Academic journal article Environmental Health Perspectives

Quantitative Microbial Risk Assessment for Spray Irrigation of Dairy Manure Based on an Empirical Fate and Transport Model

Academic journal article Environmental Health Perspectives

Quantitative Microbial Risk Assessment for Spray Irrigation of Dairy Manure Based on an Empirical Fate and Transport Model

Article excerpt

Introduction

Land application of livestock manure by spray irrigation is becoming more common in the United States (Genskow and Larson 2016). In fact, state-specific guidance documents for manure irrigation are available for Colorado, Idaho, Iowa, Missouri, Kansas, Nebraska, New Hampshire, New Mexico, North Carolina, Pennsylvania, Utah, and Wisconsin (Genskow andLarson 2016).

For manure irrigation, liquid manure in storage lagoons is pumped through pipes to the field site, where it is applied to the land surface using conventional water irrigation equipment, such as traveling gun and center pivot (Figure 1). In contrast with conventional methods of manure application before planting and after crops are harvested, irrigated manure can be applied throughout the year, including applications onto growing crops.

The expressed benefits of manure irrigation tend to be maximized at large economies of scale (Genskow and Larson 2016), and the trend of intensification in U.S. agricultural production practices includes consolidation to larger farms. For example, the percentage of dairy cows housed on U.S. farms with 1,000 or more head of cattle increased from 17% to 49% between 1997 and 2012 (http://www.nass.usda.gov/). Thus, increasing use of spray irrigation for land application of livestock manure seems likely to continue.

However, spray irrigation presents unique public health challenges relative to other technologies for manure land application. Spray irrigation equipment can apply manure for prolonged periods of time and throughout the growing season, potentially resulting in more continuous and more frequent exposure of nearby residents to aerosolized contaminants. Zoonotic pathogens are chief among the contaminants of concern in livestock manure (U.S. EPA 2013). During spray irrigation, wind may transport pathogens from application areas, leading to human exposure via inhalation or contact with contaminated vectors, food, and fomites.

While exposure to zoonotic pathogens from spray-irrigated livestock manure is plausible, the actual exposure levels and corresponding human health risks are not well understood. Concentrations of human pathogens and fecal microorganisms have been measured in air samples collected downwind from spray irrigation using untreated municipal wastewater (Katzenelson and Teltch 1976, U.S. EPA 1980). The incidence of enteric infections was higher among the residents of 77 communities that used partially treated domestic wastewater for spray irrigation than in residents of 130 communities that did not (Katzenelson et al. 1976). However, exposures and risks for spray-irrigated municipal wastewater may differ from those for spray-irrigated livestock manure because municipal wastewater contains human pathogens not found in livestock manure and undergoes treatment that differs from livestock fecal material.

We are aware of only two peer reviewed studies that estimated health risks posed by exposure to zoonotic pathogens in land-applied dairy manure, including a study that generated risk estimates for center pivot irrigation of dairy manure based on an air dispersion model (Dungan 2014) and one that estimated risks based on an empirical fate and transport model for coliphage spiked into groundwater that was sprayed using conventional tanker equipment (Brooks et al. 2012). Both studies used quantitative microbial risk assessment (QMRA), but their risk estimates varied over 10 orders of magnitude, and neither included data from empirical measurements during full-scale dairy manure spray irrigation.

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Our objective was to estimate the human health risk from spray irrigation of dairy manure for residents with dwellings near spray-irrigated fields. We measured air concentrations of zoonotic pathogens and bovine commensal microorganisms downwind of traveling gun and center pivot spray irrigation equipment during full-scale irrigation events. …

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