Structure of weathered and fractured peridotites of New Caledonia: from field data to groundwater modellingJeanpert J.1, Join J-L.², Maréchal J-C.3, Genthon P.4, Sevin B.1, Iseppi M.1, Robineau B.1

1 Geological Survey of New Caledonia, Department of Industry, Mines and Energy, Nouméa, New Caledonia. E-mail: julie.jeanpert@gouv.nc2 University of La Réunion, St Denis, La Réunion

3 French Geological Survey, BRGM, France4 IRD, HydroSciences Montpellier, France

Paper number:

4345

References[1] Cluzel D, Aitchison JC, Picard C. 2001. Tectonic accretion and underplating of mafic

terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific):

geodynamic implications. Tectonophysics 340: 23-59

[2] Join J-L, Robineau B, Ambrosi J-P, Costis C, Colin F. 2005. Système hydrogéologique d'un

massif minier ultrabasique de Nouvelle-Calédonie. Comptes-Rendus Géoscience 337: 9.

DOI: 10.1016

[3] Lei S. 1999. An analytical solution for steady flow into a tunnel. Ground Water 37 no. 1:

23-26.

[4] Dewandel B, Jeanpert J, Ladouche B, Join J-L, Maréchal J-C. in prep. Inferring the

heterogeneity of crystalline aquifers and evaluating transmissivity or hydraulic conductivity

fields, an attempt from a detailed water table map.

AcknowledgementsThe authors would like to thank Koniambo Nickel SAS and Vale NC for providing the deep

boreholes cores.

This work was supported by the Center for Technological Research « Nickel and its

Environment » (Centre National de Recherche Technologique, CNRT « Nickel et son

environnement ») as part of the program HYPERK (2013-2016).

FRACTURED PERIDOTITES AND HYDRAULIC CONDUCTIVITY

FRACTURE DISTRIBUTION AND HYDRAULIC CONDUCTIVITY FROM 1D DATA – MASSIF OF KONIAMBO AND MASSIF DU SUD

HYDRAULIC CONDUCTIVITY FROM 3D OBSERVATIONS : THE CHROMICAL TUNNEL – TIEBAGHI MASSIF

GROUNDWATER MODELLING : THE CASE OF TIEBAGHI MASSIF DISCUSSION AND CONCLUSION

Four 200 m deep boreholes cores of the Massif du

Koniambo (Pz3 to Pz6), and one of Massif du Sud

(10EX0178), are studied and fracture distribution is

described : single fracture and highly weathered

and/or fractured zone are counted (Fig. 3).

Fig. 3 : Fracture counting on 5 deep boreholes cores.

From the five boreholes observations, peridotites

are highly and homogeneously fractured (mean

fracture intensity equals 2,81 fract./m) among the

same massif and between them.

95 values of hydraulic conductivity by Lefranc test

between Packer are available on the 4 boreholes of

Koniambo massif (Fig. 4).

LogK varies between -8.5 and -2.9.

A correlation between fracture distribution and hydraulic conductivity is not clear but data show that :

• mean hydraulic conductivity is logK = -6 in the 200 m deep peridotites of Koniambo massif;

• most of observed fractures on cores are closed in situ and most probably are impervious (serpentinite veins);

• more permeable zones (logK = -5) correspond to highly weathered peridotites.

Fig. 4 : Variation of LogK with

depth (depth from fine and coarse

saprolites interface).

ULTRAMAFIC ROCKS AND WEATHERING

The Peridotite Nappe obducted on New

Caledonia in Late Eocene [1]. Since its

emersion in Oligocene, a deep

weathering profile developed and now

covers the fractured and serpentinized

peridotites. The ultramafic rocks

outcrop over more than one third of

New Caledonia on scattered massifs all

over the island (Fig. 1).

The weathering profile consists in, from

top to bottom, one hard layer of

ferricrete, a semi-impervious layer of

laterites (also termed ‘red laterites’) and

saprolites (‘yellow saprolites’ or ‘fine

saprolites’), and a saprock that consists

of coarse saprolites and a fractured zone

within slightly weathered peridotites,

Fig. 2.Fig. 2 : Regolith profile developed on peridotite in New Caledonia.

Massif du

Sud

Tiébaghi

Massif

Koniambo

Massif

Fig. 1 : The Grande Terre of New Caledonia

(SW Pacific) with ultramafic formations.

pictured in green. Studied massifs are localized :

Tiébaghi, Koniambo and Massif du Sud.

Considering fracturing of peridotites and the fractured bedrock as a constitutive part of the aquifer is a new way of

building the aquifer model as till now substratum of the aquifer was considered a few meters deeper that the bottom

of the saprock layer [2].

A tunnel of an old chrome ore exploitation on the Tiebaghi

Massif (Fig. 5) gives an access to groundwater flow in the

fresh rock peridotites.

Fig. 6 results from stream gauging by dilution on several

fractures outflow. The law built by Lei [3] gives a value of

hydraulic conductivity from the outflow values.

Using topographic elevation, the correlation between

hydraulic conductivity and capping thickness results in a

linear law (Fig. 7) and shows that the hydraulic conductivity

of the fresh peridotites decays with depth.

Fig. 5 : Chromical tunnel in the Tiebaghi Massif.

Fig. 6 : Location of underground fracture outflow and

hydraulic conductivity estimation on topographic profile.

Fig. 7 : Linear law between logK and capping

thickness for the fresh peridotites of the Tiebaghi

Massif. The red point corresponds to a higher value, result of an

issue of field measurement, a direct inflow due to an old

quarry above, or a more weathered fractured zone (as seen

on Fig. 4).

FRACTURED PERIDOTITES AND HYDRAULIC CONDUCTIVITY

A numerical model is built with Feflow®.

Boundary conditions are :

• Hydraulic head = elevation downstream;

• Fluid-flux = 0 laterally and upstream ;

• Recharge = 100 mm/yr.

The geological observations (Fig. 8) are used

to build a multilayered model with given

hydraulic conductivities (Fig. 9).

Geometry and distribution of K within the

profile are consistents with boreholes data

and hydrodynamic investigations.

The results (Fig. 10) from steady flow

simulation in a vertical confined 2D aquifer

produce both a realistic piezometric and

hydraulic head distribution and the

observed springs at the top of the bed rock

in the breaking slope of the Tiebaghi

Plateau. Fig. 10 : Results of the Feflow 2D model.

Fig. 9 :

Multilayered

and

conductivity of

Tiebaghi

model.

Measurements and modelling of fractures and hydraulic conductivity of

peridotites show that :

• Fracture is intense and homogeneous in peridotites;

• Hydraulic conductivity of peridotites depends on depth and varies

between logK=-6 and logK=-8;

• 2D modelling confirms the multilayered aquifer and the importance

of bedrock thickness and conductivity;

• Tiebaghi Massif model is consistent with measured hydraulic

conductivity and hydraulic head distribution;

• 3D modelling will evaluate anisotopy and homogeneity of

conductivity.

Fig. 8 : Geological structure of Tiebaghi aquifer.