Pathogen Pathways − Contamination of water bodies via artificial drainage
4. Experiment 1: Pathogen contamination of artificial drainage - natural rainfall events coinciding with grazing (Objective 3)
4.1 Overview
Plots A and B were grazed by dairy cows overnight on 24 September 2003 (grazed plots, 12 h grazing period). The timing of this grazing event on Plots A & B, in context with the whole experimental period of this study, is shown in Figure 6. Less than an hour after the cows were removed from the grazed plots on 25 September 2003 a natural rainfall event started. Plots C and D had last been grazed 19 days prior to this rainfall event (grazed 19 days prior). The rainfall event was 3 hours in duration (6–9 am) and delivered 11 mm of rain. The rainfall was sufficient to generate subsurface drainage, but was not intense enough to cause surface runoff.
Figure 6: Natural and simulated rainfall and drainage and runoff volumes for plots A and B, for the experimental period (24 September-15 October 2003) associated with this study
4.2 Sample collection
The drainage event on 25 September 2003 was sampled to determine the presence of Campylobacter in water draining from dairy pasture. Three drainage water samples were collected from each plot for thermophilic Campylobacter and E. Coli determinations. Two samples were taken as the drainage rate increased and one just after peak flow (Figure 7). Only one sample was collected from each plot for screening and enumerating of Cryptosporidium oocysts and Giardia cysts.
Figure 7: Times (T1-T3) of sampling for Campylobacter and E. Coli determinations and their corresponding drainage flow rates for each of the four plots sampled on 25 September 2003
4.3 Results
Cryptosporidium oocysts and Giardia cysts
One drainage sample was collected from each plot on the 25 September 2003 to test for Cryptosporidium oocysts and Giardia cysts. Only one sample (Plot C – grazed 19 days prior) contained Cryptosporidium oocysts (8 oocysts/100 L). Giardia cysts were present in the samples collected from plot B (grazed overnight) and plots C & D (grazed 19 days prior) and measured 5, 73 and 9 cysts/100 L of sample, respectively.
At a subsequent natural rainfall event, on the 29 September 2003, surface runoff occurred on three (Plots A, B & C) of the 4 plots. A runoff sample was collected from each of these three plots to test for Cryptosporidium oocysts and Giardia cysts. Cryptosporidium oocysts were not present in any of the samples, however, Giardia cysts were present in the samples from plots A & C, measuring 25 and 12 cysts/100 L, respectively.
Campylobacter
Enumeration of thermotolerant Campylobacter by the most probable number (MPN) technique identified that thermotolerant Campylobacter were present in all the drainage water samples from all the plots (i.e. both grazed overnight and grazed 19 days prior; Table 2). In hindsight, a further dilution of the drainage sample from the grazed plots was necessary to obtain an absolute result for MPN. Populations ranged widely with the highest populations typically occurring in the drainage from the grazed plots (plots A and B). However, the prevalence of Campylobacter in the two grazed plots were not well matched, being nearly an order of magnitude different at T1 and T2, which were the samples taken as the drainage rate was increasing (Figure 7). The cumulative drainage volumes (Table 2 and Figure 7) and flow rates for all plots were similar, therefore, it is concluded that the difference in Campylobacter prevalence in drainage from plots A and B were caused by variability in the Campylobacter populations in and on the soil and not differences in plot hydrology.
Table 2: Campylobacter concentrations (MPN) in drainage water following a natural rainfall event (25 September 2003)
| Plot | Campylobacter concentration (MPN/100 mL) | |||
|---|---|---|---|---|
| Sampling Time 1 | Sampling Time 2 | Sampling Time 3 | ||
| Grazed overnight | A | >1.1 x 102 (0.18)* | >1.1 x 102 (0.68) | >1.1 x 102 (1.70) |
| B | 9.3 (0.14) | 2.4 x 101 (0.70) | 1.1 x 102 (1.73) | |
| Grazed 19 days prior | C | 2.0 (0.06) | 2.4 x 101 (0.56) | 4.6 x 101 (1.93) |
| D | 9.3 (0.07) | 1.5 x 101 (0.60) | 2.4 x 101 (1.95) | |
*Values in parenthesis represent cumulative drainage (mm) at time of sampling.
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