On the recognition of areas with problematic sulphidic sediments using multi-disciplinary data

Abstract

Increased concentrations of several trace elements have been observed in water draining areas with sulphidic sediments along the Baltic Sea coast of Sweden and Finland. Some of these elements are harmful to the water environment. The increased leaching of trace elements is often caused by antrophogenic activities, such as ditching, which cause oxidation of the sulphides and thereby acid soil conditions. It is therefore important to recognise areas with sulphidic sediments and, if possible, avoid land use, which causes sulphide oxidation. The aim of this study was to find time effective methods, which can be used to delineate areas with sulphidic sediments. Three areas in Sweden with known occurrences of sulphidic sediments were studied. Data obtained from the mapping program at the Geological Survey of Sweden (SGU) were first studied. Selected sites were thereafter visited and geoelectrical measurements were carried out by the use of several instruments. The results imply that areas with sulphidic sediments are reflected as areas with high trace element contents on SGUs biogeochemical maps, which are based on analyses of trace elements in aquatic plants. The same deposits are often shown as gyttja clay on the maps of Quaternary deposits. Furthermore models derived from airborne electromagnetic measurements show that areas with sulphidic sediments often have a lower electric resistivity than surrounding sediments. Also the results from ground geoelectrical measurements suggest that the sulphidic sediments have a lower resistivity than surrounding fine-grained deposits. That interpretation was verified after stratigraphical studies and analyses of the sulphur contents of the sediments. It is therefore suggested that data from ground geophysical measurements can be used for a detailed delineation of sulphidic sediments, whereas data obtained from airborne geophysical and geological maps give an overview over potential areas with these sediments. The absolute resistivity of the sulphidic sediments varies, however, between the three investigated areas. Complementary corings and stratigraphical descriptions are therefore always necessary to verify the occurrence of these sediments.

Appendix 1

EnviroMT

The EnviroMT has been mainly developed at the Department of Earth Sciences of Uppsala University. The instrument uses RMT (RadioMagnetoTelluric) method. RMT is an electromagnetic method that makes use of the signal in the frequency range between 15 and 250kHz from long-distance radio transmitters. Two components of the electric field and three components of the magnetic field of the signals are measured at the same time. The ratio between the horizontal electric and the horizontal magnetic field components for each measuring frequency is directly related to the electrical resistivity of the ground. For a homogenous half space the resistivity, ρ, is given by

$$ \rho = \frac{1} {{2\pi f\mu _{0} }}{\left| {\frac{{E_{x} }} {{H_{y} }}} \right|}^{2} $$

where f is the frequency, μ ₀ the magnetic permeability of free space, E _x the electric field component in x direction and H _y the magnetic field component in y direction. x and y directions are dependent on the measuring coordinate system. X is usually in the magnetic north and y in the magnetic east direction. The signal at lower frequencies penetrates deeper into the ground and letting us to model the ground resistivity with depth.(The measured electric and magnetic fields at each frequency and station are transformed into the resistivity and phase. The measured values are then used to model the resistivity variations along and under the profile. The spacing for EnviroMT measurements was 10m along the profiles. The EnviroMT has been used in earlier studies dealing with the stratigraphical distribution of Quaternary deposits (Bastani 2001; Pedersen et al. 2005).

Lund imaging system

The ABEM Lund Imaging System (Dahlin 1996) is a multi-electrode resistivity system. In the dc resistivity method an electrical current is injected into the ground and the resulting potential distribution is measured. By altering the distances between the electrodes, information about resistivities on different depths is obtained. In the multi-electrode system a large number of steel electrode are placed along the profile and connected to multi-core cables with one take-out for each electrode. The systems use an electrode selector, a switching unit that is controlled by the computer to select the appropriate electrodes for each measurement.

A large number of electrodes (up to 64) were placed at the ground and the resistivity at different depth was then automatically measured along the profile. An electrode separation of 5m were used for all profiles.

OhmMapper

The Geometrics OhmMapper is a capacitively coupled resistivity meter that measures the electrical properties of the ground without the use of electrodes as in traditional resistivity surveys. This technique is based on the capacitive coupling mechanism between sensors and the ground, in contrast to resistivity techniques that are based either on galvanic coupling, e.g., dc resistivity or inductive coupling electromagnetic EM techniques (Kuras et al. 2006). The ground resistivity is continuously measured along profiles, which admits a fast collection of data. This instrument has proved to be useful for detailed mapping of the ground resistivity (Walker and Houser 2002). The data were acquired 4 times per second (ca. 25cm station spacing) that provided a high resolution along the profiles. The data used here contain measurements with four transmitter-receiver separation (2, 5, 10, and 20m).

Resistivity probe

The resistivity probe has an electrical sensor, which measures the resistivity in a soil volume with a radius of a few centimetres. The probe is manually pressed through the sediments. The resistivity were measured every 25cm and in some areas it was possible to reach a depth of more than 10m. The probe was developed by Puranen et al. (1997) at the Geological Survey of Finland (GTK) and has earlier been used to study the properties of sulphidic sediments (Suppala et al. 2005).