1. Delivery of Faecal Contamination to Waterways by Surface Runoff – Large Scale Rainfall Simulator Experiments

1.1 Introduction

There is good evidence, both within New Zealand (Wilcock et al. 1999; Donnison et al. 2001) and from elsewhere (Baxter-Potter and Gilliland 1988) to indicate that grazing livestock are an important, diffuse source of faecal contamination to freshwaters. This contamination of pastoral waterways may be predominantly caused by the delivery of faecal material to a watercourse in surface runoff and, where livestock have access to a stream, direct deposition of faecal material.

Whilst cattle behavioural studies can address direct deposition delivery by surface runoff is more difficult to quantify. A few studies have determined microbial concentrations in surface runoff (Doran and Linn 1979), but have failed to quantify the microbial load delivered to waterways by this mechanism. Furthermore, little is known about the variation of microbial delivery by surface runoff with soil, slope, rainfall characteristics, and stock numbers. This lack of information limits 1) quantification of the relative importance of surface runoff as a transmission pathway for faecal contamination and 2) assessment of the impact of mitigation measures (e.g. riparian retirement) that will potentially reduce delivery by surface runoff.

To improve understanding of the delivery of microbes to waterways by surface runoff, experiments were conducted under Objective 2 of the PTRRP using a large scale rainfall simulator located upon hill-country pasture. The experiments aimed to quantify microbial delivery under heavy rainfall and examine the variation of delivery with stock history.

1.2 Methodology and site description

The large-scale rainfall simulator (Photograph 1.1) encompasses 1050 m²of a topographically-convergent hillslope in the Pukemanga catchment, within the Whatawhata Research Station, west of Hamilton. The simulator consists of 13 rainfall stands at 9 m spacing. Each stand consists of a 5 m high pole with 4 closely spaced upward-spraying sprinklers mounted on a frame. The design rainfall rate is 35 mm/hr, providing a total application rate of 10.2 L/s, applied over approximately 1 hour during each experiment. This application rate and duration corresponds approximately to an 8-year recurrence interval.

Mean slope angle at the site is 18° , and the hillside is underlain by a yellow-brown earth soil (Kaawa hill soil). Vegetation is predominantly ryegrass-clover. No artificial borders were used around the area encompassed by the simulator since surface runoff converged and flowed naturally into a headwater stream. A 10m wide wing-wall dug 50 mm into the ground was used to guide the flow to a flume (Photograph 1.2) at the outlet of the simulator area. Flow rate was measured using a 25 L tipping bucket. Samples of the outflow were collected manually throughout an experiment and taken immediately to the laboratory for microbial analysis. Five experiments (Table 1.1) were conducted before and after grazing by sheep in both winter and summer. To ensure comparable antecedent moisture conditions between experiments, the simulator was (if necessary) run one day prior to each event in order to wet up the soil.

Photo 1.1: The rainfall simulator and experimental site.

Photo 1.2: The flume and catchment outlet.

Event	Description	Stock History	Load	Conc.
1	Summer pre-graze pre-graze	No grazing for 8 weeks prior	9 x 10⁸	6 x 10³
2	Summer post-graze	Grazed for 8 days prior	4 x 10¹¹	3 x 10⁶
3	Winter pre-graze	No grazing for 7 weeks prior	2 x 10⁹	1 x 10⁴
4	Winter post-graze	Grazed for 4 days prior	6 x 10¹¹	6 x 10⁶
5	Winter post-graze	No grazing for 2 weeks prior	2 x 10¹⁰	1 x 10⁵

Table 1.1: A description of the rainfall simulator events. Load is total number of bacteria in the outflow (MPN) over the duration of an experiment and Conc. is the event mean concentration (MPN/100 mL).

1.3 Results

Time-series outflow and outflow E. coli concentrations for each event are shown in Figure 1.1. These generally illustrate an initial flush of relatively high bacterial concentration, prior to the attainment of peak flow. This is followed by a gradual decline in concentration over the remainder of the event. A clockwise hysteresis, apparent during event 3 (Figure 1.2), indicates that concentrations are higher on the rising than the falling limb of the hydrograph. This suggests that a gradual washout or depletion of the catchment store of bacteria occurs as the event progresses.

The total number (load) of bacteria washed across the outflow flume during an event varied between 9 x 10⁸ and 6 x 10¹¹ MPN (Table 1.1). 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.

Both the loads and event mean concentrations of bacteria (Table 1.1) correlate 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 bacterial die-off on the catchment surface and is described by a power relationship (Figure 1.3). The rapid initial die-off indicates that event mean concentration declines by about 2 orders of magnitude within 15 days. It is not possible to discriminate the influence of season upon bacterial dynamics from these results although it is likely that die-off is enhanced in summer due to greater sunlight and lower levels of soil moisture.

1.4 Discussion

Experiments using the LRS have shown that surface runoff is a key delivery mechanism for faecal contamination under grazed hill-country pasture, during very large rainfall events.

Event mean surface runoff E. coli concentrations during the experiments ranged between 10³ to 10⁶ MPN/100 mL, whilst concentrations observed during high flow in a nearby catchment outlet (the Upper Managaotama, sampled for bacterial water quality by AgResearch) typically peak at >10⁵ MPN/100 mL. This comparison provides further evidence that surface runoff is at least a key delivery process (and perhaps the dominant one) during large events. However, its importance under low to moderate rainfall currently remains un-quantified. If further research shows that appreciable faecal contamination is only delivered by surface runoff during large events then, clearly, for mitigation measures (primarily riparian buffer strips) to be effective they must be capable of significant microbial attenuation during large events. As a consequence, one key objective of the buffer experiments (Section 2) was to explore the impact of flow rate upon microbial attenuation.

Figure 1. 1 Time-series outflow (L/s, 2nd Y axis) and outflow E. coli concentration (MPN/100 mL, Y axis) for the five rainfall simulator events. Units of time are minutes after rainfall began.

Figure 1. 2 Clockwise hysteresis in E. coli concentrations during event 3, whereby values are higher on the rising than the falling limb of the hydrograph.

Figure 1. 3 The decline in event mean concentration of E. coli with the number of days since sheep were removed from the hillside, prior to an event

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