Executive Summary
Objective 2 of the Pathogen Transmission Routes Research Programme (PTRRP) focused upon two areas of related research:
- faecal contamination of surface runoff; and
- riparian attenuation of faecal microbes.
1. Faecal Contamination of Surface Runoff
A large-scale rainfall simulator (LRS) has been used to quantify the delivery of microbes to a hill-country stream via surface runoff. The LRS encompasses 1050 m2 of a steep grazed hillside (18° ) within the Whatawhata research station, west of Hamilton. The design rainfall rate is 35 mm/hr providing a total application rate of 10.2 L/s, applied over 1 hour during each experiment. This application rate and duration corresponds to an 8-year recurrence interval. Surface runoff generated by the LRS converges naturally and flows directly into a headwater stream. At the point of convergence, flow was measured and samples of runoff collected for E. coli analysis. Five experiments were conducted before and after grazing by sheep in both winter and summer.
The total number (load) of bacteria washed across the outflow flume during an event varied between 9 x 108 and 6 x 1011 MPN. Since the outflow drains into a headwater stream these loads represent a substantial hillside delivery (9 x 105 to 5 x 108 MPN/m2) of E. coli direct to the stream network. The LRS experiments have shown, therefore, that surface runoff is a key delivery mechanism for faecal contamination under grazed hill-country pasture, during large rainfall events.
Both the loads and event mean concentrations of bacteria correlated with the recent stock history prior to each experiment. Events undertaken immediately following sheep removal gave a mean concentration of approximately 106 MPN/100 mL. This fell to 105 MPN/100 mL 2 weeks after sheep removal, to 104 MPN/100 mL 7 weeks after removal, and to 103 MPN/100 mL 8 weeks after removal. The decline in event mean concentration with the number of days since sheep removal is explained by die-off on the catchment surface. A T90, the time taken for the original E. coli concentration to decline by 90%, of 4-7 days was derived from these results. This rapid die-off supports the assertion that applying dairy effluent to land, rather than direct to streams, improves bacterial water quality.
2. Riparian Attenuation of Faecal Microbes
Ongoing field studies have continued to determine the ability of buffer strips to trap Campylobacter and E. coli washed in by surface runoff. Sloping grass plots (downslope length 5 m, width 2 m) were irrigated with clean water (using a sprinkler system) to generate steady surface runoff. 20 L of farm dairy effluent, artificially contaminated with C. jejuni was then applied to the saturated surface, at the top of the plots, over 2-3 minutes. Irrigation with clean water was continued for 40-60 minutes. Both surface and subsurface outflow at the lower end of the plots was sampled for microbial analysis. Some experiments involved the application of cowpats (rather than liquid effluent) to the plots. Faecal material from the cowpats was washed into the plots by both the sprinkler and rainfall simulators.
Flow rate had a clear impact upon the recovery (the percentage of the applied microbe recovered in the outflow, i.e. the inverse of attenuation) of microbes over a 40-minute period. At high flow, recovery varied between 16% and 62% for E. coli, and between 13% and 89% for Campylobacter. In contrast, recovery at slow flow ranged between <1% and 5% (i.e. attenuation is > 95%) for both microbes. The relationship between flow rate and microbial attenuation raises important implications for buffer strip design, particularly if appreciable faecal contamination is only delivered by surface runoff during large events.
Under fast flow, subsurface flow contributed between < 1% and 13% of the total number (surface and subsurface combined) of E. coli recovered in the outflow. For Campylobacter this figure ranged between < 1% and 24%. Under some plots, therefore, the Hamilton clay loam enabled a substantial number of microbes to bypass surface vegetation within subsurface flows. This response is likely to be more marked under soils with better drainage, particularly those characterised by macropores.
During all the experiments, recovery of applied water was low (typically < 30%) and clearly a substantial proportion contributed to the wetting up of deeper soil horizons (the surface layers were saturated prior to each experiment). The fate of microbes carried within the non-recovered water is unknown; both movement to deep groundwater and die-off within the soil water may occur.
Outflow microbial concentrations from cowpats were typically at least an order of magnitude lower than those from effluent (despite similar numbers applied, and similar flow rates). In contrast to effluent, typically, cowpat concentrations continued to increase over the duration of an experiment, suggesting that as a pat becomes steadily more saturated over time, so the rate of break-up increases.
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