Transport of MS2 phage, Escherichia coli, Clostridium perfringens, Cryptosporidium parvum and Giardia intestinalis in a gravel and a sandy soil (AGGREGATION CH 9)
To define protection zones around groundwater abstraction wells and safe
setback distances for artificial recharge systems in water treatment,
quantitative information is needed about the removal of micro-organisms
during soil passage. Column experiments were conducted using natural
soil and water from an infiltration site with fine sandy soil and a river bank
infiltration site with gravel soil. The removal of phages, bacteria, bacterial
spores, and protozoan (oo)cysts was determined at two velocities and
compared with field data from the same sites. The microbial elimination
rate (MER) in both soils was generally >2 log, but MER in the gravel soil
was higher than that in the fine sandy soil. This was attributed to enhanced
attachment, related to higher metal-hydroxides content. From the high
sticking efficiencies and the low influence of flow rate on MER it was
deduced that straining played a significant role in the removal of
Escherichia coli and Cryptosporidium parvum oocysts in the gravel soil. Lower
removal of oocysts than the 4-5 times smaller E. coli and spores in the fine
sand indicates that the contribution of straining is variable and needs
further attention in transport models. Thus, simple extrapolation of grain
size and particle size to the extent of microbial transport underground is
inappropriate. Finally, the low MER of indigenous E. coli and Clostridium
perfringens observed in the soil columns as well as under field conditions
and the second breakthrough peak found for Cryptosporidium and spores in
the fine sandy soil upon a change in the feedwater pH indicate a significant
role of detachment and retardation to microbial transport and the difficulty
of extrapolation of quantitative column test results to field conditions