Flooding of the ground used to cultivate crops can usually be classified into one of two categories. First, and most common, is when sustained, heavy rainfall saturates land and water begins to pool on the surface of soil being used to cultivate crops. Flooding caused by soil saturation may cause a reduction in crop yield and potentially may even kill crops. However, the saturated soil flood water is essentially rainwater and therefore it is unlikely to contain toxic chemicals or pathogenic biological agents that can cause human illness (US-FDA, 2011). However, please note that saturated floodwater is not guaranteed to be clear of harmful substances.
A less common form of more-severe flooding occurs when surface waters such as streams or rivers are unable to contain the water from heavy, sustained rainfall and overflow across land that is being used for crop cultivation. Surface water overflows are quite likely to contain contaminants that can cause human illness (Geldriech, 1996). Sources of pathogenic microorganisms include livestock manures on pastures washed into surface waters, the release of incompletely-treated sewage by water companies whose treatment plants are swamped by excessive volumes of rainfall, overflows from domestic septic tanks and livestock manure stores (Bergholz et al., 2016). Castro-Ibanez et al. (2015) report there is an enhanced likelihood of Salmonella isolations from crops, water and soil when leafy greens were flooded. It is important to note that if the contamination source is livestock or wildlife, then the principle human health hazard is bacterial in nature. However, if the contamination includes human faeces, then it is possible the contamination will also include viruses such as norovirus that can also cause illness. Norovirus is the single most common cause of gastroenteritis and food borne illness worldwide (Patel et al., 2009). In addition to biological hazards, flood water can contain toxic chemicals such as heavy metals, pesticides, fuel residues and industrial process wastes carried from upstream (Casteel et al., 2006).
There have been very few studies that have determined the duration of contamination in flooded soils. Bergholz et al. (2016) determined from drag swab results that contamination declined slowly over 238 days since the flood cleared. The prevalence of E. coli on swabs dropped by around 75% over the winter, but remained quite stable during milder temperatures from day 44 to day 238. In contrast E. coli was difficult to isolate from soils shortly after a flood event in North Carolina (Casteel et al., 2006). It is not clear from the papers whether differences in testing methodology was the reason for the apparently contradictory reports.
During the discussion of their findings, Bergholz et al. (2016) noted that there is a limited amount of research describing the survival of biological agents and other hazards on crops that have been exposed to flood water. Bergholz et al. (2016) believe this lack of information is the reason why the current advice of the United States Food and Drug Administration is that if an edible part of a crop has come in contact with flood water, the crop should be considered contaminated and unsuitable for human consumption (US-FDA, 2011).
Bergholz, P.W., Strawn, L.K., Ryan, G.T., Warchocki, S. and Wiedmann, M. 2016. Spatiotemporal Analysis of microbiological contamination in New York State produce fields following extensive flooding from Hurricane Irene. Journal of Food Protection. 79, 384-391.
Casteel, M. J., M. D. Sobsey, and J. P. Mueller. 2006. Fecal contamination of agricultural soils before and after hurricane-associated ﬂooding in North Carolina. J. Environ. Sci. Health A Tox. Hazard. Subst. Environ. Eng. 41, 173–184.
Castro-Ibanez, I., Gil, M. I., Tudela, J. A. and Allende, A. 2015. Microbial safety considerations of flooding in primary production of leafy greens: A case study. Food Research International, 68, 62-69.