values are converted by an analogue/digital converter, using a non-linear ... pictures were stored in the TIFF graphic format with a resolution of 200 dpi. All.
Tracer Technologies/or Hydrological Systems (Proceedings of a Boulder Symposium, July 1995). IAHSPubl.no. 229,1995.
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Particle characteristics and their significance in the identification of suspended sediment sources T. UDELHOVEN & W. SYMADER Department of Hydrology, University of Trier, PO Box 3824, D-54286 Trier, Germany
Abstract The precise importance of the properties of suspended particles in controlling the transport of particle-associated contaminants remains uncertain. However, there is little doubt that the characteristics of particles can be used to derive information about particle source. As many particle-associated contaminants are supply controlled, the question of particle source is crucial. There is, however, no general agreement about the characteristics that should be considered. Furthermore, there is still a great demand for simple methods, especially for routine use under dry weather conditions when suspended particle concentrations are small. In addition to grain size distribution, loss on ignition and turbidity, the determination of particle colour also provides a fast and easy approach. After filtering of the suspensions through glass microfibre filters the dried filter residues are scanned by a colour scanner. Examination of the digital pictures is feasible with standard methods of digital image analysis. This method yields a quantitative description of a sample's colour that can be compared and can be used as an independent input into quantitative models.
INTRODUCTION In recent years many studies have shown that both suspended particles and pollutant contamination are primarily supply controlled (Imeson et al., 1984; Walling, 1984; Umlauf & Bierl, 1987; Symader et al., 1994). A common procedure for the detection of particle source uses suspended particle characteristics, including physical, chemical and also more recently biological properties. These are utilized in the classical fingerprinting approach (Walling & Kane, 1984), as well as in the modified approach proposed by Strunk (1992) and Symader & Strunk (1992). However, there is still no general agreement as to which particle characteristics are most important in terms of the particle-associated contaminant load, because many theoretical and experimental approaches do not hold under natural conditions. For example the generally accepted theoretical influence of particle size and particulate organic carbon on the load of organic contaminants (Karickhoff, 1979), could not be identified by Umlauf & Bierl (1987) and Symader et al. (1994) in field investigations. Furthermore, there are major methodological problems in dealing with suspended solid concentrations in the range of only some milligrams per litre. There is still a great need for easy and efficient techniques for routine examination. Examples of commonly applied physical methods include measurements of turbidity, loss on ignition, particle size, particle shape and
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various microscopic techniques. The use of loss on ignition as a measure of a sample's organic content is, however, especially problematic in calcareous drainage basins. One characteristic that has been more or less neglected until the present, is a quantitative description of the suspended particle colour. But in the literature of hydrological remote sensing, there are many examples of applications of spectral pattern measurements. They range from examination of spatial and temporal variations in suspended particle concentration to the identification of chlorophyll, phaepigments and organic substance in oceans and lakes (Tassan, 1990; Home & Wrigley, 1975; Grunwald et al, 1988; Kondratyev & Pozniakov, 1990; Farmer et al, 1983; Morel & Bricaud, 1980). Furthermore, grain size distribution (Bhargava & Mariam, 1991; Novo et al, 1989), mineral composition (Choubey, 1994; McKim et al, 1984), colour and iron oxide content (Torrent etal, 1983; Stoner & Baumgardner, 1981) have a strong impact on the spectral reflectance of suspended solids, sediments and soils.
METHODS AND MATERIALS The gravimetric determination of suspended particle concentration is a commonly used method. A known volume of river water is drawn through a weighted glass microfibre or membrane filter, which is dried and weighed again. The dried filter residue can provide information about the suspended particle concentration and also its colour. Different residue colours are as obvious as the colour changes observed during flood events, which indicate the activation of different particle sources (Strunk, 1992; Grimshaw & Lewin, 1980). A simple way to measure colour of the residue is to scan the dried filters with the aid of a colour scanner. The digitized colour values, which contain the red, green and blue colour information for the residue, can be stored using one of the common graphic formats, for example TIFF or PCX. The scanning operation works as follows: The integrated light source of the scanner with a defined colour temperature illuminates the sample. A special filter system separates the reflected light into its three component colours and leads the focused light beams to a CCD-element. The measured voltage values are converted by an analogue/digital converter, using a non-linear calibration equation, into discrete number values. An eight bit resolution for each colour, that is a total colour depth of 24 bit, provides 256 X 256 x 256 = 16.8 X 106 colours. In contrast, the human eye can "only" differentiate approximately 200 000 colours. In order to compare colour information for different suspended particle samples, the measurement conditions must be hold constant. The scanner must work with a fixed adjustment and, furthermore, filter type and filter residue weights should be always the same. Figure 1 illustrates the relationship between the values for the three colours and residue weight for a single suspended particle sample. It is obvious, that the relationships are different for each colour. The thinner the residue, the more its colour is influenced in a non-linear manner by the underlying white filter. For this reason, the scan procedure for a suspended particle sample at a given concentration always provides a relative and not an absolute colour measurement. This is true if the filter is completely covered up to a residue weight of 40 mg. For colour comparison of different samples at low particle concentrations we propose a defined filter residue weight of 10 mg, but the use of any other concentration is also possible. At this concentration, the filter
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