Wastewater Reuse in Irrigated Agriculture: Global Perspectives on Water Quantity, Quality, and Exposure to Health Risks
Safe, equitable water reuse is a fundamental component of resilient water systems both in the western United States and around the world. As the dominant water user, agricultural water use is central to this discussion. Urban water infrastructure sits at the interface between health risk mitigation and the quantity and quality of water received by downstream agricultural water users. For the vast majority of the world, wastewater receives little to no treatment before being discharged to surface waters, often resulting in irrigation water quality that is far lower than the standards set for direct reuse schemes. Even when wastewater is treated, urban wastewater return flows can constitute a significant portion of the water available in a given region. In both instances, there is a great need for improved knowledge on both the extent of these practices and the efficacy of different risk mitigation strategies.
The first portion of this dissertation addresses this knowledge gap through the development of a spatially explicit, global assessment of the extent and characteristics of wastewater use in irrigated agriculture. GIS-based models incorporating global data on irrigated croplands, hydrography, urban extents and populations, water and sanitation coverage, water availability, and terrain were used to develop these estimates. The first component of this analysis quantified the global extent of urban and peri-urban croplands, finding that 130 Mha or 60 percent of all irrigated croplands are located within 20 km of urban extents. Thirty-five percent of these croplands are irrigated compared to 17.7 percent of non-urban croplands. Cropping intensity in these urban croplands was also 32 percent higher than non-urban croplands, alluding to the potential economic significance of these croplands. The degree of health risks posed via the indirect or de facto reuse of wastewater depends on concentrations of pathogens in the irrigation water. However, global water quality monitoring data is scant and current water quality models are both coarse and uncertain. Given these considerations, this analysis instead opted to use wastewater return flows and levels of wastewater treatment as proxies for wastewater dependence and irrigation water quality. This analysis found that nearly 26 percent (35.9 Mha) of irrigated croplands are located in a catchment where wastewater return flows constitute more than twenty percent of available water. Of these wastewater dependent irrigated croplands, 29.3 Mha are located in countries where less than 75 percent of wastewater receives some form of treatment. These same catchments are home to some 1.37 billion urban residents. These figures provide some of the first global-scale estimates of the magnitude of the role wastewater reuse plays in meeting the water and food needs of people around the world and will hopefully contribute to the on-going discussion on resource recovery and reuse and the scale-up of wastewater treatment in rapidly urbanizing cities.
While understanding the scale of agricultural water reuse is important in the guidance of planning decisions, the health risks associated with this practice are realized on farms and in markets and households. The second component of this dissertation focuses on a case study in Dharwad, India to better understand the relationship between irrigation water quality, food and farmer safety. This study interviewed 29 vegetable growers and collected 330 water, soil, and produce samples from their farms and local markets. These samples were analyzed for both culturable E. coli and five diarrheagenic E. coli pathotype gene targets. Selected farms were divided roughly evenly amongst those using wastewater versus those using borewell water. Culturable E. coli were detected in all sample types except borewell water. This suggests the presence of additional sources of contamination beyond irrigation water source on farms. At least one pathotype gene target was detected in all sample types from farms irrigating with wastewater, but only on produce samples from farms using borewell water for irrigation. Greens were the most contaminated class of crops on both types of farms. Enterotoxigenic and enteropathogenic E. coli were the two most common strains of diarrheagenic E. coli detected. Concentrations of culturable E. coli showed a strong positive association with the detection of ETEC and the count of pathotype gene targets detected in water, soil, and greens samples, suggesting that, in this context, E. coli was a good indicator of the presence of at least some pathotypes of diarrheagenic E. coli. These findings provide new insights into how exposure to diarrheagenic E. coli varies on farms and forms the basis of future risk assessment modeling work. These findings also allude to a need for further characterization of concentrations of actual pathogens, particularly on different types of produce, and the role of non-irrigation related sources of contamination on farms such as inadequate sanitation facilities, livestock, and wildlife.
Access to wastewater for irrigation makes important contributions to global crop production, but can introduce exposure to health risks when levels of treatment are low. This dissertation provides initial insights into the scale of agricultural water reuse and explores the heterogeneity of exposure that occurs on farms and in markets. As the Sustainable Development Goals emphasize, safe disposal, treatment, and reuse are all important components of complete access to sanitation. This dissertation examines the scale and diversity of ways in which agricultural water reuse is practiced. Planning practices incorporating the vast existing indirect reuse of wastewater are an important component of safe, equitable management of scarce water resources and complete sanitation.