Pathogen Pathways – Riparian Management II
3. Riparian attenuation of faecal microbes
3.1 Introduction
There is a widespread perception that fencing to exclude livestock from stream channels and a proportion of riparian land is likely to be the most effective measure in reducing faecal contamination by grazing cattle. Not only does this prevent the deposition of faecal material directly to streams and near channel contributing areas, but also the dense vegetation associated with riparian buffer strips (RBS) reduces the momentum of overland flow, promoting entrapment of faecal material (and other particulates). However, although many studies of sediment and nutrient entrapment in RBS have been conducted, understanding of microbial processes within them is poor, limiting evaluation of their effectiveness. To improve understanding, MAF Policy funded an initial field study of riparian attenuation of faecal microbes during the financial year 2001/02 (Collins et al. 2002), followed by a second study under Objective 2 of the PTRRP (Collins et al. 2003). Both these studies were conducted upon experimental plots established at the Ruakura farm campus, Hamilton. Objective 5 of the PTRRP has extended the Ruakura experimental work further, addressing one specific objective:
- To complete an assessment of the impact of flow rate upon microbial attenuation through conducting “fast flow” experiments, providing a comparison with results from earlier Ruakura experiments run at “intermediate” and “slow” flow rates.
The Ruakura plots are underlain by a Hamilton clay loam, characterised by high bypass flow (McLeod et al. 2003). Bypass flow occurs when water (and entrained contaminants) flow through soil cracks, and worm and root holes, rather than through the small soil pores of the matrix. As a consequence there is likely to be limited beneficial interaction (e.g., filtration of microbes) with the soil. Riparian attenuation is therefore likely to vary with soil type and in order to develop riparian guidelines that are widely applicable, further experiments have been conducted upon an Allophonic Soil on a farm at Tirau in the Waikato. This soil has developed in loamy volcanic tephra and is characterised by low bypass flow, with a matrix thought to be very efficient at filtering pathogens. It therefore provides a clear contrast with the clayey Hamilton soil, facilitating a second objective within the riparian attenuation study.
- To determine the impact of a contrasting soil type upon microbial attenuation.
3.2 Methodology
The study objectives were achieved through a series of plot experiments that involved the flushing of liquid dairy farm effluent into sloping grass strips, by surface runoff generated by a sprinkler. Surface and subsurface outflows at the lower end of a plot were sampled for microbial analysis during each experiment. This information, coupled with a known input of microbes to each plot, enables attenuation of Campylobacter and E. coli to be quantified.
3.3 Experimental design
3.3.1 Ruakura plots
Experimental plots have been established for earlier RBS studies (Collins et al. 2002, 2003) on the Ruakura campus farm, Hamilton. Two of these plots were used during this latest study. The plots lie on a sloping paddock (8-12°) underlain by a Hamilton clay loam soil, and have not been grazed since October 2001. The plots are 2 m wide and 5 m long. During the experiments one of the plots had short grass (7-10 cm) to simulate recently grazed pasture, while the other had long grass (30-37 cm) to simulate a grass buffer strip.
Each plot was bound along its sides by sheet metal inserted approximately 5 cm into the soil, to minimise lateral flow of water out the plot. At the lower end of each plot a trough was dug. Sheet metal was pushed horizontally into the exposed soil face, along the width of the plot, approximately 5 cm below the soil surface. Foam sealant was applied from the sheet metal upwards to just below the ground surface, across the width of the plot. This allowed collection of surface flow. The metal sheet overhung open plastic piping into which flowed surface runoff generated on the plot. This piping sloped laterally so that runoff flowed out of the lower end. At a depth of about 30 cm (corresponding to a clay-rich layer that impeded vertical movement of water) a second sheet metal strip was pushed horizontally into the exposed soil face, along the width of the plot. The metal strip overhung plastic piping and this design collected subsurface runoff draining out of the exposed soil face between depths of 5 and 30 cm. Water was applied to the top of each plot using a “sprinkler” which comprised a hosepipe, to which was attached a closed plastic tube that lay across the top of a plot. Ten holes were drilled in the plastic tube to ensure an even application of water to the plot. A water meter was attached to the hosepipe to determine input rates. The water supply was subject to changes in pressure. Effluent was obtained from a local dairy farm, and stored overnight at 10°C. A strain of C. jejuni, isolated from dairy land drainage sediments, was added to the effluent.
3.3.2 Tirau plots
Four plots (two short grass, two long grass) were also established at the Tirau study site, on an 8-10° slope, using the same experimental design as that adopted for the Ruakura experiment. However, since the Tirau site is a privately owned farm (as distinct from the research farm at Ruakura), it was not acceptable to introduce Campylobacter to the effluent. Instead, samples were analysed for E. coli only.
3.4 Experimental procedure
Two sets of experiments (Table 2) were conducted to address the objectives of the study. During each set, experiments were conducted on two (Ruakura) or four (Tirau) plots.
Table 2: Overview of the two sets of experiments
| Expt. | Num | R/T | Purpose | Date | Q_In | Q_Out |
|---|---|---|---|---|---|---|
| 1 | 2 | R | Objective A (flow) | 15/10/03 | 12.9 – 13.3 | 3.5 – 8.4 |
| 2 | 4 | T | Objective B (soil type) | 11/02/04 | 11.5 -12.6 | 2.4 – 4.7 |
Notes:
Num - the number of individual plots used in each set
R/T - the location (Ruakura or Tirau)
Q_In and Q_out - (surface plus subsurface) give the range of input and output flows (L/min) from the plots under each set of experiments
3.4.1 Experiment 1
Experiment 1 involved the application of effluent to two of the Ruakura plots. The inflow rate of water was about 13 L/minute, sufficient to saturate the soil and generate overland flow. Additionally, this rate provided a contrast to the slow (5-6 L/min) and intermediate (10 L/min) rates applied during objective 2. The outflow rate was steady and similar to that seen during heavy rainfall. Once the plots had attained a hydrological steady state by pre-watering, the hosed water was temporarily stopped and 20 L of dairy effluent was applied to each plot using watering cans fitted with a distributor to ensure an even application. Effluent was applied evenly across the width, and 0-30 cm above the top end, of each plot, over a period of c. 2 minutes, the time taken for the sprinklers to apply 20 L of water. This practise minimised the changes in outflow from a plot whilst the hosed input of water was temporarily stopped. Once effluent application was completed, the water supply was turned back on. The plastic pipe (attached to the hosepipe) was placed above the band of effluent enabling surface runoff to wash down through the effluent. Water was applied for 40 minutes. Surface and subsurface outflow from the lower end of each plot was measured at designated intervals using a graduated plastic container held beneath the collection pipe. One-litre samples of outflow were also collected at designated intervals for microbial analysis. Samples of background outflow (prior to the addition of effluent), and of the effluent, were also taken for microbial analysis. All samples were placed in a chilly bin and transported to the laboratory within one hour of collection. Analysis was completed within six hours of collection.
3.4.2 Experiment 2
Experiment 2, conducted upon the Tirau plots, used the same procedure as that for experiment 1, with the application of a fast inflow rate of 13 L/min to four plots.
3.4.3 Microbial analysis
E. coli analysis
E. coli were analysed using a commercial MPN technique involving Colilert and QuantitrayTM (IDEXX, USA). Trays were incubated at 35°C for 24 hours and E. coli identified under UV light (366 nm). The concentration of E. coli was determined from MPN probability tables supplied by the manufacturer.
Campylobacter analysis
Samples were analysed for C. jejuni using a 5-tube (i.e., 5 tubes per dilution) two-stage MPN technique (MIRINZ Manual 2001). A ten-fold dilution series was inoculated into Campylobacter medium. The concentration of confirmed Campylobacter in the original sample was determined by reference to five-tube MPN probability tables.
3.5 Results
3.5.1 Objective A
To determine the impact of flow rate upon microbial attenuation – Experiment 1
Both concentrations of E. coli and Campylobacter in plot surface outflow during experiment 1 exhibited an initial peak, within 10 minutes of effluent application, followed by a gradual decline (1-2 orders of magnitude) for the remaining 40 minutes of the experiment. This pattern is consistent with an initial first flush, followed by the depletion of a finite store of faecal material. It is also a pattern consistent with earlier experiments conducted during objective 2 of the PTRRP.
Microbial recovery (the percentage of the applied microbes recovered in the outflow, i.e., the inverse of attenuation) and outflow data from experiment 1 has been pooled with that from earlier experiments. These combined results (Figures 4 and 5) show that flow rate has a clear impact upon microbial recovery, with a relationship evident between percent recovery and outflow rate, for both E. coli and Campylobacter. At low flow microbial recovery is less than 5% but at high flows, ranges between 15% and 100%. The relationship between flow rate and microbial recovery 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 this relationship is specific to the Hamilton clay loam, a soil characterised by high bypass flow.
3.5.2 Objective B
Application of a fast flow rate to the Tirau plots (11.5-12.6 L/min) resulted in E. coli recovery rates of 40%, 41%, 47%, and 96% respectively for each of the four plots. Concentrations in outflow peaked between 5 and 10 minutes after effluent application, and declined steadily thereafter for the remainder of the experiment (Figure 6).
No clear difference in E. coli recovery (40%-96% at Tirau; 41% and 100% at Hamilton) was apparent between plots on the two differing soil types. This was probably due to the limited number of experiments conducted and uncertainties associated with the experimental methodology. Clearer evidence of a difference in soil processes between the two sites is provided by the hydrological data. Under fast flow at Ruakura (Hamilton clay loam) outflow rates were 3.5-8.4 L/min and included a substantial subsurface contribution. At Tirau, for the same inflow rate, surface outflows were 2.4-4.7 L/min, and no subsurface flow was recorded on any of the four plots. These hydrological differences provide some evidence that the Allophonic Soil is devoid of bypass flow and is better suited for soaking up both water and microbes.
Figure 4: The relationship between the percent recovery of applied Campylobacter and outflow rate
Figure 5: The relationship between the percent recovery of applied E. coli and outflow rate
Figure 6: Outflow (surface runoff) E. coli concentrations over time upon the four Tirau plots
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
Contact this person
