Reply [to “Comment on ‘An evaluation of mathematical models of the transport of biologically reacting solutes in saturated soils and aquifers’ by Philippe Baveye and Albert Valocchi”

Abstract

Widdowson [this issue] has raised a number of interesting points about our evaluation of mathematical models of the transport of biologically reacting solutes in saturated soils and aquifers [Bayeye and Valocchi, 1989]. He has summarized his remarks in the four points listed toward the end of his comment. In the following paragraphs, we reply to each of these points sequentially. The first point concerns the similarities existing between models I and II. As formulated by Bayeye and VaIocchi [1989], these two models are conceptually very different; the first involves no assumption whatsoever on the microscopic distribution of bacteria or on any geometrical feature of the biomass (e.g., cell size), whereas the second assumes that the cells are grouped into microcolonies having a constant volume. Therefore, these models cannot be said to be identical, as asserted by Widdowson [this issue]. It is, however, possible to transform model II in such a manner that the resulting formulation is essentially equivalent to that of model I. Widdowson et al. [1988] have suggested such a transformation. In the capacitancelike parameter C = (Ds/•5) Ncz'rc 2 of model II [see Bayeye and Valocchi, 1989, Table 1], one may lump the terms Nc,rr• into a single parameter, a, representing the area of the external surface through which the growth-limiting substrate diffuses into the biomass. Even though it still involves a microscopic parameter (/5), the resulting model does not require any assumption on the bacterial distribution in the pore space and thus subscribes to the philosophy underlying model I. However, since it no longer involves microcolonies explicitly, it is not conceptually equivalent to model II. Widdowson [this issue], in his second point, echoes some of the questions raised by Bayeye and Valocchi [1989] concerning the applicability of the biofilm concept in subsurface environments. Further evidence in this respect has been recently obtained in sand columns by P. Vandevivere and P. Baveye (Saturated hydraulic conductivity reduction caused by strictly aerobic bacteria in sand columns, submitted to Soil Science Society of America Journal, 1990). As observed on scanning electron micrographs, the distribution of bacterial cells in the pore space appeared highly heterogeneous, without any noticeable pattern. In particular, relatively large portions of the surface of the sand grains were entirely devoid of adsorbed cells. Even in clogged layers, where bacterial concentrations were found to be as high as 10 •o cells per cubic centimeter of porous medium, no biofilms could be observed.