5, before cooling and degassing [49].

7.4. Decreasing Cr(III) Precipitation in the Presence of [B](OH)4]− Ions, a Potential Buffer It has been suggested that adsorption–desorption reactions may be the only significant mechanism influencing the fate of B in water, depending on the pH of the water and the B concentration in the solution, with the greatest adsorption being recorded at pH 7.5–9.0 [63]. Additionally, the dependence of the Cr sorption extent on the solution composition, which decreases from pure water to seawater, with no evidence for Cr(VI) sorption in seawater, Minerals 2021, 11, 160 10 of 15

is probably due to the competition between Cr(VI) and other anions for sorption sites [64]. – In addition, the B(OH)3 is acidic because of its reaction with OH from water, forming the − complex [B(OH)4] and releasing the corresponding proton left by the water auto-protolysis:

− + B(OH)3 + 2 H2O [B(OH)4] + [H3O] (2)

The pKa value (=−log(Ka) of boric acid has been determined to be pKa = 8.98 [59]. Due to a relatively high pKa, boric acid has limited dissociation at neutral or low pH values. Additionally, it has been emphasized [65] that borate may be the only ion which inhibits the Cr(VI) reduction to Cr(III) [66]. The Cr(VI) reduction/removal under alkaline conditions was investigation experimentally, applying the efﬁcient reduction of Cr(VI) by UV/sulﬁte process, and it was indicated that after the addition of borate buffer (pH = 9.2) the precipitation of Cr(III) was inhibited, due to the formation of a Cr(III)–borate complex [66]. Thus, the formation of the Cr(III)–borate complex tends to inhibit the precipitation of Cr(III) and facilitate the stability of Cr(VI), although under acidic conditions the Cr(VI) is effectively reduced into Cr(III) and is precipitated [67]. The negative relationship between the Ca/Mg mass ratio and Cr(VI) concentrations (Figure4d) may suggest that the reduction of the Cr(VI) to Cr(III) precipitation is facilitated by the presence of Ca ions [66].

7.5. The Cr Isotopes (δ53Cr Values) as an Evidence of the Cr Redox Processes The oxidation of Cr(III) to Cr(VI), and subsequent back-reduction of Cr(VI) processes, have been shown to produce signiﬁcant Cr isotope fractionation [68]. During reduction, the lighter isotopes are preferentially reduced, resulting in an enrichment of 53Cr relative to 52Cr values in the remaining Cr(VI) pools. This enrichment is measured as the change in the ratio of 53Cr/52Cr, and is expressed as δ53Cr values in units per mil (‰) relative to a standard [39,60]. Thus, during the transport of the mobilized Cr(VI) in various aquifers, low Cr(VI) concentrations and elevated positive δ53Cr values imply reductive processes, probably due to the presence of Fe(II) and/or organic matter [18,38,46,69]. The Cr isotope data for groundwater from the C. Evia and Assopos–Thiva Basins [38,46,70] imply oxidative mobilization of Cr(VI) from the ultramafic host rocks, and successive back- reduction of the Cr(VI), as is exemplified by the recorded range of the δ53Cr values between 0.8 and 1.99‰. In addition, a series of leaching experiments carried out for ultramafic rock (natural outcrops) from C. Evia, have shown δ53Cr values ranging between 0.56 and 0.96‰ for the rock leachates (Table2). Applying the available literature data concerning the redox reaction between Cr(III) and Cr(VI) and the Cr isotope fractionation during Cr redox reactions, the stability of Cr(VI) in contaminated coastal groundwater seems to be consistent with the oxidizing role of H2O2 in the presence of high B concentration in seawater (around 4.5 mg/L) [59]. Specifically, the potential Cr(III) oxidation by H2O2 in the presence of abundant − tetra-hydroxyborate [(B(OH)4] complex at pH > 7.5 in contaminated coastal groundwater may be reflected in the preferential reaction of heavier Cr isotopes, leaving the reactant enriched in isotopically light Cr (Figure4[ 60,71,72]). Thus, the positive trend between B and Cr(VI) recorded in coastal groundwater from C. Evia (Figure4a), and the negative δ53Cr–Cr(VI) and δ53Cr–B trends for coastal groundwater from C. Evia and Assopos Basin seawater (Figure 5a), may point to the role of borate in seawater on the stability of Cr(VI) in coastal groundwater from the cultivated C. Evia and Assopos–Thiva Basin. Therefore: (i) the low δ53Cr values, − high Cr(VI) concentrations and the dominance of [B(OH)4] ions, as a buffer, potentially imply oxidative and alkaline pH (pH > 7) conditions, and decreasing precipitation kinetics of Cr(III) [59,60]; and (ii) elevated δ53Cr values potentially imply reductive processes, probably due to the presence of organic matter (Figure4d), which serves various functional groups as electron donors for Cr(VI) reduction. In general, during the transport of the mobilized Cr(VI) in various aquifers, natural attenuation processes may be facilitated.

7.6. Implications of the Salinity Effect in Coastal Groundwater Contaminated by Cr(VI) According to the structural data on the Cr(III) and Cr(VI) forms and their soluble products [27], Cr(III) at pH ≥ 4 may form insoluble polymeric products (Figure1), while Minerals 2021, 11, 160 11 of 15

in the presence of organic matter, the formation of stable Cr(III) complexes with small organic molecules can increase their mobility and maintain the solubility of Cr(III) even at 2− neutral pH [27]. Additionally, the chromate anion (CrO4 ) is the predominant form of Cr(VI) in dilute solutions at neutral pH [27]. Under pH conditions 7.5–8.2 and Eh 0.36–0.41 V, reduction of the Cr(VI) to Cr(III) recorded at an aquifer past-contaminated by Cr(VI) (up to 4500 g/L), due to industrial activities in 1997, in the Friuli Venezia Giulia Region (N. Italy). Subsequently, it was naturally attenuated, and the contamination completely disappeared in 2003 [18]. However, the abundance of Cr(VI) in coastal groundwater (Table2), contaminated by Cr(VI) by either human activities and/or natural processes, is a potential risk, causing serious constraints to food production. The lack of the of seawater effects in groundwater in the case of Friuli Venezia Giulia Region, northern Italy, as exempliﬁed by the relatively low TDS and Na concentrations in those aquifers [73] and in turn in borate ions, may indicate that the reduction of Cr(VI) to Cr(III) is not inhibited, due to the absence of borate ions. Assuming that, in coastal groundwater, the Cr(VI) concentrations reaching values − of hundreds µg/L Cr(VI) (Table2) may be facilitated by the presence of the [B](OH) 4] ions, a potential buffer [66], efforts are needed toward the: (i) prevention and/or minimization of the contamination of aquifers by Cr(VI), which are inﬂuenced by the intrusion of seawater; (ii) prevention of coastal aquifers within the zone threatened by changes in sea level, due to climate change. The knowledge of the contamination sources, the presence of hotspots, the degree and extent of the groundwater and soil contamination, monitoring of water salinization processes, expressed by GIS (geographical information system) and risk analysis (mapping) for every coastal country would facilitate optimization of agricultural management strategies; (iii) protection from the consequence of the plant/crops irrigation with contaminated water and the bio-accumulation of toxic elements by plants, which in turn are transferred into food chain, an important threat for human health and ecosystems [74]. Efforts need to apply ways to decrease the seawater intrusion and/or the Cr(VI) concentration, such as the use of organic matter during its cultivation [75]. For example, the application of the natural organic material leonardite, which is an oxidized form of lignite, as a land management technique, seems to be a cost-effective method consistent to related protocols for the protection of the soil quality [75]. Europe accounts for roughly 40% of global lignite reserves; the top European producers include Germany, Greece, Poland, and the Czech Republic [76].

8. Conclusions The compilation of the present and available databases/literature on the composition of coastal groundwater, and the Cr mobility as a function of the salinity led us to the following conclusions: • Although the Cr concentrations in seawater are less than 0.5 µg/L, the Cr(VI) concentrations in coastal groundwater affected by the intrusion of seawater reached values of hundreds of µg/L; • There is a pronounced positive trend between B and Cr(VI) and a negative trend between δ53Cr values and B concentrations for coastal groundwater and seawater; • The negative relationship between the Ca/Mg mass ratio and Cr(VI) may suggest that Ca facilitates the Cr(VI) reduction and Cr(III) precipitation; • Despite the precipitation of Cr(III) under alkaline conditions, the potential formation of a Cr(III)–borate complex in contaminated coastal groundwater tends to inhibit the precipitation of Cr(III) and facilitate the stability of Cr(VI); • The abundance of Cr(VI) in coastal groundwater may be related to the presence − of [B](OH)4] ions, a potential buffer, which signiﬁcantly inhibit Cr(VI) reduction to Cr(III). Minerals 2021, 11, 160 12 of 15

Author Contributions: Conceptualization and methodology: M.E.-E., G.D.E. and I.-P.D.E.; software and validation of data: I.-P.D.E. and G.D.E.; writing—original draft preparation: M.E.-E., G.D.E. and I.-P.D.E. All authors have read and agreed to the published version of the manuscript. Funding: The Municipality of Oropos, Greece (Grand No AK 70/3/9997) and the University of Athens (Grant No KE 11078). Data Availability Statement: The study did not report any additional data of those included in the presented Tables and cited references. Acknowledgments: Many thanks are expressed once again to the University of Athens, the Mu- nicipalities of Oropos and of Dirfys-Messapia (Greece) for the financial support on the soil and groundwater contamination in Greece. Many thanks are due to the anonymous reviewers for their constructive criticism and suggestions to an earlier version of the paper. Conflicts of Interest: The authors declare no conflict of interest.

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