B17001: A review of the use of water in UK agriculture and the potential risks to food safety

Study Duration: May 2001 to February 2002

Contractor: ADAS Consulting Limited

There are a large number of management practices on UK farms that involve the use of water in the production of food for human consumption (e.g. irrigation, produce washing, pesticide applications and livestock drinking). The water sources used and the nature of the practices employed vary from farm to farm and from region to region. The variations in both the use and source of water influences the potential risks to food safety, which may arise from the contamination of produce by micro-organisms carried in the water supply.

One of the primary uses of water in agriculture is for crop irrigation. Previous research has established a link between the application of irrigation water containing pathogens and the incidence of disease in the human population or increased frequency of pathogen isolation on harvested produce. This link is supported by additional studies, which have demonstrated the ability of pathogens such as E. coli O157 to persist on lettuce, watermelon and cantaloupe after harvesting. In the UK, irrigation has become increasingly important in the production of salad and field vegetable crops. The area irrigated in England alone has risen from 11,040 hectares in 1987 to 27,310 in 1995. Irrigation is used to help ensure that produce meets the quality parameters set by retailers. Estimates of the value of irrigation water supply to agriculture suggest that the absence of irrigation would result in a £433m per annum loss of output value.

The majority of the water used for irrigation is abstracted directly from rivers or underground aquifers. Some surface waters are stored in on-farm reservoirs, but almost half of the water used is abstracted from surface waters during the summer months. A proportion of the flow of many of these rivers is derived from the out-flow of sewage treatment plants, a situation which can be exacerbated by low river flows in the summer, when the demand for irrigation is at its peak. Therefore, in some areas of the UK, a proportion of irrigation water applied could be considered to be treated wastewater. Additionally, rivers and streams may further be contaminated by run-off from agricultural land and holdings, and faecal waste from wildlife. As irrigation water is rarely treated before use, any pathogens it contains will be applied directly to crops. There are few data available to describe the microbiological status of irrigation water, but those data that are available indicate that irrigation water may pose a hazard to food safety. These data contrast with the Engelburg guideline limit of <1000 faecal coliforms per 100ml for irrigation water. Guidelines of the US Environmental Protection Agency are even more stringent, with an upper limit of only 2.2 faecal coliforms per 100ml.

The quality of irrigation water, however, is not the only issue that will affect the risks to food safety. Many crop species receive irrigation water, but the timing of irrigation and the method of application vary from crop to crop. Irrigation timing is important, as the interval between the last application and crop harvest will affect the survival of any pathogens deposited on edible parts of the crop. The method of application may also be important as some systems apply water directly to the soil surface (e.g. soil surface drippers), thus avoiding direct contact with the harvested parts of the plant.

Many vegetable and salad crops are processed in on-farm washing plants prior to dispatch. Produce washing is a particularly important potential source of contamination of fresh produce; these processes have been linked previously to outbreaks of food poisoning. Produce washing often involves the use of non-potable water supplies and it is not uncommon for water to be recycled. Although potable water is always used for the final rinse, the practices used in washing plants have the potential to cause cross contamination of produce should the original water source be of low quality or recycled water become contaminated. The use of water in hydro-coolers to reduce the temperature of produce before dispatch is also of relevance. As high volumes of produce pass through such systems, there is the potential for cross contamination, should the water become contaminated or disinfection procedures fail.

Water is extensively used in the preparation of pesticide sprays (fungicides, insecticides and herbicides), which are applied to the majority of crops, and a large proportion of crops receive foliar or liquid fertiliser application. Therefore the sources and potential for contamination of water used in these sprays needs to be considered. Livestock enterprises also use agricultural water for livestock drinking, general washing of machinery and in milking parlours. The quality and sources of the water used needs to be established so that the potential risks to food safety can be assessed. Due consideration also needs to be taken of the current grower/buyer relationship and the influence this may have on farming practices. In particular, the requirements placed on farmers by the retail trade through various 'farm assurance' schemes should be evaluated. In order to respond to retailer demand, growers need to adopt practices which allow some degree of flexibility in the time of harvest and the quality of the product. The potential impact of these market forces on food safety should be considered.

The overall objective of the project is to review current on-farm water use and agricultural water management practices, and to assess the associated risks to food safety.

Information on agricultural water sources (including surface, ground and recycled water supplies), water use and irrigation management practices and available data on pathogen levels in agricultural waters will be collated. The quantities of water used in UK agriculture will be estimated for each of the identified water uses including irrigation, produce washing, drinking supplies for livestock, pesticide and fertiliser applications, and glasshouse production (hydroponic, peat-based and soil grown). This information will be used to assess the risks to food safety from the presence of pathogenic micro-organisms in farm water supplies. In addition, current legislation and guidance documents relating to on-farm water use will be reviewed and their implications for food safety assessed.

The final report will summarise all the information gathered and, where possible, assess the potential risks to food safety. The report will also identify management practices which may be improved, or new practices which may be adopted, to reduce the risk of food contamination.

The following objectives will be carried out:
Objective 01: Undertake a literature review to assess the potential for the contamination of food by waterborne pathogens.
Objective 02: Assess how the source and management of water used in agriculture may affect pathogen loading and survival.
Objective 03: Identify farming practices which involve the use of water and to gather reliable data of the scale and extent of use.
Objective 04: Identify farming practices which may influence the risk of food contamination by pathogens.
Objective 05: Review existing guidelines for the control of food contamination by waterborne pathogens.
Objective 06: Assess the impact of future changes in agricultural water use on the risks to food safety.
Objective 07: Report on the use of water in UK agriculture.

Pathogens in agricultural water
The results indicated that water from river/streams, ponds/lakes and springs/wells is susceptible to faecal contamination by run-off from land occupied by livestock and sewage treatment plants. Indeed the levels of pathogens, assessed by faecal coliform numbers, exceeded 10⁴ colony forming units (cfu)/100ml in some river waters. However, it is unlikely that borehole water will contain pathogens as the potential for direct faecal contamination is limited.

The prediction and assessment of water quality
Faecal contamination of surface waters is a continuous process, which gives rise to temporal and spatial variation in water quality. As the factors that affect water quality are variable, the prediction of pathogen loads at a given abstraction point is problematic. Water sources such as on-farm reservoirs and deep boreholes are unlikely to exhibit fluctuation in pathogen loads as they are less susceptible to significant contamination.

Irrigation management
The quality of water used for crop irrigation is governed by the quality of water available in the local environment and therefore varies from farm to farm. The quality of water available cannot be significantly improved by farmers through management practices as the factors which determine quality are largely external to their farms. On-farm storage or treatment of water to improve quality is costly and rarely used.

The majority of irrigation water is applied as a spray above the crop (95%) and this increases the risk of contamination of the edible parts of the crop and entry of pathogens into the food chain. Soil surface drip irrigation is less common (<5%) but avoids wetting the aerial parts of the crop, and therefore reduces the risk of pathogen transfer to crops.

The schedule for irrigation varies with crop type, crop water demand, soil texture and rainfall. These factors determine the amount of irrigation water applied and the interval between the last application of water and harvesting. The harvest interval may impact upon food safety, as the number of pathogens on crop surfaces are likely to decline over time due to the effects of UV radiation and desiccation.

The crops at most risk of pathogen transfer into the food chain are ready-to-eat crops as these are often consumed raw. Risks are lower for other crops which are cooked before consumption.

Produce washing and cooling
There is a risk of cross contamination during the early stages of the washing process as the process involves the re-circulation of wash water. This risk is minimised where potable waster is used in the final rinse or where chlorinated waster is used during the hydro-cooling phase.

Pesticide and fertiliser application
Results indicate that mains water is used for application, as high quality water is required to ensure spray nozzles do not block. As a consequence, the risk of pathogen transfer by this route is minimal.

Final report is available from the FSA Library and Information centre. To obtain a copy, please contact the Enquiry Desk, Dr. Elsie Widdowson Library and Information Services, Food Standards Agency (020 7276 8181/8182 or at library&info@foodstandards.gsi.gov.uk).

Contact: For any enquiries concerning this research project, please contact the relevant Programme contact or email science@foodstandards.gsi.gov.uk