Engineering Geology for Developing Countries - Proceedings of 9th Congress of the International Association for Engineering Geology and theEnvironment. Durban, South Africa, 16 - 20 September 2002 - J. L. van Rooy and C. A. Jermy, editors

Géologie de l'Ingénieur dans les Pays en voie de Développement - Comptes-rendus du 9ème Congrès de L'Association Internationale de Géologie del'Ingénieur et de l'Environnement. Durban, Afrique du Sud, 16 - 20 septembre 2002 - J. L. van Rooy and C. A. Jermy, éditeurs

ISBN No. 0-620-28559-1

HYDROGEOLOGICAL CHARACTERIZATION OF MAIELLACARBONATE AQUIFER BY THERMOMETRIC METHOD

L. Tulipano1, G. Sappa1 and G. Pietrelli1

ABSTRACT: The Maiella massif rises in the Region of Abruzzo in central Italy. Its approximatemeridians are elongated in form.

The hydrogeological interest regarding the technical and scientific nature of the study of this carbonateaquifer is essentially due to the fact that it is the heart of an important subsurface water basin which is thesource of a total average flow of 7 m3/s. This is clearly a water resource that constitutes an environmentalpatrimony that, because of its consistency, should be appropriately managed and protected. In the secondplace, and not of lesser importance, the Maiella is the only significant relief in central-southern Italy that hasnot a face on Tirreno sea. Within the research study being carried out, the results of the application oftemperature as a natural, physical tracer of the subsurface water circulation to this important hydrogeologicalunit are presented in this report. The model that has been derived from this study is presented in thefollowing pages. It points out how the use of this tracing method has considerable potential in contributing tothe comprehension of the running of hydrogeological systems. This work actually shows the results of thetemperature-measuring processes performed and the innovative indications that this seems to supplyconcerning the running of this complex, hydrogeological system.

RÉSUMÉ: Le massif de la Maiella, situé dans la région de l’Italie centrale, les Abruzzes, est le siège d’unimportant bassin hydrographique souterrain d’òu jaillit un debit moyen supérieur a 7 m3/s. Dans cette étudeon a presenté les résultats de l’application de la température comme un tracage phisìque naturel de lacirculation hydrographique souterraine à cette importante unité hydrogeologique.

Le modèle qui en derive, presenté dans le pages suivantes, met en évidence comme l’emploi de cetteméthodologie de tracage a de remarquables capacités et contribute la compréhension du fonctionnament dessystémes hydrogéologiques.

GEOLOGICAL AND HYDROGEOLOGICAL FRAMEWORK

With an elongated form in its approximate meridians, the Maiella extends for about 420 Km2 and isseparated from the broad, deep Caramanic gorge by the M. Morrone massif in the West. In this versant, theMaiella presents a vast, uniform face, with very steep gradients that originate from the highest reliefs, suchas Mount Amaro (2795 m asl), Mount Acquaviva (2787 m asl) and Pesco Falcone (2646 m asl). Theappearance of the other versant is quite different: first of all, the slope is very inferior; then, from the crest,deeply creviced, uniform valleys fall away in radial lines (figure 1).

The most evident feature of the succession of rocks that currently constitute the powerful complex of theMaiella is the stratification (Fraternali G., 1996-1995).

The depositional units of the Maiella have been dominated by marine systems in which the bathymetryhas varied from a minimum of 0m (conditions of emersion) up to several hundred meters (pelagicconditions). The transition between these two conditions is performed by means of more or less steep slopesthat have developed in an east-west direction. During the upper Cretaceous period, the separation of theMaiella area from emerged reliefs able to supply terrigenous sediments, the tropical and subtropical climateand the presence of persistent, subarea conditions favoured the formation of reddish, paleosoil thicknesses

1. Dept. Of Hydraulics, Transportations and Roads – University “La Sapienza”

known as bauxite which are present today in the central-southern sector of the Maiella (Crostella A.,Lanzavecchia S., 1966).

Table 1. Principle springs with height and discharge

Spring Height (m asl) Aver. Ht (l/s)Lavino 150 1200Val di firo 285 700Gruppo del Verde 415 3500Acque Vive 450 900

Moving to the North and West, laterally to continental conditions, low-water marine environmentsdevelop. These ambients are also characterised by extensive lagoons, whose predominant deposits were sandand calcareous ooze, composed of microscopic particles of calcium carbonate. The continuation ofconditions favourable to the deposition of this kind of sediment produced the formation of carbonateplatforms with thicknesses of several hundred meters and extending over an area several tens of squarekilometers. Today these deposits constitute a large part of the reliefs of Cima delle Murelle as well as thoseof Mt. Amaro. Further N, there has been a transition from low-sea conditions with limited circulation to thatof seaconditions, that are still low but with open circulation. The latter conditions have been characterised bystrong marine currents that have carried large quantities of nutritional resources in the form of microscopicorganisms to the platform, thus favouring the development of building organisms (Coral, Rudista,Echinoderms, Bryozoa, etc.) that gather in colonies and form so-called organogenic barriers or reefs. The

Figure1. Layout Arcview with TIN 3D of the area with thermopluviometric stations,springs divided into flow classes and National Park borders.

organogenic barriers or reefs extend along strips that are a few kilometers wide and several tens ofkilometers long. (Crescenti U., 1969) This results in an external limitation of low-sea environments, herebymarking the beginning of very deep environments. The latter environments, assimilable at more or less deep,extended slopes, are characterised by the presence of deposits composed of flows of detrital material in thinlayers composed of planktonic microrganisms. Conditions in deep environments have also favoured thebuild-up of microscopic particles of silice scattered in water, which form centimeter levels of varicolouredchert.

These conditions lasted throughout the Cretaceous period and up to the beginning of the Palaeogeneperiod, resulting in a levelling off of the topographic differences. During most of the Palaeocene period, theentire area was covered by an extensive, sandy blanket. At the end of the Palaeocene period, the areas

Figure2. “Lithology” View reproducing the geology of the area, completely redesigned on aGIS Arcview layout as a polygonal shape

previously characterised by sedimentation were subjected to a sudden, vast emersion (Parotto M. andPraturlon A., 1975).

Starting from the Oligocene period, the area of the Maiella was again characterised by marinesedimentation, although only during the Miocene period did this extend to areas previously above sea level.The end of the Miocene period marked the beginning of another phase of profound geological changes thatconcerned the entire Apennines. This period witnessed the “uprooting” and upheaval of the carbonate plateof the internal massifs of Latium and Abruzzo that began advancing towards the East, hereby causing evidentmodifications in the paleogeography of the entire zone (Antonucci A., Bevilacqua E. , 1994).

In the Maiella area, two markedly different environments were created this way: the first, that borders thepresent mountain to the N, E and S, is defined by evaporitic features with gypsums and argillaceousformations; the second, that borders the present western versant of the Mariella along the whole Caramanicodepression, is defined by much deeper bathymetric features.

The Pliocene period witnessed the “birth” of the present configuration of the Maiella. The entire area roseabove sea level until it reached its present dome-shaped form. It is composed of an anticline restricted on thewest by a majestic fault that runs to the eastern border of the Caramanico depression.From a structural point of view, the general lines of the Maiella constitute an anticline with a vast, nucleuscropping out that is composed of prevalently large-scale, permeable, carbonate formations: Jurassic-Cretaceous in the central-southern part of the Altofondo in Abruzzo, and Cretaceous-Palaeogenic in thenorthern part of the transition toward the Umbrian basin and in the south-eastern part toward the basin inMolise (Bonarelli G., 1951).

This structure along the entire border of the Maiella then dips 10°-20° towards N-W, N, E, S-E below theimpermeable, marly formations of the Miocene period, overlapping the underlying formations in regular,stratigraphic succession. On the N, N-E, E and, partially, S-E versants, clays from the Pliocene periodoverlap each other in transgression to the anticlinal structure of the Maiella, integrating and completing theimpermeable, peripheral belt formed by the clay-marly formations from the Miocene period. (figure 2)

The mountain group of the Maiella appears to be hydrogeologically well-marked; therefore, it can beconsidered a distinct, clearly defined hydrogeological unit, divisible in one more permeable, southern part(platform facies) and one less permeable, northern part (transition facies).

This variation of facies, although marking a sharp differentiation in hydrogeological behaviour towardsthe infiltration and method of subsurface water circulation, does not seem to represent an subsurface waterdivide. This is probably because the more permeable soils of the southern areas tend to drain the northernareas. (Bertini T. and Manfredini M., 1982).

Towards the southern border, the limit of the hydrogeological unit of the Maiella coincides with ansubsurface water divide established by the Dolomitic basement of the carbonate succession of Mt. Porrara.Four (4) springs, with sizeable flows gush from the hydrogeological unit of the Maiella, which is quiteclearly defined along its entire perimeter. These springs are located at rising heights from N to S: in the N,the Lavino spring, that runs into the Pescara river by means of the Lavino river; in the N-E, the Val di Forospring, that feeds the river of the same name; in the E, the spring of the Verde, which is part of the catchmentbasin of the Sangro river; in the S-E, the spring of the Acque Vive that runs into the Sangro river by meansof the Aventino river (Demangeot j., 1965).

TEMPERATURE AS A NATURAL TRACER OF GROUNDWATERS

In the study of heat conveyance through interstitial fluids, geothermology considers such heat conveyanceas a disturbance in the natural, conductive flow (Forster C., Smith L. ,1986) . Therefore, all of the theoreticalinvestigations on this subject are concerned with analysing the entity and extension of the sphere of influenceof the disturbance with regard to structural constraints of the embedding medium, hydrodynamics of theflow and meteorologics of the site in question (Smith L. and Chapman D.,1983) .

The use of temperature as a conservative, physical tracer is based on the elementary, physical suppositionthat seepage water is found at atmospheric temperatures and, during subsurface circulation, comes intocontact with rocky masses that host the aquifers, hereby establishing a thermal balance with them. (Tulipano,1988). Given the ratio between the specific, reciprocal capacities, the water tends to assume the temperatureof the rocky formation it has flowed through: the longer the water remains in the aquifer, the more variationin its temperature (Bianchini and Sappa, 1999).

The tracing of lines, performed on horizontal and vertical sections of the aquifer, at equal temperaturesthat are opportunely defined, may sometimes unequivocally indicate the trend of the principal, subsurfacewater flows. Otherwise, such flows could remain hidden inside the rocky mass. As is known, water seepsfrom recharge areas and begins its underground path. Depending on the time it remains in contact with therocky body, its temperature tends to balance out with the host rock. Thus, in examining the trend ofisotherms, the presence of strikes at lower thermal gradients is the sign of a major flow velocity presenttherein compared to the surrounding areas.(Forster C and Smith L., 1986).

This means that the moving water does not remain in contact with the rock for enough time to acquire abalance in temperature and, consequently, its temperature will vary much less compared to seepage water. Itcould be stated that it remains “fresher” than immobile water, or slow-moving water, because subsurfacewater will tend to preserve its original temperature, i.e. the atmospheric temperature at the time and place ofseepage. As verified in karstified masses, if the passage through the wall rock is rapid, there will be even lessvariation in temperature and thermal tracing will be even more significant (Cotecchia V., Tavolini T. andTulipano L., 1978).

That is why the use of temperature for thermal tracing is considered a useful choice, since it belongsnaturally to the hydrogeological system being studied and is able to supply elements of interpretation that aredifficult to acquire using other investigative systems Grassi D. and Tulipano L., 1983).

CONSTRUCTION OF THE HYDROGEOLOGICAL MODEL OF THE MAIELLA BY ATHERMOMETRIC METHOD

The starting point of this study was constituted by the subdivision of the aquifer (hypothesised in previousstudies [Celico, P. 1976]) into very distinct parts: the two northern sub-zones that feed the Lavino springs inthe NW and the zone that feeds the Foro spring in the NE, and the central sub-basin, which, in turn, has beensubdivided into two groups: one feeding the Verde springs, the most consistent of the entire aquifer, and theother being responsible for feeding the Orta torrent. Last of all, there is the southern sub-basin whose aquiferfeeds the Acquaviva springs in the Municipality of Taranta Peligna. Moreover, from the comparativeanalysis of lithology and soil usage, it has emerged that the zone where the infiltration coefficient is higher iswithout doubt located in the central-eastern part of the Massif, west of the main ridge, between the MacchiaLunga Valley and Mt. Amaro in the South, and the deep Valley of the Tre Grotte (Three Grottos) and theMaielletta in the North. The whole zone of high mountains is where the calcareous formations are most bare,fissured and exposed to very intense disgregative action due to the combined effect of mechanical factors(rain, wind, differential expansion due to diurnal and seasonal cycles) and chemical factors due to theaggression of cold, carbonate waters. We could imagine this area being more raised, that of feeding the entireaquifer, and also being the zone of the deep, upper deep-valleys where the minor, karstic forms denselydistributed on the terrain are most evident: karren, holes and grikes. Furthermore, observation of the spatialdistribution of the hypogeous classified cavities specifically assigns the thickest density present on theaquifer with considerably developed grottos (Grotta dei Tre Portoni and Grotta dell’Inferno, both with 140 mof development and Grotta dei Faggi with 310 m of development) to this large, areal quadrilateral.

Therefore this initial hypothesis should be subjected to thermal verification, based on the processing of161 thermometric measurements carried out in the same number of water points (Min. LLPP, ServizioIdrografico: “Le Sorgenti Italiane - elenco e descrizione” – Vol. IX “Abruzzo - 1964), whose reliability hasbeen tested through 25 experimental measurements effected between October 2000 and March 2001. Thefinal objective was that of constructing, at significant altimetrical levels, horizontal sections in order toidentify the areal distribution of the temperature. The principal meaning of these sections resides in the factthat they supply useful indications about the evolution of the water circulation at various depths.

The first operation completed in the study area was that of tracing sections that were closest to the waterpoints with temperature measurements available on the terrain (figure 3).

These sections were processed based on a digital model of the terrain with grid-spacing at 75 meters andthe subsequent construction and interpretation of the geological sections that constituted the basis fordefining the vertical, thermometric sections.

The interpretation that followed this preliminary operation represented to a certain extent the “body” ofthe whole work. It dealt with designing the isothermic lines on the profiles of the above mentioned sections

(figure 4 and 5), hereby trying to connect the water points to the same temperature, taking opportuneconstraints into account, such as: the mark of the gradient (obviously positive towards the earth’s interior and

Figure 4. Vertical, geological-thermometric section C-C’

more or less distorted by the convective transfer carried out by groundwater discharges), the curve of theselines, that must not become excessive, and the presence of strongly discontinuous, tectonic areas, which can

Figure3. Layout Arcview vith 13 thermometric sections and pointswhere the interpolation of temperature has been measured (the yellow

points are vertical intersactions between two sections).

Figure5. Vertical, geological-thermometric section H-H’

induce a distortion of double isotherms in the thermal field if cataclasized or milonitized materials arepresent. In the case of compact limestones (present in the cited central-eastern zone of the Massif), tectoniccataclasis phenomenon are witnessed since the rocky block is compact and fractured giving rise to minorbodies without minute disgregation; this favours the discharge, hereby raising permeability. Thus, if thefracture clearly rises up to the surface (sub-vertical fault), this indicates a potential, preferential, rechargepoint, which effects the evidently deep thermal field of the cold, feeding pockets. In the other case,milonitization is a process that mainly concerns the calcarenites and marly limestones and has an inverseelement with respect to the previous one since the tectonic action produces strips of degraded products with afine granulometry that have the effect of drastically lowering the permeability, impeding the groundwaterdischarge and influencing the design of the thermography as in an increase of the local, geothermic gradient.

The next step entailed the site of a series of vertical “pullings”, on each geological and thermometricprofile, that would function as references for the compilation of sections horizontal to the three chosenheights (200m, 500 m and 1000 m).

The choice of this site was guided by the fundamental criterion of the spatial positioning of three levelsthat would intersect the average height of the four, main springs (Lavino, 152 m; Valle del Foro - S.Eufemia, 280 m; Verde, 415 m; and Acquaviva, 450 m) with the addition of a level at 1000 m that, other

Figure 6. Horizontal thermometric section at aheight of 200 m s.l m.

Figure 7. Horizontal thermometric section ata height of 500 m s.l.m.

than the situations of the low andmedium hillside, would also“photograph” the theometric state athigher heights since one of the aimsof this work is that of qualitativelyestimating the site of possiblefeeding areas of thehydrogeological basin of theMaiella The latter hosts a good 61springs (on 161 catalogued springs)at heights of over 1000 m, i.e. agood 38% of the total, therefore, ascan be simply deduced, it isunthinkable to neglect theinformation relating to theseheights.

At this point, three horizontal,thermometrical sections, reported infigures 6,7 and 8, were constructedthat corresponded to the altimetricheights of 200 m, 500m and 1000 masl; these sections significantlydescribe the thermal and thus, also,the hydrodynamic state of theaquiferous groundwater as a whole.

The comparative examination ofthe three sections allowed forreaching the conclusion that it isreasonable to hypothesise about anoticeable modification in theoriginal hydrogeological model inso far as, based on thethermometrical measurements thatseem to supply some additionalinformation regarding the past,there are 4 and not 5hydrogeological sub-basins, aspreviously hypothesised, intowhich the entire aquifer of theMaiella seems to be divided. This might clearly lead to significant modifications in the choice of method forprotecting these important, subsurface, water resources.

CONCLUSIONS

This work has granted the formulation of an original model of the function of the hydrogeological systemof the Maiella through the processing and interpretation of over 200 thermometric measurements The resultof this experiment has, first of all, pointed out the significance of temperature as a natural, physical tracer inso far as it genetically belongs to the hydrogeological system being studied. At the same time it is able tosupply indications concerning subsurface water circulation that cannot be obtained using differentinvestigative methods. In the second place, the reconstruction of the hydrogeological model on athermometrical basis has led to defining a circulation method of subsurface waters in the aquifer of theMaiella that is noticeably different from past hypotheses. This aspect assumes particular significance indefining criteria for the protection of this important, subsurface water resource.

Figure 8. Horizontal thermometric section at a height of 1000 ms.l.m.

REFERENCES

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Bonarelli G. (1951) La Maiella (Appennino Centrale). Boll. Serv. Geologico d’Italia, LXXI (1947-48-49),nota 6, p. 59-76. Roma.

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