The biological clogging of natural porous media, often in conjunction with physical or chemical clogging, is encountered under a wide range of conditions. Wastewater disposal, artificial groundwater recharge, in situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The present review provides an overview of the techniques that are used to study clogging in the laboratory, or to monitor it in field applications. After a brief survey of the clogging patterns most commonly observed in practice, and of a number of physical and chemical causes of clogging, the various mechanisms by which microorganisms clog soils and other natural porous media are analyzed in detail. A critical assessment is also provided of the few mathematical models that have been developed in the last few years to describe the biological clogging process. The overall conclusion of the review is that although information is available on several aspects of the biological clogging of natural porous media, further research is required to predict its extent quantitatively in a given situation. This is particularly true in cases that involve complicating factors such as predation or competition among organisms.

Bacterial reductions of the saturated hydraulic conductivity of natural porous media appear to be caused by a wide range of mechanisms, few of which have been carefully studied. Nevertheless, a number of mathematical models have been developed in recent years to describe the microbial clogging process, based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. In the present study, two independent sets of experimental data available in the literature are used to test the existing bioclogging models. To broaden the scope of the assessment, an additional model, initially developed to describe the deep filtration of suspended colloids, is also included in the comparisons. Analysis of the experimental data reveals a clear relationship between the texture of a porous medium and the ability of a given level of biomass to reduce its saturated hydraulic conductivity; at equal biomass, clogging is much more pronounced in fine-textured materials than in coarse-textured ones. In addition, the results of the model comparisons suggest that none of the existing models can predict satisfactorily the saturated hydraulic conductivity reductions observed in fine sands, whereas they fare somewhat better in coarser materials. It is argued that this inadequacy of existing models is due to the continuous biofilm assumption on which they are founded. Indeed, a simplistic model that assumes the biomass to be distributed as plugs instead of as continuous biofilms produces quantitatively much improved predictions of the saturated hydraulic conductivity reductions. Reference is made to the consequences of this observation in terms of future research.

The extent to which a methanogen can clog sand columns was examined: two permeameters packed with clean quartz sand were sterilized, saturated with water, inoculated with Methanosarcina barkeri and percolated under upward flow conditions. After approx. 5 months, the hydraulic conductivity of the sand had decreased to 3% and 25% of the highest values measured earlier. At that point, gas-filled regions in the sand were clearly visible through the transparent walls of the permeameters, and methane bubbles were continuously released from the columns into the effluent. Scanning electron microscopy observations and biomass assays indicated that cell mass accumulation did not contribute significantly to the observed decrease of the hydraulic conductivity. This decrease was therefore attributed to pore blocking due to the entrapment of methane bubbles.

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During the last two decades, various non-governmental organizations have strongly encouraged Bolivian farmers in the Altiplano region near Lake Titicaca to resume the ancestral agricultural practice of constructing raised fields. In addition to improved drainage and possibilities this practice affords for sub-irrigation, advocated benefits of this system traditionally include frost mitigation and high crop yields. Until recently, reliable data to assess the extent of these benefits were unfortunately lacking. In this context, field experiments on raised fields were designed and carried out at two locations in the Bolivian Altiplano to obtain reliable potato yield and temperature data. Observed yields ranged from 2.73 to 10.80 t ha -1 at the first site, where salinity caused significant yield variability (R 2 = 0.79). At the second site, yields per raised platform varied between 8.25 and 33.45 t ha -1 . However, comparable yields were obtained in flat control plots in spite of a mid-season frost, and the minimum temperatures differed only by 1ºC in the conventional plots relative to the raised fields. These results suggest that, under the experimental conditions, the potential benefits of raised fields in terms of frost mitigation or increased yields might only be observable in exceptionally bad years, when extreme frosts wipe out entire potato crops on conventional fields. Nevertheless, it is argued that in spite of these marginally supportive observations, raised-field agriculture may still be a viable option for farmers to consider if the water-filled channels between the raised fields are managed for fish and fertilizer production. Key words: raised fields, productivity, frost mitigation, crop LIMITAÇÕES POTENCIAIS PARA PRODUTIVIDADE DA BATATA EM SOLOS DE CAMPOS ELEVADOS NO LAGO TITICACARESUMO: Nas últimas duas décadas várias organizações não governamentais entusiasmaram lavradores bolivianos da região do Altiplano, próximo ao Lago Titicaca, a reassumir a prática ancestral de construir campos elevados. Além de sua drenagem melhorada e das possibilidades de implementar a sub-irrigação, as vantagens desse sistema tradicional incluem a mitigação da geada e altas produtividades. Até recentemente não havia dados para avaliar estes benefícios. Neste contexto, experimentos realizados em campos elevados foram planejados e executados em dois locais dentro do Altiplano boliviano, para obter dados confiáveis em relação à produtividade da batata e das temperaturas reinantes. As produtividades observadas variaram de 2,73 a 10,80 t ha -1 no primeiro local, no qual a salinidade provocou uma variabilidade significante na produtividade (R 2 = 0,79). No segundo local, as produtividades nas plataformas elevadas variaram de 8,25 a 33,45 t ha -1 . Entretanto, produtividades comparáveis foram obtidas nas áreas controle, mesmo na presença de uma geada no meio da estação e de uma diferença de apenas 1ºC entre as parcelas convencionais e as elevadas. Estes resultados sugerem que nas condições dos experimentos, os benefícios...