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.

Cold soil temperatures, seedling diseases, and soil crusting may limit stand establishment of early-season muskmelons (Cucumis melo L.). We tested the ability of seed and soil treatments to overcome these factors and improve seedling emergence. The seed treatments were seed priming (6 d at 25 °C in aerated 0.3 M KNO3 solution followed by drying) to improve the rate of germination at low temperatures, and metalaxyl [N-(2,6-dimethylphenyl)-N-(methoxyacetyi) alanine methyl ester] fungicide (Apron 25W) to prevent damping-off diseases. The soil treatments were spot applications of soil drenches containing metalaxyl fungicide (100 ;tg-~ R idomil 2E), an anticrostant [2% Naico 2190, (Nalco Chemical Corp, Carson, CA)] or both fungicide and anticrustant. In laboratory tests at 18 °C, both germination rate and final germination were markedly improved by seed priming in 'PMR 45', 'Magnum 45', 'Topmark', and 'Topscore' plants. Seedling emergence from sterilized soil in flats under ambient outdoor temperatures (7-23 °C) was also improved by seed priming. Seed priming resulted in more rapid emergence or increased final emergence in five of seven field trials in two locations. Anticrustant applications to the soil covering the seed consistently improved stand establishment, particularly in badly crusted soils. Metalaxyl application to the seed or soil generally improved emergence, but the effect varied with cultivar, location, and planting method. None of the treatments significantly influenced final fruit yield. The combination of seed priming, fungicides, and anticrustants could allow lower seeding rates of expensive hybrid seed while achieving earlier emergence and adequate plant densities in earlyseason muskmelon crops.

In polluted soil or ground water, inorganic nutrients such as nitrogen may be limiting, so that Monod kinetics for carbon limitation may not describe microbial growth and contaminant biodegradation rates. To test this hypothesis we measured 14CO2 evolved by a pure culture of Acinetobacter johnsonii degrading 120 micrograms 14C-phenol per ml in saturated sand with molar carbon:nitrogen (CN) ratios ranging from 1.5 to 560. We fit kinetics models to the data using non-linear least squares regression. Phenol disappearance and population growth were also measured at CN1.5 and CN560. After a 5- to 10-hour lag period, most of the 14CO2 evolution curves at all CN ratios displayed a sigmoidal shape, suggesting that the microbial populations grew. As CN ratio increased, the initial rate of 14CO2 evolution decreased. Cell growth and phenol consumption occurred at both CN1.5 and CN560, and showed the same trends as the 14CO2 data. A kinetics model assuming population growth limited by a single substrate best fit the 14CO2 evolution data for CN1.5. At intermediate to high CN ratios, the data were best fit by a model originally formulated to describe no-growth metabolism of one substrate coupled with microbial growth on a second substrate. We suggest that this dual-substrate model describes linear growth on phenol while nitrogen is available and first-order metabolism of phenol without growth after nitrogen is depleted.

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