Mass physical properties of muddy intertidal sediments: some applications, misapplications and non-applications

Mass physical properties of sediments are increasingly receiving attention outside the traditional fields of soil mechanics, geotechnical engineering and engineering geology because they are being recognised as important physical process-response parameters in sediment dynamics, benthic ecology, microbiology and biogeochemistry. In this study systematic relationships between bulk density, water content and sediment composition are presented for a variety of geographic environments. In all cases high correlations between these parameters are observed, all regions showing characteristic trends reflecting local environmental conditions. In this context, absolute water content is shown to be a universal master variable by means of which differences between individual environments can be normalised. It is postulated that relationships between water content and any other sediment parameter can be established by generating calibrations validated by carefully selected data bases which cover local ranges of sediment composition. Such site-specific calibrations can be used in regional and inter-regional modelling exercises. Thus, a universal negative relationship between absolute water content (W(a)) and dry bulk density (BD(d)) of common terrigenous material is expressed by the equation BD(d) = 2.6596369 - 0.0886164W(a) + 0.0088041W(a)(1.5) - 0.0002594W(a)(2) (r = - 0.991, n = 112). An extensive literature survey reveals that the term "concentration", which refers to a mass per unit volume, is frequently confused with the term "content" which refers to a mass per unit mass. It is demonstrated that this widespread malpractice has been responsible for serious misinterpretations of otherwise perfectly good data because quantitative comparisons are being made between parameters having different physical dimensions. In other cases, it has prevented the recognition of well-correlated relationships, resulting in incomplete arguments or unfounded speculations. In view of this, we advocate a stricter adherence to the correct use of these defined terms. We also propose that the terms "enrichment" or "impoverishment" be used when increases or decreases in content are meant, whereas the terms "condensation" or "dilution" should be used to indicate increases or decreases in concentration. Finally, we contend that more attention should be paid to the use of volumetric, rather than the traditional gravimetric units in sedimentary ecology, including biochemistry and ecotoxicology. (C) 2000 Elsevier Science Ltd. All rights reserved.