Studio idrogeologico

ADDITIONAL INFORMATION ON THE ROLE AND THE RELATIOSHIPS AMONG
THE HYDROGEOLOGICAL UNITS IN THE VICANO-CIMINI MTS DISTRICT

Abstract:

The aim of the present stage is the definition of the main hydrogeological units in the region
between the Vicano-Cimini Mts Volcanic District (DVVc) and the sedimentary deposits of the
River Tiber Valley. The study was carried out into two phases: the collection of scientific literature
information and the collection of stratigraphic information of wells deriving from the L.464/84
database at the Department of Land Resources and Soil Protection of APAT for the drawing of
hydrogeological sections.
In the study area volcanic rocks widely crop out and subordinately, along the drainage
directions, Plio-Pleistocene sedimentary deposits occur. The volcanic rock deposition in the Latium
region, in which the study develops, started from Upper Pliocene, along a trough (the so called
“main graben” belt) parallelly disposed with respect to the Tyrrhenian coastline. The volcanic rocks
occurring in the study area belongs to the Vicano-Cimini Mts Volcanic District (DVVc), formed by
eruptions derived from a central volcanic apparatus. The DVVc (800-90 ky) is composed of
different volcanic products with time: at first either latitic and trachi-basaltic either tephriticphonolitic/
phonolitic-tephritic lavas of the “high-K series” were erupted; in the final stage, wide
pyroclastic rocks were emitted (the so called A, B, C and D Ignimbrite formations).
The sedimentary units cropping out in the study area are constituted by silty sand and sandy clay
of the Chiani-Tevere Formation (Gelasian-Santernian) and by gravel and sand deposited in the
conglomeratic sedimentary basin of Civita Castellana (Neogenic-Quaternary).
In the study area the hydrogeological unit of the Vicano-Cimini Mts, in the western sector, and
the hydrogeological unit formed by the sediments deposited in the paleo-course of the River Tiber,
in the eastern sector, were defined. The hydrogeological reservoir of the two previous sectors is
mainly represented by volcanic rocks and by gravel and sand of the stratigraphic unit of Civita
Castellana (UCC), respectively, while the impermeable bedrock is constituted by sandy clay and silt
formation. The quite variable morphology of the impermeable sedimentary bedrock top surface
controls the features of the basal aquifer.
Three hydrogeologic sections were realized in the Middle River Tiber Valley district, mainly
constituted by Vicani volcanic products, with the aim to defining the hydrogeological features of the
local volcanic and sedimentary units and their hydraulic relationships. The constructed sections allow
to understand the geometry and the stratigraphic relationships among the volcanic and sedimentary
formations and the aquiclude bodies.
In agreement with the conclusions of a previous study, the hydrogeological information provided
by the constructed sections have defined that the regional basal aquifer is hosted by the Tephriticphonolitic
Ignimbrite formation and it has an eastwards flowing direction through the gravel and sand
of the UCC and subordinately of the topping Graffignano Unit. The deep and tight valleys occurring
in the study area control the hydraulic circulation in the proximity of the creeks, where the aquifer
feed the streams, which flow in the hydrogeologic basin of the Treia River. Finally, the surface and
groundwater divide role of the impermeable clay and silty sand formations was evidenced.