Release of Cryptosporidium and Giardia from dairy calf manure: impact of solution salinity.

Studies were initiated to determine the release behavior of Cryptosporidium oocysts and Giardia cysts from dairy calf manure to waters of various salinities. Experiments were conducted by sprinkling a particular aqueous solution over a manure disk and collecting the runoff water. Effluent concentrations of manure and (oo)cysts were initially several orders of magnitude below their starting concentration in the manure, after continued application of water the concentrations gradually decreased, and then exhibited persistent concentration tailing. Solution salinity significantly affected the shape and magnitude of the manure and (oo)cyst concentration curves. Increases in solution salinity tended to decrease the manure and (oo)cyst concentrations at a particular time. This was attributed to a stabilization of manure by compression of the double layer thickness between negatively charged components of the manure phase. Calculated release efficiencies of the (oo)cysts (relative to manure release) also decreased with increasing solution salinity. Experimental observations indicate that onlythe surface layer of manure was depleted of finer manure materials and (oo)cysts and that the manure will act as a long-term source of contamination. A conceptual model to describe and predict manure and (oo)cyst release rates and cumulative loading for the various solution salinities was proposed and applied to the experimental data. The calibrated model yielded a reasonable description of the experimental results.