The physical properties of clay allow to consider argillaceous formations as geological barriers to radionuclide migration in high-level radioactive-waste isolation systems. As laboratory simulations are short term and numerical models always involve assumptions and simplifications of the natural system, natural analogues are extremely attractive surrogates for the study of long-term isolation.
The thermally altered clays of the Orciatico area (Tuscany, Central Italy) represent an interesting natural model of a heat source which acted on argillaceous materials. The study of this natural analogue was performed through detailed geoelectrical and soil-gas surveys in order to define both the geometry of the intrusive body and the gas permeability of a clay unit characterized by different thermal alteration degrees. In particular, soil-gas radon and carbon dioxide distributions highlighted that the clay sequences, in spite of their thickness and plasticity, if fractured and metamorphosed, form a lesser impermeable barrier for naturally migrating gas

Lombardi, S.

Voltattorni, N.

Earth-prints Repository

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Selagite outcrop
Boundary of the
resistive complex
LEGEND
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Rn (Bq/l)
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Unaltered host clays
SOIL GAS GEOCHEMISTRY AS PERMEABILITY TRACER OF THERMALLY 
ALTERED CLAYS AT ORCIATICO  (Tuscany, Central Italy)
Introduction
The Orciatico clay study was part of an EC project aimed to study 
natural analogues of the thermo-hydro-chemical and thermo-hydro-
mechanical response of clay barriers. The main idea was to achieve 
information and experimental data regarding the long-term stability of 
permeability and plasticity of the clays affected by high temperatures. 
Natural analogues were identified in clay formations near contact with 
volcanic intrusions. Though available data do not allow exact evaluations 
of depth, many features of the Orciatico igneous body (widespread 
glass, highly vesicular peripheral facies etc.) point to a shallow 
emplacement, comparable with that reasonably forecast for a repository. 
Not even exact definitions of the temperature of magma at the moment 
of emplacement are feasible. Only some evaluations can be proposed: 
from its distinctly femic composition temperatures over 800°C may be 
assumed for the alkali-trachytic magma intrusion (Leoni et al., 1986; 
Hueckel and Pellegrini, 2002). These values are much higher than those 
expected around a radiowaste container (up to 300°C, according to 
Dayal and Wilke, 1982}; therefore, as to the thermal aspects the 
Orciatico magmatic body and its metamorphic aureole must be regarded 
as an extreme condition model of a radiowaste repository and probably 
it can be mainly used to demonstrate a worst case.
Methodology
A total of 1086 soil-gas samples was collected in the Orciatico area. A first survey was 
performed collecting 486 samples along a regular grid with a sampling density of about 500 
samples/km2. A total of 600 samples (monthly surveys, from April to September 1998)  were 
collected in two small zones within the studied area: the first one called “Casa Petroia” area, 
characterized by altered clays and the second one, “background” area, situated in the 
unaltered clays (Fig. 1). The aim of the monthly surveys was to monitor possible variations of 
soil-gas concentration due to weather conditions. 
A total of 20 electrical soundings (VES) and 8 dipole electrical tomography (DET) profiles was 
carried out across the intrusive body and in the clays characterized by several thermal and 
mechanical alteration degrees. 
Results and discussion
The radon, as well as the CO2 contour line maps, Fig. 2, show that highest values (222Rn> 25 Bq/l, CO2>2 %,v/v ) occur in the south-western part of the studied area (characterized by the presence of Selagite outcrop) and along a narrow belt, with direction NNW-SSE, 
where metamorphosed clays (named “Termantite”) are present. Furthermore, anomalous values occur in unaltered clays especially in correspondence of the boundary of the resistive complex supposed on geoelectrical results. 
All over the north-eastern sector, in non metamorphosed clays, radon and carbon dioxide values are very similar to background values reported in literature (Rn: 10-15 Bq/l; CO2: 0.5 %,v/v). 
Figure 3 shows two merged probability plots elaborated to highlight different concentrations on Selagite outcrop, Termantite (metamorphosed clays) and intact (unaltered) clays: it is well evident a general decreasing trend of the measured concentrations in the three 
different sites, showing the lowest values in the un-metamorphosed clays. Results from monthly surveys (from April to September 1998) showed (Fig. 4) that soil-gas concentrations remain constant confirming that they are directly correlated with clay permeability. 
Geoelectrical surveys were performed in order to identify the geometry of the trachite-alkaline intrusive body (Selagite) outcropping a few hundred meters NE of the village of Orciatico. A total of 20 vertical electrical soundings (VES) and 8 dipole electrical tomography 
(DET) profiles have been performed across the intrusive body. The VES defined three electrical layers: the first and third layers have a resistivity between 5 and 10 ohm*m and are attributed to Pliocene clays, while the intermediate layer has a resistivity between 15 and 60 
ohm*m and is correlated with the Selagite intrusion, near the outcrop, and with the clays that underwent modification because of the intrusion effects. The metamorphosis degree of clays is function of their distance from the intrusion, and, clearly, highest metamorphosed 
clays are near Selagite outcrop. 
Comparison between geochemical and 
geoelectrical results
As Rn and CO2 values seem to decrease gradually from 
Selagite outcrop towards un-metamorphosed clays, 
soil-gas data set was projected along two longitudinal 
lines coinciding with one geoelectrical profile that 
highlights a quick resistivity decrease both in the 
vertical and in the horizontal way. 
Figure 5 shows polynomial regression (3rd degree) of 
Rn and CO2 values plotted against the distance from a 
reference point. Graphs highlight a slight decreasing 
trend of radon (continuous line) and carbon dioxide 
(dashed line) soil-gas values towards the NE, from 
Selagite outcrop until un-metamorphosed clays. Highest 
CO2 and Rn values overlap and were found between 
Selagite outcrop and the first resistive limit, in a narrow 
belt characterized by a high alteration degree and, 
probably, by an intense shallow fracturing.
Conclusions
The results of this study provided specific information about soil-gas permeability on the Orciatico clay units characterised by different degrees of thermal alteration. This research represents the first study performed in thermally and mechanically altered clays and results 
demonstrated that the method gives interesting information also in clays that apparently did not undergo to mineral and geotechnical variations. Radon and carbon dioxide soil-gas anomalies seem to be concentrated in zones where Selagite outcrop and thermally altered clays 
may be inferred: regional trend (that excludes possible local effects on soil-gas concentrations) of soil-gas distribution highlights the strong difference of radon and carbon dioxide concentrations over Selagite and over unaltered clays.
These distributions are interpreted as being due to intense shallow fracturing of clays along the inferred laccolith boundary: the intrusion of the laccolith caused thermo-hydro-chemical and thermo-hydro-mechanical stress and contact metamorphism in the clay. Soil-gas results 
in the Orciatico area were supported by detailed geoelectrical survey: the main conductive lineaments representing highly permeable zones within the clays, were outlined in an area where highest Rn and CO2 soil-gas anomalies occur. This study allowed to highlight the role of
soil-gas technique for the identification of secondary permeability in a clay sequence. In particular, at Orciatico site, this secondary permeability was found along the boundary of the resistive complex forming a lesser impermeable barrier for naturally migrating gas. It means 
that a clay sequence, if  affected by thermal alteration, can strongly modify its characteristics and the effect is not always visible through traditional investigative methods.
Acknowledgments
This research was funded by EC project (Contract FI4W/CT95/0014, “Natural Analogue of the Thermo-hydro-chemical and Thermo-hydro-mechanical Response of Clay Barriers”):the authors would like to thank all partners available for exchanging information in the framework of the project.
Figure 1. Geological sketch map of the Orciatico igneous body (from Conticelli et 
al., 1992) and soil-gas surveys. A first survey (July-September 1997)  was 
performed along a regular grid with a sampling density of about 500 
samples/km2 (dark dots). The two dashed rectangles indicate two monitored 
zones (from April to September 1998)  in order to study seasonal influence and/or 
local factors (i.e. seasonal vegetation) on soil-gas distributions.
1) Pliocene Quaternary sediments; 2) Miocene lacustrine and marine sediments; 
3) thermally metamorphosed rocks (Termantite); 4) Orciatico orendite; 5) faults 
and inferred faults. 
Figure 2. Radon and carbon dioxide distributions in soil-gases. Anomalous values (CO2 >2 %,v/v, Rn >25 Bq/l) are in correspondence of the boundary of the resistive complex supposed 
on geoelectrical results. 
Lombardi° S. and Voltattorni* N.
°Fluid Chemistry Lab., University “La Sapienza” , Piazzale Aldo Moro n° 5, 00185 Rome (Italy)           : salvatore.lombardi@uniroma1.it
*Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata n° 605, 00143 Rome (Italy)        : nunzia.voltattorni@ingv.it
Normal Expected Value
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-2 -1 0 1 2 3
Unaltered host clays
Thermally altered clays
(Termantite)
Selagite outcrop
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Distance (km)
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CO2
Rn
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Resistive limit
Legend
Figure 3. Merged probability plots of CO2 and 222Rn. The plots were elaborated in order to 
highlight the gas concentration differences on Selagite outcrop, methamorphosed clays and 
intact (un-methamorphosed) clays. There is a general decreasing trend of the measured 
concentrations in the three different sites, showing the lower values in the un-
metamorphosed clays. 
Figure 5. Comparison between polynomial regression (3° degree) map and geoelectrical
profile n°4. Radon graph (continuous line) highlights a general slightly decreasing trend 
of soil-gas values towards the NE, from Selagite outcrop until un-metamorphosed clays. 
The same behaviour is well evident also for CO2 polynomial regression (dashed line).
Figure 4. Polynomial regression (5° degree) of radon and carbon dioxide 
concentrations at the "casa Pietroia" area during six month monitoring (April-September 
1998). The left axis represents carbon dioxide values (straight line) whilst on the right 
axis there are radon values (dashed line). Graphs highlight a general decreasing trend 
of soil gas values towards NE. Results from these monthly surveys showed that in spite 
of seasonal variations, soil gas concentrations remain constant.
"background"
area
"Casa Pietroia"
area
0             0.2 km
6
References
•Conticelli, S., P. Manetti and Menichetti, S. (1992), Mineralogy, geochemistry and Sr-isotopes in orendites from South Tuscany, Italy: constraints on their genesis and evolution. Eur. J. Mineral., 4, 1359-1375.
•Dayal, R. and Wilke, R.J., 1982. Role of clay minerals as backfill in radioactive waste disposal. Proc. Int. Clay Conf. Bologna]Pavia, 1981, pp. 771--787.
•Hueckel, T., and Pellegrini R. (2002). Reactive plasticity for clays:application to a natural analog of long-term geomechanical effects of nuclear waste disposal. Engineering Geology, 64, 195-215.
•Leoni, L., Polizzano, C, and Sartori F. (1986). Nuclear waste repositories in clays: the Orciatico methamorphic aureole analogy. Applied Clay Sequence, 1, 385-408.
University of Rome
“La Sapienza”
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Soil-Gas Geochemistry as Permeability Tracer of Thermally Altered Clays at Orciatico (Tuscany, Central Italy)

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Soil-Gas Geochemistry as Permeability Tracer of Thermally Altered Clays at Orciatico (Tuscany, Central Italy)

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