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TECHNICAL REPORTS

Ground Water Quality

Distance and Flow Effects on Microsphere Transport in a Large Gravel Column

Institute of Environmental Science & Research Ltd., P.O. Box 29181, Christchurch, New Zealand

^* Corresponding author (murray.close{at}esr.cri.nz)

Received for publication July 26, 2005. Consumption of microbially contaminated ground water can cause adverse health effects and the processes involved in pathogen transport in aquifers need to be understood. The influences of distance, flow velocity, and colloid size on colloid transport were examined in homogenous pea-gravel media using an 8-m column and three sizes (1, 5, and 10 µm) of microspheres. Experiments were conducted at three flow rates by simultaneously injecting microspheres with a conservative tracer, bromide. Observed concentrations were simulated with CXTFIT and analyzed with filtration theory. The results demonstrate that colloid concentration is strongly log-linearly related to transport distance (as suggested by filtration theory) in coarse gravels, similar to our previous field studies. In contrast, the log-linear relationship is often reported to be invalid in fine porous media. The observed log-linear relationship is possibly because straining is negligible in the coarse gravels investigated. This has implications in predicting setback distances for land disposal of effluent, and suggests that setback distances in gravel aquifers can be estimated using constant spatial removal rates (f). There was an inverse relationship between transport distance and colloidal concentration, but not with temporal attachment rate (k_att) and collision coefficient (). Increases in flow velocity result in increasing colloidal recovery, k_att and but decreasing f. Increases in sphere size result in decreasing colloidal recovery with increasing k_att, f, , and velocity enhancement. Diffusion is the dominant collision mechanism for 1-µm spheres (81–88%), while settling dominates for 5- and 10-µm spheres (>87%), and interception is very small for all spheres investigated.

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