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 phys ical process-response parameters in sediment dynamics, benthic ecology, mic robiology and biogeochemistry. In this study systematic relationships betwe en 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 r eflecting local environmental conditions. In this context, absolute water c ontent is shown to be a universal master variable by means of which differe nces between individual environments can be normalised. It is postulated th at 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-spec ific calibrations can be used in regional and inter-regional modelling exer cises. Thus, a universal negative relationship between absolute water conte nt (W-a) and dry bulk density (BDd) of common terrigenous material is expre ssed by the equation BDd = 2.6596369 - 0.0886164W(a) + 0.0088041W(a)(1.5) - 0.0002594W(a)(2) (r = - 0.991, n = 112). An extensive literature survey re veals 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 re sponsible for serious misinterpretations of otherwise perfectly good data b ecause quantitative comparisons are being made between parameters having di fferent physical dimensions. In other cases, it has prevented the recogniti on of well-correlated relationships, resulting in incomplete arguments or u nfounded speculations. In view of this, we advocate a stricter adherence to the correct use of these defined terms. We also propose that the terms "en richment" or "impoverishment" be used when increases or decreases in conten t are meant, whereas the terms "condensation" or "dilution" should be used to indicate increases or decreases in concentration. Finally, we contend th at more attention should be paid to the use of volumetric, rather than the traditional gravimetric units in sedimentary ecology, including biochemistr y and ecotoxicology. (C) 2000 Elsevier Science Ltd. All rights reserved.