Factors that influence water quality

• Some fresh produce growers abstract water and store it for use during the growing season.
• Proper water storage can help improve water quality. Improper storage, such as keeping water in unfenced reservoirs exposed to the open sky, introduces risks, including contamination from surface run-off originating from livestock-farmed land and direct contact with livestock or wildlife.
• Research has shown that while some pathogens (e.g. Campylobacter) found in stored water are not associated with increased infection rates, others (e.g. Cryptosporidium) show a strong connection. 
• Rainfall is recognised as a significant risk factor to water quality, likely due to microbial contamination from run-off entering water systems.
• Regular cleaning or replacement of storage containers can help mitigate these risks. Additionally, the use of suspended shade cloth covers on exposed reservoirs has been shown to significantly reduce bacterial levels and improve overall water quality.

Water use and quality play a vital role in preventing food contamination throughout all stages of production. Enhanced water management practices and improved water quality can significantly reduce food contamination incidents (1). In the UK, the fresh produce sector frequently abstracts water from rivers during the winter, when precipitation is highest, and stores it in reservoirs or tanks for use during spring and summer. Storage, in itself, is not a harmful practice. In fact, under optimal storage conditions, water with high microbial or pathogen loads can see significant reductions in contamination levels (2). Many water companies regard water storage as a low-tech form of primary treatment. Moreover, some reports suggest that installing periodic reservoirs to manage surface streams and rivers is beneficial, even when water is initially contaminated (3). However, if water is stored improperly in unfenced, open reservoirs, many of the same issues affecting surface waters, such as contamination from livestock and wildlife (including birds and insects), still apply (figure 1).

Given the key role wildlife and livestock play in contaminating surface waters and inadequately protected reservoirs, several studies have examined the extent to which zoonotic agents are transmitted via water. These investigations have shown that different pathogens behave in different ways and contribute variably to human foodborne illness. For example, Campylobacter jejuni is the leading cause of bacterial foodborne illness in the UK. A study conducted in Cheshire involved sampling a 10 km by 10 km area, where water and environmental samples, including soil and foliage, were tested for Campylobacter spp. (4). C. jejuni was the most prevalent species across all animal faecal samples, appearing in 11% of samples from non-avian wildlife and 36% of cattle faeces. It was already well-established that birds, including migratory species, are carriers of Campylobacter (5). In the Cheshire study, C. jejuni was found in 15% of all surface water samples. However, over the study period, Campylobacter levels in water did not correlate with increases in human illness, leading the authors to conclude that “The observed pattern is not consistent with large-scale transmission [of Campylobacter] attributable to watercourses.” A later study confirmed that water contributes minimally to human foodborne illness from Campylobacter (6). In contrast, the same study found that Cryptosporidium in water, including mains supplies, private water sources and surface waters, was implicated in 69% of all recorded outbreaks of cryptosporidiosis.

An increasing body of research supports the view that rainfall events can elevate bacterial loads in stored rainwater (7-10). The exact mechanism remains unclear, as fresh rainwater typically contains very low levels of bacteria. One plausible explanation, particularly for observations made in Australia, is that the surfaces used to collect rainwater were not clean, leading to microbial contaminants being washed into the water store. Another possibility is that over time, dust and soil accumulate as sediment within water stores. When new rainwater enters with enough force, it can disturb this sediment, releasing nutrients and trapped bacteria, thereby increasing bacterial levels temporarily. Routine cleaning, or replacement of butyl liners, for small-volume storage tanks (up to 50,000 litres), combined with a microbiological testing regime, can help prevent the build-up of sediment and microbial contamination.

A study examined the effect of suspended shade cloth covers (SSCCs) on the quality of irrigation water stored in reservoirs (11). Over a year-long trial in Spain, four water storage sites were monitored, two of which were covered with a black polyethylene SSCC, while the other two remained uncovered. A key finding was that the SSCC significantly reduced photosynthetic activity in the water, thereby limiting algal blooms. The use of covers led to an 82% reduction in Escherichia coli and faecal coliforms, attributed both to the reduced organic matter in the water (due to suppressed algal growth) and to lower water temperatures. In warm climates, covering irrigation water stores resulted in a significant improvement in water quality mostly by reducing evaporation and thus preventing a concentration effect of harmful substances/organisms from occurring.