Water sources and storage

• Irrigation water sources can be assessed along a spectrum of microbiological risk, based on their origin.
• The least risky sources include mains water, deep borehole water, and desalinated water. In contrast, the riskiest sources include wastewater and water from industrial processes.
• Below is an overview of different water sources, with relevant information on each.
• Some fresh produce growers abstract water and store it for use during the growing season.
• If stored appropriately water quality can improve over time, if stored inappropriately stored water can present with microbiological risks.

This section considers only the inherent safety of water sources before any treatment is applied. Other guidance materials address the benefits and methods of water treatment. While much of the referenced literature relates to drinking water standards, irrigation water is not always required to meet the same criteria and there is still a lack of peer-reviewed studies specifically focused on irrigation water.

There is a widely accepted gradient of microbiological safety associated with irrigation water sources, evaluated at the point of collection. Stored water is assessed separately. As illustrated in Figure 1, mains water and desalinated seawater are regarded as virtually free from microorganisms, making them the safest options (1, 2). By contrast, surface waters and recycled industrial wastewater pose the greatest risk of microbial contamination (3-5). Numerous studies report that heavy rainfall can significantly deteriorate the microbiological quality of various water sources (1, 4, 6). However, following rainfall, immediate irrigation is less likely to be needed.

Microorganisms are naturally present in virtually all environments, including water. Untreated water typically contains a higher microbial load, which can fluctuate based on numerous environmental conditions. Water treatment processes aim to reduce microbial content through physical or chemical means, thereby enhancing hygiene.

Mains water is considered the safest source for irrigation due to stringent disinfection protocols and routine testing. In the UK, the Drinking Water Inspectorate  has regulated potable water quality since 1990, in Scotland the Drinking Water Quality Regulator for Scotland (DWQRS) is the responsible entity (7). Additional periodic surveillance is also conducted by public health authorities, including tests for pathogens such as Helicobacter pylori (8). This robust regulatory and testing framework ensures consistently high microbiological quality. Moreover, UK law makes it a criminal offence for water companies to supply potable water that fails microbiological standards. While rare, some breaches do occur (7, 9); for instance, in 2023 the DWQRS reported two microbiological incidents, one classed as serious and one as significant, although only a small number of customers were affected (217 and 333, respectively) (7). 

Desalinated water is typically sourced from seawater and processed to remove salt, using either evaporation or reverse osmosis. Evaporation uses high temperatures that also kill microorganisms, while reverse osmosis filters water through membranes with pore sizes too small for bacteria. Both methods effectively decontaminate the water, making desalinated water a safe, albeit costly, option for irrigation (10, 11). However, the presence of pollutants such as oils and surfactants in source seawater may reduce the microbiological quality of the final product.

Private supplies like springs and deep boreholes are regulated in Scotland by the DWQRS and in England by the Drinking Water Inspectorate. However, data on their microbiological quality is limited, as testing results are not typically made public. When constructed properly, boreholes drilled to depths over 30 metres, fitted with a liner (cannula), and sealed with cement, they are generally regarded as very safe. Water from such aquifers tends to have low microbial counts (12), and the cement seal prevents contamination from surface runoff. Nevertheless, periodic maintenance is necessary to ensure continued safety. Cases of aquifer contamination have been documented, although they remain rare (13).

Rain and clouds originate from evaporated water, making them initially low in microbial content. However, as rain descends, it may collect airborne bacteria. If stored, especially in poorly maintained cisterns or tanks, microbial populations can increase. In Canada, a study found total coliforms in 31% and faecal coliforms in 13% of 360 rainwater storage samples (14). In Australia, a 2008 outbreak of Salmonella enterica serovar Typhimurium was linked to contaminated rainwater tanks (15). Evans et al. and Hamilton et al. reviewed hazards associated with rainwater harvesting, especially from rooftops (16, 17). Contaminants such as insects, bird droppings, and dust are also applicable to commercial systems in the UK. Water quality can be improved by risk assessing roof contamination sources, implementing hygiene and maintenance controls, and treating stored water before use. Hamilton et al. (2019) also describe Solar disinfection could be a possible treatment method in these setting, though its effectiveness under UK sunlight conditions is uncertain (17).

Shallow boreholes and wells can be safe but are more prone to contamination than deeper sources. A study by Richardson et al. analysed 35,000 water samples in England and found a seasonal pattern for Escherichia coli contamination: lower risk from January to May and higher risk from June to September (18). Other risk factors included livestock proximity and high rainfall, which may increase contamination through runoff.

Private wells and shallow boreholes have been linked to Shiga toxin-producing E. coli infections in both the UK (19) and North America (20).

Surface waters (rivers, canals, lakes) pose a high risk for fresh produce irrigation due to potential contamination from wildlife and livestock, domestic sewage (e.g. septic tanks), and industrial discharges (1, 21, 22). Between 1992 and 1999, at least 46 disease outbreaks in the US were linked to surface waters, affecting nearly 3,000 people1.  Rainfall increases contamination through sediment disruption and combined sewer overflows1. Surface water abstraction should be avoided during or immediately after flooding.

In rural areas, around 3% of the Scottish and 1% of the English population rely on private drinking water supplies, some of which come from surface water. These are regulated (DWI in England and the DWQRS in Scotland) but not maintained by water authorities.

The effects of climate change and extreme weather, such as flooding, can elevate the risk of pathogen contamination from sewage, livestock waste, and run-off, especially in dry, capped soils (23). Temperature changes caused by extreme precipitation can affect pathogen survival time and/or promoting multiplication of some pathogens (23). It is well established that contamination of surface water is seasonally elevated, especially in autumn and winter with increased precipitation (21).

Figure 2 below shows sheep with unrestricted access to a river, an example of contamination risk. Other livestock near water sources have been implicated in disease outbreaks previously. Even light rain can transport E. coli from faeces over long distances, particularly on steep terrain (24, 25).

Tyrell et al. and Pandey et al. note that rivers are both irrigation sources and recipients of treated sewage (1, 3). Panton et al. confirmed this through evidence of chemical pollutants (26). Astrom et al. found that sewage outflows contain human viruses, E. coli, and protozoa like Cryptosporidium (4).

Park et al. (2013) found that using pond water for spinach irrigation significantly increased E. coli contamination risk (27). Won et al. demonstrated seasonal variation in E. coli levels across four water sources, with higher levels after rain, particularly in canals (28). Won et al. underscore the importance of accounting for the bacterial fluctuations by testing the water at different times, especially if river water is used or immediately after rainfall (28).

In June 2020, the EU introduced Regulation 2020/741 on the reuse of urban wastewater for agricultural irrigation. Full implementation took place in 2023.

The regulation defines four water quality classes (A–D), each suited to different crop types and irrigation methods (see Table 1). Where a crop falls into more than one category, the strictest standard applies.

Table 1: Reclaimed water classes and permitted agricultural uses and irrigation application method.

Compliance criteria for each class are defined in Table 2, including parameters such as Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and turbidity (NTU).

Table 2: Reclaimed water quality criteria for agricultural irrigation. Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and turbidity (NTU).

Regulation 2020/741 is consistent with the EU Commission’s guidance on microbiological risks in fresh produce (2017/C 163/01), particularly regarding good hygiene practices during primary production.

Water use and quality play a significant role in food contamination at all stages of the production process. Improved water management and the resulting improvements in water quality can greatly reduce the risk of food contamination (29). In the UK, the fresh produce industry commonly abstracts river water during the winter, when rainfall is highest, and stores it in tanks or reservoirs for use during the spring and summer growing seasons. Water storage, in itself, is not a problematic practice. In fact, when carried out under suitable conditions, the levels of microbial indicators and pathogens in contaminated water can decline substantially (30). To achieve this, storage must prevent contamination by wildlife, including birds, livestock, and airborne sources such as dust.

Many water companies view storage as a form of low-tech primary treatment. Moreover, there is evidence that managing surface streams and rivers through the installation of periodic reservoirs can be beneficial, particularly when these reservoirs help hold back contaminated water (31).

Nonetheless, many of the factors previously discussed in relation to microbial contamination in surface waters, also apply to poorly managed stored water. When water is held in open, unfenced reservoirs, the main concerns remain surface runoff from livestock-farmed land and direct contamination by livestock and wildlife, including birds and insects.