- Flooding from rivers and streams bursting their banks poses the greatest risk to crops.
- After flooding, pathogens on crops begin to decline over time. However, the first two weeks post-flooding are the most critical in terms of risk.
- Use of flood-prone land for cultivation should be avoided. If such land must be used, the construction of gullies and drains is recommended.
Climate change and extreme weather events, e.g. intense rainfall over a short time, can contaminate soils and surface waters with organisms pathogenic to humans. These pathogens may come from septic tanks, sewers, livestock on elevated ground, or livestock waste stores (1).
Unusually dry soils may become compacted (i.e.capped). When heavy rainfall follows a drought, runoff becomes more severe, increasing the risk of contamination (1). Additionally, temperature shifts associated with heavy rain can influence pathogen survival times or promote their growth, affecting food safety (1).
Flooding of cropland usually falls into two categories:
- Soil saturation from sustained rainfall - This is the most common form. Prolonged rain saturates the land, causing water to pool on the surface. This may reduce crop yield or kill plants. Since this water is mainly rainwater, it is generally unlikely to carry harmful chemicals or disease-causing agents (2). That said, it is not guaranteed to be safe.
- Overflow of rivers or streams - This more severe form occurs when surface waters cannot handle the volume of rain and spill onto cropland. This type of flood is far more likely to contain organisms pathogenic to humans (3). Possible sources include livestock manure washed off pastures, sewage overflows from water treatment plants overwhelmed by rain, domestic septic tanks, and livestock waste stores (4).
Increased Salmonella levels in crops, soil, and water following flooding of leafy greens have been reported previously (5). If the contamination comes from livestock or wildlife, bacterial pathogens are the primary concern. However, if human faeces are present, viruses like norovirus may also be involved. Norovirus is the most common global cause of gastroenteritis and foodborne illness (6).
Floodwater may also carry toxic chemicals, including heavy metals, pesticides, fuel residues, and industrial waste from upstream sources (7).
It is recommended not to use land prone to flooding for the cultivation of crops (1). Where use is unavoidable, gullies and drains should be installed. Another approach is to grow crops whose edible parts are elevated above ground level, making them less susceptible to floodwater (1). There is evidence that flooding increases the presence of Salmonella, pathogenic E. coli, and other enteric pathogens in affected fields (8, 9). It has been noted that pathogen testing of leafy greens before harvest can be ineffective (1), likening it to finding a needle in a haystack. As a result, current food safety guidance does not focus on pre-harvest pathogen testing. The consensus is that rigorous sampling strategies are impractical and uneconomical. The recommended approach is to follow preventive intervention strategies rather than relying on testing alone.
Only a few studies have examined how long flood-related contamination persists in soil. One study found contamination declined slowly over 238 days post-flood. E. coli levels on swabs dropped by 75% during winter but remained stable during milder temperatures between days 44 and 2384. By contrast, another study found it difficult to isolate E. coli from soil shortly after a flood in North Carolina (7). It is unclear whether the differing findings are due to variations in testing methods or other reasons. There exists a lack of research on how long pathogens and other hazards survive on crops after flooding4. Because of this knowledge gap, the U.S. Food and Drug Administration advises that if the edible part of a crop contacts floodwater, the crop should be considered contaminated and unsuitable for consumption (2). While the EU has no specific regulations on flooding, guidance does exists. According to the European Commission's notice on microbiological risks in fresh produce, crops with edible parts that contact floodwater within two weeks of harvest should not be consumed raw (10). If the flooding occurs more than two weeks before harvest, or if the crops will be processed, a site-specific risk assessment should be carried out (10).