Current conditions of saltwater intrusion in the coastal Rhodopeaquifer system, northeastern Greece
Christos Petalas*, Vassilios Pisinaras, Alexandra Gemitzi, Vassilios A. Tsihrintzis,Konstantinos Ouzounis
Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering,School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece
The geological, hydrogeological and hydrochemical regimes of the coastal Rhodope aquifer system, northeasternGreece, are described. The aquifer system includes two aquifers within coarse grained alluvial sediments. Bothvertical and lateral saline water intrusion occurs, usually caused by over pumping. Water has been pumped from theaquifer system at an ever-increasing rate for many years. Water samples for chemical analyses were obtained from36 productive wells and from 5 research wells at several depths to cover the entire study area. The EC and chlorideconcentration distribution clearly illustrate the large extent of saline water intrusion in the aquifer system of the studyarea. Although the ionic content of groundwater of the study area is highly variable, the dominant anions are HCO3
!
and Cl! and the dominant cations are Na+ and Ca2+. Water in the saline parts of the confined aquifer is generally ofthe Ca2+-Cl! type. Evidence of cation exchange and reverse cation reaction between fresh and saltwater in theRhodope aquifer system are reflected in the Piper diagram and the expanded Durov hydrochemical diagram,respectively, both for productive and research wells. The results of this study show that the development of a strategyfor managing the aquifer system is vitally necessary.
Keywords: Coastal aquifers; Hydrochemistry; Ion exhange; Salinization
1. Introduction
The intrusion of saltwater into coastal aquifersis a widespread phenomenon that gradually
*Corresponding author.
causes the problem of groundwater salinization.It is especially severe in Mediterranean regionswhere semiarid conditions lead to high-pumpingextraction rates and low recharge [1–3]. Saltwaterintrusion endangers future exploitation of coastal
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aquifers [4]. The problems associated with naturalsaline groundwater, saltwater intrusion, and up-coning of saltwater in pumping wells are of majorconcern in many coastal aquifers of Greece [5,6].Both vertical and lateral intrusion occurs, usuallycaused by over pumping. High salinity of waterresources severely affects most agricultural crops,seriously reduces yields and damages crops withlow tolerances to salt, while wells pumping waterwith salinity concentration levels that surpassdrinking water standards cease production.
Many of the fundamental flow and transportprocesses that take place when saltwater intrudes,flows and reacts with the aquifer matrix are stillpoorly understood, mainly due to the naturalhydraulic and chemical heterogeneity of thematrix formations [7,8]. These heterogeneities,which vary both spatially and temporally, andappear in virtually all aquifers, lead to prefer-ential pathways and non-uniform chemicalinteractions, which ultimately control ground-water quality.
In this study, the hydrogeological and hydro-chemical characteristics of the coastal aquifersystem of Rhodope are investigated, with the aimto contributing to the understanding of theprocesses which affect groundwater quality dueto over pumping that takes place in the area andto the lack of proper management practice.
2. Materials and methods
2.1. Study area description
The coastal aquifer system of Rhodope islocated in northeastern Greece, east of Vistonislagoon (Fig. 1). Salt water intrusion occurs in thisaquifer because of flows induced by extensivefreshwater withdrawals. Petalas [9] and Petalasand Diamantis [10] listed several saline sourcesthat affect water quality in the Rhodope coastalaquifer, which, however, may not be directlyrelated to saltwater intrusion. These include
among others, displacement of old salinegroundwater from the adjacent saline aquifer ofVistonis lagoon. Water has been pumped fromthe Rhodope aquifer system at an ever-increasingrate for many years. During the last 25 years,withdrawal has exceeded the maximum quantityof water that can be continuously with-drawnfrom the Rhodope aquifer system without adverseeffects [11]. The problem is intense particularlyin Glykonerion area adjacent to the inlet ofVistonis lagoon (Fig. 1).
The study area has a low relief and mostlygentle slopes. The regional annual precipitation,one of the sources of freshwater recharge of thecoastal aquifer, ranges from 329–844 mm. Theaverage annual precipitation is 628 mm. Mostprecipitation is lost through runoff and evapo-transpiration. The climate of the area is Mediter-ranean with hot and dry summers.
Withdrawal has exceeded the safe yield [9],particularly in the Glykonerion area adjacent tothe inlet of Vistonis lagoon (Fig. 1). Recently,water levels have declined at an annual rate of1–2 m, causing a reversal of the groundwatergradient. The lagoons of the wider area supportbrackish or saltwater, are in some cases hyper-saline, and indicate an environment that in recentyears was actively transgressed by the sea. Thelargest of the earlier mentioned lagoons, Vistonis,has brackish water, extends on the western part ofthe area, is in hydraulic connection with theaquifer system of the study area and it is believedthat it plays a significant role in the evolution(alteration/degradation) of the groundwaterquality.
The lithology of the aquifer sediments is verycomplex. The occurrence, movement, storage,and chemical character of groundwater in theregion is affected by numerous geologic factors,individually or in combination, which have to beevaluated in detail with respect to the hydro-geologic parameters of permeability and porosity,to have reliable results.
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Fig. 1. Location map of the study area, indicating groundwater level monitoring wells, groundwater quality monitoringwells and the five research wells.
2.2. Investigation of geological and hydro-geological conditions
A detailed investigation was conducted in thestudy area, based on the existing geological, andhydrogeological data, and on the construction offive fully-cased exploratory wells open only atthe lower confined aquifer. These wells werelocated along a WE direction adjacent to theVistonis lagoon inlet, and were used to collectwater samples at discrete points in the aquifer andto monitor groundwater level. The screens ofthose wells were installed only in the confinedaquifer of the study area and the upper semi-confined aquifer was sealed using cement. Auniformly distributed network of 18 productivewells was utilized along with the 5 newlyconstructed research wells for groundwater levelmonitoring. Groundwater level measurementswere made using a Solinst Model 101 water level
meter. Prior studies of the aquifer system of thestudy area include the work of Petalas [9], con-stituting a hydrogeological and hydrochemicalstudy of the wider area, part of which is the studyarea. Transmissivity, storage coefficient and hori-zontal hydraulic conductivity were estimated bymeans of 10 aquifer pumping tests [9].
2.3. Groundwater sampling and analysis —groundwater chemistry
In order to establish groundwater qualitycharacteristics, groundwater samples were col-lected on a monthly or bimonthly basis fromseveral depths in the research wells. Groundwatersamples were collected with a Solinst pointsource bailer. The design and construction of thebailer, in combination with the appropriate usage,assures that no contamination takes place. Thebailer is lowered to the desired sampling depth on
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a support line. As the bailer is lowered, both ballvalves open, allowing water to flow through thesampler. On reaching the sampling depth, thebailer is raised using the support line. The weightof water and upward movement of the bailer keepboth ball valves closed. The top ball valveprevents the sample in the bailer from mixingwith water at higher levels in the borehole.
Also, a regional hydrochemical survey wasperformed in July 2006, in the middle of thepumping period, when considerable groundwaterquantities were extracted. Water samples forchemical analyses were obtained from 36 care-fully selected productive wells to cover the entirestudy area.
The methods of sample collection and preser-vation used are specified in Title 40, Code ofFederal Regulations, Part 136 by the U.S. Envi-ronmental Protection Agency [12]. Water wasonly taken from boreholes that were pumped fora significant amount of time. Conductivity (EC),temperature (T) and pH were measured in thefield in order to acquire representative values ofambient aquifer conditions. Standard methodswere used for chemical analyses [13]. Na+, K+,Ca2+, Mg2+, NO3
!, and SO42! concentrations were
determined witn a Dionex ICS-3000 ion chroma-tography system. Alkalinity was determinedbased on the titration method (SM 2320) and Cl!
based on 4500-Cl-B method.Careful quality controls were undertaken to
obtain a reliable analytical dataset with an ionicbalance error less than 5% by checking blanksamples, calibration standards and duplicate ortriplicate sub-samples. The percentage recoveryof ionic species in standard samples was in the95–105% range.
In order to distinguish and group the ground-water of the study area into several chemicalgroundwater types, the expanded Durov diagram[14] and Piper diagram [15] were used, whichfacilitate interpretation of the evolutionary trendsand hydrochemical processes occurring in thegroundwater system.
3. Results and discussion
3.1. Geological, hydrogeological conditions andthe conceptual model of the aquifer system
The stratigraphic sequence in the study arearanges from the Eocene to the late Miocene age,and is composed mainly of interbedded marl, silt,clay, sandstone, and conglomerate. The aquifersystem consists mostly of upper Miocene alluvialdeposits. At the base of the sequence is a wide-spread thick clay of late Miocene age. Overlyingthe clay throughout most of the region is anextensive alluvial deposit laid down in a braided-river sedimentary environment. The most impor-tant feature in the paleogeographic evolution ofthe study area since the late Miocene time, is thepresence of a braided-river system in whichcoarse-grained sediments were deposited. Fine-grained sediment content (e.g., lacustrine depo-sits) is negligible (about 3–5%). Little of the claymineralogy of these sediments has been studied,and only in some cases where continuous drill-core was available, have clay minerals beendirectly identified. About 60–80% of the upperMiocene sediments overlying the braided riversystem (confined aquifer) in the study area areinterbedded sandstone, siltstone, and clay in bedsranging from a few millimeters to tens of metersthick.
The tectonic evolution of the study area isdirectly related to the tectonic uplift of theRhodope massif (of the Alpine age) and thedevelopment of the underlying Xanthi–Komotinisedimentary basin (of the Tertiary age). UpperMiocene beds were deposited in the eastern partof this basin. During the late Miocene time, pro-nounced rotational and compressional eventsresulted in general uplift of the region, accom-panied by differential subsidence along northwestand northeast-trending normal faults; this activityhas continued into recent time [16]. Several faultsin the study area are important controls on theoccurrence and movement of groundwater in thearea [9].
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A lower confined aquifer is present in coarse-grained alluvial sediments in the upper Miocenepart of the Rhodope alluvial system. An uppersemiconfined aquifer, which commonly containssaline groundwater, is present in alluvial sedi-ments (mainly sandstone) in a limited area alongthe coast. The semiconfined aquifer is separatedfrom the underlying confined aquifer by a thickclay unit. Three types of pumping wells occur inthe study area, namely, screened wells that areopen to either the confined or the semiconfinedaquifers, but not both, and screened wells open toboth the confined and semi-confined parts of theaquifer system. Well depths range from 50–140 m. Groundwater recharge to both aquifers isby infiltration of surface runoff of the streamscrossing the area, from direct percolation of rain-fall, from groundwater inflow along the westernand northeastern boundaries and from infiltrationof irrigation water. Most recharge occurs duringDecember–April. However, most of the annualprecipitation is lost through runoff and evapo-transpiration. Recharge from the semi-confinedaquifer into the underlying confined aquifer doesnot occur under natural conditions, but it mayoccur artificially because of leakage throughimproperly plugged boreholes. Estimated re-charge to the regional flow system within theupper Miocene clastic sediments averages about44 mm/y, based on a simulation of the flowsystem using a finite-difference computer model[9]. Isotopic analysis of the groundwater showsthat the surface runoff that recharges the upperMiocene aquifer system originates mostly fromareas about 200–400 m above sea level [9].
Pumping-test results show that despite itsheterogeneity the alluvial aquifer has a narrowrange of transmissivity and hydraulic conduc-tivity values [9]. The hydraulic conductivity ofthe upper Miocene aquifer system in the coastalarea ranges from 8.3×10!4 to 8.7×10!2 m/swhereas the storativity (S) ranges from 1.55×10!5
to 1.18×10!3 [9]. In the case of the semi-confinedaquifer, the respective hydraulic conductivity
ranges from 3.5×10!4 to 2.4×10!3 m/s and thestorage coefficient from 1.25×10!5 to 3.4×10!3.The pumping tests were performed only in theconfined aquifer, in fully penetrating wells withpiezometers as observation wells. The distancesof the piezometers from the pumping wells rangefrom 100–250 m.
A saline phreatic aquifer of the Quaternary agethat ranges in thickness from 80–120 m exists atthe inlet of Vistonis lagoon. This aquifer consistsmostly of semi-consolidated sandstone with thinintercalations of clay and siltstone, and it is inhydraulic communication with the Rhodopeaquifer system [9].
The above-mentioned aquifers are hydrau-lically connected with the saline phreatic aquifer[9] at the inlet of Vistonis lagoon. This phreaticaquifer is directly recharged from saltwater ofVistonis lagoon. Therefore, due to over pumping,saltwater from the phreatic aquifer intrudes inboth the semi-confined and confined aquifers,forming thus a dispersed interface between fresh-water and saltwater. The vertical relationsbetween those hydrogeologic units are shown inseveral generalized hydrogeologic sections pro-duced by interpolating data obtained during theresearch well construction (Fig. 2).
Long-term measurements of the water levelelevations in numerous wells [9] penetrating theupper Miocene aquifer system showed that waterlevel declined over the recent years by at least30 m due to intensive pumping.
Figs. 3 and 4 reveal that the hydraulic head isseriously reduced in the study area due to overpumping of groundwater and a permanentregional cone of depression has been formed inthe potentiometric surface of the aquifer system,western of Porpi. High hydraulic heads at thewestern boundary of the aquifer system show anincreased recharge of the upper Miocene aquifersystem. The aforementioned cone resulted inseawater intrusion into the aquifer system fromthe Southern boundary of the study area and insaltwater intrusion from Vistonis lagoon at the
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Fig. 2. Aquifer layer profile based on well logging from the five research wells.
Fig. 3. Groundwater level map on 30/3/2006 (above meansea level).
western boundary. Fig. 4 illustrates the ground-water level distribution in the study area on1/4/1993. Comparing with that of 30/3/2006(Fig. 3), it is obvious that groundwater level iscontinuously decreasing, leading to saltwaterintrusion.
Average groundwater inflows in the aquifer ofthe study area were about 12.4×106 m3 [9] and
Fig. 4. Groundwater level map on 1/4/1993 (above meansea level).
consist of groundwater recharge to both aquifers,mainly from the adjacent Kompsatos River fanaquifer, rainwater infiltration and irrigationreturns. Average groundwater outflows are attri-buted to pumping for irrigation and are estimatedat 13.8×106 m3. For this a safe yield groundwatervolume of about 1.4×106 m3 is proposed for theaquifer of the study area. This groundwater safe
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yield value would not maintain a sufficienthydraulic head to impede saltwater intrusion inthe aquifer but it represents a value which wouldmaintain current conditions in the aquifer withoutfurther quality deterioration.Numerous screenedwells drilled to both the confined and semi-confined parts of the aquifer system, exist in thestudy area. The depths of these wells range from50 to 140 m. Vertical recharge may locally occurin the form of leakage, and artificially throughimproperly plugged boreholes from the semi-confined aquifer downwards to the underlyingconfined aquifer because the lower aquifer sys-tem is overexploited in comparison to the uppersemi-confined aquifer.
Table 1 summarizes a statistical overview ofthe chemical analyses of groundwater samples.An electrical-conductivity value greater than1000 µS/cm was considered to be indicative ofsaltwater intrusion [9] and was used for mappingcontaminated areas. The electrical conductivity of
saline groundwater normally ranges from 1,010–14,300 µS/cm. The electrical conductivity ingroundwater samples from the freshwater well-fields (confined aquifer) ranges from 554–940 mS/cm. The pH of fresh groundwater in thestudy area normally ranges from 7.47–7.95 (meanvalue 7.69); pH of saline groundwater rangesfrom 6.77–8.12 (mean value 7.30). Groundwatertemperature ranges from 18.4–24.6EC, reflectinga higher than normal geothermal gradient (greaterthan 1EC per 33 m of depth).
The EC (Fig. 5) and chloride concentration(Fig. 6) distributions clearly illustrate the salt-water intrusion in the aquifer system of the studyarea. The assumption that the main point of entryof saltwater into the Rhodope aquifer system isthe inlet of Vistonis lagoon seems to be validbecause of the fact that, in the western part ofthe study area, EC values were up to14,300 µS/cm and chloride concentrations wereup to 5,158 mg/L.
The concentrations of various ions in ground-water at the sampling sites are highly variable;however, many similarities also exist. Althoughthe ionic content of groundwater of the study areais highly variable, the dominant anions are HCO3
!
Table 1Descriptive statistics from chemical analyses of 36 groundwater samples
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Fig. 5.Distribution of EC (µS/cm) in the aquifer systemof the study area in July 2006.
and Cl! and the dominant cations are Na+ andCa2+. Water in the saline parts of the confinedaquifer is generally the Ca2+–Cl! type. The SO4
2!
concentration is normally low. The intrusion ofsaline water is reflected in the high ionic contentof groundwater [17].
Most chemical evolution of subsurface wateroccurs in the unsaturated zone, between the landsurface and the water table [18]. Dissolution ofcalcite is the main source of HCO3
! in ground-water. Cation exchange of Ca2+ for Na+ on Na-montmorillonitic clay results when excess Ca2+
from calcite dissolution is available in infiltratingwater [9]. Exchange of Ca2+ for Na+ results in anincrease in dissolved Na+ and further dissolutionof calcite. Through the mechanism of cationexchange, high concentrations of Na+ are gene-rated in groundwater. High Na+–HCO3
! water atdepth in the study area is largely a result of theprocesses of sulfate reduction in the presence ofcoal beds [9]. In the coal-bearing strata of theconfined aquifer, where recharge is throughpredominantly clayey and silty flood-basin sedi-
Fig. 6. Distribution of Cl (mg/L) in the aquifer system ofthe study area in July 2006.
ments, Na+ is released from montmorilloniticclays. Dissolution of gypsum, a common mineralin those sediments, results in additional Na+–Ca2+
exchange. Where sand and silty sand are pre-dominant in the recharge area, the dominant ionsin solution are Ca2+ and HCO3
! [9]. The dominantclay mineral of the overlying confined-aquifersediments in the study area is sodium mont-morillonite.
Evidence of cation exchange and reversecation reaction between fresh and saltwater in theRhodope aquifer system is reflected in the Piperdiagram (Fig. 7) and the expanded Durov dia-gram (Fig. 8), respectively. Use of these data faci-litates interpretation of the evolutionary trendsand hydrochemical processes occurring in thegroundwater system. The significance of each ofthe nine fields on the expanded Durov diagram isas follows: Field 1 indicates recharging waters;fields 2 and 3 indicate ion exchange; fields 4 and5 indicate waters exhibiting simple dissolution ormixing; field 6 indicates probable mixing influ-ences; fields 7 and 8 indicate that groundwaters
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Fig. 7. Piper diagram of groundwater samples obtained from productive wells in the study area.
may be related to reverse ion exchange; and field9 indicates end-point waters [19]. Ion exchangebetween aquifer matrix and intruding saltwatermixing between freshwater composition and sea-water composition, and local simple mixing ofintruded saltwater with freshwater along bore-holes are reflected by the alignment of samplesparallel to the arrows a1, a2, and a3, respectively,in the diamond shaped field of the Piper diagram[20], as it is shown in the small diamond-shapedfield of Fig. 7. The expanded Durov diagram, incontrast to the Piper diagram, better displayshydrochemical types and some processes [19].The arrows in Fig. 7 indicate possible processpaths, such as ion exchange or dissolution.
The freshwater and saltwater of the coastalplain undergo chemical processes that result invarious identifiable hydrochemical facies. Fresh-
water generally is low in dissolved solids and hasno dominant anions or cations. When saltwaterintrudes into a freshwater aquifer, an exchange ofcations occurs and Na+ is taken up by theexchanger (clay), and Ca2+ is released; thus waterquality changes from NaCl-rich to CaCl2-richwater [21].
The exchange of calcium and sodium betweenaquifer rock matrix and intruding saltwater is welldisplayed in the Piper diagram (Fig. 7). This pro-cess results in an increase in calcium and adecrease in sodium in the groundwater in both theconfined and semiconfined aquifers. In a fewlocalities in both aquifers, sodium-saturatedconditions exist. Mixing of saline and freshwateris also suggested by the alignment of severalsamples parallel to the Ca+Mg axis in the dia-gram. Groundwater samples plotted in the field of
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Fig. 8. Expanded Durov diagram of groundwater samples obtained from productive wells in the study area.
Piper diagram (Fig. 7) where Na+K as well asSO4+Cl range from 0–50% indicates freshwatersamples from the recharge area located at thenortheastern part of the study area.
As shown in Fig. 8, most groundwatersamples fall in fields 7 and 8; in field 7, Cl! andCa2+ are dominant, and in field 8, Cl! is dominantor no dominant cation is evident. These analysesmay reflect reverse ion exchange of Na+–Cl!
waters. Reverse ion exchange, in which Na+
reduces preferentially for Ca2+, is a common
feature of waters on the fringe of saline intrusion[22]. Sample 36 located in field 9, in which Na+
and Cl! are dominant, represents endpoint water.Samples 32 and 37, located in the fifth field,indicate waters exhibiting simple dissolution ormixing and possibly reverse ion exchange.Generally, the samples can be classified in twomajor groundwater groups. The first groupincludes 7 groundwater samples of the hydro-chemical types Na+–HCO3
!, Ca2+–HCO3!, whereas
the second group, including all the other samples,
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has the hydrochemical types Na+–Cl!, and Ca2+–Cl!. Both groups show ion exchange [22]. Thepresence of Ca2+–Cl! and Na+–HCO3
! hydro-chemical types indicates salinization and remedi-ation of the groundwater quality phenomena thatmay be attributed to ion exchange processesbetween the ions Ca2+, Mg2+ and Na+ [10,21]. TheDurov diagram (Fig. 8) clearly indicates that bothaforementioned competitive processes take placesimultaneously. Thus, the salinization process iscertified in groundwater samples from wells,located in the southern and south-western coastalzone, while the inverse phenomenon takes placein the central and northern area because of thepresence of recharging waters, and consequentlythese constitute recharge areas.
Significant nitrate concentrations were mea-sured in the aquifer of the study area. Mean valueis almost equal to the upper recommended con-centration value of 25 mg/L proposed by [23].The nitrate concentration of 14 samples washigher than 25 mg/L and two of them exceededthe maximum allowed nitrate concentration of50 mg/L. Agricultural practices, including the useof fertilizers containing nitrogen and cattle feed-ing operations, and sewage from septic tanksystems are the important sources for this kind ofcontamination [24].
3.2.2. Research wells
Groundwater sample analyses obtained fromresearch wells at several depths showed a widerange of almost all measured parameters. The firstresearch well (DUTH1) located close to the inletof Vistonis lagoon showed a wide range in pH,EC values and Na+, K+, Ca2+, Mg2+ and Cl!
concentrations. EC values ranged between 1,420and 27,400 µS/cm. The EC profile (Fig. 9) clearlydisplays the intrusion of saltwater from the inletof Vistonis lagoon as EC increases from1410 µS/cm at a depth of about !0.5 m amsl to23,000 µS/cm at a depth of about !58 m amsl andreaches the maximum value at !88 m amsl
(28,900 µS/cm). The temporal pattern of ECdistribution at the first research well shows atrend to increase from 30/3/2007 to 7/6/2007especially in the lower depths due to groundwaterlevel decrease from pumping. The same depthand temporal pattern is observed for Na+ and Cl!
(Figs. 10, 11). The values of pH ranged between6.61 and 8.44 with a trend to decrease as depthincreases (Fig. 12).
The second research well (DUTH2) showedlower salt content in relation to DUTH1 well forthe same depths but is still high as EC reached20,100 µS/cm. Apart from pH (Fig. 12), all theother measured parameters, including EC, Na+,K+, Ca2+, Mg2+ and Cl! concentrations, increasedas depth increased (Figs. 9–11). The temporalpattern of EC distribution at the first researchwell shows a trend to increase from 30/3/2007 to10/7/2007, especially in the lower depths. Thesame depth and temporal pattern is observed forNa+ and Cl! (Figs. 10, 11).
The third research well (DUTH3) showedlower salt content in relation to DUTH1 andDUTH3 wells for the same depths, but salt con-tent is still high as EC reached 17,800 µS/cm.Apart from pH (Fig. 12), all the other measuredparameters, including EC, Na+, K+, Ca2+, Mg2+
and Cl! concentrations, increased as depthincreased (Figs. 9–11). The temporal pattern ofEC distribution at the first research well shows awider range on 30/3/2007 than on 1/5/2007,especially in the lower depths. The same depthand temporal pattern is observed for Na+ and Cl!
(Figs. 10, 11).One sample was taken from the fourth
research well (DUTH4) at !88 m amsl, exceptfrom 10/7/2007 where one more sample wastaken from groundwater level depth. Salt contentwas significantly decreased compared to DUTH1,DUTH2 and DUTH3 results, as EC rangedbetween 1,750 and 2,650 µS/cm, Na+ concen-tration ranged between 116.2 and 160 mg/L, Cl!
concentration ranged between 431 and 826 mg/Land Ca2+ concentration ranged between 184.8 and
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Fig. 9. EC (µS/cm) profiles of the research wells. Dots correspond to the sampling points several depths in each well.
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Fig. 10. Na+ (mg/L) profiles of the research wells. Dots correspond to the sampling points several depths in each well.
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Fig. 11. Cl! (mg/L) profiles of the research wells. Dots correspond to the sampling points several depths in each well.
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Fig. 12. pH profiles of the research wells. The dots correspond to the sampling points several depths in each well.
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Fig. 13. Piper diagram for research well data.
288.9 mg/L. On the contrary to DUTH1, DUTH2and DUTH3, EC and Na+, Ca2+, Mg2+ and Cl-
concentrations decreased from 30/3/2007 to7/6/2007 and then increased again.
The chemical analysis results for samplesobtained from the fifth research well (DUTH5)showed that the saltwater intrusion did not reachthat part of the study area, as EC values rangedbetween 520 and 744 µS/cm. Although EC andmost of ion concentrations increased with depth,the salt content level was still low (Figs. 9–11).
The exchange of calcium and sodium betweenthe aquifer rock matrix and intruding saltwater iswell displayed in the Piper diagram of theresearch well groundwater samples (Fig. 13).The rich in sodium groundwater samples ofDUTH1 well become rich in calcium in DUTH2,DUTH3 and DUTH4 wells with a decreasingionic content as one moves to the east. Ground-water samples of DUTH5 well are located in thefreshwater fields of the Piper diagram, sincesamples with Na+K as well as SO4+Cl ranging
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Fig. 14. Expanded Durov diagram for research well data.
from 0–50% indicates samples from the fresh-water fields or from the recharge area located atthe northeastern part of the study area. Mixing ofsaline and freshwater is suggested by the align-ment of DUTH1, DUTH2 and DUTH3 ground-water samples parallel to the Ca+Mg axis in thediagram.
The expanded Durov diagram is also illus-trating the groundwater salinization processes inthe study area (Fig. 14). DUTH1 groundwatersamples are located in the 9th field, reflectingendpoint water where Na+ and Cl! are the domi-nant ions. This reflects saltwater intrusion fromthe Vistonis inlet. DUTH2, DUTH3 and DUTH4samples are located in the 7th and 8th fieldswhere Cl! and Ca2+ are the dominant ions,
indicating reverse ion exchange, in which Na+
reduces preferentially for Ca2+ [22]. The hydro-chemical composition of DUTH5 groundwatersamples is completely different, as they arelocated in the 3rd or 6th fields of the expandedDurov diagram where ion exchange processestake place.
4. Management plans
The optimum economic and environmentallyeffective development of the Rhodope coastalaquifer system requires an integrated approach inorder to maintain sustainability. The absence of amanagement plan is obvious in the study area aswater users, and particularly the farmers, install
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new illicit productive wells without any priorenvironmental impact assessment, at regionsclose to the coastline where the saltwater entersthe mainland.
Based on the results of the field hydrogeo-logical and hydrochemical investigation, themajor problems are the over pumping resulting insaltwater intrusion and the pollution from anthro-pogenic sources. Therefore, proper managementof this aquifer is needed. Furthermore, therelatively recently agreed EU Water FrameworkDirective (2000/60/EC) requires that the preser-vation of “good” quality water be achievedthrough adoption of river basin managementplans, including groundwater. Best ManagementPractices (BMPs), defined by the US Environ-mental Protection Agency as “methods, measuresor practices implemented to meet non-pointsource control needs” [25] have received con-siderable attention over the last years both inUSA and in Europe [26], and have been widelyproposed and applied in watersheds to reducenon-point source pollution. The southeasternRhodope aquifer management authorities have toapply BMPs adapted to the local environmentaland socioeconomic conditions of the area.
BMPs should be applied urgently to the entirestudy area in order to control over pumpingthrough groundwater saving techniques. Also, agroundwater quantity management plan is parti-cularly necessary in drought periods, such as thatof 1981–1993. These BMPs may include [27–30]: (1) the increase of use of drip irrigationsystems; (2) the increase of water use efficiencyin irrigation networks by installing closed pipe-lines for water distribution; and (3) the use ofmulch and tillage techniques, as a water savingpractice at the farm level. When the appropriatewater saving techniques are applied, the mostimportant BMP is the establishment of properirrigation scheduling, based on weather forecastand timely measurements or estimations of soilmoisture content and crop water needs. A numberof devices, techniques and computer aides are
available to assist in determining when water isneeded and how much is required. Effectivescheduling requires knowledge of: (1) soil water-holding capacity; (2) current available soil mois-ture content; (3) crop water use or evapotrans-piration (ET); (4) crop sensitivity to moisturestress at current growth stage; (5) irrigation andeffective rainfall received; (6) availability ofwater supply; (7) length of time it takes to irrigatea particular field. Training of young farmers inirrigation management techniques is also helpfulin water savings and nutrient pollution reduction.Also, the application of artificial recharge pro-jects in the study area could help to interceptgroundwater salinization. Finally, modelling ofthe aquifer system and an optimization schemecould also help in order to locate appropriateplaces for well construction and maintain theoptimum pumping rate.
The groundwater chemical analyses indicatedthat pollution levels of the local aquifer systemhave been increased over the last years, withseveral pollutants such as NO3
! exceeding therecommended drinking water limits. Based onthese findings, non-point pollution sources of thestudy area, which include mainly fertilizers andagricultural waste disposal, should be controlledby the local authorities with BMP application. Itis essential to manage nitrogen application, tomaximize crop growth and economic return,while protecting water quality.
Proper fertilizer type, rates, and timing are thekey factors to maximize plant uptake and mini-mize losses. The following are a few of the manyBMPs for N fertilizer use [27–30]: (1) sample soilto a minimum depth of about 0.6 m, and pre-ferably to the effective rooting depth, to deter-mine residual NO3
!; (2) establish yield goals foreach field based upon the previous five-yearaverage, plus a modest increase of about 5%;(3) credit all sources of available N toward cropN requirements, that is, organic matter and pre-vious crop residues, irrigation water nitrate, soilnitrate and manure; (4) use slow release N ferti-
C. Petalas et al. / Desalination 237 (2009) 22–4140
lizers and nitrification inhibitors as appropriate;(5) split N fertilizer into as many applications aseconomically and agronomical feasible; and(6) avoid fall application of N fertilizers, especi-ally on sandy soils which exists in some harvestfields in the study area. Managing irrigation is notonly a water saving technique, but also helps tominimize transport of chemicals, nutrients, orsediments from the soil surface or the immediatecrop root zone. Irrigation water is the majortransport mechanism for contaminants such asnitrate. Beneficial use of irrigation water implieskeeping it free from pollutants that might affectquality. Methods to help minimize groundwatercontamination due to deep percolation of waterinclude: (1) scheduled irrigation as describedabove; (2) improved application efficiency byirrigation equipment upgrade; (3) timing of theleaching of soluble salts to coincide with periodsof low residual soil nitrate; (4) reduction ofirrigation rates to ensure that no runoff or deeppercolation occurs during chemical application.
5. Conclusions
Groundwater salinization is one of the mostcommon environmental problems in Greece, anddefinitely the most serious groundwater qualitydegradation factor of coastal aquifers. The over-exploitation of the Rhodope coastal aquifersystem for the satisfaction of anthropogenic needsand especially for irrigation has resulted indramatic decline of groundwater level and thus insaltwater intrusion mainly from the inlet ofVistonis lagoon. The hydraulic head is seriouslyreduced and a permanent regional cone of depres-sion has been formed in the potentiometricsurface of the aquifer system, western of Porpi.High hydraulic heads at the western boundary ofthe aquifer system show an increased recharge ofthe upper Miocene aquifer system.
Additional field information and well monitor-ing, which is gained from the research wells, are
needed to model the behaviour of the zone ofsaltwater intrusion more satisfactorily and toestimate the rate of movement of the fresh-water/saltwater interface. Use of data gained fromthe construction of the five research wellsresulted in a well defined conceptual approach ofthe Rhodope aquifer system.
The horizontal distribution of EC and chlorideconcentration in groundwater samples from pro-ductive wells, as well as the vertical distributionof groundwater samples from the five researchwells, illustrates clearly the saltwater intrusion inthe aquifer system of the study area. The assump-tion that the main point of entry of saltwater intothe Rhodope aquifer system is the inlet of theVistonis lagoon seems valid because of the factthat, in the productive wells of the western part ofthe study area and at the first research well, ECand chloride concentration values were signi-ficantly higher.
Evidence of cation exchange and reversecation reaction between fresh and saltwater in theRhodope aquifer system are reflected in the Piperand expanded Durov diagrams, respectively, con-structed for both productive and research wellgroundwater samples.
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