Comparing Transport of Cryptosporidium -Sized Carboxylated Microspheres in Two Undisturbed Soil Cores under Unsaturated Conditions

Cryptosporidia are opportunistic pathogens, some of which infect humans when ingested in water or food. Filtration through granular media is the primary mechanism to remove them from water. Natural soils' ability to filter the oocysts is expected to be affected by their physicochemical properties. This study used two undisturbed soil cores: an Oxisol, rich in clay minerals and iron oxides and an Ultisol, dominated by quartz minerals and limited in clay minerals and iron oxides, both dominant in the tropics and subtropics. To better evaluate the impact of macropores on the movement of Cryptosporidium parvum oocysts, the two cores were scanned using computed tomography. Local rainwater was used as the leaching solution, and fluorescent carboxylated microspheres (1.75 μm) served as surrogates for cryptosporidial oocysts. Results showed early appearance of microspheres compared with the conservative tracer, suggesting an enhanced velocity for the microspheres due to preferred flow-path structure. Higher recovery (10% versus 0.5%) of microspheres occurred in the ultisol compared with the Oxisol. This was due to the high content of quartz, the limited content of minerals, and negligible content of iron oxide in the Ultisol. The long tailing that occurred in the Oxisol suggested the release of previously attached microspheres and a possible source of groundwater contamination.