Colloid transport and filtration of Cryptosporidium parvum in sandy soils and aquifer sediments

We present theoretical and experimental work on Cryptosporidium parvum oocy sts to characterize their transport behavior in saturated, sandy sediments under strictly controlled conditions. Column experiments are implemented wi th three different sands (effective grain size: 180, 420, and 1400 mu m) at two different saturated flow rates (0.7 and 7 m/d). The experiments show t hat C. parvum oocysts, like other colloids, are subject to velocity enhance ment. In medium and coarse sands, the oocysts travel 10-30% faster than a c onservative tracer. The classic clean-bed filtration model is found to prov ide an excellent tool to estimate the degree of C. parvum filtration. Exper imentally determined collision efficiencies, alpha, range from 0.4 to 1.1. The magnitude of alpha is consistent with the known physical and chemical p roperties of the oocyst and the transport medium and compares well with, e. g., measured collision efficiencies of similarly sized E. coli bacteria. Ho wever, a significant amount of the initial deposition appears to be reversi ble leading to significant asymmetry and tailing in the oocyst concentratio n breakthrough curve. We are able to show that the observed late-time oocys t elution can qualitatively be explained by postulating that a significant fraction of the oocyst filtration is reversible and subject to time-depende nt detachment.