Pathogen Pathways – Riparian Management II
1. Executive summary
Objective 5 of the Pathogen Transmission Routes Research Programme (PTRRP) focused upon three areas of related research:
- Faecal contamination of surface runoff,
- Riparian attenuation of faecal microbes, and
- The drawing up of preliminary riparian management guidelines with respect to faecal microbes.
Faecal contamination of surface runoff
Continuing work initiated under objective 2 of the PTRRP, 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. Surface runoff generated by the LRS converges naturally and flows directly to a headwater stream. At the point of convergence, flow was measured and samples collected for E. coli analysis. Following the five experiments conducted under objective 2, a further six experiments have been conducted under objective 5. Together these 11 events encompass two simulated rainfall rates and variable antecedent soil moisture conditions. Additionally, they have been conducted between zero (stock still present) and 75 days after grazing by sheep, in both summer and winter.
The total number or load of bacteria washed across the outflow flume during an event varied between 2 × 108 and 6 × 1011 MPN. Since the outflow drains into a headwater stream, these loads represent a substantial delivery (2 × 105 to 6 × 108 MPN/m2) of E. coli direct to the stream network. Event mean concentrations varied between 3 × 103 and 6 × 106 MPN/100 mL. Both the load and event-mean concentration of bacteria declined exponentially with the time elapsed since the paddock was last grazed.
Riparian attenuation of faecal microbes
Ongoing field studies (on the Ruakura farm campus, Hamilton) 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. Twenty litres 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. Results using a fast flow rate under objective 5 have been combined with those from earlier experiments (objective 2) using intermediate and slow flow rates. Pooling these results shows that flow rate (or hydraulic loading) has a clear impact upon the recovery (the percentage of the applied microbe recovered in the outflow, i.e., the inverse of attenuation) of microbes. At low flow, microbial recovery is ≤ 5% (i.e., entrapment is ≥ 95%) but at high flows, recovery ranges between 15% and 100%. This finding raises important implications for buffer strip design, particularly if appreciable faecal contamination is only delivered by surface runoff during large events. However, it is important to note that these results are specific to the Hamilton clay loam, a soil characterised by high bypass flow.
Grass plot experiments have also been undertaken upon a farm near Tirau in the Waikato. These plots are located upon an Allophonic Soil characterised by a predominance of flow through the soil matrix. The Tirau soil therefore provides a clear contrast to the Hamilton clay loam (underlying the Ruakura site) with its high rates of bypass flow.
No clear difference in E. coli recovery was apparent between plots on the two differing soil types. This was probably due to the limited number of experiments conducted (two at Hamilton, and four at Tirau under the fast flow rate) and uncertainties associated with the experimental methodology. However, strong hydrological differences were apparent. For the same fast inflow rate, surface outflow on the Tirau soil was much lower than that of the Hamilton clay loam. Additionally, no subsurface flow was measured upon the Tirau plots, this contrasted with the large volumes recorded at Ruakura. These hydrological differences provide evidence (supporting that from other studies) that the Allophonic Soil at Tirau is likely to be much more efficient at attenuating microbes.
Preliminary riparian guidelines
Brief guidelines, as discussed in this report, have been established describing riparian buffer effectiveness with respect to faecal microbes. These account for variations in efficiency with slope, soil type and buffer width.
Contact for Enquiries
Phil Journeaux
Manager
North Island Regions
Sector Performance Policy
MAF Policy
Private Bag 3123
Hamilton
NEW ZEALAND
Phone: +64 7 957 8313
Fax: +64 7 957 8315
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