Evaluating the Transport of Bacillus Subtilis Spores As a Potential Surrogate for Cryptosporidium Parvum Oocysts.

The USEPA has recommended the use of aerobic spores as an indicator for Cryptosporidium oocysts when determining groundwater under the direct influence of surface water.  Surface properties, interaction energies, transport, retention, and release behavior of B. subtilis spores were measured over a range of physicochemical conditions, and compared with reported information for C. parvum oocysts.  Interaction energy calculations predicted a much larger energy barrier and a shallower secondary minimum for spores than oocysts when the solution ionic strength (IS) equaled 0.1, 1 and 10 mM, and no energy barrier when the IS=100 mM.  Spores and oocysts exhibited similar trends of increasing retention with IS and decreasing Darcy water velocity (q_w), and the predicted setback distance to achieve a six log removal was always larger for spores than oocysts.  However, low levels of observed spore and oocyst release significantly influenced the predicted setback distance, especially when the fraction of reversibly retained microbes (F_rev) was high.  An estimate for F_rev was obtained from large release pulses of spore and oocyst when the IS was reduced to deionized water.  The value of F_rev always increased with q_w, whereas an opposition trend for F_rev with IS was observed for spores (decreasing) and oocysts (increasing).