Testing the waters

The quality of the irrigation water with respect to chemical, microbial and physical properties affects crop quality and health. Growers producing edibles are also concerned about the risk of spreading waterborne human pathogens during production and postharvest. Many growers resolve to use water from public water systems to lower the risk of foodborne illnesses and avoid the hassle of testing and treating water. However, growers must always monitor the chemical parameters of irrigation water to grow high-quality crops. This article explains why growers should test “clean” water.

Public drinking water must meet the Environmental Protection Agency (EPA) drinking water standards established in the Safe Drinking Water Act. EPA sets regulatory limits for microbial contaminants, among others. The Food and Drug Administration (FDA) indicates that “water that meets the microbial standards for drinking water is considered safe and sanitary” and is recommended in the Good Agricultural Practices (GAP) and Good Handling Practices (GHP) certification guidelines. The Food Safety Modernization Act (FSMA) waives microbial testing of water if the source comes from a public water system and has a certification of treatment and sampling. Hence, many growers adopt public drinking water for irrigation and postharvest. However, public water facilities inject chlorine, also an EPA-regulated contaminant, to control microbes in drinking water. Chlorine in irrigation water can be toxic to crops.

Chlorine chemistry

Chlorine is an effective germicidal agent for removing pathogens from water. Chlorine is added to water as a gas, liquid (e.g. sodium hypochlorite, AKA bleach) or solid (e.g. calcium hypochlorite), or generated via membrane electrolysis. All chlorine sources react with water and form hypochlorous acid (HOCl). Further dissociation of HOCl will result in hypochlorite (OCl^-) and hydrogen (H⁺) ions. The sum of HOCl and OCl^- is known as free chlorine; both are sanitizing agents. Hypochlorous acid is the strongest form of chlorine sanitizer.

Hypochlorous acid reacts with nitrogen-containing compounds, both organic and inorganic, to form chloramines. Chloramines are a combined chlorine form. Chloramines have a lower disinfection efficacy and longer residual effect than free chlorine.

The sum of free and combined chlorine is total chlorine. Growers can measure all forms of chlorine with colorimetric kits.

Phytotoxicity

In separate experiments, our team from the University of Connecticut and researchers at the University of Guelph and the University of Florida, have established that most container-grown crops can be irrigated with up to 2 ppm (or mg/L) free chlorine without causing phytotoxicity (Fig. 1). Target doses to control plant pathogens and phytotoxicity thresholds vary by crop-pathogen combination.

Chlorine demand is the difference between the initial (applied) and residual (measured after a given contact time) concentration. The organic matter in the substrate reacts with chlorine and exerts chlorine demand. Hence the recommendations for container-soilless media cannot be directly applied to hydroponically grown crops.

We tested the sensitivity of lettuce to chlorine in hydroponic production. We observed reduction in plant weight when the concentration was as low as 0.5 ppm free chlorine (Fig. 2).

The phytotoxicity symptoms caused by chlorine on hydroponically grown young lettuce plants can be confused with root rot or nutrient deficiencies (Fig. 3). In contrast, the symptoms in mature plants are not very distinctive (Fig. 2). For this reason, sending symptomatic (and healthy) plants to a diagnostic clinic and monitoring the chemistry of nutrient solutions is an important part of the diagnosis.

Testing and treating the waters

The maximum chlorine level allowed in drinking water is 4 ppm. Public water treatment facilities can change chlorine residual levels, reaching up to 4 ppm combined or free chlorine, without notifying the end-user. Therefore, growers using public drinking water must include chlorine in their standard water-testing practice.

Hanna Instruments, Hach and similar companies have developed kits that can be used to measure chlorine in-house.

We do not know yet the phytotoxicity thresholds of free or combined chlorine for most hydroponically grown crops. For this reason, we recommend that growers measure total chlorine.

Growers using public water should have a water treatment option to remove chlorine from the water. The options include activated carbon filters, sodium thiosulfate and aeration.

Take-home message: No matter what, test the chemical parameters of your irrigation water!

Rosa (rosa.raudales@uconn.edu) is an assistant professor at the University of Connecticut and Cora and Juan are Ph.D. students at the University of Connecticut.

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