Analysis of the geo-environmental conditions as morphological evolution factors of the sand-clay series of the Tiber valley and Dunarobba forest preservation

Directorate-General For Research And Innovation European Commission

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Nuclear energy and safety

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• Publié par: DIRECTORATE-GENERAL-FOR-RESEARCH-AND-INNOVATION-EUROPEAN-COMMISSION
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 12 Mo

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ISSN 1018-5593
European Commission
nuclear soieni
Ü ¡tf
and technoloa
Analysis of the geo-environmental
conditions as morphological evolution
factors of the sand-clay series of the Tiber
valley and Dunarobba forest preservation European Commission
nuclear science
and technology
Analysis of the geo-environmental
conditions as morphological evolution
factors of the sand-clay series of the Tiber
valley and Dunarobba forest preservation
Edited by
G. Valentini, S. Lombardi, F. Bozzano, G. Scarascia Mugnozza
University of Rome 'La Sapienza'
Piazza Aldo Moro, 5
1-00185 Rome
Contract No FI2W-CT92-0121
Final report
This work has been performed under a cost-sharing contract with the European Atomic
Energy Community, in the framework of its fourth R&D programme on
'Management and storage of radioactive waste' (1990-94)
Part A, Task 4: Disposal ofe waste
Directorate-General
Science, Research and Development
1997 EUR 17479 EN A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int.)
LEGAL NOTICE
Neither the European Commission nor any person acting on
behalf of the Commission is responsible for the use which might be made of the
following information
Cataloguing data can be found at the end of this publication
Luxembourg: Office for Official Publications of the European Communities, 1997
ISBN 92-827-9710-4
© European Communities, 1997
Reproduction is authorized, except for commercial purposes, provided the source is acknowledged
Printed in Luxembourg ABSTRACT
The excavation of clay for bricL· production in a quarry close to Dunar ob ba
(Umbria Region, Central Italy) brought to light a significant number of still in life
position sequoia tree trunks. Their outstanding state of preservation led to consider the
site as a natural analogue; therefore, the undertaken researches were aimed at
analyzing the characteristics of the soil masses which have been protecting the trunks of
the Dunarobba Fossil Forest until present and at evaluating the effects which have
been induced by vulnerability factors acting at the boundaries of the hosting system.
Geophysical investigations, soil gas prospecting, geological and structural
surveys, as well as logs from mechanical boreholes allowed to reconstruct the evolution
and the present boundary conditions of the satellite basin where Dunarobba Forest is
located. Such a basin is filled up by Plio-Pleistocene alluvial deposits, about 100 m
thick, which unconformably overly the carbonatic Meso-Cenozoic bedrock.
On the western bank of this basin the trunks of Dunarobba Forest formed during
Upper Pliocene. The trunks, classified as Taxodioxylon gypsaceum, were buried still in
life position with a rate of buring of about 4 m/1000 yrs, as suggested by the fades
analysis of the surrounding deposits.
Results from hydrogeological and geochemical investigations show that the
circulating waters concentrate along the main vertical faults and then move upward,
because of the high dissolved gas pressure, to meet the low permeability masses
represented by the Plio-Pleistocene deposits. The main causes of such a low hydraulic
conductivity (3*10~10 - 8*10'12 m/s) can be related to the physical properties of these
deposits, to their high density conditions, and to their overconsolidation state. Such
factors contribute in preventing the migration of circulating waters and the chemical
exchange among the different levels and inside each one of them. Therefore each
sedimentary horizon behaved as a closed geochemical system as shown by the different
values of 8/Sr/å6Sr isotope ratios in the carbonate fraction of samples collected at
various depths along the drilled sequence.
All the above mentioned factors, accompanied by a high content of organic matter
in the examinated continental soils, contributed in creating a reducing environment that
allowed the trunks preservation and guaranteed the low geochemical mobility of some
natural radionuclides such as U, Th and Ra. GENERAL CONTENTS
- OBJECTIVES AND SCOPE 1
- WORK PROGRAM
- STRUCTURE OF THE REPORT
SECTION 1 - THE STRATIGRAPHIC, DEPOSITIONAL,
GEOMORPHOLOGICAL AND STRUCTURAL
CHARACTERISTICS OF THE SEDIMENTS
WHICH CONTAIN THE FOSSIL FOREST OF
DUNAROBBA (FFD) 3
SECTION 2 - HYDROGEOLOGY - WATER SYSTEM, THE
ORIGIN, EVOLUTION AND GEOCHEMICAL
CHARACTERISTICS OF FLUID PHASES 105
SECTION 3 - STUDIES OF THE NATURAL BARRIERS IN THE
DUNAROBBA FOREST AREA 141
- OVERALL CONCLUSIONS 213
ν OBJECTIVES ANP SCOPE
In the continental deposits within the Tiber basin, very old (some My) vegetal remains often
outcrop in different areas (Umbemde, Todi, Dunarobba and Spoleto); in some stratigraphie levels they
show the onginal characteristics, in other levels they have been transformed into lignite-like deposits.
The objective of this research was the définition of geological, hydrogeological and geochemical
processes which allowed the preservation of the unaltered wood from many tree trunks in Dunarobba
area, located near the village of Avigliano (Temi, Central Italy).
As the Dunarobba Fossil Forest represents an outstanding example of natural analogue, the
final scope of such a research is the evaluation of the here proposed methodological approach in
assessing the confinement conditions of geological sites for radioactive waste disposal.
WORK PROGRAM
The study was organized in three main fields:
a) reconstruction of the geological and structural setting of the sedimentary basin;
b)n of the hydrogeological setting and studies of the fluid phases (water and gas);
c) studies of the natural barriers in the Dunarobba Forest area.
In the first field, the geological and structural setting and the morphological evolution have been
analyzed. The data come from surface mapping in the south-western part of the Tiber basin and from
geophysical (gravimetric and geoelectrical), soil gas prospecting and local geomechanical
investigations.
Regarding the second field, hydrogeological, geochemical and geophysical surveys have been
carried out in order to understand the groundwater circulation and the water-rock interactions in the
Dunarobba forest area. Furthermore, in order to assess the effectiveness of the Dunarobba sandy-
clayey sequences (Fosso Bianco Unit) as a natural barrier to fluid migration, soil gas surveys and a gas
injection test have been performed.
In the third field, the spatial identification of the natural barriers limiting groundwater flow
within the Dunarobba Forest area is dealt with. With particular regard to this point, detailed
lithostratigraphic reconstructions and hydraulic conductivity investigations of the deposits surrounding
the Dunarobba Forest are considered of great importance in relation to the diffusion of natural
radionuclides. According to these goals, research on the solid phase of the clayey-sandy deposits was
planned. Investigations using convergent geotechnical, mineralogical and geochemical methodologies
have been performed. Further, analyses of the organic matter qualify the preservation state of the
Dunarobba Forest tree trunks.
STRUCTURE OF THE REPORT
Referring to each research field, the acquired data are synthesized in the following pages,
organized in three main sections:
Section 1 : The stratigraphie, depositìonal, geomoiphological and structural characteristic of the
sediments which contain the Fossil Forest of Dunarobba.
Section 2: Hydrogeology - water system. The origin, evolution and geochemical characteristics
of fluid phases.
Section 3 : Studies of the natural barriers in the Dunarobba Forest Area.