Removal of plant pathogenic bacteria during agricultural managed aquifer recharge to produce irrigation water and protect plant health

Fresh water is an essential resource for life. Yet, overexploitation of water resources, rainfall shortages, and a decrease in water quality by pollution increase water scarcity worldwide. Since agricultural production consumes about 70% of the available fresh water resources, it is particularly vulnerable for water scarcity. Therefore, this research investigated agricultural aquifer storage, transfer and recovery (ASTR) as nature-based solution to store fresh water in the subsurface from where it can be reused for irrigation. The source water used for infiltration may carry plant pathogens that have to be removed during the storage. Otherwise, the recovered water poses a threat to crop production if pathogens are still present in the irrigation water. Consequently, an understanding of pathogen die-off and removal under relevant conditions found in aquifers is required to predict changes in water quality and protect plant health. In my research, I focussed on the removal of three plant pathogenic bacteria during ASTR: Ralstonia solanacearum and the Soft Rot Pectobacteriaceae, Dickeya solani and Pectobacterium carotovorum. They affect a broad variety of crops with hosts ranging from potato to flower bulbs, both being important cash crops worldwide and particularly in the Netherlands.
An ASTR pilot site located in North Holland was investigated where tile drainage water (TDW) is collected from a 10 ha agricultural field and infiltrated into a sandy, anoxic, and originally brackish aquifer. The TDW can mix with surface water where the selected pathogens are regularly detected. ASTR uses separated wells for infiltration and abstraction of the recharged water. This creates a soil passage and forces the water to flow through the porous medium (sand layers) of the aquifer. Water microcosms and column experiments were used to simulate the aquifer processes in the laboratory and analysed pathogen removal during ASTR.
The results showed that the die-off in the water phase depends on the residence time and ranged between 1.3 to 2.7 log10 after 10 or 60 days for R. solanacearum, respectively. A subpopulation of the bacteria persisted for a prolonged time at low concentrations which may pose a risk if the water is recovered too early. However, the soil passage within the aquifer proved to be highly effective in removing the bacteria by attachment (18 log10 after 1 m). Together with results of dose-response experiments where I studied the effect of contaminated irrigation water on potato plants, all results were ultimately combined in a quantitative microbial risk assessment (QMRA). QMRA is a useful (water) management tool to evaluate the treatment steps of water reclamation technologies and support decision-making processes. As a result of this PhD work, ASTR can be considered a natural treatment system to remove bacterial plant pathogens and provide safe irrigation water.