Pathogen Pathways – Riparian Management II
4. Preliminary riparian management guidelines
4.1 Introduction
Results from the PTRRP and elsewhere have shown that a range of factors influences the performance of RBS with respect to the entrapment of faecal microbes. Entrapment efficiency of a RBS can vary markedly and is strongly dependent upon local, site-specific factors. In order to aid the interpretation of this information, a set of riparian guidelines has been developed. Currently these are brief and preliminary in nature. However, it is proposed that further development of them occurs during year 3 (objective 9) of the PTRRP to provide a more comprehensive set of management guidelines.
4.2 Review of information
Whilst numerous studies have been conducted to investigate RBS performance with respect to sediment and nutrients, very little information exists with respect to microbes. To date only five papers on this topic have been found in the international literature, and these do not examine the variation of efficiency with a given factor. Additionally, work conducted under this program represents the only study undertaken within New Zealand. Given the general scarcity of information, this review also encompasses key findings with respect to sediment and particulate nutrients. Justification for this is twofold:
- Microbes can become attached to soil particles and when they do so their short-term fate within a RBS will mirror that of the particle they are adhered to.
- Whilst entrapment efficiencies for sediment (and particulate P) cannot be directly transferred to microbes (since at least some microbes are likely to be transported unattached) the relative impact of a range of factors upon sediment entrapment may be broadly applicable to microbes.
4.2.1 Slope
A composite of data from studies conducted using differing methods at varying locations suggests that slope angle is a key factor in determining sediment entrapment within RBS (Dillaha et al. 1989; Magette et al. 1989; Peterjohn and Correll 1984; Young et al. 1980). Stronger evidence is provided by the work of Dillaha et al. (1988) who compared sediment removal under differing slopes with all other factors constant, deriving an inverse relation between slope angle (6°-9°) and sediment entrapment (50%-90%). Whilst no equivalent data is yet available for microbes (i.e., studies upon variable slope angles have not been conducted), the plot studies at Ruakura and Tirau have shown, upon a slope of 8°, that microbial entrapment occurs provided flow rates are not excessive. Studies elsewhere indicate that there is likely to be an optimum range of slope angle at which RBS attenuate both sediment and microbes. On steep hill-country, convergence of surface and subsurface flows occurs upon the hillside, leading to the development of wetlands that drain directly to the stream network. These are permanently (or near-permanently) saturated features that respond rapidly to rainfall with high discharge velocities and channelised flow. Typically outflow rates from these wetlands (Collins 2004) far exceed those generated upon the Ruakura plots at which little or no microbial entrapment was shown to occur. Establishing RBS at the lower end of these wetlands is, therefore, unlikely to trap microbes in surface runoff, particularly during storm events. Some, limited, field evidence supports this assertion (Collins 2002).
Cattle are not attracted to the larger and deeper of these wetlands, presumably for fear of entrapment. However, the smaller and shallower of these wetlands have been shown to attract cattle for lush grazing and considerable faecal material has been observed to be deposited directly upon them (Collins 2004). Consequently, fencing to exclude cattle from the smaller wetlands is likely to reduce their levels of faecal contamination and, therefore, those of the stream network.
The primary role of RBS is the entrapment of pollutants washed in by surface runoff. Consequently, they are generally likely to be bypassed upon flat or gently sloping land where the vertical movement of pollutants down through the soil horizons dominates. Additionally, in order to reduce surface ponding and aid infiltration, low-lying pastoral land is likely to be underlain by artificial drainage that feeds a network of open drains discharging directly into streams. Subsurface drains have been shown to be susceptible to high levels of faecal contamination (Ross and Donnison 2003). Furthermore, studies within the Toenepi catchment, Waikato, have shown that the loss of nitrogen at the catchment outlet can be accounted for by the sum of all drainage inputs to the stream network (R. Wilcock, pers. comm.). In other words, the presence of RBS at Toenepi would generally be ineffectual at attenuating nitrogen delivery to waterways. It is likely that this also applies to the entrapment of faecal microbes, although, in contrast to soluble nitrogen, filtration probably attenuates microbes as they infiltrate down through the soil horizons. This process, however, differs from entrapment within buffer strips. Muscott et al. (1993) in a comprehensive review of the role of buffer zones in improving water quality, also note the minimal impact RBS have upon artificially drained land. Ongoing studies from the PTRRP at Massey University have, however, shown that surface runoff can be generated upon drained land. Work proposed for next year plans to quantify the importance of this relative to drainage water, under varying rainfall rates.
4.2.2 Buffer width
Data from studies comparing multiple width buffers in the same location (Dillaha et al. 1988; Dillaha et al. 1989; Magette et al. 1989; Peterjohn and Correll 1984; Young et al. 1980, Mander et al. 1987; Vought et al. 1994) have shown that sediment and Total Phosphorus entrapment (removal rates between 53% and 98%) increase with increasing buffer width (4.6 m to 27 m). Additionally, Young et al. (1980) observed a linear decrease in total coliform concentration with increasing (0-25 m), cropped buffer width. Results from the earlier Ruakura plots studies (Collins et al. 2002) showed that increasing plot length (equivalent to buffer width):
- Decreased peak flow.
- Increased the time taken for peak microbial concentration in outflow to occur.
- Decreased peak microbial concentrations.
All three effects indicate that the longer plot length (5 m versus 1 m) led to a greater entrapment of microbes, although this wasn’t directly quantified due to the high level of uncertainty associated with the 3-tube MPN technique that was used.
4.2.3 Soil type
Soil drainage properties influence RBS performance. Free draining soils minimise the generation of surface runoff, both on the hillside and within a buffer. Under these conditions RBS entrapment may be high relative to the amount of material washed in. However, if surface runoff generation is too infrequent then it is questionable as to whether buffer strips are a cost-effective mitigation measure. Poorly drained clay-rich soils promote the generation of surface runoff and, provided flow rates are not excessive, RBS are likely to trap faecal microbes. However, such soils can also be characterised by macropores (e.g., cracks and worm holes) that promote rapid vertical flows that bypass the soil matrix, providing little attenuation of microbes. The Hamilton clay loam underlying the Ruakura experimental site exhibited high rates of bypass flow that reduced the microbial trapping efficiency of overlying grass strips (Collins et al. 2003). In contrast, Allophonic Soils are characterised by good drainage (little or no bypass flow) and, effective filtration of infiltrating microbes, McLeod et al. (2001).
Filter strips are best able to remove coarse particles from overland flow, with a larger reduction in transport energy being required to deposit clay-sized particles within a strip (Collier et al. 1995). Whilst E. coli may be approximately as large as a clay particle, most other bacteria and viruses are considerably smaller. Their entrapment within a RBS may therefore require either low flow rates or attachment to soil particles.
4.2.4 Table of guidelines
Using the review of information provided in sections 4.2.1-4.2.3, a preliminary table of riparian efficiency has been drawn up (Table 3).
Table 3: Preliminary table of RBS efficiency
| Slope Angle | Relative RBS Efficiency | Notes |
|---|---|---|
| Flat to gently sloping land often drained | Generally, RBS may not be effective since vertical movement of water and pollutants dominates. High intensity rainfall, however, can generate substantial surface runoff and faecal loads. Average Efficiency; although this rating remains uncertain until ongoing studies are complete. | Poorly, imperfectly drained soil is often artificially drained. |
| Rolling land | Efficiency rating encompasses low through to high, and is dependent upon buffer width and soil type. Allophonic Soils are effective in attenuating faecal microbes and, in combination with the dense vegetation of an RBS, provide a high efficiency of attenuation. High bypass flow soils limit RBS effectiveness to average, or low efficiency. | Efficiency also varies with flow rate, which is a function of rainfall, antecedence, slope, and soil. |
| Steep land | Low efficiency, limited by topographical convergence that provides high flow velocities and channelised flow. For buffers to be effective they are likely to need to extend some distance upslope, following flow pathways. Exclusion of stock from critical source areas (e.g., wetlands, flow pathways) is an appropriate mitigation measure. | Stock are wary of large deep wetlands. Fencing of smaller shallower wetlands may, however, yield improvements in water quality. |
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