2009 Geometric Mean 2010 Geometric Mean
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compare it to in New York, but interestingly one of the samples contained generic E. coli (2 MPN/100 mls). Several samples from TN were taken from irrigation equipment. In the New York data set these samples were removed because in many cases there were samples taken from the source water at the same time. Since there were fewer TN samples, the samples taken from irrigation water equipment were included in the analysis. TN water samples were analyzed for the same parameters as the NY samples but modified mTec (EPA 1603) protocol was used for quantification of generic E. coli resulting in colony-forming units (CFU) per 100 mls. Despite the different protocols, the results of NY and TN were compared on a one to one basis. This is supported in the scientific literature as noted by Cho et al, 2010 when they reported a positive relationship between CFU and MPN estimates.
Overall, TN ground water had a geometric mean of 1 CFU/100 mls, while TN reservoir water had a geometric mean of 5 CFU/100 mls and running water had a geometric mean of 38 CFU/100 mls. One hundred percent of the time, the ground water samples were below the 126 MPN/100 mls when allowing a one to one comparison. Reservoir water met the 126 MPN/100 mls standard 93% of the time with only 4% of the samples exceeding the 235 MPN/100 mls single upper limit standard. These percentages are consistent with those seen in NY water sources. TN Running water samples met the 126 MPN/100 mls standard 75% of time with only 6% of the samples being higher than 235 MPN/100 mls standard. The percentage of samples achieving the 126 MPN/100 mls standard is comparable to NY while the percentage of samples that exceed the 235 MPN/100 mls limit is approximately half of what was seen in NY. A possible explanation for this variation is that TN running water samples accounted for the smallest analyzed sample size at 16 samples. This issue of small sample size also caused an anomalous result when attempting to access seasonal variation.
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Analysis of TN running water data in the fall (September 22-December 20) resulted in an excessively high geometric mean of 255 CFU/100 mls. This is the result of calculating the geometric mean with only three samples. In general, a five sample minimum is preferred for calculating a geometric mean. The three samples were 860, 160, and 120 CFU/100 mls, all taken from the same water source on the same day, what is interesting to note is the variation and the fact that one of the samples would meet the 126 MPN/100 mls standard and the 160 CFU/100 mls sample does not exceed the 235 MPN/100 mls limit. These results likely would not have caused the grower who was testing the water to do anything, while the 860 CFU/100 mls result exceeds even the most liberal EPA recreational water standards of 575 MPN/100 mls for “Infrequently Used Full Body Contact” water and it also exceeds the upper limit in the
CSFSGPHLLG standards (576 MPN/100 mls) for water that does not contact the edible parts of the plant such as water delivered through a drip system (USEPAAWQS, 1986; CSFSGPHLLG, 2010). With a clear understanding of these issues, this data was included in the comparison of seasons and states found in Figure 3-3. Unlike NY, TN running water samples are not always higher than the TN reservoir samples. In NY, E. coli counts in both running and reservoir water are highest in the summer (June 21-September 21), while spring (March 20-June 20) resulted in the highest counts in both reservoir and running if you discount the fall running data due to the issues discussed earlier in this section. To be confident in the TN data, more samples are needed to make the evaluation more meaningful. The NY data shows seasonal trends in both running and reservoir water with the changes more pronounced in the running water from season to season. Encouraging participation in the database from more states would allow for additional data analysis and may reveal trends that are regionally and/or seasonally dependent.
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Figure 3-3. New York and Tennessee data was analyzed by season. Spring is represented by
samples taken between March 20-June 20, summer is represented by samples taken between June 21-September 21, and fall is represented by samples taken between September 22-
December 20. No samples were taken in winter (December 21-March 19) and NY samples from 2009 and 2010 were combined for this analysis.
Impact of Sampling Location
For all water collections, the sample location was noted. Most samples were taken directly from the source water, but there were several opportunities in the 2010 NY sampling season to collect water samples directly from irrigation equipment. The idea behind sampling from irrigation equipment was to collect water closest to the point of use and get the best possible data on the quality of water that was contacting the plant. Data generated in this study found that this may not be the best way to determine source water quality but may be relevant to
27 7 97 17 59 6 26 29 24 3 255 2 0 50 100 150 200 250 300 Spring: Running Spring: Reservoir Summer: Running Summer: Reservoir
Fall: Running Fall: Reservoir
M PN o r C FU / 100 m ls
Season and Source