6. Management of Water Quality
A number of management strategies are available to improve water quality or to limit deterioration of water quality. Some strategies, such as fencing and reticulation, are already used on New Zealand farms, and in this section, we consider the possibilities.
6.1 Fencing
The literature noted earlier has indicated some of the potential to reduce contamination of waterways by preventing access of stock to stream channels and parts of riparian zones. While permanent fencing would be preferable, Collins (2002) noted alternatives such as less intensive farming practices, consideration of drainage practices, and optimising the timing of effluent application to land. With dams, Lardner (1999) stated that it is good practice to ensure that banks of a dam are vegetated, as plants with spreading root systems stabilize the slopes preventing slumping and increasing the useful life of the dam. Similarly, permanent vegetation around a dam (eg. in runways feeding the dam and surfaces delivering run-off water to the reservoir) helps to protect water quality by trapping sediment and nutrients.
New Zealand hill country farms frequently utilise dams as a major source of water supply. Hence the impacts of direct access to water are highly relevant to these properties and strategies such as fencing would appear to provide major opportunities to enhance water quality. Fencing and planting of riparian areas are also components of a long-term strategy to discourage development of cyanobacterial blooms in water bodies. Over time, this approach will lead to increased shading and reduced nutrient inputs (from livestock effluent, bank erosion and runoff from agricultural land), both of which will reduce the suitability of a water body for cyanobacterial growth. Fencing also keeps livestock away from the edges of farm dams and rivers where cyanobacterial scums or benthic mats tend to accumulate and become accessible to livestock.
6.2 Treatments and watering systems
6.2.1 Trough cleaning
Frequent cleaning of troughs should offer a cleaner source of water for stock. However LeJeune et al. (2001) actually found that recently cleaned troughs had higher coliform counts and that cleaning interval had no significant effect on the E. coli counts, but both E. coli O157 and Salmonella spp. tended to be isolated more frequently in the less recently cleaned troughs. The ability of E. coli O157 and Salmonella spp. to survive in other aquatic environments suggests that, once introduced, these bacteria may persist and possibly proliferate as endogenous flora within the troughs, whereas recently cleaned troughs would be less likely to harbour these particular strains of bacteria until they are recontaminated from an outside source. It is likely that the method of cleaning and the ability for stock to recontaminate would influence results.
Certain devices (e.g. Troff Tops®) are promoted as being designed to limit the ability for animals to recontaminate troughs, hence providing cleaner drinking water. However, there is only anecdotal evidence as to their effectiveness and the reduction in access by sunlight and its strongly bactericidal ultra-violet component (Sinton et al. 2000), indicates that a comprehensive analysis would be required to assess the potential benefits.
6.2.2 Aeration of dams and troughs
Aeration may also be used to enhance water quality in dams or troughs. Dissolved oxygen (DO) is essential to aquatic organisms in a dam. In waters with low oxygen levels, the production of 'swamp gases' such as hydrogen sulphide and methane is likely to occur, and phosphorus is more rapidly released from decomposing sediments. Increased phosphorus concentrations in water favour the growth of algae, including toxic blue-green species. In most dams, the decomposition of dead plants and animals uses up oxygen more rapidly than it can be absorbed from the atmosphere. Similarly, if the circulation of water in the dam is not complete, a layer of poorly-oxygenated (water) will develop at the bottom and noxious gases may be produced at depth.
DO content, and also water temperature and pH, vary greatly, with characteristic seasonal and diurnal fluctuations. For example, during sunny weather in January (mid-summer) in New Zealand studies, the DO content ranged from about 10-20% of saturation in the early morning hours (2am till 6am) then increased to a peak of about 140% around 3pm before falling again. The DO content is a key factor in the inactivation of enterococci and E. coli (as well as F-DNA and F-RNA phages) and research has shown that the combined effects of pH, DO and sunlight exposure will greatly reduce the level of microbial contamination (Davies-Colley et al. 1999).
6.2.3 Water treatment
Various methods of treatment (disinfection, reduction of dissolved salts, etc) to produce higher quality water are available, but most have their limitations, especially for delivery to animals at pasture. The methods include filtration, coagulation (to precipitate suspended solids), reverse osmosis, chlorination, and oxidation (Solsona 1996). More recent developments in the area of disinfection include SODIS (solar water disinfection) and SOPAS (solar pasteurisation) processes which rely on the synergistic effects of solar radiation and thermal water treatment (Wegelin and Sommer 1996; Sommer et al. 1997). Before considering treatment, it must first be established whether it is the microbiological or the chemical quality of the water that is of concern or whether other factors, such as palatability due to faecal contamination are of greater significance. In addition, the cost to benefit ratios of such processes are questionable except where a real problem is present, such as in some situations where there may be very high concentrations of TDS.
Chlorination offers a method for disinfection of the drinking water, but the presence of organic matter compromises the impact of chlorine, so that its impact may be very limited (Adhikari et al. 2002, WQRC 1991). Also chlorine breaks down in the presence of sunlight (UV) and chlorine gas readily leaves the water, reducing the bactericidal effects (Anon 2002).
The application of algicides (commonly copper sulphate) is one approach that is often employed to control cyanobacterial growth and to prevent or reduce the level of cyanobacterial toxins. However, algicides should be used with caution as they cause cells to die and lyse, thus releasing toxins (and also taste and odour compounds) that are predominantly held within live cyanobacterial cells. This means that algicides are best used when cell numbers are low as a preventative measure. However if cyanobacterial blooms are treated, livestock should be prevented from using the water body until sufficient time has passed for toxin degradation to occur. Algicides also have environmental impacts (eg. copper is not biodegradable, and can accumulate in lake sediments) that should be considered before using this approach.
6.2.4 Watering devices
Where animals do not have access to dams or streams, water must be moved to troughs or other watering devices. Along with conventional power sources (electricity, solar, wind), animal nose-pumps often an alternative; with such pumps, water is delivered to a bowl when an animal pushes the pump lever with its nose (Buchanan 2002) but there are no reports as to their effect on water quality. Such devices offer another means of minimising contamination by drinking animals.
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