monitoring of seawater intrusion in a karst aquifer … · 2003. 12. 6. · terrestrial heat flow,...

First International Conference on Saltwater Intrusion and Coastal Aquifers— Monitoring, Modeling, and Management. Essaouira, Morocco, April 23–25, 2001

MONITORING OF SEAWATER INTRUSION IN A KARST AQUIFER

(APULIA - SOUTHERN ITALY)

M. Maggiore1, G. Raspa2, L.Sabatelli3, D.Santoro2, O.Santoro4, M. Vurro5

1 Università di Bari – Italia 2 Università di Roma – Italia 3 SEA s.c.r.l. Fasano (Brindisi) – Italia 4 Regione Puglia – Italia 5 C.N.R. – Istituto di Ricerca Sulle Acque (Bari) – Italia

ABSTRACT

Geological and hydrogeological investigations have been carried out in a

typical area of the murgian coastal aquifer located in Apulia (Southern Italy) between the town of Fasano (Brindisi district) and the Adriatic Sea.

The aquifer consists of platform cretaceous limestones covered with a thin layer of plio-quaternary calcarenites. The carbonate rocks is bedded, jointed and subject to karst phenomena. Freshwater lying on seawater flows along preferential pathways where rock permeability is greatest, depending on the highest fracturing degree. Moreover, it is evident the agreement between the mainly joints or faults and superficial karst channels, locally called “lame”.

The study area is about 40 km2 and the shape is almost square. Ten wells have been drilled to control seawater intrusion to the depth where calcareous layers contain brackish and salt water. Logs of Temperature, Salinity and Dissolved Oxygen have been carried out in these wells.

The obtained data have allowed to represent the distribution of recorded parameters, and particularly of temperature-salinity, along various cross-sections.

Using some multivariate geostatistical techniques the correlations among the values of the physico chemical parameters have been carried out and modelled. The model has been used to reconstruct and monitor seawater intrusion phenomena and the preferential pathways in the aquifer. INTRODUCTION

In Apulia, mesozoic carbonate rocks outcrop in very large areas and extend under the ground for about 5000 m [Ricchetti et al., 1988]. They are mainly constituted of platform limestones and they are layered, jointed and subject to karst phenomena.

The carbonate units define very important aquifers; the main hydrostructures are Gargano, Murge and Salento.

The aquifer’s base level corresponds to sea level and freshwater overlays saltwater. Next to the coast, seawater intrusion is exalted by groundwater over-exploitation, because of intensive agricultural use. The investigated area is located in a typical coastal zone of the Murge, between the town of Fasano (Brindisi district) and the Adriatic Sea (Fig.1).

The orography presents moderate elevations and gently slopes to the sea; the internal area is connected with the hills of Murge plateau.

Concerning soil use, the area is characterized mainly by agricultural activity: in particular, 40% is devoted to olive trees and 30% to vegetables. Groundwater monitoring as to seawater intrusion has been realized in an area of about 40 km2, representative of coastal murgian aquifer. The monitoring network consists of 46 wells, 4 coastal springs and a weather station.

The first investigation [Maggiore et al., 1999] has been performed by means of periodic hydraulic measurements and monitoring of physical and chemical parameters (pH, salinity, temperature, dissolved oxygen, RedOx potential).

The main aim of this paper are: - to report and to analyse the results of temperature and salinity measurements carried

out in ten wells, deep as far as 200 m. Figure 2 shows the monitoring network and the location of the ten investigated wells;

- to reconstruct these data using geostatistical techniques in a 3D domain [D'Agostino et al, 1997; Raspa, 2000]

Figure 1 - Location of the study area

GEOLOGICAL AND HYDROGEOGICAL FEATURES

Mesozoic limestones, referred to the Cretaceous age [Maggiore et al.,1978; Luperto Sinni&Borgomano,1989], outcrop in the western part of the study area (higher zone) and are covered of thin layer of quaternary calcarenites, in the lower part. This last unit outcrops with continuity from the Adriatic Sea to an altitude of 80 m above s.l. (Fig. 3).

Alluvial and colluvial deposits are located at the bottom of karst channels,

locally named “lame”. These channels are placed along a fracture system which develops perpendicularly to the coast line.

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Ω 6Ω 6Ω 6Ω 6

Ω 7Ω 7Ω 7Ω 7Ω 8Ω 8Ω 8Ω 8

Ω 10Ω 10Ω 10Ω 10Ω 17Ω 17Ω 17Ω 17

Ω 18Ω 18Ω 18Ω 18

Ω 49Ω 49Ω 49Ω 49 Ω 31Ω 31Ω 31Ω 31

Ω 3Ω 3Ω 3Ω 3

Ω 1Ω 1Ω 1Ω 1

Ω 2Ω 2Ω 2Ω 2Ω 4Ω 4Ω 4Ω 4

Ω 9Ω 9Ω 9Ω 9

Figure 2 – Sampling points network

Figure 3– Schematic geological map

Carbonatic sands Alluvial deposits Calcarenites Limestones Quarries Altitude (m a.s.l.) Antropogenic areas

Stratigraphic data from 10 boreholes and from several wells allowed to reconstruct the roof of calcareous substratum, the thickness of quaternary calcarenites and the main hydrogeological structures.

The limestones constitute a large aquifer; freshwater, overlying seawater, flows along preferential pathways where rock permeability is the greatest, depending on fracturing and karst processes. The most important discharge zone is represented by a group of coastal springs which are aligned along a distensive fault [Maggiore et al.,1999], as figure 4 and figure 5 show.

The aquifer is not confined; hydraulic head ranges from 0.5 m to 3.0 m (Fig.6). The piezometric surface puts in evidence the influence of the fracture system on groundwater flow.

Figure 4 – Schematic hydrostructural map

Figure 5 – Hydrogeological cross-section

Figure 7 shows hydraulic head versus time in two wells (W1, W17) located about 1 km far from the coast line and the rain distribution inside the study area. The trend of the values appears to be quite similar and it is well related to the rain distribution. During the monitored period, the variation of the piezometric head has been about 0.50 m and, in particular, the most significative increase has been evaluated in winter 1997, because of the intense and well distributed rain.

Groundwater salinity ranges from 2.0 g/l (inner areas) to 10.0 g/l (next to the coast). These values were measured on water samples, collected during pumping. Figure 8 shows some zones, along preferential flow pathways, where seawater intrusion is higher.

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Ω 4Ω 4Ω 4Ω 4Ω 2Ω 2Ω 2Ω 2

Ω 1Ω 1Ω 1Ω 1

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Ω 31Ω 31Ω 31Ω 31Ω 49Ω 49Ω 49Ω 49

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Ω 8Ω 8Ω 8Ω 8Ω 7Ω 7Ω 7Ω 7

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Ω 18Ω 18Ω 18Ω 18

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Figure 6 – Piezometric surface (m a.s.l.) – November 2000

Figure 7 – Piezometric levels (W17, W49) and rain during the monitored period

During the monitoring period, the salinity trend of the spring S1 is rather different

from the other coastal springs that have the same behaviour among them. Moreover, the trend of springs S2, S3, S4 is similar to the one of well 17. The

maximum value of salinity (15.7 g/l) was measured in spring S1 in September 2000. Figure 9 underlines the correlation between rain and salinity detected in the water belonging to the four coastal springs and to the well 17; the lowest salt content is recorded after a rainy period.

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10.0

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Ω 49Ω 49Ω 49Ω 49 Ω 31Ω 31Ω 31Ω 31

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Ω 1Ω 1Ω 1Ω 1

Ω 2Ω 2Ω 2Ω 2Ω 4Ω 4Ω 4Ω 4

Ω 9Ω 9Ω 9Ω 9

Figure 8 –Salinity contour lines (g/l) – November 2000

Figure 9 – Groundwater salinity and rain during monitored period

TEMPERATURE AND SALINITY STRATIGRAPHIES

Ten wells have been drilled in order to monitor groundwater Their depths range from about 100 m to about 200 m and get to limestone aquifer. Logs of physico-chemical parameters have been carried out in all the wells.

The stratigraphies of salinity explain that groundwater is contaminated by saltwater which has been recorded in the wells W1, W5, W10 (Fig. 10).The salinity values range from 0.5 g/l to 10.0 g/l in the shallower layers of groundwater and maximum values have been recorded in wells W5 and W7, according to the salinity distribution shown in figure 8. The vertical distributions of salinity show a stepped shape, related to local anisotropy of the aquifer. This is particularly evident in W5 and W10 (Fig.10). The thickness of transition zone has been measured in wells W1, W5 and W10, ranging from 10 m to 20 m. The top of the transition zone ranges from 70 m (near the costal line) to 80 m below sea level (inner areas). This evidence justifies the existence of several submarine springs [Accerboni&Mosetti,1969].

The values of temperature, ranging from 16,5 °C to 18,5 °C, are typical of groundwater located in the Adriatic part of Murge aquifer [Cotecchia et al.,1978]. The temperature logs show the influence of groundwater flow on the terrestrial heat flow (Fig.10). In particular, it is evident the role played by the movement of infiltration water (recharge zone) and by different velocity of water occurring in the aquifer layers that take to almost zero the geothermal gradient. In the upper zone of the water body, temperature gradients are very low (

GEOSTATISTICAL ANALYSIS Multivariate geostatistical techniques have been carried out in order to spatially

correlate in a 3D domain the data, obtained by logs. This analysis allows to define a global structure of the considered parameters.

The first step of such analysis is the evaluation of the experimental semivariograms in a 3D space exploration. The evaluation of experimental semivariograms was achieved by considering the main direction of the water flow, its perpendicular one and the vertical (Table 1).

Direction ϕ ϑ 1 90 150 2 90 60 3 0 0

The spatial variability of parameters along direction 3 (vertical) is clearly not

stationary, as parabolic behaviour of the experimental semivariograms underline (Fig.11). This not stationary spatial law is also observed along direction 2 (Fig.12).

To get more information among the parameters, a correlation analysis has been carried out: temperature appears to be correlated to salinity (R2=0.90) (Fig.13). In particular, there are three groups of measured points: the first regards the existing correlation in the freshwater zone, the second and the third ones underline different spatial correlation structures, referring to the transition and saltwater zone.

Table 1 – Polar coordinates of the considered directions

Figure 11 - Experimental semivariograms along direction 3 of temperature, salinity and dissolved oxygen, respectively

Figure 12 - Experimental semivariograms along direction 2 of temperature, salinity and dissolved oxygen, respectively

The modelling of the two correlated variables (T, S), moreover with two trends, should require a non stationary geostatistical approach. This kind of modeling is too heavy, if compared with a mere reconstruction of the physical and chemical parameters of the water body. Therefore, the stationary underlying semivariograms have been identified using a simple procedure consisting in selecting those variogram parameters which minimize the experimental cross-validation variances.

The resulting experimenthal semivariogram suggests two spatial structures: the former is anisotropic (zonal type), having the vertical as zonality direction and the latter is isotropic in the horizontal plane with a missing vertical component.

Due to the adopted variability model, the reconstruction of the three variables has been performed using kriging for Dissolved Oxygen and cokriging for Temperature and Salinity [Wackernagel,1995; Raspa,1999].

The geostatistical model and parameters which minimize cross-validation variances have been reported in Table 2.

Variable Vertical structure Horizontal structure Temperature spherical with a range of 7.5 m

without nugget effect Spherical with a range of 3200 m Without nugget effect

Dissolved Oxygen linear with nugget effect linear with nugget effect

Salinity spherical with a range of 7.5 m without nugget effect

Spherical with a range of 3200 m Without nugget effect

By means of geostatistical analysis, a numerical procedure has allowed to obtain a 3D space reconstruction of the three monitored parameters. These reconstructions have been reported in figures 14, 15, 16. The distributions of salinity, temperature and dissolved oxygen show a good accordance at different depths.

In particular, the salinity and temperature distributions (Figg.14,15) are able to detect two spatially orientated structures: the former, having NE-SW as direction, represents an intrusion preferential way for seawater; the latter, having W-E as direction,

Figure 13 – Scatter plot of Temperature vs. Salinity and Temperature vs. Dissolved Oxygen

Table 2. Spatial structures of the variability geostatistical model

puts in evidence a structure referred to the recharge and discharge local mechanisms of upper layers of the water body .

a

ash

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

W4

W3

W2

W1

2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

4524500.00

4525000.00

4525500.00

4526000.00

W10

W9

W8

W7

W6

W5

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2722500.00 2723500.00 2724500.00 2725500.00

4524000.00

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A ’

As regard the investigated deptre more evident in the snapshots colloc

Moreover, the comparison betw

nd 35 m b.s.l. (Fig.14) and the dynaimilar shape of the contour lines. Thiaving limited thickness, are also the m

Figure 14 – Salinity contour linesb.s.l.) and vertical distribution of salsections A-A’ and B-B’

A’ B

hs, one can observe how the salinization prated at 5 m and 75 m below sea level.

een the salinity distribution at the deepnesmic salinity distribution (Fig.8) underlines s evidence shows how the most productiveost exploited ones.

at different depth (5,15,25,35,45,55,65,7inity (from 0 to 75 m b.s.l.) along lines of cr

B

ocesses

s of 25 a quite levels,

5 m oss-

Furthermore, the geostatistical temperature reconstruction shows that the level collocated at 35 m b.s.l. is characterised by minimum gradient in the (x,y) plane. This fact, referring to the greatest mobility and mixing of the freshwater in such level, is also well indicated by the vertical cross-sections (Fig.15) [Tulipano&Fidelibus, 1989].

Dissolved oxygen seems to have a quite different behaviour from the other analysed parameters. As matter of fact, its distribution does not give us more information about the saltwater intrusion mechanisms (Fig.16). On the contrary, this natural tracer emphasises how the recharge and discharge processes, referring the spatial structure previously identified, are going on.

W10

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2722500.00 2723500.00 2724500.00 2725500.00

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2722500.00 2723500.00 2724500.00 2725500.00

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Figure 15 – Temperature contour lines at different depth (5,15,25,35,45,55,65,75m b.s.l.) and vertical distribution of salinity (from 0 to 75 m b.s.l.) along lines ofcross-sections A-A’ and B-B’

A A’ B B’

The analysis of dissolved oxygen distribution, referring to the depths ranging from 35 to 55 m b.s.l., confirms that more oxygenated water flows on those levels as the inversion of the dissolved oxygen gradient shows. CONCLUSIONS

Groundwater monitoring assumes a strategic importance for water resources protection as from seawater intrusion, as from anthropogenic pollution. These monitoring strategies are very relevant in regions, as Apulia, where water supply is constituted mainly by groundwater.

2722000.00 2723000.00 2724000.00 2725000.00

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Figure 16 – Dissolved oxygen contour lines at different depth(5,15,25,35,45,55,65,75 m b.s.l.) and vertical distribution of salinity (from 0 to 75m b.s.l.) along lines of cross-sections A-A’ and B-B’

A B’ A’ B

The data have allowed to represent the distribution of recorded parameters, and particularly of temperature-salinity, along various cross-sections.

The results of investigations show that several water layers, having different salt concentrations, characterise the aquifer and the seawater intrusion. They are related mainly to the development of joints and karst phenomena. The distributions of salinity values seem to be different from the expected ones and this depends on the fractured system.

The temperature-depth profiles show that geothermal heat is redistributed by mobility of water. The significant irregularities on the distribution of temperature depend on various factors: anisotropy of carbonate-rock aquifer; vertical groundwater movements; convective movements due to differences of density between fresh-water and sea-water.

Temperature measurements permit to obtain hydrologic information and to reveal the main structural features of the aquifer.

Using some multivariate geostatistical techniques the correlations among the values of the physico chemical parameters have been carried out and modelled. The preferential pathways of the aquifer are well reconstructed by various slices, both vertical and horizontal. Furthermore, this analysis allows to reconstruct the structure of the whole system in a 3D domain. The salinity distribution, as confirmed by geostatistical method, shows how the most productive levels, having limited thickness, are the more exploited ones.

This observation suggests to adopt appropriated strategies to monitor, to protect and to manage such water body. REFERENCES

Accerboni,E. and Mosetti,F., “Localizzazione dei deflussi di acqua dolce in mare; ricerche eseguite lungo la costa adriatica della Puglia,” Quaderni de “La Ricerca Scientifica”, 58, 213-225, 1969.

Cotecchia. V. , Tadolini, T. and Tulipano, L., “Groundwater temperature in the Murgia karst aquifer (Puglia-Southern Italy),” In: Int. Symp. On karst Hydrology, Budapest, 1978.

D'Agostino, V., Greene, E., Passarella,G. and Vurro,M., " Spatial and Temporal Study of Nitrate Concentration in Groundwater by Means of Coregionalization," Environmental Geology, 36,285-295,1997.

Domenico, P.A. and Palciauskas, V.V., “Theoretical Analysis of Forced Convective Heat Transfer in Regional Ground-Water Flow,” Geol. Soc. of Am. Bull., 84, 3803-3814,1973. Luperto Sinni, E. , Borgomano, J. , “Le Crétacé supérieur des Murges sud-orientales (Italie Méridionale) : stratigraphie et évolution des paléoenvironnements,» Riv.It.Paleont.Strat. ,95(2), 95-136, 1989. Maggiore, M., Ricchetti, G. and Walsh, N., ”Studi geologici e tecnici sulle pietre ornamentali della Puglia – il Filetto rosso ionico di Fasano,”Geologia Applicata e Idrogeologia,13, 335-345,1978. Maggiore, M., Sabatelli, L., Santoro, O., Tanzarella, V. and Vurro, M., ”Primi risultati della sperimentazione di un sistema di monitoraggio in acquifero carsico costiero,” Quaderni di Geologia Applicata, eds. Pitagora - Bologna, 3.245-3.254, 1999. Mongelli, F., Ciaranfi, N., Tramacere, A., Zito, G., Perugini, P., Squarci, P. and Taffi, L., “Contributo alla mappa del flusso geotermico in Italia: misure dalle Marche alla Puglia,”. In: 2nd Conv. Ann. Gruppo Naz. Geofisica della Terra Solida, Roma, 1983 Mongelli, F. and Pagliarulo,P., ”Influence of water recharge on heat transfer in a semi-infinite aquifer,” Geothermics, 26(3), 365-378, 1997.

Raspa, G., “Il ruolo della Geostatistica nella modellizzazione ambientale,” Acqua e Aria, 3, pp. 89-98, 2000. Ricchetti,G., Ciaranfi, N., Luperto Sinni, E., Mongelli, F. and Pieri, P., “Geodinamica ed evoluzione sedimentaria e tettonica dell’avampaese apulo,” Memorie della Società Geologica Italiana, 41, 57-82, 1988. Tulipano, L. and Fidelibus, M.D., “Temperature of groundwaters in coastal aquifers: some aspects concerning salt-water intrusion,” 10th SWIM Ghent (Belgium) 1988, Natuurwet.Tijdschr. ,70, pp.308-316, 1989. Wackernagel, H., “Multivariate geostatistic,” Berlin, pp.256,1995.

Keywords: coastal aquifer, saltwater intrusion, monitoring, geostatistic, temperature, dissolved oxygen

Corresponding author: Prof. Michele Maggiore, Dipartimento di Geologia e Geofisica, Università di Bari, Via Orabona - 4, 70125 Bari (Italy). Email: [email protected]

1 Università di Bari – Italia2 Università di Roma – Italia3 SEA s.c.r.l. Fasano (Brindisi) – ItaliaABSTRACT

GEOSTATISTICAL ANALYSISEmail: [email protected]

monitoring of seawater intrusion in a karst aquifer … · 2003. 12. 6. · terrestrial heat flow,...

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