CONCETTUALE DELL’ACQUIFERO DI - GIT · 3) Fans, subdivided in 3 ... functions, variogram.. • 3D spatial distribution of ... Studio idrogeologico per la bonifica del piano della

GIT – Geology and Information Technology 8a Riunione del Gruppo di Geologia informatica

Sezione della Società Geologica Italiana Chiavenna (So), 17 - 19 giugno 2013

MODELLO IDROGEOLOGICO

CONCETTUALE DELL’ACQUIFERO DI

FONDOVALLE VALTELLINESE

A. Merri, T. Bonomi, L.Fumagalli, S.Dubricich*

Dip. di Scienze dell’Ambiente e del Territorio e di Scienze della Terra,

Università degli Studi di Milano-Bicocca

* Provincia di Sondrio, Settore Agricoltura, Ambiente, Caccia e Pesca



Context: scientific collaboration between University of Milano-

Bicocca and the Province of Sondrio.

Goal:

• Collect, codify and organize well and borehole information.

• Define the hydogeologichal structure of the alluvial valley

aquifer.

• Characterize the hydrogeologichal properties of the

recostructed hidrostratigraphic units

Location:

Lower part of

Valtellina, from

Como lake to

Tirano (70 km

length)

CHIAVENNA

SONDRIO



WORKFLOW



Morphology: Longitudinal profile

• Plain and step shape

100

200

300

400

500

m slm

0 10 20 30 40 50 60 Km

Morbegnol.Como Talamona Rohn SernioSondrio

Tirano



Morpology: Alluvial and debris fan characterization

• Fans constitute a quite continuous belt at the slope feet

• Fan deposits are eteropic and interfingered with the surrounding

alluvial and lacustrine sediments

• Fans can be divided into tree groups:

– Alluvial fan (outlet of the main lateral valleys), characterized by gentle slope,

remarkable vertical extension and usually high permeable material.



Morphology: Alluvial and debris fan characterization

– Detrital, debris and slide fan, characterized by modest feeding basins, step

slope, and usually made up by diamicton with high fine material content.

Permeability of these bodies is poor.

– Mixed fan, characterized by with intermediate properties between the

previous 2 groups.

• Detrital fans are the most lateral extended, and at least in tree

cases dammed the Valley



Data collection

About 500 well (mainly shallow) coded and stored in

hydrogeological DB (www.tangram.samit.unimib.it)

Well depth

8

58

191

106

53

23 18 166 2 5 6 1

0

50

100

150

200

250

N.D

.

<10

10-19.9

20-29.9

30-39.9

40-49.9

50-59.9

60-69.9

70-79.9

80-89.9

90-99.9

100-10

9.9

>=11

0

well depth (m)

nu

mb

er

of

well

s

• Location

• Administrative information

• Technical data

• Coded stratigraphy



Data collection

• 300 geognostic survey

• 250 Vertical Electrical Sounding (V.E.S), often reaching

depth over 200 meters

• 1 high resolution seismic line: Teglio section



Methodology

Teglio seismic line • Substrata depth ~ 500m

• Clinostratified and

chaotic basal sediments

(1-6)

• DSGSD on the N side (7)

• Highly deformed

sediments on the sliding

wedge front (8)

• Anticline deformation of

deposits (9)

• Lacustrine sediments

below 180m, thickness

> 100 (9)

• Coarse sediments from

180 to 40 m

• Silt, sand and peat s.

from 40 to 10 m

• Alluvial deposits from 10

m to the top.



Methodology: from Teglio line to aquifer structure

1. Basal body: glacial diamicton and gravitative sediments

2. Thick lacustrine body, infill of the ancient extension of the Present

Como Lake (200 m slm). No data on the maximum lake extent,

speculated near Tirano.

3. Sudden increase of the paleolake level until 260-270 m asl, interpreted

as the result of a landslide obstruction event: Tartano/Talamona Fan,

30 km downstream. Local transgressive trend.

4. Sealing of the lacustrine depression by a deltaic sequence.

5. Fluvial and fluvioglacial deposition.



Methodology: from Teglio line to aquifer structure

6. Slide event on the Ponte detrital fan Obstruction of the Valley,

formation of a lake (40/50 m. max depth, 10 km long).

7. Thick coarse body downstream the fan for the accumulation of the

fan eroded materials

8. Alluvial fan at the mouth of the main lateral valleys (for ex. Sondrio),

create local aquifer bodies eteropic with the surrounding lacustrine

and alluvial sediments



3D section of the aquifer

between Tirano and Sondrio



Methodology: definition of the bodies geometry

Geometry of the main hydrostratigraphic units have been reconstructed on the

basis of the stratigraphic available data.

4 main unit identified and geometrically defined:

3. Sandy silt body

4. Basal coarse body

1. Phreatic aquifer

2. Fans




1) Phreatic aquifer: this unit is mainly composed of alluvia and fluvial

sediments. Base: roughly interpolated form the top of the lacustrine layer




2) Sandy silt body: filling of the ancient Como Lake

4)coarse basal body




3) Fans, subdivided in 3 groups: alluvial (dark red); detrital (brown) and

mixed (light red).



From stratigraphic log to hydrogeological properties

(porosity and permeability)

• Query of the stratigraphic DB: the

stratigraphic log is sampled at user

defined intervals. Fraction content of

each component is extracted for each

layer.

• DB of property for pure stratigraphic

component

• Sample properties: calculate as log and

simple weighted mean of the actual

stratigraphic fraction.

Weighted log mean of k Weighted mean of n

LEVEL PERMEABILITY LEVEL POROSITY

Weighted log mean……

…………………….

Weighted mean…….

……………….



Metodology: definition of the bodies geometry

GOCAD: 3D geologic object modeling software

Stratigraphic data imported as textural and/or hydrogeological properties



Methodology: hydrogeological aquifer model: from

geometry to unit properties characterization

• Stratigraphic log: translated in

georeferenced k and n values,

sampled every 1 meter

• Statistical analysis of data,

descriptive stats, distribution..

• 3D geostatistical analysis:

trend, spatial correlation

functions, variogram..

• 3D spatial distribution of

parameters: 3D kriging and

Gaussian simulation

• Independent simulations for

every hydrogeological unit



Results: superficial unit



Results: lacustrine unit



Results:

lacustrine unit

Results: grid layers



Results: aquifer sections



Conclusions

• Well data and other available stratigraphic information have been

collected and reorganized.

• Hydrogeological conceptual model of the aquifer have been

defined.

• Hydrostratigaphic bodies have been characterized in term of

permeability and porosity.

Developement

• New data are needed for the validation of the model, especially for

the deeper sectors of the aquifer

• A flow model is under construction



Thank you for attention!



References

• Bonomi T. (2007). Database development and 3D modeling of textural variation in

heterogeneous, unconsolidated aquifer media: application to the Milan plain. Computer &

Geosciences 35 (2009) 134-145

• Bonomi T, Cavallin A, De Amicis MGM (1995). Un data base per pozzi: TANGRAM.

QUADERNI DI GEOLOGIA APPLICATA, vol. 3, p. 461-465

• Colombera, L. e Bersezio, R. (2011). Impact of the magnitude and frequency of debrisflow

events on the evolution of an alpine alluvial fan during the last two centuries: responses to

natural and anthropogenic controls. Earth Surf. Process. Landforms 36, 1632–1646 (2011)

• De Franco R, Biella G, Caielli G, Berra F, Guglielmin M, Lozej A, Piccin A, Sciunnach D.

2009. Overview of high resolution seismic prospecting in pre‐Alpine and Alpine basins.

Quaternary International 204: 65–75.

• Petrucci F, Careggio M, Cavazzini R. 1982. Indagini geofisiche sul fondovalle valtellinese

(dal lago di Como a Teglio). Geografia Fisica e Dinamica Quaternaria 5: 207–218.

• Scesi L, Pellegatta T. 1984. Studi geologici sulla Valtellina: fondovalle tra Colico e Fusine.

Le Strade 1215: 399–416.

• Scesi L.(1982) – Studio idrogeologico per la bonifica del piano della Selvetta (Sondrio). Le

Strade, Anno LXXXIV, Casa Editrice la Fiaccola, Milano.

• www.tangram.samit.unimib.it, Copyright©2013 Unimib, ’Università di Milano-Bicocca.

Riferimenti: Bonomi T., Cavallin A., De Amicis M., 1995.

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CONCETTUALE DELL’ACQUIFERO DI - GIT · 3) Fans, subdivided in 3 ... functions, variogram.. • 3D spatial distribution of ... Studio idrogeologico per la bonifica del piano della