Bacteroides species are promising indicators for differentiating livestock and human fecal contamination in water because of their high concentration in feces and potential host specificity. In this study, a real-time PCR assay was designed to target Bacteroides species (AllBac) present in human, cattle, and equine feces. Direct PCR amplification (without DNA extraction) using the AllBac assay was tested on feces diluted in water. Fecal concentrations and threshold cycle were linearly correlated, indicating that the AllBac assay can be used to estimate the total amount of fecal contamination in water. Real-time PCR assays were also designed for bovine-associated (BoBac) and human-associated (HuBac) Bacteroides 16S rRNA genes. Assay specificities were tested using human, bovine, swine, canine, and equine fecal samples. The BoBac assay was specific for bovine fecal samples (100% true-positive identification; 0% false-positive identification). The HuBac assay had a 100% true-positive identification, but it also had a 32% false-positive rate with potential for cross-amplification with swine feces. The assays were tested using creek water samples from three different watersheds. Creek water did not inhibit PCR, and results from the AllBac assay were correlated with those from Escherichia coli concentrations (r 2 ‫؍‬ 0.85). The percentage of feces attributable to bovine and human sources was determined for each sample by comparing the values obtained from the BoBac and HuBac assays with that from the AllBac assay. These results suggest that real-time PCR assays without DNA extraction can be used to quantify fecal concentrations and provide preliminary fecal source identification in watersheds.

The health risks of As exposure due to the installation of millions of shallow tubewells in the Bengal Basin are known, but fecal contamination of shallow aquifers has not systematically been examined. This could be a source of concern in densely populated areas with poor sanitation because the hydraulic travel time from surface water bodies to shallow wells that are low in As was previously shown to be considerably shorter than for shallow wells that are high in As. In this study, 125 tubewells 6−36 m deep were sampled in duplicate for 18 months to quantify the presence of the fecal indicator Escherichia coli. On any given month, E. coli was detected at levels exceeding 1 most probable number per 100 mL in 19−64% of all shallow tubewells, with a higher proportion typically following periods of heavy rainfall. The frequency of E. coli detection averaged over a year was found to increase with population surrounding a well and decrease with the As content of a well, most likely because of downward transport of E. coli associated with local recharge. The health implications of higher fecal contamination of shallow tubewells, to which millions of households in Bangladesh have switched in order to reduce their exposure to As, need to be evaluated.

ter supply wells. Many of the approaches for dealing with microbial contamination, such as establishing mini-This paper attempts to introduce the work described in this special mum setback distances between wells and septic fields section on colloid transport within a more general perspective of the evolution of our understanding of the importance of colloids in (onsite wastewater disposal systems) for individual homes, subsurface systems. The focus will be on the transport of colloidal were based on empirical evidence of contaminant filtraparticles in natural (i.e., chemically and physically heterogeneous) tion and are still in use today (TDEC, 2002). Other geological settings because the complexity imposed by these situations studies focused on the role of clay migration from surrepresents the greatest challenge to current and future understanding.face to deeper horizons in soil diagenesis (Jenny and Great progress has been made in addressing many of the key questions Smith, 1935;Thorp et al., 1957). Mechanistic studies of related to colloid transport. However, as in most areas of science, colloid transport originally focused on issues of wasteincreased knowledge also serves to reveal new and more complex water filtration and controlling aquifer permeability. challenges that must be addressed. Yao et al. (1971) described colloid removal for water filtration in terms of two rate-limiting steps: (i) the physical processes of diffusion, interception, and gravitation

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Field values of horizontal hydraulic conductivity measured in the upper 1.5-5.5 m of a weathered and fractured clay-rich till were strongly influenced by smearing around piezometer intakes, which occurs during augering, and by the physical scale of the measuring device. Values measured in conventional augered piezometers were typically 1-2 orders of magnitude lower than those measured in piezometers designed to reduce smearing. Measurements of hydraulic conductivity in small-scale seepage collectors or piezometers, which typically intersect fewer than 10 fractures, vary over a much greater range, 10-l0 to 10-6 m/s, than large-scale values based on infiltration into 5.5-m-deep trenches which intersect thousands of fractures (range 10 -7 to 3 x 10 -7 m/s). Values of hydraulic fracture aperture, 1-43/zm, and fracture porosity, 3 x 10 -5 to 2 x 10 -3 , were calculated using the cubic law with fracture orientation/distribution measurements and the small-scale hydraulic conductivity measurements. This paper provides the first reliable determination of the magnitude and spatial distribution of hydraulically derived fracture parameters in a clay deposit. The absence of such data has, until now, severely limited the application of quantitative groundwater flow and contaminant transport models in this type of deposit. through the fractures; (2) diffusion into the immobile, or less mobile, pore water of the blocks of clay matrix (or porous rock) between fractures, which is often referred to as matrix diffusion [Foster, !975; Grisak and Pickens, 1980; Grisak et al., 1980]; and (3) retardation processes such as sorption, precipitation and biodegradation in both the fractures and the matrix. Several analytical or numerical techniques including those by Sudicky and Frind [ 1982] and Sudiclcy and McLaren [1992] have been developed to quantitatively assess flow and contaminant transport in fractured porous media. The application of these models to fractured clays, for research or field engineering purposes, has been limited by the scarcity of data concerning the critical factors of fracture aperture (or width of opening) and fracture orientation and spacing. Aperture values cannot be measured directly in the field or laboratory because they are sensitive to disturbance caused by sampling and because of their small expected size (micrometers to hundreds of micrometers). Values can be indirectly determined based on measurements of hydraulic conductivity, fracture spacing and fracture orientation. These are referred to as "hydraulic apertures" and are generally calculated with the cubic law [Snow, 1969] which is based on an analogy to flow between two smooth parallel plates. More complex representations of flow through fractures, which include factors such as surface roughness and aperture variations along fractures [Tsang, 1984; Brown, !987], have been suggested. Development and application of models which include fracture roughness and aperture variation in fractured clays are severely limited by the absence of data.