Water Sources

Water Sources and Relative Safety

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. 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. Although the information gap is improving, there is a lack of peer-reviewed scientific information relating to irrigation.

It is generally considered that there is a sliding scale of microbiological safety associated with different water sources used for the irrigation of crops assessed at the point of acquisition.  Water that has been stored is considered further into the assessment. As shown in Figure 1, mains water and seawater which has been desalinated (had the salt removed) are considered to be almost free of microorganisms and are therefore considered the safest sources (Pandey et al., 2014; Allende and Monaghan, 2015). Surface water and recycled wastewater from industrial processes have the highest potential for contamination (Tyrell et al., 2006; Astrom et al., 2009; Uyttendaele et al., 2015). A commonly reported theme from a number of publications (Astrom et al., 2009; Pandey et al., 2014; Balleste et al., 2020) is that immediately after significant rainfall, the microbiological quality of a number of water sources can decline significantly. It is acknowledged that after rainfall, irrigation of field crops is less likely to be required immediately. 

Figure 1: The relative risks associated with different water supplies

Microbes are found naturally in every conceivable environment on the planet, including water. Waters that have not been treated, normally have a higher microbiological load, which fluctuates dependent on multiple environmental factors. Water treatments are sanitary processes that aim to improve hygiene standards by the physical or chemical removal of microbes.’

Mains Water 

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, 2020) has been responsible for the quality of all potable (drinkable) water supplied by water companies. Furthermore, in addition to the statutory testing, there are periodic surveys of water undertaken for pathogens such as Helicobacter pylori by Public Health Authorities (Watson et al., 2004). In combination, these testing regimes appear to function as an effective mechanism for monitoring water quality. The disinfection procedures and the cleanliness of the distribution system 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 is very rare, there are instances where drinking water quality does not meet statutory criteria at the point of delivery to the consumer (Gray, 2008; DWI, 2020). In the first quarter of 2020, there were 44805 tests undertaken for mains water across the UK. The test results showed there were zero failures for Escherichia coli and seven (0.015%) failures for coliforms.

Desalinated Water

Desalinated water tends to be abstracted from the sea and is then treated to remove the salt. There are two main treatment processes used in the UK which are, evaporation, 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 remove bacteria making desalinated water a safe, if expensive, choice for irrigation (Rakhmanin et al., 1982; Zuluaga-Gomez et al. 2020). 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; Zuluaga-Gomez et al. 2020).

Deep borehole water

Responsibility for private water supplies such as springs and deep boreholes are regulated in Scotland and in England by the Drinking Water Inspectorate. 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 30 metres depth, are fitted with a cannula (liner) and the gap between the canula and the soil/rock is backfilled with cement. The water contained within an aquifer at 30 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. However, periodic maintenance is required to prevent contamination of boreholes. Deep borehole water is a generally safe source for irrigation, although there are now some documented instances where an aquifer has become contaminated (Motlagh et al., 2020).

Rainwater

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 Australia, an outbreak in 2008 involving nearly 30 people was linked to rainwater stored in tanks that had become contaminated by Salmonella enterica serovar Typhimurium (Franklin et al., 2009). There has been some review of the hazards relating to how rainwater is captured by Evans et al. (2006) and Hamilton et al. (2019). Although the focus is on harvest from building and glasshouse rooves, many of the hazards identified (e. g. insects, bird droppings, dust) have relevance for commercial systems and UK growers. The quality of rain-harvested roof water can be improved by Risk assessing potential contamination to roof areas used for water capture, maintainence and hygiene controls and finally treating water before use conventional water treatment. There is a technology called solar disinfection (SoDis) summarised by Hamilton et al. (2019), although it is unclear if the intensity of sunlight in the UK is sufficient to drive a SoDis unit.

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 these 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. There were no other significant differences for the remaining months. High rainfall and the presence of livestock near the supply were also identified as risk factors; presumably by the mechanisms described below for surface water. There have been reports that private wells and shallow boreholes used for drinking water are a risk factor for infections by Shiga toxin producing E. coli, both in the UK (Elson et al., 2018) and in North America (Reynolds et al., 2020).

Surface waters

Surface waters from rivers, canals and lakes are considered risky for use as irrigation sources for fresh produce. The risks stem largely from the potential for contamination by wildlife and livestock manures upstream or in close proximity to where the water is abstracted (Pandey et al., 2014; Falardeau et al., 2017). Domestic sewage (septic tanks or commercial treatment plants upstream of abstraction points) and any industrial activity can also increase microbial load in surface waters (Amalfitano et al., 2018).  Between 1992 and 1999 in the USA, surface waters caused at least 46 disease outbreaks affecting nearly 3000 people, with several fatalities (cited by Pandey et al., 2014). An important point made by Pandey et al. (2014) is that surface water contamination increases during significant rainfall for a variety of reasons, including sediment churning and the release of untreated sewage by water companies. It is prudent not to abstract water for irrigation from rivers in flood. 

Surface water can also uncommonly be used as a private drinking water supply (approximately 3% of the Scottish population and 1% of English Population uses a private water supply for drinking water. These supplies can originate from a number of sources including lochs, burns or boreholes and are often found in more rural areas. Private water supplies are not provided or maintained by Water Authorities. In England these supplies are regulated by the Drinking Water Inspectorate and in Scotland.)

Related to the hazard of abstracting surface waters during periods of heavy rainfall is flooded land (Gil et al., 2015). Climate change and extreme weather events (e. g. exceptionally heavy rainfall) may cause the contamination of soils and surface waters with potential human pathogens originating from septic tanks, sewers, and livestock waste stores (Gil et al., 2015). Soils that are unusually dry can become compacted (e. g. capped) and subsequent heavy rainfall after a period of drought can result in more severe run-off and an increased risk of some types of contamination (Gil et al., 2015). Furthermore, temperature changes caused by extreme precipitation can also directly influence the safety of the food chain by changing pathogen survival time or even by promoting multiplication of some pathogens (Gil et al., 2015). It is now well established that contamination of surface waters can be seasonally elevated, with highest counts in autumn and winter as a consequence of higher rainfall (Falardeau et al., 2017).

A not uncommon cause for surface water contamination in the UK is shown in Figure 2 below, which shows sheep with unfettered access to the river that was also used as their drinking water supply. Other livestock in close proximity to water have been implicated in disease outbreaks (Yiannas, 2021).

Figure 2: Sheep grazing with unrestricted access to a nearby riverĀ 

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) and Pandey et al. (2014) both report that rivers are 'both an important source of irrigation water and the recipient of treated urban wastewater'. Panton et al. (2020) report evidence that sewage treatment works can deposit materials directly into rivers in the UK, identified by chemical rather than microbiological pollution. Astrom and colleagues (2009) report that outflows from sewage treatment plants can be a significant source of human viruses, E. coli and protozoa such as Cryptosporidium in rivers. 

Park et al (2013) investigated the farm management and environmental factors on pre-harvest spinach contamination with generic E. coli as an indicator of faecal contamination. Spinach farms were visited, and spinach samples (n=955) were collected and tested from 12 farms along with survey information such as farm-related management and environmental factors. Significant risk factors for spinach contamination with E. coli were the use of pond water for irrigation.

Estuary and coastal waters are also frequently contaminated with bacteria from urban runoff and sewers (Pandey et al., 2014; Panton et al., 2020). The potentially poor water quality associated with seawater is exacerbated by a potential for the presence of sediment in seawater. Sediment improves the survival chances for a number of bacteria, including E. coli and faecal coliforms (Pandey et al., 2014). Pandey et al. (2014) further review that around 16% of faecal indicators in coastal waters are from sewers during heavy rainfall. In estuaries, the primary source of contamination is urban point sources including released, untreated sewage, which accounts for 12% of estuary contamination (Pandey et al., 2014).

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 ten times more E. coli than water in reservoirs. Furthermore, E. coli numbers in canals were significantly increased during precipitation and in the 24h after a heavy rainfall event (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 (Falardeau et al., 2017). 

Waste water from sewage treatment and industrial processes

In June 2020, a new regulation relating to urban wastewater recycling, specifically targeting agricultural water use came into force for the EU. The implementation of regulation 2020/741 is staged, with full compliance and adoption targeted for 2023. The basis of the regulation is safe and sustainable use of reclaimed waters for crop irrigation and other agricultural activities. In brief, four classes of water are created and designated A-D, with A being the highest quality source. Crops are designated a minimum quality of water and a suitable application method that can safely be used for cultivation as shown in Table 1.

Table 1: Classes of reclaimed water quality, permitted agricultural use and irrigation application method. If a crop meets the description of more than one category, the strictest conditions should be applied.

Class A is the highest quality water. There are compliance criteria for each class of reclaimed water to be used for irrigation as shown in Table 2.

Table 2: Reclaimed water quality criteria for agricultural irrigation. BOD is biological oxygen demand, TSS is total suspended solids, NTU is nephelometer turbidity units.

Given that regulation 2020/741  is an EU law, it is not surprising that there is a high degree of alignment with the EU Commission notice on guidance document on addressing microbiological risks in fresh fruits and vegetables at primary production through good hygiene (2017/C 163/01) water standards.

References (click a reference to read it (where it's available); some require purchase from the publisher)

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