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Soil

Description of the Problem

The soil in the city of Rome can be treated from several important points of view. First of all, the soil is the substrate studied by a specific approach to the problem of the knowledge of the landscape, the geological and geomorphological approach.

Secondly, the first layer of soil, studied by the science called pedology, is the intermediate environment among bedrock, water atmosphere, and biosphere. In this layer, chemical compounds are allocated, both organic and inorganic, which, by means of complex interaction processes, allow the growth and reproduction of both natural vegetation and crops. The knowledge of pedology is then essential for managing resources in order to develop the agricultural sector. Moreover, it is in this layer that archeaological objects are buried, therefore knowledge of it is important in archaeological excavations as it is a significant point of reference.

Thirdly, the soil is exposed to environmental impacts due to different phenomena: chemical pollution (because of solid waste disposal, emissions from cars and heating systems in residential areas, chemical products used in agriculture), compaction, erosion, and salinisation

And finally, because of the action of water, the soil is exposed to several phenomena: groundwater and interflow create the phenomenon of landslides; groundwater causes underground caves, which sometimes create hollows in the surface of the ground; and last but not least, the action of sea currents erodes the littorals, combined with the diminution of the solid transport along the rivers due to the construction of dams and barrages.

With reference to the first of the above four points, the Roman landscape can be considered a complex one. The analysis of such complexity requires knowledge of the constituent factors, among which are geological and geomorphological characteristics, which shape the landscape in specific ways. Geology and geomorphology become then an instrument of landscape analysis, which allows one to make distinctions between different formation processes of landscape units, such as shores and dunes, coastal plains, alluvial valleys, terraces, hills, plateaus, volcanic craters, and calderas, which can be composed. The landscape units are then the constituents of three types of landscape which characterise the Roman territory, the Tuscia and Albani hills, the Campagna Romana (Roman countryside), and the coastal plain. Therefore soil knowledge, particularly the pedology, lithology, and geomorphology of soils, contributes significantly to the preservation of the landscape values, which is developed under National Laws 1497/39, 431/85 as well as under Law 394/91 concerning the national system of parks and reservations, and under Regional Law 29/97 concerning the regional system.

Although of less importance for the Rome territory, knowledge of geolithology and geomorphology allows one to manage the geological risks, such as the seismic risk and the volcanic risk. In the Rome case, these risks were important in past geological areas but are not considered of primary importance at the moment. Nevertheless, recent research, based on data about recent earthquakes in central Italy, seems to demonstrate a more consistent seismic risk even for the Rome area.

As for the second of the above four points, the Rome territory is the object of intensive urban development, with a consumption of agricultural soil of about 200 km² in the last twenty years. Such a pace of soil consumption is of course dangerous because of the loss of productive capacity, but we have to consider that the agricultural sector is not very developed in our area. The main danger consists in the loss of soil permeability, which increases the flood risk, decreases the quantity of groundwater (available for water consumption and, in a broader sense, contributing to an equilibrium of water content in the soil), and lowers the base flow in rivers and streams during the dry season. The selection of soils for urban development, according to the importance they have in the water cycle and/or as productive input in agriculture, is an important tool for managing and minimising the impact of urban development itself.

As for the support of knowledge during archeaological excavations, pedologic studies have been undertaken during the development of several archeaological campaigns, such as that for the Centocelle area, a non-built area previously occupied by a military airport and now in the process of urban development in the framework of the "Sistema Direzionel Orientale" (SDO) business district.

As for the third point, the soil is exposed to several environmental impacts. The most important impacts in the case of Rome are a result of erosion; salinisation for a limited area corresponding to the previously existing coastal lagoons of Saline and Stagno Ostiense (near Ostia); for some part of the coastal area, the reduction of the watertable and the substitution of it with saline watertable; and chemical pollution.

The soils most vulnerable to erosion are the pre-volcanic ones, that is soils belonging to the pre-volcanic platform on which the volcanic products were spread during the eruptions. They generally characterise the sides of the valleys and show a higher degree of erosion where the texture is thin.

Chemical pollution, in a city like Rome with a low concentration of industrial plants, is mainly due to emissions from cars and heating systems in residential and office buildings. Such pollution therefore shows higher degrees of chemical pollution in the central areas and along the main streets and urban highways. The main pollutants are lead, zinc, and cadmium.

As for the fourth point, disasters due to the action of water in the soil, Rome is affected by the phenomenon of landslides, particularly in the areas of urban development with steep slopes. Generally, the landslides start either because of an increase in the weight on the soil or because of a change, due to new construction, in the soil morphology or hydrology, occurring mostly in clay or sandy soils. Another phenomenon that results from the action of water in the soil is soil collapse; this occurs because of the pressure of the building weight on flat areas, characterised by low geomechanical properties and by the presence of underground caves. In the city of Rome, there have been about sixteen examples of disasters attributable to the action of water in the soil.

Lastly, the erosion of the littoral, due, as previously mentioned, to the construction of dams and barrages, caused a receding effect in the coastal line of about 150 m at the Tiber River estuary and of several tens of meters in correspondence to the urban area of Ostia, assessed during the last twenty-five years of measurements.

Data Sources

Regarding the geological and stratigraphical characteristics, in 1995 the Geological Survey of Italy (Servizio Geologico Nazionale), the National Institute of Geophysics, and the Department of Geology of the III University in Rome finished a study of the geology of Rome with the support of the city of Rome. Such a study determines in detail the soil stratigraphy and is based on an analysis of about 113 samples and on observations made on a more developed network of survey stations (the databank consists of 3000 data).

The database of the hydrogeological study, realised in the same framework of the previous geological study, consists both of about 400 wells and soil perforations, available from previous research, and a specific field survey completed for the study in the last years.

For geotechnics knowledge, as for the previous sections, the data are organised in the form of a database, managed by the dBASE IV management system (Borland) on personal computers, and consist of both previous survey data, which were put inside the database in a standardised form, and of new data surveyed to integrate the existing knowledge. The specific database is made from observations of about 450 cavities found in the central area of Rome. Such observations are organised by means of a form collecting standard data (dimensions, lithology, geological unit in which the cave is located, age, eventual archeaological interest, present use, hydrogeological conditions, structural failures, structural interventions).

Such surveyed locations are georeferenced and the database is then part of a Geographical Information System. The observations are integrated with the database of about 136 phenomena of structural failures of the soil, consisting both of landslides and collapses of the ceiling of underground cavities, either from the past or evolving in the present. Most of the underground cavities were dug in the past mainly in order to mine building materials; afterwards they were used partly as churches and burying places (catacombs, cellars, etc.). Other types of underground cavities consist in aqueducts, sewers, and water tanks of the Roman period.

For the pedological classification, a survey about the structure of the top soil (pedologic) layer was developed in 1980-81. A database was built from about 1,400 field observations on the territory of the city of Rome, with a density of 1.5 observations per km², from drills of a depth of 1 m. These observations were combined with about 280 sample observations from two representative areas.

From approximately 200 samples, chemical and physical characteristics, unable to be immediately surveyed at the field stage, were determined. Furthermore, on some of these samples the plasticity index was determined, which is useful for establishing the geotechnical characteristics for the construction interventions.

For soil pollution, a survey was made in 1996 by the Enea Research Agency and the "La Sapienza" University in Rome about the lead and cadmium content in the soils and in two vegetal species (Malva Sylvestris and Bellis Perennis) inside the main green areas in Rome. Twenty areas were selected as representative of the variability from the point of view of the distance from the city central area and of the distance from the main urban streets and highways. Another parameter used in the selection was traffic intensity.

Methods

The comparative analysis of the stratigraphical data allows one to reconstruct (in more detail, compared to the previous studies) the location, depth, age of formation, and other characteristics of the lythological layers, identified by means of the lythological type, by means of recognition of the fossils of organisms inside the layers (micropaleontological analysis), and by means of mineralogical analyses.

For the hydrological studies, the data are elaborated taking into account a background knowledge of the hydrogeology at the regional scale as well as the original hydrography of the area and the works made in the Roman period to drain the central area of the city. Then attributing the area of the city to four hydrogeological complexes of soils and defining the circulation of groundwater, according to the characteristics of the aquifers and the aquitards, is carried out. A further section makes an inventory of the springs inside the city, and a last section makes a study of the chemical physical characteristics of the different aquifers and their vulnerability.

The observations of the structural failures were analysed, together with the available information about:

• the lithological units of the area ;
• the geotechnical characteristics of the soils (physical and mechanical parameters);
• stratigraphy of the area subject to structural failure.

Synthetic graphic descriptions (hystograms, pies) about the frequency distribution, among the geological units, of types of fractures and cavities and of the different types of failures, the location of drills, and their stratigraphy were made. For some cavities, a more detailed survey was developed with graphic descriptions (plans, sections), stratigragraphies, etc.

The study for the pedologic map was developed in four stages :

• the first stage consisted in a general field survey to further develop the existing framework knowledge of the geological units and the different soil types;
• the second stage consisted in a survey of two representative areas, concerning the variability of the geological types (geological formations), in order to establish in detail the relationship between the lithology, slope, duration of the pedogenetic phenomena, and soil characteristics;
• interpretation of the aerial photos (in the scale 1:35.000, year 1955), and first definition of soil units (for soil type, geomorphology, pattern, etc.);
• field survey and realisation of 1.400 drill holes.

For the chemical pollution study, several soil samples were extracted, together with several individual samples of the two target types of plants. Lead and cadmium content were determined for each location, and some statistical parameters were calculated (mean and standard deviation).

Results

The main results of the studies consist of a series of maps of the central area of Rome (about 50 km²):

• geological map;
• map of the depth contour lines of the geological layer of Mount Vatican Unit (Pliocenic period);
• two maps of the depth contour lines of the geological layers of volcanic deposits and late alluvium soils;
• map of the depth contour lines of the top soil layer, determined by the antropic activity;
• hydrogeological map of the central area of Rome, with the attribution of the soils to four hydrogeological groups (alluvial deposits, volcanic soils, pleistocenic sediments, pliocenic clays), the contour lines of the watertable depths, the types of aquifers and the direction of flow, the water springs;
• map of structural failures and underground cavities; the map locates the phenomena and establishes the types of landslides and collapses of the cave ceilings, the type of cavities, their density, and the geological units in which such phenomena take place;
• pedological map, with a soil classification of about seventy categories, divided among the main geological landscape units: continental platform, coastal terraces and ancient coastal plain, present coastal plain, great valleys, and volcanic craters. The map is going to be updated in 1978;
• geological landscape units and geosites map, identifying the landscape units, according to the geomorphological origin, and accompanied by the location of the geosites, that is natural monuments documenting past geological phenomena in a very recognisable feature.

Moreover, stratigraphical sections were developed and, together with the maps, provide complete information.

Finally, databases of the various studies were organised in tabular and graphical form. Such results are available in the form of a publication in two volumes: The Geology of Rome - The Historic Centre, one volume with the text, graphics, and photographic archive, the other with the above-mentioned maps in the scale 1:10.000 and a series of reproductions of historical maps of geological content.

Uses

The completed studies about the soil of Rome are an important support tool for identifying the geomorphological structure of the landscape and for then defining the landscape value from the geological point of view. The complex of the above mentioned studies can be used by environmental planners in the definition of the preservation areas and of the related degrees of preservation.

Knowledge of the hydrogeological characteristics of soils, the groundwater circulation (aquifers), and the location of the impervious layers (aquitards) of the confined aquifers allows one to define the areas of vulnerability of the aquifers.

The map of structural failures and underground cavities can also provide support during the study of the project of important public works and urban developments (infrastructures, public transportation, urban projects). It allows one to prevent structural failures by fixing specific requirements to the project, and in extreme cases it can be consulted for alternative locations of the project.

The pedological map was used in the past years mainly for studies about some archeaological sites and for the information provided about the age of the layers in which the archeaological items were buried. A future application could consist in support for the definiton of a permeability map, to be used in the preventative evaluation of the effect of urban development on some environmental risks (hydraulic risk).