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A study was conducted to relate leaching of a herbicide, atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4,‐diamine], and inorganic water tracers, Br− and Cl−, to the amount of deep‐percolating water produced from irrigation. Soil at the site was classified as a Dehli Loamy Sand (Mixed, Thermic, Tepic Xeropsamment) which was an unstructured sandy soil that was low in organic C content, conditions conducive to solute leaching. The relationship between depth of solute movement and amount of deep‐percolating water was measured in sprinkler, basin, and furrow irrigation methods. Soil distribution of inorganic tracers indicated that graded levels of added water treatments, which were based on reference evapotranspiration, produced corresponding increases in the depth of percolated water. Atrazine's soil distribution indicated greater downward movement in response to increases in amount of deep‐percolating water. Magnitude of leaching differed between irrigation methods and increased in the order: sprinkler < basin < furrow. Simulations using the LEACHM model provided a physically based explanation for the differences in water movement between sprinkler and basin methods. The total amount of applied water was similar at each level of percolation but sprinkler irrigations were more frequent, resulting in more evaporation and, consequently, less water available for deep percolation. Both amount and method of water application are important factors that determine pesticide movement and that, in irrigated agriculture, must be considered as integral components of pesticide management.

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Passive samplers have been shown to be an inexpensive alternative to direct sampling. Diffusion denuders have been developed to measure the concentration of species such as ammonia (NH 3 ), which is in equilibrium with particulate ammonium nitrate. Conventional denuder sampling has required active sampling that inherently requires air pumps and, therefore, electrical power. To estimate emissions of NH 3 from a fugitive source would require an array of active samplers and meteorological measurements to estimate the flux. A recently developed fabric denuder was configured in an open tube to passively sample NH 3 flux. Passive and active samplers were collocated at a dairy farm at the California State University, Fresno, Agricultural Research Facility. During this comparison study, NH 3 flux measurements were made at the dairy farm lagoon before and after the lagoon underwent acidification. Comparisons were made of the flux measurements obtained directly from the passive flux denuder and those calculated from an active filter pack sampler and wind velocity. The results show significant correlation between the two methods, although a correction factor needed to be applied to directly compare the two techniques. This passive sampling approach significantly reduces the cost and complexity of sampling and has the potential to economically develop a larger inventory base for ambient NH 3 emissions.

Although water is crucial to the performance of preemergence herbicides, pesticide performance has rarely been related to irrigation management. This 2-yr study investigated the effect that amount of irrigation water applied had on activity of simazine. Three rates of simazine at 0, 1.12, and 2.24 kg/ha were applied to a 3-yr-old nectarine orchard that was irrigated with microsprinklers. The performance of simazine was compared between irrigation treatments initially targeted to provide water at 110 (efficient) and 175% (overwatered) of crop water requirements. Simazine effectiveness was based on the survival of oat and cucumber plants that were seeded at 0, 14, 28, 56, and 84 d after herbicide application. A longer time interval to 50% survival indicated prolonged herbicidal activity. Results were consistent between years in that simazine's performance was consistently greater in efficient irrigation treatments. The greatest increases were measured at the higher simazine application rate (2.24 kg/ha); overall averages for the length of time to reach 50% survival for cucumber were 50 and 23 d and for oats were 55 and 15 d for efficient and overwatered irrigation treatments, respectively. Use of an efficient irrigation management technique could have enhanced simazine's performance through a decreased leaching of residues from the weed root zone or less chemical or biological degradation (or both). Adoption of efficient irrigation management has been identified as a best management practice to mitigate leaching of pesticide residues to groundwater in coarse soils in California. This study indicates that efficient irrigation improves simazine performance and that both factors, pesticide application and irrigation management, should be considered when developing a weed management system.