Fate of the fecal indicator Escherichia coli in irrigation with partially treated wastewater

A Critical Review of Groundwater Table Fluctuation: Formation, Effects on Multifields, and Contaminant Behaviors in a Soil and Aquifer System

The groundwater table fluctuation (GTF) zone is an important medium for the hydrologic cycle between unsaturated soil and saturated aquifers, which accelerates the migration, transformation, and redistribution of contaminants and further poses a potential environmental risk to humans. In this review, we clarify the key processes in the generation of the GTF zone and examine its links with the variation of the hydrodynamic and hydrochemistry field, colloid mobilization, and contaminant migration and transformation. Driven by groundwater recharge and discharge, GTF regulates water flow and the movement of the capillary fringe, which further control the advection and dispersion of contaminants in soil and groundwater. In addition, the formation and variation of the reactive oxygen species (ROS) waterfall are impacted by GTF. The changing ROS components partially determine the characteristic transformation of solutes and the dynamic redistribution of the microbial population. GTF facilitates the migration and transformation of contaminants (such as nitrogen, heavy metals, non-aqueous phase liquids, and volatile organic compounds) through colloid mobilization, the co-migration effect, and variation of the hydrodynamic and hydrochemistry fields. In conclusion, this review illustrates the limitations of the current literature on GTF, and the significance of GTF zones in the underground environment is underscored by expounding on the future directions and prospects.

Irrigation of Olives with Reclaimed Wastewaters and Deficit Strategies Affect Pathogenic Bacteria Contamination of Water and Soil

This study aimed to evaluate pathogenic bacterial contamination of the water-soil-plant system in potted olive trees irrigated with reclaimed wastewater. Desalinated water (DW) obtained by treating municipal wastewater (SW) and reclaimed water (RW) obtained by mixing SW with the brine (BR) produced by DESERT technology (tertiary treatment by ultrafiltration, active carbon and reverse osmosis) were used. Two different irrigation regimes were compared: full irrigation (FI) and regulated deficit irrigation (RDI). During two irrigation seasons the concentrations of Escherichia coli, enterococci, spores of sulfite-reducing Clostridia (SRC) and Salmonella spp. were monitored in water, soil and fruit samples. Microbial concentrations in DW were always below the threshold for reuse in agriculture, while RW showed the highest level of contamination for all observed parameters. RDI management appeared to increase the soil content of SRC spores with respect to FI. Sporadically low SRC spore contamination was recorded in some fruits only in 2018, regardless of the irrigation source, probably because of accidental contamination during sampling or post-harvest handling. This study encourages the creation of a better regulatory framework reference, with specific guidelines for the use of RW as part of integrated environmental systems for the management of sustainable agriculture.

Vertical distribution and affecting factors of Escherichia coli over a 0-400 cm soil profile irrigated with sewage effluents in northern China

Quantification and evaluation of the spatial distribution and the primary factors that affect Escherichia coli (E. coli) distribution in soils is important to assess soil pollution and potential contamination of groundwater. However, little information is available on distribution of E. coli in deep soil layers. To analyze the spatial distribution and factors affecting E. coli over a 0-400 cm soil profile, soil samples were collected from two land use type in the sewage irrigation fields. The primary factors dominating the spatial distribution of E. coli were quantified by the model of principal component analysis with multiple linear regression (PCA-MLR). The results indicated that the number of E. coli under cropland decreased greatly with soil depth. The average number of E. coli over the 0-400 cm profile under forestland was 49 × 10⁴ colony-forming unit/g (cfu/g), which was significantly higher than that under cropland (20 × 10⁴ cfu/g). For forestland and cropland, the average number of E. coli at depths of 300-400 cm decreased by 85% and 88%, respectively, compared to that at depths of 0-100 cm. The presence of E. coli at the depths of 300-400 cm was at high level (forestland: 3 × 10⁴ cfu/g; cropland: 2 × 10⁴ cfu/g) for the potential risks of shallow groundwater. The PCA-MLR model estimated that the factors of soil organism, soil salt and land type use contributed 28%, 29% and 43%, respectively, to the distribution of E. coli. According to the Monte Carlo simulation, the average number of E. coli over the 0-400 cm profile was 46 ± 17 × 10⁴ cfu/g in the sewage irrigated area, and the interval distribution with a probability of 95% varied between 14 × 10⁴ cfu/g and 78 × 10⁴ cfu/g. The findings of this study are useful for understanding negative effects of sewage irrigation on pathogens in deep soil and are critical to assess the potential risks of groundwater pollution.

Application of QMRA to MAR operations for safe agricultural water reuses in coastal areas

A pathogenic Escherichia coli (E.coli) O157:H7 and O26:H11 dose-response model was set up for a quantitative microbial risk assessment (QMRA) of the waterborne diseases associated with managed aquifer recharge (MAR) practices in semiarid regions. The MAR facility at Forcatella (Southern Italy) was selected for the QMRA application. The target counts of pathogens incidentally exposed to hosts by eating contaminated raw crops or while bathing at beaches of the coastal area were determined by applying the Monte Carlo Markov Chain (MCMC) Bayesian method to the water sampling results. The MCMC provided the most probable pathogen count reaching the target and allowed for the minimization of the number of water samplings, and hence, reducing the associated costs. The sampling stations along the coast were positioned based on the results of a groundwater flow and pathogen transport model, which highlighted the preferential flow pathways of the transported E. coli in the fractured coastal aquifer. QMRA indicated tolerable (<10^-6 DALY) health risks for bathing at beaches and irrigation with wastewater, with 0.4 infectious diseases per year (11.4% probability of occurrence) associated with the reuse of reclaimed water via soil irrigation even though exceeding the E. coli regulation limit of 10 CFU/100 mL by five times. The results show negligible health risk and insignificant impacts on the coastal water quality due to pathogenic E. coli in the wastewater used for MAR. However, droughts and reclaimed water quality can be considered the main issues of MAR practices in semiarid regions suggesting additional reclaimed water treatments and further stress-tests via QMRAs by considering more persistent pathogens than E. coli.

The fate of pathogens in treated wastewater-soil-crops continuum and the effect of physical barriers

Secondary treated wastewater (TWW) could provide a cheap and sustainable alternative to potable water (PW) irrigation and ensure food security, especially in arid and semi-arid regions. However, TWW may pose a health risk by introducing pathogens to the irrigated soil and crop, and especially to irrigated vegetables that are eaten raw. To avoid contamination, national and international authorities have mandated the use of physical barriers, such as drip irrigation and plastic mulch, to separate the irrigation water and the crops. Although the barriers are mandated, it is not clear whether they prevent contamination of crops. To evaluate the role of barriers on crop safety, cucumbers and melons were cultivated in a field irrigated with TWW or PW with the application of barriers including surface and subsurface drip irrigation and plastic mulch. Over 500 samples of water, soil and the model crops (surface and tissue) were collected during two growing seasons and used to monitor fecal indicator bacteria and pathogens using culture dependent and independent methods. The results showed that there were no statistically significant differences in both the fecal indicator and the pathogen abundance between treatments in either the soil or the crops, regardless of the water quality or barrier applied, even though TWW supported higher diversity and abundance of indicators and pathogens than PW. Moreover, the microbial communities detected in the irrigated soils and crops could not be linked to the irrigation water. The obtained results suggest that irrigation with TWW does not result in fecal pathogen contamination of the irrigated soil or crops.

Monitoring Opportunistic Pathogens in Domestic Wastewater from a Pilot-Scale Anaerobic Biofilm Reactor to Reuse in Agricultural Irrigation

Wastewater reuse for agricultural irrigation in many developing countries is an increasingly common practice. Regular monitoring of indicators can help to identify potential health risks; therefore, there is an urgent need to understand the presence and abundance of opportunistic pathogens in wastewater, as well as plant phyllosphere and rhizosphere. In this study, an anaerobic biofilm reactor (ABR) was developed to treat rural domestic wastewater; the performance of pollutants removal and pathogenic bacteria elimination were investigated. Additionally, we also assessed the physicochemical and microbiological profiles of soil and lettuces after wastewater irrigation. Aeromonas hydrophila, Arcobacter sp., Bacillus cereus, Bacteroides sp., Escherichia coli, Legionella sp., and Mycobacterium sp. were monitored in the irrigation water, as well as in the phyllosphere and rhizosphere of lettuces. Pathogens like B. cereus, Legionella sp. and Mycobacterium sp. were present in treated effluent with relatively high concentrations, and the levels of A. hydrophila, Arcobacter sp., and E. coli were higher in the phyllosphere. The physicochemical properties of soil and lettuce did not vary significantly. These data indicated that treated wastewater irrigation across a short time period may not alter the soil and crop properties, while the pathogens present in the wastewater may transfer to soil and plant, posing risks to human health.

Importance of soil texture to the fate of pathogens introduced by irrigation with treated wastewater

World-wide water scarcity is urging the use of treated wastewater (TWW) for irrigation but this practice may have adverse effects on soil and crop contamination due to the introduction of potential microbial pathogens. The objective of this study was to evaluate the potential health risks caused by TWW irrigation of soils differing in their texture, i.e., soil particle fractions including sand, silt and clay. We predicted that the presence of fecal indicator bacteria (FIB) and pathogens would not be linked to TWW irrigation, yet their abundance would be favored by the smallest soil fraction (~2 nm, e.g., clay) as it provides the largest surface area. To test our hypotheses, culture dependent and independent techniques were used to monitor the presence, abundance and source of FIB and microbial pathogens (bacteria and protists) in water (TWW and potable water) and three irrigated soil types (clay, loam and loamy-sand) in a field study spanning two years. The results showed that FIB and pathogens' abundance were significantly different between water types, yet these differences did not carry to the irrigated soils. The abundance and presence of FIB and potential opportunistic or obligate human pathogens did not significantly differ (p > 0.05) between TWW and potable water irrigated soils. Moreover, the source of the FIB and potential pathogens could not be linked to irrigation with TWW. Yet, soil type significantly altered the potential pathogens' diversity (p < 0.05) and abundance (p < 0.05), and differences were affected by clay content, as predicted. The results gave no indication for potential adverse health effects associated with the application of TWW but demonstrated that clay has a particular stabilizing effect on the potential presence of microbial pathogens.