The use of dye tracers (1994) allowed a detailed study of the phenomenology of solute transport in field soils, thereby revealing that preferential flow as a consequence of various soil structural features was the rule rather than the exception.
It is important to recognise that soil structure on a visible scale is changeable, especially during wetting and drying cycles (Brewer 1964). These cycles help to break down clods of soil and produce finer aggregates (McLaren and Cameron. 1996). Planes in soil materials originate primarily through shrinking and swelling during wetting and drying. Large planes would originate under very dry conditions (Stirk 1954).
There has been considerable experimental success in batch studies, and in studies involving systems without macropore flow, such as repacked soil columns. In structured soils, however, preferential flow can have a major impact on leaching. As a result, some fundamental problems and questions remain unanswered for specific regions and their soil types.
It is believed that soils under grasslands have a larger proportion of macropores than under arable farming, hence are more susceptible to preferential flow. This is because of greater inputs of structure-building o.m. under grasslands, and the disturbing of soil structure under arable farming. This phenomenon can be seen in other relevant research on transport of chemicals (Heathwaite and Dils 2000; Toor et al. 2004a) and bacteria (Joergensen et al.
1998).
2.3.4 Effects of season and irrigation regime on bacterial
transport
Seasonal variation is controlled by temperature on the one hand, and the water balance (rainfall vs. actual evapotranspiration ET) on the other hand. For bacteria transport from land-applied animal waste over short periods, water inputs and outputs, i.e. weather and irrigation practices are addressed more effectively by changing soil moisture, which also generates changes in soil structure. The dynamic changes of soil structure with regional weather conditions (season) and irrigation schemes certainly influence the transport of water, solute and pollutants. There is only little previous research on the relation between seasonal variation of soil moisture regime and structure, and their impacts on water, solute
Chapter 2 16 and pollutant transport to receiving groundwater. Heathwaite (2000) identified a seasonal variation in P leaching, with more P in leachate in summer which could be related to climate and land management activities.
Summer rainfall in eastern areas of New Zealand is generally much less than the potential evapotranspiration, thus summer leaching is typically minimal or absent in normal conditions. However, bacterial leaching in summer can occur through macropores under irrigation or intensive heavy rainfalls (Figure 2.1).
M ay-05 Jun-05 Jul-05 Aug-05 Sep-05 Oct-05 Nov-05 Dec-05 Jan-06 Feb-06 M ar-06 Apr-06 0.00 1.00 2.00 3.00 4.00 5.00 6.00 R an ifal l an d E T ( m m d -1 ) Date Rainfall ET
Figure 2.1 Monthly average rainfall and ET in the irrigation season of 2005/2006
An investigation report from Waikato in the North Island found no significant difference in faecal contamination between summer (samples collected between October and March) and winter (collected between April and September) (Collins 2002). There is no similar report on seasonal variation for the South Island or the Canterbury area.
A few studies have focused on the effect of initial soil wetness on preferential flow, though with conflicting results. Soil water content and hydraulic loading rate are important factors in the velocity of downward migration of bacteria and also influence the number of bacteria moved to depth (Hegde and Kanwar 1997; Stevik et al. 1999). It is known that high hydraulic loading rate increase the water velocity through larger pores (Bouma et al.
1974) and reduces the exchange between mobile and less mobile water. High water flow rates result in more irregular flow patterns as compared with low flow rates, and
consequently the degree of lateral and longitudinal dispersion is higher under high flow rates (Dekker and Ritsema 1996). Hamdi (1994) also found for a conservative tracer that there is higher initial risk of groundwater contamination under ponded irrigation than under sprinkler irrigation, and the variability was much greater for the ponded plots than for the sprinkler plots. This implies that the irrigation scheme is a critical factor for controlling (or preventing) bacteria from travelling to the deeper soil profile and groundwater.
There are two main irrigation regimes for pastoral use in New Zealand: flood irrigation (border dyke) and spray irrigation. Previous solute transport research suggested that flood irrigation results in lower nitrate concentrations in the leachate than spray irrigation, due to the greater dilution of soil solution nitrate by the larger volume of irrigation water applied (Di et al. 1998). However, no relevant research has been done for bacterial leaching in Canterbury soils.
Dairy farms need recommendations for better performance to reduce environmental risk. Landcare Research (Hamilton) researchers have investigated bacterial transport within short periods (a few days) with spray irrigation at a constant rate for several New Zealand soil types from both North and South Island. However, further work is needed to find quantitative relationships between irrigation practices, soil properties and microbial leaching under general field practices.
Several publications describe the physical process of bacterial transport as equivalent to simple colloids transport (Bouwer and Rittmann 1992; Albinger et al. 1994). However, because bacteria are living organisms, their transport in soil is more complex than is the case for abiotic colloid (Ginn et al. 2002).