Researchers: Thomas Harter, Rob Atwill

Recently, several large waterborne outbreaks of human cryptosporidiosis have raised concern about the occurrence of the protozoal pathogen, Cryptosporidium parvum, in drinking water. C. parvum has also been found in groundwater samples. In reaction to widespread public attention, state and federal health and environmental agencies have begun to require increased monitoring of C. parvum in drinking water systems. High levels of C. parvum have been associated with the occurrence of livestock and concentrated animal facilities among others. Municipal and rural water suppliers are becoming increasingly aware of potential sources of C. parvum in their watersheds. In recent decisions, they have begun to take measures to control or even eliminate known potential sources from entire watersheds. However, research and knowledge on the fate of C. parvum in the environment is widely lacking. In particular, there is almost no systematic research regarding the transport mechanisms of C. parvum in soil and groundwater. Neither has there been a systematic effort to provide evidence on whether and where the organism is likely to occur in groundwater.

We have implemented bench-scale experiments (see figure) to better understand the transport properties of C. parvum and to provide preliminary data for the development of a more extensive research program.. Outside its mammal host, C. parvum occurs in form of an oocyst (see figure) that is approximately 5 µm in diameter (5,000 oocysts need to be lined up in a chain measuring 1 inch). The results indicate that the transport behavior of the infectious C. parvum oocyst is similar to that of other colloids (colloids are small particles that are larger than molecules but smaller than approximately 10 µm. Examples of colloids are clay particles, protozoa, bacteria, and viruses). The experiments were conducted with different sands (fine, medium, and coarse sand) and at different water filtration velocity (fast to immitate infiltration during a rain, and slow to immitate water movement in an aquifer). In the coarse sand (sand particles with a diameter of approximately 1/12 of an inch), approximately 70% of the oocysts passed through the sand column (see figure), during rapid infiltration, whereas only 10% passed through the column with the slower groundwater velocity. The finer the sand particles, the fewer oocysts passed through the sand column. In the fine sand (sand particles with a diameter of approximately 1/100 of an inch) less than one percent passed through the column with the rapid infiltration velocity. The remainder of the oocysts were filtrated and sorbed within the sand column. We made several interesting observations that we will be looking at closer: in the coarse sand, the oocysts travelled approximately 20% faster than the average water molecule, because their tortuous travel path through the sand column is restricted to the coarsest pores where the water travels faster. This phenomenon is referred to as "velocity enhancement" and is well-known in colloid science. standard colloid filtration theory with a "sticking coefficient" of 1 predicts the breakthrough concentrations of oocysts within an order of magnitude. in contrast to predictions by the colloid filtration model, we observed low, but measurable concentrations of oocysts long after we had begun flushing the column with uncontaminated water. In the fine sand, oocyst concentrations at levels near the initial peak persisted for over 100 pore volumes of column flushing indicating a slow release of oocysts trapped in the fine sand.

Table 1: Average relative velocity enhancement and mass retained in the soil column for each experiment. The relative total recovery is the sum % mass eluted (equal to the relative steady-state concentration at the bottom of the column) and the % mass retained. The sticking coefficient, , is computed after determining all other parameters independently. CS: coarse sand, MS: medium sand, FS: fine sand. ()* indicates questionable value due to experimental difficulties.

Also see:

Harter, T., S. Wagner, E. R. Atwill, Colloid transport and filtration of Cryptosporidium parvum in sandy soils and aquifer sediments, Env. Science and Technology, 34(1), 62-70, 2000. (pdf file and supplement for personal use only).

Atwill, E. R., L. Hou, B. M. Karle, T. Harter, K. W. Tate, R. A. Dahlgren. Transport of Cryptosporidium parvum oocysts through vegetated buffer strips and estimated filtration efficiency, Applied and Environmental Microbiology 68(11), pp. 5517-5527, 2002.(pdf file for personal use only).

Searcy, K. E., A. Packman, E. R. Atwill, and T. Harter, 2005. Association of Cryptosporidium parvum with Suspended Particles: Impact on Oocyst Sedimentation, Applied and Environmental Microbiology 71(2):1072-1078.(pdf file for personal use only).

Discussion of C. parvum removal in bank filtration