Roofing material and irrigation frequency influence microbial risk from consuming homegrown lettuce irrigated with harvested rainwater

The effect of a first flush rainwater harvesting and subsurface irrigation system on E. coli and pathogen concentrations in irrigation water, soil, and produce

Climate change is stressing irrigation water sources, necessitating the evaluation of alternative waters such as harvested rainwater to determine if they meet water quality and food safety standards. We collected water, soil, and produce samples between June and August 2019 from two vegetable rain garden (VRG) sites in Frederick, Maryland that harvest rainwater using a first flush system, and deliver this water to produce through subsurface irrigation. The raised VRG beds include layers of gravel, sand, and soil that act as filters. We recorded the average surface soil moisture in each bed as well as antecedent precipitation. All water (n = 29), soil (n = 55), and produce (n = 57) samples were tested for generic E. coli using standard membrane filtration, and water samples were also tested for Salmonella spp. and Listeria monocytogenes by selective enrichment. No Salmonella spp. or L. monocytogenes isolates were detected in any water samples throughout the study period. Average E. coli levels from all harvested rainwater samples at both sites (1.2 and 24.4 CFU/100 mL) were well below the Good Agricultural Practices food safety guidelines. Only 7 % (3/44) of produce samples from beds irrigated with harvested rainwater were positive for E. coli. E. coli levels in soil samples were positively associated with average surface soil moisture (r² = 0.13, p = 0.007). Additionally, E. coli presence in water samples was marginally associated with a shorter length of antecedent dry period (fewer days since preceding rainfall) (p = 0.058). Our results suggest that harvested rainwater collected through a first flush system and applied to produce through subsurface irrigation meets current food safety standards. Soil moisture monitoring could further reduce E. coli contamination risks from harvested rainwater-irrigated produce. First flush and subsurface irrigation systems could help mitigate climate change-related water challenges while protecting food safety and security.

Physicochemical and microbiological quality of rainwater harvested in underground retention tanks

A study was made of the physicochemical properties (among others: pH, temperature, conductivity, hardness, chlorides, dissolved oxygen, ammonia, nitrites, nitrates, manganese, iron) and microbiological quality (coliform, psychrophilic and mesophilic microorganisms) of rainwater harvested from the roofs of three large buildings and from a parking lot, stored in three large underground tanks (with storage volume from 60 m³ to ca. 200 m³), including the variability of the quality. The underground tanks were located in the city of Poznań, and were characterized by different parameters, rainwater sources, rainwater treatment processes, and types of rainwater use. Samples of rainwater harvested in these tanks were collected from March 2019 to February 2020. Physicochemical and microbiological laboratory analyses were performed to determine selected parameters which have an impact on potential treatment and disinfection methods. The results obtained underwent statistical analysis. The laboratory tests showed that the physicochemical quality of rainwater collected in the three underground tanks met the Polish and EU requirements for drinking water standards. The main problem concerns microbiological quality: the number of coliform bacteria reached a value of 19,300 CFU/100 mL, the number of psychrophilic bacteria was over 264,000 CFU/1 mL, and the number of mesophilic bacteria was over 100,000 CFU/1 mL. Also the variability of microbiological quality was very high for all tanks. Analysis of the calcium carbonate equilibrium showed that the harvested water had corrosive properties. The treatment of the investigated rainwater for potable purposes should focus mainly on biological and chemical stability.