Chapter 4.2 Hydrogeology of the city of Rome
Published:January 01, 2006
The hydrogeologic framework of the area around Rome is characterized by a Pliocene–Lower Pleistocene marine claystone bedrock, which is the major regional aquiclude underlying the shallow hydrogeologic units; the claystone bedrock is hundreds of meters thick and has a very low permeability (Ventriglia, 1971, 1990, 2002; Albani et al., 1972; Boni et al., 1988; Corazza and Lombardi, 1995a; Funiciello and Giordano, 2005; Capelli et al., 2005). The nearly impermeable bedrock is overlain by Lower to Middle Pleistocene marine to continental sediments (claystones, sandstones, and thick sequences of conglomerates), which are in turn overlain by and partly interfingered with Middle to Upper Pleistocene volcanic deposits from the Sabatini volcanic complex to the north and the Colli Albani volcanic complex to the south (Funiciello and Giordano, 2005; Capelli et al., 2005). Holocene alluvial sediments cap the stratigraphic sequence along the present-day river systems (Corazza et al., 1999; Funiciello and Giordano, 2005). Holocene eolian sand dunes cover a narrow area along the present-day Tyrrhenian coastline.
All rock sequences overlying the claystone bedrock are aquifers, the geometry and circulation of which are controlled by both the evolution with time of the paleotopographic setting and the vertical and lateral variations of lithologies, each with different permeabilities (Capelli et al., 2005).
Other locally important hydrogeologic units, only present in the urbanized areas, are the backfill deposits accumulated during 3000 yr of human civilization in the Roman area. On a regional scale, the Pliocene–Lower Pleistocene marine claystone aquiclude overlies a deep aquifer in highly deformed Mesozoic-Cenozoic carbonates (Boni et al., 1988), which is recharged from the Apennine region. Tectonic and volcano-tectonic discontinuities control groundwater flow on a local scale, as well as gas and fluid leakage from the deeper Mesozoic-Cenozoic carbonate reservoir, as evidenced by the presence of several low- to medium-enthalpy hydrothermal springs (e.g., Tivoli) (Funiciello et al., 2003; Carapezza et al., 2003; Tuccimei et al., 2006).
HYDROGEOLOGY OF THE VOLCANIC DEPOSITS
Major aquifers around Rome can be found in interbedded pyroclastic deposits (ignimbrites, surge and fall deposits, and reworked pyroclastic materials) and lavas. The volcanic aquifers are characterized by good permeability as a whole (primary permeability in tuffs and secondary in lavas), although considerable variation in permeability may be found among different deposits (Capelli et al., 2000, 2005). Fractured lavas embedded in generally lower-permeability tuffs represent preferential groundwater drainage pathways.
The volcanic deposits host many aquifers at different depths. The main aquifer is located at the base of the volcanic pile, and is confined by either the oldest low-permeability volcanic products or by the Pliocene-Pleistocene marine sediments. Above the main aquifer, many local perched aquifers can be found confined at the base by low-permeability pyroclastics and thick paleosols. Aquifers are generally unconfined because of the lateral discontinuity of impermeable layers, although confined aquifers, especially thick lava-flow units, can be found locally (Capelli, et al., 2005).
To the east and south of the Tiber River, volcanic deposits belong to the Colli Albani volcanic complex, and the main aquifer flows radially outward from the volcano reaching the southern and eastern suburbs of Rome. The river system and springs in the area, which supply much of the potable water in the Roman area, gain water from the main aquifer. Examples include the Acqua Vergine spring, which presently has a discharge of 600 L/s (and was 1200 L/s during classical Roman times) and drains water from lavas, or the many springs utilized by the Appio and Augusto aqueducts of the ancient Romans, which discharged more than 1000 L/s (presently ∼600 L/s). In the city center, a number of small springs are fed by water from a localized shallower aquifer (important for the local history and economy) (Coppa et al., 1984; Pisani Sartorio and Liberati, 1986; Corazza and Lombardi, 1995b). North of the Aniene River, some springs with few liters per second of discharge gain water from the aquifer that flows toward the Tiber and Aniene Rivers.
West of the Tiber River, volcanic deposits mainly belong to the Sabatini volcanic complex, and thin southward toward Rome. The aquifer flows radially away from the volcanic complex, i.e., from the northwest (where it reaches the surface at the Bracciano and Martignano lakes) to the south-southeast (Boni et al., 1988; Ventriglia, 1990; Capelli et al., 2005). Some important springs are located in the northern sector of the Roman area, where the volcanic pile is thickest and the main aquifer is more substantial. Smaller springs (<0.5 L/s) are related to perched, local aquifers. Many water wells have been drilled in the area that drain water from the main aquifer, with resulting specific capacity of up to tens of liters per meter of water-table drawdown.
ANCIENT SPRINGS OF ROME
The presence of water under Rome and its use from springs and water wells may have been one of the main factors influencing the siting of the early city. During classical Roman time, the many springs within the city and the surrounding area were well known and utilized (Lanciani, 1881; Pisani Sartorio and Liberati, 1986; Corazza and Lombardi, 1995a, 1995b). Some of them still exist, and they are an invaluable archaeologic and natural treasure to be preserved. These springs, together with water wells and the Tiber River, were the only available water in Rome for a long time. Water catchment structures are found in the city center, as well as in the surrounding areas, confirming the abundance of water in this territory during ancient times.
Collection of water from local springs and water wells was abandoned after the Romans began to build aqueducts, and some of the springs were transformed into sacred sites. Most of the Roman engineering patrimony was lost with the fall of the Roman Empire. The barbarian Vitige knocked down the aqueducts in 537 A.D., and the city was forced to rely on local springs and old catchments, although most were unusable or backfilled with manmade debris. Between the sixteenth and the seventeenth centuries, searches for new sources of groundwater were sponsored by the papal administrations. Springs (active presently or in the past) that gain(ed) water from volcanic deposits, mostly tuffs, are listed below:
Acque Sallustiane . The Sallustiane springs were located in the valley between the Pincio and Quirinale hills, and flowed into the Amnis Petroniae stream. The springs gained water both from the volcanic deposits and the underlying Pleistocene conglomerates, which cropped out at the base of the valley, but are presently completely buried by debris.
Acqua di S. Felice . The S. Felice fountain, which is located in the gardens of the Presidential Quirinale Palace, receives its water from a small spring that is likely the ancient Fons Cati.
Acque Fontinali-Fonte del Grillo . Some springs collectively known as Acque Fontinalis outcropped along the southern slope of Quirinale hill, where the Fontinalis door of the Serviane walls was located.
Lupercale . Water from this spring was renowned during Roman time, as it was consecrated to Lupercus Faunus (the one who keep wolves away) and was used during ceremonies of the Lupercalia feasts. It is known to be the first spring used for sacred ceremonies in ancient Rome. The location of the spring is uncertain, but the geology of Colle Palatino suggests that it emerged from the Tufo Lionato formation, a zeolitized ignimbrite from the Colli Albani volcano.
Fonte di Pico . The Fonte di Pico was named by Ovidius and was located in a cave at the base of Colle Palatino, facing the Tiber River. It probably disappeared when the present-day river embankments were built up during late nineteenth century. This spring probably derived its water from the volcanic deposits, which were, in turn, drained by Pleistocene sandy-travertine deposits.
Fonte Egeria . The Caffarella Valley was renowned for the many mineralized springs and their therapeutic properties. The mineralization was the product of mixing of groundwater with upwelling, deep-seated fluids that rose in this area along volcano-tectonic fractures. The Fonte Egeria is known to have been used by Erode Attico for a Nymphaeum in his villa. The ruins of the Nymphaeum still host a spring of mineralized water.
Area of Salone
Acqua Vergine . The Acqua Vergine is composed of a number of springs located ∼10 km to the east of Rome, along the old Via Collatina, in a once swampy area. The legend by Frontinus and Plinio il Vecchio tells of a young virgin girl who indicated the spring's location to some soldiers. More likely, the origin of the name is to be ascribed to the purity of the water, which Marziale defined as gelida (icy), nivea (snowy), and cruda (sharp) to underline its freshness. The Acqua Vergine was captured by the Romans and transported into Rome via the Virgo Aqueduct, the only aqueduct of Roman time that is still functioning. The aqueduct was designed by Agrippa and inaugurated on the 9th of June, 19 B.C. Frontinus states that the aqueduct discharge was 101,000 m3/d, versus the present-day discharge of 78,000 m3/d.
PERMEABILITY OF VOLCANIC DEPOSITS
The permeabilities of volcanic deposits vary as a function of primary porosity, compaction processes, sealing, and secondary fracturing. Lava flows are generally characterized by very low matrix porosities, but are intensely fractured, either by columnar jointing or by postdepositional faulting and fracturing. Lavas have well-developed fracture systems, which result in an effective drainage system via fracture permeability. Lava flows near Rome are generally confined to paleovalleys, where they are locally up to few tens of meters thick, but lack lateral continuity. To the south-southeast of the city, a lava plateau, up to 100 m thick, has been revealed from drillings; it constitutes a major drainage pathway in that region (Funiciello and Giordano, 2005).
Pyroclastic deposits, on the other hand, show large variations in permeability. The porosity of such deposits varies as a function of granulometry, which depends on the type and intensity of explosive processes and on the mechanism of their emplacement, as well as postdepositional alteration processes.
Fallout deposits are generally well sorted and highly permeable, at least in proximal and medial areas where block- to lapilli-sized deposits are common. Fallout deposits also mantle pre-existing topography and therefore are laterally continuous, if not eroded.
Pyroclastic flow deposits emplaced by density currents (i.e., ignimbrites and surge deposits) are generally poorly sorted and have a wider range of particle sizes, with abundant fine ash (<0.064 mm). Ignimbrites and surge deposits have generally low to medium permeabilities. Individual surge deposits are also relatively thin (up to few meters) and are confined to proximal areas of the volcanic center, whereas individual ignimbrites can be up to few tens of meters thick and are widely dispersed around the volcanic centers of the Colli Albani and Sabatini volcanic complexes. The ignimbrites are massive, with pumice, scoria, and lithic clasts dispersed throughout an ashy matrix. To the south and east of the city of Rome, the main aquifer is mainly represented by the “Pozzolane rosse"–“Pozzolane nere” ignimbrite sequence (Capelli et al., 2000). The permeability of these ignimbrites, however, varies as a function of postemplacement lithification processes. For example, zeolitization is characteristic of ignimbrite units in the Roman area, including the “Tufo Lionato” from the Colli Albani (Watkins et al., 2002) and the “Tufo Rosso a Scorie Nere” from the Sabatini volcanic field. Zeolitized ignimbrites can be local aquifers if they develop a good fracture network.
The alteration of volcanic glass to halloysite may reduce the primary porosity of pyroclastic deposits, filling up fractures in both lavas and zeolitized ignimbrites. Permeabilities may also be reduced by deposition of carbonates from circulating water, especially by travertine where hydrothermal fluids mix with ground-water (Tuccimei et al., 2006).
Permeability values (cm/s) have been determined for volcanic deposits around Rome with the Lefranc test at variable head (Table 4.2.1). Transmissivity values of aquifers in volcanic terrains (m2/s × 10−3) were obtained by pumping tests (Table 4.2.2).
The Roman area is characterized by a wide range of mineral waters, generally cold, but occasionally >20 °C (ipothermal waters) (Camponeschi and Nolasco, 1982; Brondi et al., 1995). There are six plants for mineral water extraction and bottling in Rome (Tables 4.2.3 and 4.2.4), and this is probably a unique case in the world for a metropolis. The mineralization is caused in part by normal cationic enrichment and partly by mixing of shallow groundwater with deep-seated hydrothermal fluids upwelling along tectonic fractures.
Chemical and Physical Properties of Groundwater
Groundwater in the Roman area is classified as bicarbonated-alkaline-earth, with the noticeable exception of mineralized waters (Table 4.2.5).
Vulnerability of the Aquifers to Pollution
The Roman area, like any urbanized and industrialized area, is characterized by the pollution of groundwater. Potential pollutants include liquid and gaseous industrial waste, hydrocarbon depots (comprising petrol stations), solid waste depots (either controlled or uncontrolled), car parks, cesspools, injection wells, areas lacking sewage systems, and farming areas where there is potential pollution by fertilizers, pesticides, herbicides, and anti-parasites. Extensive urbanization has also considerably altered soil and surface conditions, increasing aquifer vulnerability. Construction has also introduced pollutants into the aquifers, as well as reduced the recharge area (the urbanized areas are ∼20% of the Rome Council area), therefore depleting the aquifer and its self-purification properties.
As no specific work on the vulnerability of volcanic aquifers in the Roman area has been carried out yet, we refer to the general knowledge of an aquifer to assess its vulnerability to pollution (Table 4.2.6). Unconfined aquifers in the Roman area are generally vulnerable because they are shallow and lack a significant thickness of impermeable rocks; pollutants can easily reach the aquifers. Confined aquifers are embedded in, and therefore protected by, impermeable rocks. However, the large number of water wells and catchment basins present in the area has induced an artificial hydraulic continuity between aquifers at different levels, so that confined aquifers are virtually absent, and pollutants leak from upper aquifers to deeper levels. Data from the Roman Council (Table 4.2.7) show that none of the aquifers in the area are protected from microbiological pollution, not even the deepest ones.
Figures & Tables
Tuffs - Their Properties, Uses, Hydrology, and Resources
- aquifer vulnerability
- chemical properties
- clastic rocks
- confining beds
- ground water
- igneous rocks
- Latium Italy
- lower Pleistocene
- mineral waters
- perched aquifers
- physical properties
- pump tests
- Rome Italy
- sedimentary rocks
- Southern Europe
- volcanic rocks
- water pollution
- Tiber River
- Caffarella Valley
- Salone Italy
- Palatino Hill
- Quirinale Hill