Pathogen Pathways – Soil Risk Index
5. Risk-based approach to assessment
Risk assessment forms the basis of this work. Relative risks of microbial transport associated with:
- Surface runoff
- Bypass flow through soil
- Flow through the vadose zone
are presented spatially. Relative risk associated with surface runoff and bypass flow are combined to present relative risk of microbial contamination of surface water, while relative risk associated with bypass flow and vadose zone transport are combined to present relative risk of microbial contamination of ground water.
To depict the risk classifications spatially we have used the New Zealand Land Resource Inventory linked to the Landcare Research soil database to provide information on soil or land characteristics such as slope, drainage class, depth to slowly permeable layer, hydraulic conductivity, particle-size class, nature of the soil parent material, toprock, soil structure, soil classification. The maximum scale at which the data should be used is 1:50 000.
Landcare Research has conducted research into transmission of non-pathogenic microbes, both faecal coliforms and bacteriophage (virus), through soil. In a practical setting, on a national scale, it is unlikely there is any difference between transport of pathogenic and non-pathogenic microbes.
Rainfall has not been incorporated into the risk models. Rainfall serves to accentuate the risk of many of the assessments. Antecedent soil water conditions affects runoff and runoff models use soil saturation as drivers. Although annual or monthly rainfall is well modelled over the country, soil saturation requires very detailed information on rainfall intensity, timing and soil water storage. Artificial distribution of rain-days becomes increasingly unrealistic with time, and soil-water status needs frequent updating to reality. In addition, local micro-topography can have a strong influence on water movement and storage. Consequently, this type of study is better undertaken at site specific locations. Furthermore, errors associated with the increasing use of supplementary irrigation or effluent irrigation would not be accounted for in national water storage models.
5.1 Relative risk of microbial transport via soil surface runoff
Faecal microbes can be transferred to waterways via the flow of water over the surface of the land, called runoff, and is recognised as a key indirect transmission pathway of microbes from cattle to waterways (Collins et al. 2005; Collins 2004). Runoff may occur from rainfall or irrigation of water or effluent. A map of intrinsic soil and land properties that influence runoff is critical to the identification of areas of land at risk during periods of heavy or prolonged rainfall, irrigation at inappropriate times or excessive volumes or rates.
Relative soil runoff potential (RSRP) is a relative ranking of soil and land properties that control runoff when the soil is at field capacity (Table 1). The RSRP is an ordinal classification based on readily observable soil properties. It is difficult to allocate precise numerical values to these properties on a nationwide basis as a genuinely quantitative model would require both vertical and horizontal hydraulic conductivity measurements at a range of tensions throughout the soil profile and water storage values. Microtopographic effects are also important in determining flow paths.
The RSRP classification is based on work by Farquharson et al. (1978), who generated RSRP based on water regime class, depth to impermeable horizon, permeability class above the impermeable horizon and slope class; the latter accentuated the three former factors. We have modified the boundary criteria slightly to conform to soil property classes contained within the Landcare Research National Soil Database (NSD) and the New Zealand Land Resource Inventory (NZLRI) while retaining class intent. Specific changes were made to the soil-water regime where depth-duration of water logging was translated to New Zealand drainage class; classification limits for depth to impermeable layer were translated to New Zealand class limits; permeability class above the impermeable horizon was based on measurements of hydraulic conductivity and extrapolations of those measurements. Whereas Farquharson et al. (1978) accentuated runoff classes with slope classes to a maximum slope class of >8°, we have modified the potential runoff classes to increase on slopes of 16-25° and >25° (Table 1) to reflect not only the greater percentage of steeper slopes but more importantly, the spatial variability in soil properties and micro-topography on steeper slopes. However, following Gage and Black (1979), who inferred that an intact cover of Waiohau Ash moderated the erosive effects of runoff, we have reduced the potential runoff class by one where the soil is developed in permeable, high water storage capacity tephra deposits.
Table 1: Relative soil runoff potential class in relation to soil and site properties
Figures 1 and 2 show the relative soil runoff potential for the North and South Islands respectively, while Figure 3 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map.
Figure 1: Relative soil potential runoff – North Island
Figure 2: Relative soil potential runoff – South Island
Figure 3: Relative soil potential runoff – 1:50 000 scale example
In the North Island, very low relative potential for soil runoff is associated with soils developed in tephra in the central North Island and the Taranaki regions. High relative potential for soil runoff is largely associated with hilly or steep land, especially where drainage is restricted. However, flat, poorly drained land in the greater Waikato region also has a high relative potential to generate runoff under adverse conditions. In the South Island, very low relative potential for soil runoff is predominantly associated with soils developed on low-angle alluvial plains. High relative potential for soil runoff is associated with hilly or steep land, especially where drainage is restricted. In Southland, soils with a drainage impediment have a very high relative potential for soil runoff. Table 2 shows the percentage of areas of soils in the North and South Islands and their relative potential for soil runoff.
Table 2: Percentage of areas of soils in the North and South Islands and their relative potential for soil runoff
| Relative potential for soil runoff | Percentage of land | |
|---|---|---|
| North Island | South Island | |
| Very high | 9 | 11 |
| High | 41 | 49 |
| Moderate | 23 | 18 |
| Low | 11 | 4 |
| Very low | 16 | 18 |
Knowledge gaps
High resolution digital elevation models based on laser radar would allow improved runoff prediction of flow paths including micro-topographic effects. Not all soil map polygons are adequately described. There are two areas of concern. First, those from older soil surveys where soil descriptions are inadequate or map units were envisaged to contain a range of soils. Second, where the soil survey data are based on general soil surveys of the North or South Islands, the named soil may be mapped over large geographic distance, which means the range in soil properties within the unit is likely to be wide. Many of the soils in the South Island are based on general soil survey information.
5.2 Relative risk of transport of microbes through soil by bypass flow
Soil type is a critical factor in determining the degree to which faecal microbes are delivered to waterways, strongly influencing microbial movement within surface runoff and subsurface flows under both drained and undrained conditions (Collins et al. 2005). Numerous studies have identified that soils process microbes in surface-applied animal waste in different ways (Aislabie et al. 2001; McLeod et al. 2001, 2003, 2004; Gagliardi & Karns 2000; Smith et al. 1985).
The ability of a given soil to attenuate microbial transport depends strongly on the degree to which water movement bypasses the fine pores of the soil matrix; the greater the bypass flow, the lower the microbial attenuation. Bypass flow through a soil occurs when water and entrained contaminants move through a soil through the soil cracks and large pores or worm channels rather than moving through the fine pores of the soil matrix. Under bypass flow regimes soil/water contact is minimized and the water and entrained contaminants move rapidly to depth where they are potentially available to enter drains and waterbodies.
Although it is possible to numerically model microbial transport through soil, most models require a wide range of data inputs, many of which are not known with certainty over a wide geographic area. Microbes are often attached to colloids and restricted by size to bypass flow through larger soil cracks or pores where solute transport velocity is higher. The addition of bypass flow mechanisms to the models requires inputs that are difficult to obtain (Logsdon 2002). Traditional, or easy-to-obtain measures of physical parameters such as particle size distribution, water retention or microbial sorption characteristics are unreliable on their own (Young & Crawford 2004). At worst, soil microbial sorption characteristics can give rise to incorrect outcomes in a practical setting. Preliminary work (Landcare Research, unpublished) suggested microbial sorption of clayey Netherton soil material was very high while large undisturbed soil cores of the same soil irrigated with dairy-shed effluent had high concentrations of microbes in the leachate. In contrast, allophanic soil material had a low microbial sorption (Landcare Research, unpublished) while undisturbed soil cores irrigated with dairy-shed effluent leached very few surface-applied microbes. Indeed, Taylor et al. (2004) reported that in media where micro-organisms may be excluded from the matrix the disparity between the average linear velocity of groundwater flow and flow velocities transporting micro-organisms is intensified. Results of this type of disparity have been demonstrated in New Zealand soils (Aislabie et al. 2001; McLeod et al. 2001, 2003, 2004; Sinton et al. 2000; Pang et al. 1998).
To overcome these difficulties, we have modelled potential for microbial bypass flow by ranking soils into high, moderate and low classes based on results and insights gained from microbial breakthrough curves (BTCm) (Aislabie et al. 2001; McLeod et al. 2001, 2003, 2004), soil structure and other relevant soil properties. The BTCm ranking was then linked to the New Zealand Soil Classification (NZSC) (Hewitt 1998) allowing the relative risk of rapid microbial transport through soil to be extrapolated to all of New Zealand. The key for rating NZSC classes for potential for microbial bypass flow (BPFm) is given in Table 3.
Table 3: Key for rating NZSC classes and other relevant soil features for potential for microbial bypass flow.
| NZSC feature | Level used in NZSC classification | Rating of potential for BPFm | Justification overview |
|---|---|---|---|
| Organic Soils | Order | High | Artificially drained. Humic substances compete for same binding sites as microbes (Gerba 1984) |
| Ultic Soils | Order | High | Ultic Soil analysed had high BPFm (McLeod et al. 2004). Ultic soils generally have coarse soil structure with clay coatings. |
| Granular Soils | Order | High | Granular Soil analysed had high BPFm (McLeod et al. 2004). Granular soils generally have well developed soil structure with clay coatings. |
| Melanic soil properties | Order Group |
High | Melanic Soils contain smectitic clays with high shrink/swell potential. This gives rise to strong polyhedral, blocky or prismatic structure (Hewitt 1998). |
| Podzols | Order | High | Most have pans and likely artificially drained under intensive land use. Orthic Podzols likely have channelised flow pathways from sesquioxide deposition. |
| Gley soil properties | Order Group |
High | Gley Soils analysed had high BPFm (Aislabie et al. 2001; McLeod et al. 2001). Generally coarse structure. Waterlogging for long periods results in fewer air/water interfaces where microbes are retained (Poletika et al. 1995). Drains rapidly remove water from large pores where solute velocity high. Drained soils have significant microbial transport in the subsoil (Jamieson et al. 2004) |
| Perch-gley | Group | High | Similar to above. |
| Peaty | Subgroup | High | Humic substances compete for same binding sites as microbes (Gerba 1984). Artificial drainage. |
| Mottled profile form | Subgroup | High | Waterlogged at some time of the year. Drains rapidly remove water from large pores where solute velocity high. |
| Argillic horizon | Group Subgroup |
High | Coatings block pores and channelise flow. |
| Cutanic horizon | Differentiae | High | Coatings block pores and channelise flow. |
| Slowly permeable layer (SPL) | Differentiae | High | Water often perches above a SPL,often drained under intensive land use |
| Coarse soil structure | Not in NZSC | High | Soils with more than about 30 percent of peds passing through a 10 mm sieve have high BPFm. |
| High Ksat:K-40 ratio | Not in NZSC | High | Used judiciously, this ratio can indicate when flow occurs predominantly through large pores where solute transport velocity is high. |
| Skeletal soils | Soilform | High | Conceptually, raw stones and gravels with minor sand matrix have few mechanisms to retain microbes –– transport velocities likely to be fast. |
| Firm | Group | Moderate | Firm horizons are apedal. Firm soil analysed had moderate BPFm (McLeod et al. 2003). |
| Water repellency | Not in NZSC | Moderate or high | Soils with water repellency have finger flow pathways with high solute velocity (McLeod et al. 2001). |
| Pedal | Subgroup | High | Pedal Subgroups have well developed soil structure. |
| Brown Soils | Order | Moderate | Brown Soil analysed had moderate BPFm (McLeod et al. 2003). |
| Pallic Soils | Order | Moderate | Pallic Soil analysed had moderate BPFm (McLeod et al. 2003). |
| Oxidic Soils | Order | Moderate | Well-developed structure, high clay contents, high bulk density, slow permeability, clay translocation (cutans) but may have net positive charge (Hewitt 1998). |
| Recent Soils+ | Order | Low | Recent Soils analysed had low BPFm (McLeod et al. 2001; McLeod et al. 2004). |
| Allophanic soil material | Group Subgroup |
Low unless texture group clayey | Soils containing ASM are often developed in young loamy tephra which promotes matrix flow. |
| Part Semiarid Soils | Order | Low | Aged-Argillic, Argillic and Solonetzic Groups have argillic horizons and are separated above. Immature Groups do not have argillic horizon. |
| Pumice Soils | Order | Low | Pumice Soil analysed had low BPFm (McLeod et al. 2001). |
| Allophanic Soils | Order | Low | Allophanic Soil analysed had low BPFm (McLeod et al. 2001). |
Figures 4 and 5 show the relative potential risk of transport of microbes through soil by bypass flow for the North and South Islands respectively, on flat to rolling land (slopes <15°). Figure 6 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map.
Approximately 50 percent of North Island soils on flat to rolling land have a high potential for microbial bypass flow. Large areas occur, for example, in Northland on old strongly weathered, strongly structured, clayey soils and in the Manawatu, Wairarapa, and Hawkes Bay, where soils have a drainage or structural impediment. Low-lying poorly drained soils with a high potential for microbial bypass flow occur on the Hauraki Plains and in the Waikato.
Figure 4: Relative potential risk of transport of microbes through soil by bypass flow on flat to rolling land – North Island
Figure 5: Relative potential risk of transport of microbes through soil by bypass flow on flat to rolling land – South Island
Figure 6: Relative potential risk of transport of microbes through soil by bypass flow on flat to rolling land – 1:50 000 scale example
There are large areas of Organic Soils in the Waikato that are also rated as having a high potential for microbial bypass flow. Approximately 40 percent of North Island soils have a low potential for microbial bypass flow, and large areas of these soils are associated predominantly with volcanic soils in Taranaki, central North Island and Waikato.
Similarly, approximately 50 percent of South Island soils have a high potential for microbial bypass flow. Large areas occur, for example, on the West Coast where soils with an iron pan or poorly drained soils have developed. A drainage impediment suggests many of the soils in the south and east of the South Island have a high potential for microbial bypass flow. In contrast to the North Island, <25 percent of the soils have a low potential for microbial bypass flow and are associated predominantly with Allophanic Brown Soils that, interestingly, develop not from tephra, as in the North Island, but from loess. Table 4 shows the percentage of areas of soils in the North and South Islands and their relative risk of transport of microbes through soil by bypass flow.
Table 4: Percentage of areas of soils in the North and South Islands and their relative potential risk of transport of microbes through soil by bypass flow
| Relative potential risk of transport of microbes through soil by bypass flow | Percentage of land | |
|---|---|---|
| North Island | South Island | |
| High | 51 | 48 |
| Moderate | 7 | 30 |
| Low | 42 | 22 |
Knowledge gaps
Knowledge gaps pertaining to soil map units as described in section 4.1 apply. In addition, BTCm have not been generated for many soil classes including Organic Soils, Melanic Soils, Podzols. Generally, BTCm have been generated only for one soil from a soil class. Confidence would be increased by sampling other soils from the same soil class.
5.3 Relative risk of microbial transport through the vadose zone
The vadose zone (VZ) occurs from immediately beneath the soil (≈ 1.2 m), down to the top of the groundwater table. Spatially, it is of variable depth and not permanently saturated. When rain- or irrigation-water percolates through the VZ, entrained microbes may also be transported. The thickness and nature of the VZ will affect microbial transport to the regional groundwater table. For example, fine-grained VZ material such as pumice, sand alluvium will physically trap a higher proportion of microbes than large-pore heterogeneous material such as fractured rock or gravel (Davies-Colley et al. 2003).
A national map showing attenuation of microbial transport through the VZ is one part of the model to identify vulnerable groundwater. However, there is insufficient knowledge about the hydraulic properties of the wide range of VZ materials within New Zealand to form the basis of a comprehensive and reliable spatially explicit database. Similarly, the spatial effects of weathering of the VZ material, temporal wetness characteristics, differences between viral and bacterial transport are all poorly understood on a nationwide basis.
Notwithstanding these reservations VZ material has been rated for risk of microbial transport as a first step towards a national model. The “Toprock” field from the NZLRI, which describes the first-named entire (not patchy) rock type, irrespective of any succeeding stratigraphy, has been selected to represent the VZ. One of the key determinants of risk of microbial transport is pore size which encompasses spaces between clay- to boulder-sized particles as well as fractures within hard rock. Microbes can travel much faster than the average flow velocity because they may travel as clumps or attached to colloids and so are restricted by size to larger pores where solute velocity is faster, provided that there is sufficient water to fill the pores. Rock types associated with this rapid microbial transport have been inferred from experimental data where known and supplemented by rock type attribute descriptions for other rock types (Lynn & Crippen 1991). Because of the limited data on VZ properties and processes, risk of microbial transport through the VZ has been divided into two relative classes, viz. Low and High.
Vadose zone rocks with potentially low risk of microbial transport are predominantly very loose to compact sedimentary rocks and sheared rocks of mixed lithologies, crushed argillite associations, pumiceous material and bentonitic mudstone. Deeply weathered rock of any lithology (where recorded) is also set to low as weathering increases clay content and decreases joint aperture compared to the less weathered material. Both these processes can favour retention of microbes.
Unlike the North Island, the South Island NZLRI “Toprock” field does not differentiate fine and coarse alluvium. Studies by Close et al. (2002), Pang and Close (1999), and Pang et al. (1998) in the Canterbury region of the South Island indicate some transport of microbes through coarse gravel aquifer material. To capture this type of transport, Toprock “Al” (Alluvium) for the South Island has been set to High. As a result, areas of fine alluvium that may have potentially higher attenuation of microbial transport, are incorrectly classified. The rating of each “Toprock” type is shown in Table 5.
Table 5: Relative risk of microbial transport through the vadose zone
| Toprock | Relative risk of microbial transport through the vadose zone |
|---|---|
| Igneous rocks - Extremely weak to very weak | |
| Ngauruhoe tephra | Low |
| Rotomahana mud | Low |
| Tarawera tephra | Low |
| Scoria | Low |
| Pumiceous lapilli | Low |
| Kaharoa tephra and alluvium | Low |
| Ashes older than Taupo tephra | Low |
| Quaternary Breccia older than Taupo Breccia | Low |
| Lahar deposits | Low |
| Extremely weak altered volcanics | Low |
| Igneous rocks -Weak to extremely strong igneous rocks | |
| Lavas and welded ignimbrites | High |
| Indurated fine-grained pyroclastics | High |
| Lavas and welded ignimbrites | High |
| Indurated volcanic breccias | High |
| Ancient volcanics | High |
| Plutonics | High |
| Ultramafics | High |
| Sedimentary rocks - Very loose to compact | |
| Peat | High |
| Loess | Low |
| Windblown sand | Low |
| Fine alluvium1 | Low |
| Alluvial gravels1 | High |
| Coarse slope deposits1 | High |
| Glacial till1 | High |
| Alluvium2 | High |
| Unconsolidated clays and silts | High |
| Unconsolidated sands and gravels | High |
| Sedimentary rocks -Very compact to weak sedimentary rocks | |
| Massive sandstone | Low |
| Bedded mudstone | High |
| Jointed mudstone | High |
| Bentonitic mudstone | High |
| Massive sandstone | High |
| Bedded sandstone | High |
| Weakly consolidated conglomerate | High |
| Mixed sheared lithologies | High |
| Crushed argillite association | Low |
| Sedimentary rocks - Moderately to extremely strong sedimentary rocks | |
| Argillite | High |
| Indurated sandstone | High |
| Conglomerate and breccia | High |
| Greywacke | High |
| Limestone | High |
| Metamorphic rock | |
| Semi-schist | High |
| Schist | High |
| Gneiss | High |
| Marble | High |
1 Not differentiated in the South Island - included in Alluvium.
2 Differentiated in North Island.
Figures 7 and 8 show the relative potential for microbial transport through the vadose zone for the North and South Islands respectively; Figure 9 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map.
In the North Island, large contiguous areas of potentially low microbial transport occur in the central North Island and Bay of Plenty regions, Taranaki, Manawatu, and scattered throughout Northland. Southland/Otago also has large areas of vadose zone with potentially low microbial transport.
Vadose zone rocks with potentially high risk of microbial transport are very compact to extremely strong sedimentary rocks and metamorphic rocks, both of which are thought to be well jointed.
In the North Island, large contiguous areas occur to the east and south of the Volcanic Plateau, and axial ranges from Wellington to the East Cape.
The South Island is dominated by VZ materials, with potentially high risk of microbial transport through the VZ. Although the Canterbury Plains are shown as a large contiguous area, resolution is hampered by a lack of differentiation of alluvium into fine alluvium or gravels such as occurs in the North Island.
Table 6 shows the percentage of areas of soils in the North and South Islands and their relative potential for microbial attenuation in the vadose zone.
Table 6: Percentage of areas of soils in the North and South Islands and their relative potential risk of transport of microbes through the vadose zone
| Relative risk of microbial transport through the vadose zone | Percentage of land | |
|---|---|---|
| North Island | South Island | |
| High | 49 | 88 |
| Low | 51 | 12 |
Further work is required to define the nature of VZ materials on a nationwide basis.
Figure 7: Relative risk of microbial transport through the vadose zone – North Island
Figure 8: Relative risk of microbial transport through the vadose zone – South Island
Figure 9: Relative risk of microbial transport through the vadose zone – 1:50 000 scale example
5.4 Relative risk of microbial transport to surface water
In this section, surface water is considered as open drains, streams and rivers. The relative risk of microbial transport to surface water is the combined risk of microbial transport from runoff combined with flow through the soil by bypass flow. The method to determine relative risk of microbial transport to surface water from the combinations of risk of microbial transport by runoff and through the soil by bypass flow is shown in Table 7.
Table 7: Table to determine relative risk of microbial transport to surface water from relative risk of surface runoff and relative risk of microbial transport through the soil due to bypass flow
| Relative risk of surface runoff | Relative risk of microbial transport through the soil due to bypass flow | Relative risk of microbial transport to surface water |
|---|---|---|
| High | High | Very high |
| Moderate | High | High |
| Low | High | High |
| High | Moderate | High |
| Moderate | Low | Moderate |
| Low | Low | Low |
The relative risk of microbial transport to surface water is spatially predicted for both the North and South Islands on flat to rolling land (<15°) only (Figures 10 and 11) because of the uncertainty of predicting bypass flow on steeper land. Figure 12 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map.
A high potential for both runoff and microbial bypass flow are often linked, as they depend on some shared soil properties. Essentially, runoff occurs when the soil cannot accept water at an appropriate rate. This often occurs more rapidly in soils with a drainage restriction. However these soils are often clayey and have subsoil cracks which allow bypass flow of microbes. In the North Island there appears to be a very high risk associated with clayey soils in Northland that have some drainage restriction and well-developed soil structure. Similarly, low-lying soils of the Hauraki Plains have restricted drainage, well-developed soil structure, and high microbial bypass flow potential. Soils in some areas of the Manawatu and Hawkes Bay also have these combined soil properties. In contrast, soils in the Taranaki region and central North Island have a relatively low risk of microbial transport to surface water because of their relatively low risk of microbial bypass flow and surface runoff. In the South Island, the Canterbury Plains are generally rated as having a moderate relative risk, while vulnerable areas of Southland have a very high relative risk.
Artificial drainage can increase the risk of transport of microbes to surface water. For this, and other reasons, all soils with a drainage restriction have been ranked as having a high potential for microbial bypass flow. Table 8 shows the percentage of areas of soils on flat or rolling land in the North and South Islands and their relative risk of transport of microbes from land to surface water.
Table 8: Percentage of areas of soils on flat or rolling land in the North and South Islands and their relative risk of transport of microbes from land to surface water
| Relative risk of transport of microbes from land to surface water | Percentage of flat or rolling land | |
|---|---|---|
| North Island | South Island | |
| Very high | 35 | 32 |
| High | 18 | 18 |
| Moderate | 11 | 29 |
| Low | 37 | 21 |
Figure 10: Relative risk of microbial transport to surface water – North Island
Figure 11: Relative risk of microbial transport to surface water – South Island
Figure 12: Relative risk of microbial transport to surface water – 1:50 000 scale example
5.5 Relative risk of microbial transport to ground water
The relative risk of microbial transport to groundwater is the combined risk of microbial transport through the soil by bypass flow then through the vadose zone. The risk is assessed for water tables some distance from the base of the soil which is considered to be about 1.2 m. When the water table is shallow, in essence connected to the soil, there is no vadose zone and the rating cannot be applied. Risk associated with this scenario is likely to be high. The method to determine relative risk of microbial transport to groundwater from the combinations of risk of microbial transport through the soil by bypass flow and through the vadose zone is shown in Table 9.
Table 9: Table to determine the relative risk of microbial transport to ground water from relative risk of microbial transport through the soil due to bypass flow and relative risk of microbial transport through the vadose zone
|
Relative risk of |
Relative risk of microbial transport through the vadose zone |
Relative risk of microbial transport to ground water |
|---|---|---|
| High | High | High |
| Moderate | High | Moderate |
| Low | High | Low |
| High | Low | Moderate |
| Moderate | Low | Low |
| Low | Low | Very low |
The relative risk of microbial transport to groundwater is spatially predicted for both the North and South Islands on flat to rolling land (<15°) only (Figures 13 and 14) because of the uncertainty of predicting bypass flow on steeper land. Figure 15 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map.
Figure 13: Relative risk of microbial transport to ground water – North Island
Figure 14: Relative risk of microbial transport to ground water – South Island
Figure 15: Relative risk of microbial transport to surface water – 1:50 000 scale example
Although microbial transport mechanisms through high bypass flow soils and high vadose transport materials are similar, i.e. through the larger cracks where solute velocity is faster, the soil parameters and vadose zone parameters are not as closely coupled as soil parameters in the risk to surface waters. Soils may be developed in a different parent material to the vadose zone material or the soil material may be very strongly weathered compared with the vadose zone material and as a consequence have different properties relevant to the transmission of microbes.
Relatively high risk occurs where potentially high bypass soils overlie a VZ where attenuation of microbial transport is low. In the North Island, this occurs for a range of VZ materials in Northland and the Hawkes Bay region. In the greater Waikato region both the soil and VZ material are organic and also lie within this high category. Once again, volcanic soils of Taranaki and central North Island have a relatively low risk of microbial transport from land to ground water. In the South Island there appears to be a moderate risk associated with the Canterbury Plains while a relatively high risk is indicated on much of the West Coast. Soils with a low risk are less extensive and occur in minor areas on the Canterbury Plains, inland Catlins region and adjacent to Lake Te Anau. Table 10 shows the percentage of areas of soils on flat or rolling land in the North and South Islands and their relative risk of transport of microbes from land to groundwater.
Table 10: Percentage of areas of soils on flat or rolling land in the North and South Islands and their relative risk of transport of microbes from land to groundwater
| Relative risk of transport of microbes from land to ground water | percent of flat or rolling land | |
|---|---|---|
| North Island | South Island | |
| High | 16 | 34 |
| Moderate | 38 | 39 |
| Low | 10 | 24 |
| Very Low | 36 | 3 |
5.6 Optimal width of riparian buffer strips
Dense grass vegetation in riparian buffer strips (RBS) is very effective at trapping sediment particles (Niebling & Alberts 1979; Parkyn 2004) by providing flow resistance that reduces flow velocity and consequently particulate transport capacity. As microbes are in effect particulates, we may also expect a high microbial trapping efficiency. Collins et al. (2005) demonstrated that the effectiveness of RBS in trapping microbes is influenced by factors including land the characteristics of slope-angle and soil type. Riparian buffer strip mechanisms and effectiveness have been reviewed by Parkyn (2004), who noted that for particulates, Collier et al (1995) used the Chemical, Runoff and Erosion from Agricultural Management Systems model (CREAMS) to calculate the optimal riparian strip width for attenuating overland flow. Collins et al. (2005) related microbial transport to the sand, silt and clay percentages used in the CREAMS model to generate optimal widths for riparian buffer strips to trap faecal microbes. Collins et al. (2005) classified land into three slope categories and the soil into three soil permeability categories and finally three bacterial attachment categories to calculate optimal buffer width. The three bacterial attachment categories were related to topsoil clay content with high, moderate and low clay contents representing low (≈ 40 percent), moderate (≈ 70 percent) and high (≈ 90 percent) bacterial attachment respectively (Table 11).
To obtain full nationwide spatial representation of topsoil clay content and permeability, the Landcare Research Soil database was linked to the NZLRI slope classes. Class limits for topsoil clay content of high (>40 percent), moderate (20-40 percent) and low very (<20 percent) were set to >35 percent, 18-35 percent and <18 percent respectively to conform to groupings within the soil database. Soil drainage rate (permeability) was assessed from permeability estimates for upper subsoil horizons of the soil profile.
Table 11: Optimal buffer width derived from slope, soil permeability and bacterial attachment
| Slope | Soil permeability | Bacterial attachment | Buffer width ( percent of slope length) |
|---|---|---|---|
| 0–7° | <1–4 mm/h | High | 1 |
| Moderate | 5 | ||
| Low | 9 | ||
| 5–64 mm/h | High | 1 | |
| Moderate | 2 | ||
| Low | 4 | ||
| 65–>250 mm/h | High | 1 | |
| Moderate | 1 | ||
| Low | 3 | ||
| 8–20° | <1–4 mm/h | High | 2 |
| Moderate | 7 | ||
| Low | 15 | ||
| 5–64 mm/h | High | 1 | |
| Moderate | 4 | ||
| Low | 11 | ||
| 65–>250 mm/h | High | 1 | |
| Moderate | 2 | ||
| Low | 4 | ||
| 20–>35° | <1–4 mm/h | High | 5 |
| Moderate | 15 | ||
| Low | 30 | ||
| 5–64 mm/h | High | 3 | |
| Moderate | 7 | ||
| Low | 13 | ||
| 65–>250 mm/h | High | 3 | |
| Moderate | 4 | ||
| Low | 11 |
Figures 16 and 17 show the optimal riparian buffer strip width for the North and South Islands respectively, while Figure 18 shows an example at 1:50 000 scale, overlying part of a NZMS 260 topographic map. Riparian buffer strips less than this width may provide less than optimum trapping of faecal bacteria.
Figure 16: Optimal riparian buffer strip width – North Island
Figure 17: Optimal riparian buffer strip width – South Island
Figure 18: Optimal riparian buffer strip width – 1:50 000 scale example
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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