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theses

Surface waters are an important source of agricultural water. About 58% of total surface water withdrawals are used for agricultural purposes in the US, thus the microbial quality of surface water is important for the safe production of fresh fruits and vegetables. Studies have demonstrated that surface waters are prone to contamination and may contain viral, bacterial and protozoal water and food borne pathogens. The efficacy of biological and physicochemical indicators for Salmonella in surface waters in central Florida was evaluated in chapter 2 of this dissertation. E. coli and solar radiation (SR) levels were found to significantly correlate well with Salmonella levels but the R2 were low (< 0.1). The influence of key weather variables on the survival of E. coli (surrogate for Salmonella) on the surface of oranges in grove following application of contaminated spray water was evaluated in the 3rd chapter. Weather variables relative humidity (RH), solar radiation (SR) and temperature and time were found to be significantly correlated to the log change in E. coli levels/h on the surface on an orange; in addition time was also found to correlate well with log change in E. coli levels/h. However the R2 values were found to be low (< 0.4). Logistic regression analysis showed that there was a high probability of a decline in E. coli levels at high SR and low RH levels, and a low probability of an increase in E. coli levels even at the lowest SR and the highest RH levels. Chapter 4 studies the predicted levels of E. coli on the surface of an orange under the influence of the four significant variables RH, SR, temperature and time. An empirical model based on upper and lower boundary limits for RH, SR, temperature and time variable was developed. IF logic statements, probability distributions and Monte Carlo simulations were used to describe the variability and uncertainty in log E. coli change rate for the most constricting variable. The mean predicted E. coli concentrations at day 1 after spraying were very close to the actual field trial data but the model under predicted the risk for 3, 7 and 14 days post spraying. The FDA produce rule on microbial die off rate (0.5 log/day of generic E. coli for a maximum of 4 days) was also evaluated. The rule was found to over predict actual data at day 1 and 3, but under predicted at day 7 and 14. The FDA rule for a pre harvest interval of less than 4 days is fail safe and has the potential to reduce the risk of exposure to pathogens on surfaces of pre harvest crops by restricting the use of contaminated sources of agricultural water.

ETD_7048

Ph.D.

Includes bibliographical references

by Gabriel Mootian

doi:10.7282/T3CC12T8

ETD doctoral

The author owns the copyright to this work.