Caratterizzazione idrochimica di un acquifero superficiale: il caso della circolazione idrica nei corpi di frana nella dorsale carbonatica di M. Pietralata – M. Paganuccio (Appennino Marchigiano)
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Nella presente nota sono descritti i risultati di una ricerca sulle caratteristiche idrochimiche delle acque di sorgente emergenti dai corpidi frana che interessano i fianchi della dorsale anticlinalica di M. Pietralata – M. Paganuccio. Il collasso gravitativo di gran parte deifianchi della dorsale, legato principalmente all’assetto morfostrutturale e litostratigrafico dell’area, coinvolge generalmente il substratoe ha prodotto vasti e spessi accumuli di frana, variamente dislocati e disarticolati. Le masse franate, avendo sviluppato una elevatapermeabilità secondaria, hanno costituito acquiferi propri e sensibilmente condizionato la circolazione idrica sotterranea più superficiale.Ciò ha generato numerosi punti di scaturigine distribuiti sia alla base del rilievo, ai margini dei principali corpi di frana, sia all’interno dei corpi stessi. In certi casi, invece, gli accumuli di frana probabilmente ricoprono sorgenti legate a circuiti più profondi, all’internodelle rocce del substrato. I risultati delle analisi chimiche effettuate sulle acque di sorgente nelle campagne di monitoraggio del1984 e del 2004, oltre a caratterizzare facies idrochimiche sub-superficiali direttamente legate ai corpi di frana, sottolineano propriouna loro probabile miscelazione con acque di provenienza più profonda.
Geological-geomorphological and hydrochemical studies have been carried out throughout the anticline ridge of Pietralata-PaganuccioMts. (northern Umbria-Marche Apennines, Central Italy) in order to find a possible correlation between the large landslides almostaffecting the entire mountain flanks and spring-water characteristics. The study area embraces the northernmost sector of a much longercalcareous anticline ridge extending all along the Marche Region. The Furlo Gorge, a narrow transverse canyon deeply cut by theCandigliano River (Metauro River basin), breaches the anticline ridge separating the two mountains from each other. This sector of theridge, although rather simple as for its morphostructure, is indeed one of the most important examples of such landforms so characteristicof the Umbria-Marche Apennines. Pietralata and Paganuccio Mts. consist of the mostly calcareous and marly-calcareous Meso-Cenozoic units of the Umbria-Marche Succession. The Jurassic-Cretaceous terrains, ranging from the Calcare Massiccio Fm. and theScaglia Bianca Fm., crop out in the Furlo Gorge area only, deeply cut in its turn at the core of the anticline. The flanks and the crest ofthe anticline ridge, on the contrary, are made up of Createceous-Oligocene terrains, namely Scaglia Rossa-Scaglia Cinerea andBisciaro Fms. Pietralata and Paganuccio Mts. are an explicatory example of an exhumed anticline, where the geological structure is distinctly replicated by the relief topography. Indeed, the ridge morphology almost perfectly displays both the rounded geometry of theanticline and its northwestern and southeastern plunge-out areas. Furthermore, characteristic flat top-surfaces markedly reflect thesub-horizontal/gently-dipping bedding of the hinge zone, while the mountain sides slope at angles approximating the downslope-dippinglayering values. Although the peculiar morphostructural arrangement of the ridge has been shaped by different processes, the roleof large, deep-seated mass movements is once more today among the most effective ones. Landslides, affecting both sides of theridge for the most part, find their key predisposing factors in the dip-slope layering attitude on the two sides of the anticline ridge, aswell as in the occurrence of weak marly horizons (e.g. Marne a Fucoidi Fm., marly members of Scaglia Rossa Fm., etc.),embeddedamong calcareous units in different stratigraphic positions. The occurrence of an intense local rock-fracturing is an effective additionalpredisposing factor. As a result, the most part of the anticline ridge flanks have been extensively denudated by more or less shallowlandsliding, able to highlight both lithologic contrasts of the Jurassic-Miocene “multilayer” and structural features, such as fault-zonesand bended bedding surfaces. The gravitational sagging and collapse of the anticline flanks encompass several, different field evidence.As a rule, the landslide head areas display tension cracks and more or less distinct trenches, scarps and counterslopes; the slidmass becoming more and more disrupted and chaotic downslope, where an over-100m thickness is often reached and stressed bydeep gully dissection of the landslide mass. Indeed, the substantial permeability acquired by progressive fracturing of the sliding rockbodies allows for a great number of frequently perennial aquifers and springs to occur within the slid-bodies and/or at the toe of themajor landslides. On the contrary, in some places, the landslide materials have probably blanketed some springs related to relativelydeep bedrock aquifers. Hydrochemical data show that the characteristics of the majority of the analysed waters set in the field of Ca2+ -HCO3- compositions, with two different trends, namely “A”, trending towards Na+ - Cl- compositions, and “B”, trending towards Ca2+ -SO42- compositions. A possible reason of these two trends is the mixing of superficial water circulating inside the landslide masses withwaters flowing out from deeper bedrock-aquifers. Water samples from the Pietralata Mt. area, according to the most part of watersfrom carbonatic rocks of the Marche Region, display contents in Ca2+, Mg2+ and HCO3- which are mostly due to magnesian-calcite solutionor, alternatively, to a mixing of calcite and dolomite (this latter, especially for waters from relatively deep aquifers). As regards watersaturation in calcite at different PCO2 conditions, the analysed samples display a range of values approximating those typical for soils(0.15 bar). Being such values much higher than those already reported for carbonate aquifers of Nerone Mt. (0.0003 – 0.003 bar), anear-surface water circulation can be alleged. In this respect, a similar assumption can be drawn from both the relatively low dischargesof the studied springs, and from the recurring traces of NO3- concentration. As regards this latter, in particular, its relatively highconcentration in both 1984 and 2004 samplings is likely to be related to both biologic activity and humification (nitrification) on the surface of the slid-masses, although a less important amount of NO3- from fertilizers cannot be ruled out. Moreover, a low Ca2+ - SO42-composition of the samples at issue can be related to the mixing of relatively deep waters from bedrock aquifers whose springs areblanketed by thick landslide accumulations. Finally, the observed trend “A”, towards Na+ - Cl- compositions, can be ascribed to cyclicsalts coming from near marine areas.