Geomorphology 58 (2004) 323–338 www.elsevier.com/locate/geomorph Testing the potential of geochemical techniques for identifying hydrological systems within landslides in partly weathered marls T.A. Bogaard*, J.T. Buma, C.J.M. Klawer Department Physical Geography, Faculty of Geographical Sciences, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, The Netherlands Received 19 December 2002; received in revised form 25 July 2003; accepted 7 August 2003 Abstract This paper’s objective is to determine how useful geochemistry can be in landslide investigations. More specifically, what additional information can be gained by analysing the cation exchange capacity (CEC) and cation composition in respect to the hydrological system of a landslide area in clayey material. Two cores from the Boulc–Mondore`s landslide (France) and one core from the Alvera landslide (Italy) were analysed. The NH4Ac and NaCl laboratory techniques are tested. The geochemical results are compared with the core descriptions and interpreted with respect to their usefulness. Both analysis techniques give identical results for CEC, and are plausible on the basis of the available clay content information. The determination of the exchangeable cations was more difficult, since part of the marls dissolved. With the ammonium-acetate method more of the marls are dissolved than with the sodium-chloride method. The NaCl method is preferred for the determination of the cation fractions at the complex, be it that this method has the disadvantage that the sodium fraction cannot be determined. To overcome this problem, it is recommended to try other displacement fluids. In the Boulc–Mondore`s example, the subsurface information that can be extracted from CEC analyses was presented. In the Boulc–Mondore`s cores deviant intervals of CEC could be identified. These are interpreted as weathered layers (and preferential flow paths) that may develop or have already developed into slip surfaces. The major problem of the CEC analyses was to explain the origin of the differences found in the core samples. Both Alvera and Boulc–Mondore`s examples show transitions in cation composition with depth. It was shown that the exchangeable caution fractions can be useful in locating boundaries between water types, especially the boundary between the superficial, rain-fed hydrological system and the lower, regional groundwater system. This information may be important for landslide interventions since the hydrological system and the origin of the water need to be known in detail. It is also plausible that long-term predictions of slope stability may be improved by knowledge of the hydrogeochemical evolution of clayey landslides. From the analysis, it is concluded that geochemistry is a potentially valuable technique for landslide research, but it is recognized that a lot of work still has to be done before the technique can be applied in engineering practice. D 2003 Elsevier B.V. All rights reserved. Keywords: Landslide; Geochemistry; Hydrology; Weathering 1. Introduction In the study of hydrological processes in land- * Corresponding author. Tel.: +31-302532776/2749; fax: +31- slides, little attention has been paid to techniques to 302531145. determine the origin of groundwater and groundwater E-mail address: [email protected] (T.A. Bogaard). flow within landslides. Knowledge of the groundwa- 0169-555X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2003.08.001 324 T.A. Bogaard et al. / Geomorphology 58 (2004) 323–338 ter system in landslide areas is important when (cu- (CEC) and cation composition in respect to the mulative) precipitation is related to mass movement hydrological system of a landslide area. via increases in pore water pressure. Often, a black In joint research of the EU-funded HYCOSI (Leroi, box modelling approach is adopted to relate mass 1997) and NEWTECH (Corominas et al., 1998) pro- movement to precipitation. However, without knowl- jects, CEC, exchanger composition and carbonate edge of the hydrological system it can be difficult to analyses were executed on samples of three cored set up a model for predicting changes in landslide drillings, two from the Boulc–Mondore`slandslide activity due to stabilising measures, land-use or cli- complex in France and one from the Alvera landslide mate change. To predict slope movement response to in Italy. Cation exchange capacity and exchanger precipitation it is important to know the extent of the composition were measured by two different labora- area feeding the landslide with groundwater and to tory techniques. The results were then compared with have a knowledge of the internal flow system thereby the geological descriptions of the drillings. enhancing the perceptual model of the appropriate process mechanisms. For this purpose, hydrochemical research can be 2. Drilling and laboratory methods useful. Appelo and Postma (1993) write: ‘‘Ground- water chemistry also has a potential use for tracing the A rotation, double envelope, 115 mm diameter origins and history of water. Water composition drilling technique was applied, with a cable sampler changes through reactions with the environment, and to facilitate sampling. All these drillings used local water quality may yield information about the envi- surface water as drilling fluid. Penetration of drilling ronment through which the water circulated.’’ fluid into the soil core is assumed to be negligible. In addition to hydrochemical methods, geochemi- From the Boulc–Mondore`s landslide complex cal techniques can be of value, since groundwater samples were taken from a ‘cored drilling’ (core A) chemistry and soil geochemistry interact. The high and a ‘destructive drilling’ (core B). From the former vertical resolution that can be obtained by geochem- all material is lifted undisturbed and stored in wood- ical analyses of soil profiles may not only reveal the en ‘core boxes’. The latter core was taken more broad (hydro)geological structure of a landslide, but rapidly and stored in 2 m long PVC tubes, slightly may also provide indications for zones that are im- disturbing the sample. In both cases, the total core portant for slope movement, such as slip surfaces or length was saved, 21 m of core A and 25 m of core preferential flow tracks. B. The upper 5–6 m were lost during drilling Furthermore, changes in exchanger composition of because the material was flushed with drilling fluid. subsurface materials due to interactions with ground- Plasticity of material, progress of drilling and occur- water can induce changes in soil shear strength, and rences of water-bearing layers were described during thus form another subject of geochemical investiga- drilling. In a later stage, the cores were described tions on landslides. The relation between catastrophic hydrogeologically (lamination, fractures, fissures and quick-clay landslides and freshening of pore water in secondary calcite precipitation) and subsamples were Norwegian and Canadian marine clays was already taken. Twenty samples of 5–10 cm were taken from recognized by, e.g., Bjerrum (1954) and Hutchinson the undisturbed core A and 12 from the disturbed (1961) and still receives a lot of attention (see Senne- core B. The core descriptions give information about set, 1996). The type and concentration of salts in pore disturbance of the material during drilling, which water exert a significant influence on residual shear may occur when the core cylinder is full, or when strength of e.g., flysch clay (Michaelides, 1995) and less coherent material is encountered resulting in several clay types from Italian slopes (Di Maio, 1996). twisting of the sample. This relates to local clay mineralogy. In Alvera, eight subsamples from the borehole The objective of this paper is to determine how were taken. The drilling had a total length of 24 m. useful geochemistry can be in landslide investigations. The drilled core was stored in a moist state in sealed More specifically, what additional information can be plastic bags, so some chemical alteration prior to gained by analysing the cation exchange capacity analysis may not be excluded. T.A. Bogaard et al. / Geomorphology 58 (2004) 323–338 325 A brief description is given of the laboratory It furthermore can point out differences in lithology techniques for sample preparation, determination of within a drilling, e.g., secondary calcite deposition. exchanger composition, cation exchange capacity Also, unmarked double samples (duplicates) were (CEC) and carbonate determinations. included from each core to test the reproducibility of The samples were air dried, crushed manually with the analyses of both techniques. a mortar and sieved repeatedly with a 300 Am sieve, until all material had passed the sieve. Exchanger composition and CEC were analysed with a ‘displace- 3. Background of the test sites ment after washing technique’ (Thomas, 1982) using two different salt solutions: ammonium acetate and 3.1. Boulc–Mondore`s sodium chloride. In the ammonium acetate (NH4Ac) technique, the The Boulc–Mondore`s landslide complex (Fig. 1) cations at the exchange complex are displaced with 1 was one of the study sites of the HYCOSI-project, MNH4Ac. The concentration of the cations in the which studies the effects of hydrometeorological fluid is analysed using ICP-AES. The CEC is deter- changes on slope stability (Leroi, 1997). It is situated mined by treating the sample with 1 M sodium acetate in the department Droˆme in the French pre´-Alps. The (NaAc), resulting in only exchangeable Na+, the so- area consists of Mesozoic limestone and marls and has called ‘index cation’. The sample is washed with 96% a polyphase structural history from Trias to Tertiary ethanol, removing excess Na+. The sample is treated (Bogaard et al., 2000). The main geological structure + with NH4Ac extracting all Na from the exchange is an N–S running graben crossing the landslide. In complex. Subsequently, the Na+ is measured. The the graben deposits from the Lower Cretaceous are total Na+ concentration equals the CEC. found, east of the graben there is a sequence from the The sodium chloride (NaCl) technique follows the Upper Jurassic and west of the graben an anticline same principle.