Please note that this section relates only to the relative safety of water sources before any treatments are applied. There are other help pages which deal with benefits of treatments and the assessment tool takes full account of any treatment applied to some of the water sources which tend to be associated with higher bacterial numbers which are used for irrigation. It is acknowledged that many of the publications below refer to water sources in the context of drinking water quality and that water used for irrigation may not be required to attain the same standards as drinking water. However there is a lack of peer-reviewed scientific information relating to irrigation, and care has been taken to choose only relevant publications. As a general rule, there tend to be many publications that say the same thing; to prevent pages and pages of text, we provide only a recent typical publication chosen to allow those that are interested, to track back through the cited literature.
It is generally considered that there is a sliding scale of microbiological safety associated with different water sources used for the irrigation of crops. As shown in Figure 1, mains water and seawater which has been desalinated by heating under vacuum or reverse osmosis are considered to be almost-free of microorganisms and thus considered fairly safe (Tyrell et al., 2006). Surface water and recycled waste water from industrial processes have the highest potentials for contamination (Tyrell et al., 2006; Astrom et al., 2009). A commonly-reported theme from a number of publications (Edberg, 1997; Gray, 2008; Kay et al., 2007; Collins et al., 2005; Astrom et al., 2009) is that immediately after rainfall, the microbiological quality of a number of water sources declines significantly. It is acknowledged that after rainfall, irrigation is less likely to be required.
Mains water is considered the safest water source to use for irrigation because it is subjected to rigorous disinfection and testing procedures. Since 1990 in the UK, the Drinking Water Inspectorate (DWI) has been responsible for the quality of all potable water supplied by water companies. Furthermore, in addition to the statutory testing, there are periodic ad hoc surveys of water undertaken for pathogens such as Helicobacter pylori by Public Health Authorities such as the Health Protection Agency (Watson et al., 2004). In combination, these testing regimes appear to function as an effective a mechanism for monitoring water quality, the disinfection procedures and the cleanliness of the distribution system and provides a reliable basis for the microbiological quality of mains water. An additional incentive for the supply of high quality water is that it is a criminal offence for a water company to provide potable water of inadequate microbiological quality. Although it's very rare, there are instances where drinking water quality does not meet statutory criteria at the point of delivery to the consumer (Gray, 2008).
Desalinated water tends to be abstracted from the sea and is then treated to remove the salt. The are two main treatment processes used in the UK which are evaporation (under normal atmospheric pressure or part vacuum) and reverse osmosis. Temperatures that are hot enough to evaporate water under any pressure are an effective decontaminant for water. In addition, the membranes used for osmosis-based treatment have pore sizes that are too small for bacteria to pass through. Effectively, both of the methods used to remove salt also kill bacteria making desalinated water a safe, if expensive, choice for irrigation (Rakhmanin et al., 1982). The presence of any pollutants in the seawater such as oils and surfactants can diminish the microbiological quality of the finished water (Rakhmanin et al., 1982).
Deep borehole water
Responsibility for private water supplies such as springs and deep boreholes tends to be delegated from the DWI to local authorities. There is less information relating to the microbiological quality of such sources because there is no obligation for any test results or treatments undertaken on these supplies to be made public. In general, deep boreholes are considered to be exceptionally safe if they are drilled to more than 25 metres depth, are fitted with a canula (liner) and the gap between the canula and the soil/rock is back-filled with cement. The water contained within an acquifer at 25 m or lower will typically have very low microbiological counts (Edberg et al., 1997) and by sealing the borehole with cement, the opportunity for surface runoff contamination of the water source is minimised.
Clouds and rainwater are formed from evaporated liquids that have re-condensed at altitude and so both are almost entirely free of microbiological contamination. As rain falls through lower altitudes however, it will pick up small numbers of bacteria from the air which may multiply if the water is stored before application. Information on contamination in rainwater storage cisterns appears to be sparse for the UK, but in Canada one study reported total and faecal coliform contamination in 31% and 13% respectively of the 360 samples collected (Despins et al., 2009). In the Australia in 2008, an outbreak involving nearly 30 people was linked to rainwater stored in tanks that had become contaminated by Salmonella Typhimurium (Franklin et al., 2009). A review of the hazards relating how rainwater is captured has been published by Evans et al. (2005). Although the focus is on Australian domestic dwelling, many of the hazards identified (e.g. insects, bird droppings, dust) have relevance for commercial systems and UK growers.
Shallow borehole water and wells
Although shallow boreholes and wells can serve as clean sources of irrigation water, there are a number of risks associated with their use. Richardson et al., (2009) collected the test results from almost 35,000 water test samples in England and analysed the results for significant risk factors. The analyses revealed that there was a seasonal risk for the isolation of E. coli from private supplies. Between January and May, there was significantly less risk of contamination of the supply as compared with June to September. High rainfall and the presence of sheep near the supply were also identified as risk factors; presumably by the mechanism described for below for surface water.
Surface waters from rivers, canals and lakes are considered risky for use as irrigants for fresh produce. The risks stem largely from the potential for contamination by wildlife and livestock manures upstream of where the water is abstracted. A not uncommon case for the UK is shown in Figure 2 below which shows sheep with unfettered access to the river that is also used as their drinking water supply.
Although most animals are unlikely to deposit waste in the area where they drink, studies have shown that even light rain falling on fresh faecal material can transport E. coli significant distances overland. The steeper the slopes of any hills above a river or lake, the greater distances that bacteria from animal manure can be transported (Collins et al., 2005). In addition to livestock-derived contamination, Tyrell et al. (2006) report that UK rivers are 'both an important source of irrigation water and the recipient of treated urban wastewater'. Astrom and colleagues (2009) believe that outflows from sewage treatment plants can be a significant source of human viruses, E. coli and protozoa such as Cryptosporidium in rivers.
Won and colleagues (2013) investigated how bacterial indicators changed over the course of one year in four surface water sources. E. coli counts in water fluctuated across the different seasons. Water collected from irrigation canals contained approximately one order of magnitude higher numbers than water in reservoirs. Furthermore, E. coli numbers in canals were significantly increased during and following heavy rainfall events (Won et al 2013). Won underscores the importance of accounting for bacterial fluctuations by taking water samples for testing at different times, especially if water is to be used (or abstracted) from a river during or just after a rainfall event.
Waste water from sewage treatment and industrial processes
As discussed above, waste water from sewage treatment plants is not acceptable for use as an irrigant for ready to eat crops under any circumstances. In addition to bacterial contamination, a principal concern of water from human-derived waste is the presence of viruses able to infect humans. In general, it is unusual (but not unheard of) for livestock viruses to be infectious to humans and so viruses from livestock wastes are not a significant cause for concern. Tree et al., (2003) report that chlorine is applied routinely in UK sewage treatment plants to lower bacterial counts and to help the outflows from such plants meet statutory microbiological criteria. However, under the laboratory conditions set up by Tree to mimic treated outflows, inactivation of some RNA-based viruses was not satisfactory.
Industrial process water is a blanket term which covers a wide variety of materials. There are too many classes of this type of water for each case to be discussed individually; but as a general rule, this sort of water is also unacceptable for use as an irrigant. The issues with industrial process-derived water fall into three main categories. The first is that the water may be directly contaminated with human pathogenic bacteria. For example, water that is used to wash potatoes which have been contaminated with wildlife droppings in the field may not cause problems for potatoes since they will almost certainly be cooked before consumption. But recycling the wash water without adequate treatment for the irrigation of crops that are not cooked before consumption may cause food borne illness. A second concern is that industrial waste water may contain nutrients (e.g. water from brewing processes) which could allow the multiplication of any pathogenic bacteria that colonise the water. Finally, unless the exact chemical composition of the water is known, there is a possibility that materials such as tannery wastes could contain unacceptable concentrations of toxic materials such as heavy metal salts (Scholz et al., 2005).
References (click a reference to read it (where it's available); some require purchase from the publisher)
Astrom,J., Pettersson,T.J.R., Stenstrom,T.A. and Bergstedt,O. (2009) Variability analysis of pathogen and indicator loads from urban sewer systems along a river. Water Science and Technology 59, 203-212.
Edberg,S.C., LeClerc,H. and Robertson,J. (1997) Natural protection of spring and well drinking water against surface microbial contamination .2. Indicators and monitoring parameters for parasites. Critical Reviews in Microbiology 23, 179-206. (This reference is too old to be available electronically)
Ferguson,C.M., Davies,C.M., Kaucner,C., Krogh,M., Rodehutskors,J., Deere,D.A. and Ashbolt,N.J. (2007) Field scale quantification of microbial transport from bovine faeces under simulated rainfall events. Journal of Water Health 5.1, 83-95.
Franklin,L.J., Fielding,J.E., Gregory,J., Gullan,L., Lightfoot,D., Poznanski,S.Y. and Vally,H. (2009) An outbreak ofSalmonella Typhimurium 9 at a school camp linked to contamination of rainwater tanks. Epidemiology and Infection 137, 434-440.
Kay,D., Watkins,J., Francis,C.A., Wyn-Jones,A.P., Stapleton,C.M., Fewtrell,L., Wyer,M.D. and Drury,D. (2007) The microbiological quality of seven large commercial private water supplies in the United Kingdom. Journal of Water and Health 5, 523-538.
Leifert,C., Ball,K., Volakakis,N. and Cooper,J.M. (2008) Control of enteric pathogens in ready-to-eat vegetable crops in organic and 'low input' production systems: a HACCP-based approach. Journal of Applied Microbiology 105, 931-950.
Rakhmanin,Y.A., Talaeva,Y.G. and Nikitina,Y.N. (1982) De Contaminating Effect of Various Methods of Sea Water de Salinization During Its Chemical Pollution. Gigiena i Sanitariya 15-18. (This reference is too old to be available electronically)
Richardson,H.Y., Nichols,G., Lane,C., Lake,I.R. and Hunter,P.R. (2009) Microbiological surveillance of private water supplies in England - The impact of environmental and climate factors on water quality. Water Research 43, 2159-2168.
Tyrrel,S.F., Knox,J.W. and Weatherhead,E.K. (2006) Microbiological water quality requirements for salad irrigation in the United Kingdom. Journal of Food Protection 69, 2029-2035.
Watson,C.L., Owen,R.J., Said,B., Lai,S., Lee,J.V., Surman-Lee,S. and Nichols,G. (2004) Detection of Helicobacter pylori by PCR but not culture in water and biofilm samples from drinking water distribution systems in England. Journal of Applied Microbiology 97, 690-698.
Whipps,J.M., Hand,P., Pink,D.A.C. and Bending,G.D. (2008) Human pathogens and the phyllosphere. Advances in Applied Microbiology, 64, 183-221.
Won, G., Kline, T.R., and LeJeune, J.T. 2013. Spatial-temporal variations of microbial water quality in surface reservoirs and canals used for irrigation. Agricultural Water Management, 116, 73-78