Optimal strategies for monitoring irrigation water quality and the development of guidelines for the irrigation of food crops

Optimal strategies for monitoring irrigation water quality and the development of guidelines for the irrigation of food crops

Jan. 1, 2015 - Dec. 31, 2015

$117,202.00

Marc Verhougstraete, Ph.D.
University of Arizona

Kelly R. Bright, Ph.D.

Final Report

Publication

The quality of irrigation water drawn from surface water sources can vary greatly. This is particularly true for waters that are subject to intermittent contamination events such as runoff or direct entry of livestock upstream of use. Such pollution in irrigation systems increases the risk of food crop contamination. A single sample does not adequately characterize the risk potential present in large irrigation systems often utilized in the Southwestern US. Furthermore, current approaches are based on recreational water practices and have little scientific basis for irrigation waters. The goals of this project are to define optimal monitoring strategies for irrigation water quality and develop guidelines for the irrigation of food crops. We propose five key objectives that will improve our understanding of potential risk associated with fecal contamination in irrigation systems. These objectives include spatial and temporal analysis of Escherichia coli over multiple seasons and irrigation canals in Yuma, Arizona. Following water quality analysis, guidelines and best management practices will be developed for growers/producers to follow during the monitoring and use of irrigation water systems for food production. These guidelines will better characterize risks from microbial pathogen contamination in irrigation waters and aid in risk reduction practices for agricultural water.

Technical Abstract

Little information is known about irrigation water safety and the criteria that should be followed to determine whether the water should or should not be used to irrigate food crops. Water used for irrigation flows beneath and through diverse geographies prior to the point of extraction, accumulating potentially harmful pathogens along the way. The United States Department of Agriculture Agricultural Marketing Service requires only that “water quality is known to be adequate for the crop irrigation method and/or chemical application.” A single sample is often used to estimate the microbial quality of irrigation water, assuming equal representation for multiple fields. However, water research undertaken in other surface waters has routinely demonstrated significant changes in microbial concentrations on short spatial and temporal scales. Therefore, better characterization of the microbial quality of irrigation water systems is needed to reduce risk at the point of irrigation, water extraction and application. The goal of the proposed study is to assess the spatial and temporal occurrence of microorganisms in irrigation water systems to determine the most appropriate sampling strategies that aid in risk reduction practices for agricultural water used for food crops.

 All water samples will be collected in irrigation canals in the Yuma Irrigation Water District in Yuma County, AZ and will be assayed for total coliforms, Escherichia coli, turbidity, pH, temperature, conductivity, and salinity. Four specific objectives will address critical gaps in current irrigation water quality knowledge: 1) to determine the appropriate time of day for irrigation water monitoring, samples (n = 160) will be collected at various times in the morning and the afternoon; 2) to determine the appropriate sample collection point, samples (n = 180) will be collected at twelve points across the canal transect (at the water surface, 2’ below the surface, and 4’ below surface at the following locations: the left bank, ¼ of the distance of the canal width from the left bank, ¼ of the distance of the canal width from the right bank, and the right bank); 3) To address the transport of microorganisms in irrigation canals, samples (n = 210) will be collected at major canal headwaters and downstream at equal spatial intervals (e.g., every mile) based on canal-specific discharge rates; 4) to determine if it is more appropriate to collect a single sample, multiple samples, or composite samples, this objective (n = 120 samples total) will involve testing: a) a single sample from a single collection point; b) multiple samples collected from the same canal stretch (assayed individually and expressed as a geometric mean), and c) multiple samples composited into a single sample prior to laboratory testing to obtain one concentration.

 A fifth specific objective will be defining an overall sampling strategy to produce the most relevant data for determining the risks of microbial pathogen contamination of food crops via E. coli contaminated irrigation waters. This information will be used to develop guidelines and best management pract ces for growers/producers for the monitoring and use of irrigation water systems for irrigating food crops.