Irrigation and Wash Water Testing for Salad Growers: A UK Guide

Why irrigation water is back on the agenda in Britain

For years water safety on leafy salad was talked about as an American problem. Two UK outbreaks changed that.

In August to October 2022, 259 confirmed cases of STEC O157:H7 were linked to mixed salad leaves including iceberg lettuce. The published investigation traced the most plausible cause to UK-grown lettuce contaminated during a flooding event: 146.2 mm of rain fell on 16 and 17 August 2022, after a long drought, on a growing region with sheep grazing nearby. The authors describe contamination through floodwater, irrigation water drawn afterwards, and soil and dust at harvest.[1]

In 2024 came the UK’s largest recorded STEC O145 outbreak: 293 confirmed cases between May and November, 129 hospital admissions, 11 cases of haemolytic uraemic syndrome and two deaths. UKHSA’s report, published in December 2025, points to UK-grown Apollo leaf lettuce in pre-packed sandwiches. It could not confirm which grower supplied the leaf or which fields it came from. One irrigation water sample from a grower in the investigation did test positive for Shiga toxin genes, although the strain was not the outbreak one.[2]

The pattern in 2022 is the one that should worry growers after a dry summer like this one: a long drought, then a heavy storm that moves whatever has built up on grazed ground into ditches, reservoirs and fields. If you draw on surface water, the weeks after the first big rain of autumn are when your water is most likely to be at its worst. Our UK rainfall by region tracker is one way to see when that rain has arrived near you.

The rules that actually apply to UK growers

Food hygiene law

The retained food hygiene regulation requires growers of plant products “to use potable water, or clean water, whenever necessary to prevent contamination” (Regulation (EC) No 852/2004, Annex I, paragraph 5(c)). It does not put a number on it. The numbers come from assurance schemes and customers.

Red Tractor Fresh Produce

The Red Tractor irrigation standards (version 5.1, February 2025) are the ones most UK salad growers are audited against.[3] The key points:

• PR.IG.1: water used in crop production (irrigation, mixing fertiliser and plant protection products, crop and equipment washing) must be tested at a frequency set by your risk assessment and in line with the Water Matrix Appendix. The test is for E. coli, and the thresholds and frequency depend on your crop category.
• PR.IG.1.2: analysis must be done by a UKAS-accredited lab (or another ISO 17025 lab) with E. coli in water inside its scope of accreditation. A cheap field kit does not count for your records.
• PR.IG.2 (key standard): untreated sewage water must not be used.
• PR.IG.6: you need a valid licence or permit for any groundwater or surface water you irrigate with.

Leafy salads eaten raw sit at the high-risk end of the Water Matrix, so they carry the tightest limits and the most frequent testing. Read the Water Matrix Appendix in the current standards manual for your crop, and check your customer’s code as well: several retailers set their own, stricter, critical values, and Red Tractor expects you to know them.

Abstraction licences

In England, taking more than 20 cubic metres (20,000 litres) a day from a river, reservoir feed or borehole usually needs an Environment Agency abstraction licence; 20 cubic metres or less a day, as a single operation, does not.[4] Trickle irrigation lost its exemption when the New Authorisations rules came in on 1 January 2018, so do not assume drip gets you out of licensing. We cover licences, trading and winter-fill reservoirs in the UK water resources and abstraction guide.

Boreholes feeding a packhouse

This one catches people out. If a borehole, well or spring supplies water that ends up in food (washing leaf, making ice, packhouse hand-wash), it is normally a private water supply under the Private Water Supplies (England) Regulations 2016. Supplies to food production premises fall into the commercial category, which your local authority must risk-assess and monitor, sampling at the point where the water is used in the business.[5] Scotland, Wales and Northern Ireland have their own equivalent regulations. If you wash product in borehole water and your council has never been out, ring them before an auditor asks.

Know your water sources

Every source carries a different risk, and most holdings use more than one through the season.

• Mains water arrives at drinking water standard. The risk is what happens after the meter: storage tanks without lids, dead legs in old pipework, backflow from a fertiliser injector.
• Boreholes are usually the cleanest untreated source if the headworks are sealed and the borehole is well away from slurry stores, septic tanks, muck heaps and livestock. Shallow wells and poorly capped boreholes can pick up surface water after heavy rain.
• Winter-fill reservoirs settle and improve over time, but they are open to birds, wildlife and whatever ran into the fill source when it was pumped.
• Rivers, drains and ditches are the highest-risk source. Anything upstream (grazing, sewage works discharges, road run-off) ends up in your pipe. If you irrigate a crop eaten raw from surface water, plan on treating it.
• Harvested rainwater from roofs picks up bird droppings. Treat it as surface water, not as clean.
• Recirculated wash water carries soil, plant juice and whatever was on the product. It needs serious treatment before it goes anywhere near a crop again.

The water risk assessment Red Tractor asks for is simply this list written down for your holding: each source on a map, what could contaminate it within reach (neighbouring stock, manure storage, flood routes), how it gets to the crop, and what you do about it.

Testing: what, where and when

What. Generic E. coli is the test the schemes use. It does not tell you a pathogen is there; it tells you faecal contamination has happened recently, which is what makes STEC, Salmonella and the rest possible. Add a chemical panel (pH, nitrate, turbidity, and metals if you have reason to worry) when you first bring a source into use and whenever something changes. pH and turbidity matter because they decide whether chlorine and UV treatment will work.

Where. Sample at the point of use (the irrigator outlet, the drip line, the tap feeding the wash line), not only at the source. A clean borehole feeding a dirty header tank gives you a clean result and a contaminated crop.

How. Use the sterile bottles your lab supplies, follow their sampling instructions exactly, keep samples cold and get them to the lab within the holding time they give you. A late sample is a wasted sample.

When. At the frequency your crop category and risk assessment set, and also:

• before a new source goes into use, and at the start of each season;
• after flooding, or heavy rain following a dry spell, on any surface-water source;
• when stock go onto land next to a source or its catchment;
• after any repair or change to storage, pipework or treatment kit;
• after any result above your investigate level.

If a result comes back high, the order is: stop using that water on crops eaten raw, find the cause, treat or switch source, retest until you are back under your limit, and write down each step. The auditor will want to see the corrective action as well as the result.

Treatment options: an honest comparison

Filtration

Filtration comes first in almost every system. Screen, disc and sand media filters take out the sediment and organic matter that block drippers and, more importantly, shield bacteria from UV and use up chlorine. Filtration on its own is not disinfection. Backwash water is a waste stream, so check how you are allowed to dispose of it.

UV

UV disinfection adds no chemicals and leaves no residues, and it deals with the protozoa (Cryptosporidium in particular) that chlorine at normal doses barely touches. It has three weaknesses. It only works on clear water, so it needs good pre-filtration. Lamp output falls with age, so log run hours and replace lamps on the maker’s schedule. And it leaves nothing behind in the water, so a dirty storage tank downstream undoes it.

Chlorine and chlorine dioxide

Chlorine is still the most common treatment for irrigation water and wash lines because it is cheap and easy to measure. What trips people up is the chemistry. Free chlorine exists as hypochlorous acid (the form that kills bacteria) and hypochlorite ion (far weaker), and pH decides the balance. At about pH 7.5 it is roughly half and half; the lower the pH, the more is hypochlorous acid; above pH 8 most of it is doing very little. A test strip reading of free chlorine tells you the chemical is present, not that it is working. That is why wash lines are run on pH plus ORP (oxidation-reduction potential), which measures how oxidising the water actually is.

Organic matter uses chlorine up, so a dose that holds in clean water collapses in a tank full of soil and leaf fragments. And there is a residue issue: chlorine-based treatments leave chlorate, which has maximum residue levels in food under retained Regulation (EU) 2020/749, between 0.05 and 0.7 mg/kg depending on the product.[6] Over-dosing to be safe can put you over an MRL. Set your targets with your wash system supplier and your customer’s technologist, and validate them; do not copy a number from the internet, including from this page.

Chlorine dioxide is widely used on UK irrigation systems and wash lines as an alternative. It is less affected by pH and also deals with biofilm in pipework, but it needs proper dosing equipment and monitoring.

Ozone and reverse osmosis

Ozone is a strong, fast oxidiser that breaks down to oxygen, which makes it attractive for packhouse wash water. It corrodes ordinary fittings, has to be generated on site, decays within minutes and leaves no lasting protection. Reverse osmosis strips out almost everything, but it is expensive to run and leaves a concentrated brine to get rid of. For field irrigation, neither usually makes sense. They belong in a processing hall with engineers to run them.

Wash water in the packhouse

The wash tank or flume is where a small problem becomes a big one. One contaminated batch going through a tank whose sanitiser has run out can spread contamination through everything that follows. The job of the wash water is to stop that cross-contamination; it will not clean up a contaminated crop.

Three things decide whether it does that job: the sanitiser level, pH and the organic load. In practice:

• monitor pH and ORP continuously, with alarms, and calibrate the probes on a schedule you can show;
• change water on organic load, not on the clock. A muddy day after rain needs more changes than a dry one;
• pre-rinse dirty product so the heavy soil does not reach the sanitised tank;
• give line staff the authority to stop the line when readings drift out of range, and write that into the procedure;
• if you recirculate water, validate the treatment (filtration plus UV as a minimum, often more) with microbiological testing, not just a supplier’s brochure.

Wash-line engineering at processor scale is outside what I do day to day, so treat this section as the questions to ask your supplier and customer, not a specification.

The pathogens behind the rules

• STEC (including O157 and O145): the organisms behind both recent UK lettuce outbreaks. Very small doses make people ill, and HUS can cause kidney failure, particularly in children.
• Salmonella: survives for long periods in water and in biofilm inside pipes, tanks and drains.
• Listeria monocytogenes: the particular risk for bagged and prepared salad because it keeps growing at fridge temperatures. It lives in soil and damp places, so floor drains and cold stores need proper cleaning.
• Cryptosporidium and Cyclospora: protozoa that chlorine at normal wash concentrations does little against, which is the main argument for UV.
• Norovirus: mostly a people problem (sick workers, poor toilet and hand-wash provision) rather than a water one, but contaminated water can carry it.

Cold chain matters here as well. The legal limit for cold food in England, Wales and Northern Ireland is 8°C or below, and the Food Standards Agency recommends running chillers at 5°C or below to leave a margin.[7] Most salad customers expect product colder than that, and fast. The sooner leaf is down to temperature after cutting and washing, the less time anything on it has to grow.

In the field

• Drip or trickle keeps water off the leaf and is the lowest-risk way to irrigate a crop eaten raw.
• Overhead irrigation close to harvest wets exactly the part you sell. If you have to use it, especially from surface water, leave as long a gap as you can before cutting and make sure that water has been tested recently.
• Treat water before it enters the irrigation system, not part-way down it.
• Keep stock and muck away from sources and from fields close to harvest, and watch flood routes. The 2022 outbreak is the case study for why.
• After a flood, assume the crop it reached is contaminated until your risk assessment and your customer say otherwise.

Records the auditor will ask for

• a map and list of every water source and what it is used for;
• the written water risk assessment and the date it was last reviewed;
• every lab result, with sample point, date and lab;
• corrective actions for every result over your limits, and the retest that closed each one;
• treatment system maintenance: filter backwash, UV lamp hours, dosing calibration, probe calibration;
• wash-line monitoring logs and water change records;
• abstraction licence and, where it applies, private water supply paperwork.

Quick reference