Geochemical mapping using a geomorphologic approach based on catchments

Abstract

The Extended Sample Catchment Basin (ESCB) mapping technique, discussed in this paper, can be used to display the spatial distribution of geochemical variables measured in stream sediments taking into consideration the geomorphologic settings and the hydrographic patterns of surveyed areas. This approach is based on the association of an area of statistical representativeness with each sample, and on the assumption that the concentrations measured in the stream sediments can be considered as average reference values for this area.

ESCBs can be easily identified considering the position of the sampling points within the hydrographic network and using the confluences between the streams of highest rank as break points for representing changes of the geochemical background. This approach, different from the traditional geostatistical and deterministic ones, does not consider the Euclidean distance among the sampling points as a measure of geochemical similarity but only refers to their functional relationship along the streams (following the water and the sediment flow) to measure their proximity.

ESCBs can be seen as a specific development of previous techniques based on catchments and proves to be especially useful for supporting land planning in a preliminary survey phase while it is not specifically suitable for the identification of point sources of geochemical anomalies.

Due to the fractal nature of the hydrographic network, all the procedures can be driven in a GIS environment by using digital terrain models apart from their spatial resolution.

Introduction

Stream sediments are commonly used in environmental and exploration geochemistry to measure the average concentration of chemical elements in soils and rocks over wide areas and to identify possible sources of anomalies. A number of mapping techniques, such as point symbol maps, areal maps, worm diagrams and contour maps, have been developed to display the geochemical characteristics of stream sediments and their spatial representativeness (Govett, 1983).

The spatial behavior of geochemical variables has been also investigated by means of geostatistical techniques based on autocorrelation principles (Matheron, 1965, Wackernagel, 1995, Webster and Oliver, 2001). Geographically weighted regression techniques have also been developed to meet the necessity to partition the study area into more homogeneous sectors (e.g. Fotheringham et al., 2000). The efficiency of the above-mentioned techniques is closely dependent on the selection criteria of the loadings to be used for weighting the contributions of the points where the variables have been measured. Some relatively recent approaches, based on the conversion of spatial distribution of variables into the frequency domain, rely on multifractal models to separate anomalies from background values (Cheng et al., 1994, Cheng et al., 1996, Cheng et al., 2000, Cheng, 1999). Fractal geometry has also been used to characterize geochemical landscapes by means of stochastic simulation techniques in the case of low spatial resolution of the geochemical data sets available on stream sediments (Rantitsch, 2001).

Even if these mapping techniques prove to be helpful to build mathematical models of the spatial distributions of the variables and to identify subtended processes at different scales, they only partially or indirectly face the problem of the geographic origin of stream sediments and of the close dependence of their composition from the local geomorphologic processes. Catchments have been used in a number of scientific papers to take into account the provenance area of sediments for developing models of geochemical dispersion (Moon, 1999), mapping anomalous concentrations (Rantitsch, 2004) but also for simply representing geochemical baselines of selected areas (e.g. Bonham-Carter et al., 1987, Ódor et al., 1997, Spadoni et al., 2003). In a previous work Spadoni et al. (2004) emphasized the usefulness to adopt “sample catchment basins” (SCB) as map units. This geographically based approach produces choropleth maps where catchments areas are assumed to be represented, on average, by the same concentrations of the nearest sample downstream. The resulting maps prove to be useful in defining the source areas of geochemical concentrations and, in case, of localized geochemical anomalies. This technique also avoids “mathematical interference” between neighbouring samples. This is a very important aspect especially when neighbouring samples are representative of different catchment systems. However, the SCB approach is not able to produce statistical inference and also shows some remarkable inconsistencies at the closing points of sub-catchments. According to this technique, the concentration value at the sampling point is considered representative only of the catchment upstream. As a consequence, sampling points act as break points in the maps and as conceptual breaks in illustrating the spatial distribution of the variables. It means that the concentration values estimated on the two sides of sampling points (upstream and downstream) are different. This incongruence is of primary importance when facing the problem of a more realistic spatial representation of the geochemical processes especially when considering that the selection of sampling points placement along the hydrographic network is generally due only to practical considerations (e.g. site accessibility) and the need to follow predefined rules of stochastic sampling.