UC Davis

Rice production in California is a thriving, multi-billion dollar industry, with the Sacramento Valley alone producing over 95% of the nation’s short- and medium-grained rice. The sustainability of this industry is threatened, however, by the rapid evolution of herbicide resistant weeds spurred by long-term monoculture and a lack of herbicides with alternative modes of action. One agent proposed to ameliorate the lack of chemical control options is oxyfluorfen (OXY). OXY is a broad-spectrum, diphenyl ether herbicide that disrupts chlorophyll synthesis through inhibition of the protoporphyrinogen oxidase (protox) enzyme. It has demonstrated effectiveness against rice weeds, including weedy rice (Oryza sativa f. spontanea), a pest for which no herbicide is currently registered, and resistance to its mode of action has not been reported by rice growers. However, OXY is not currently registered for use with rice and its use in- or near-aquatic resources is currently prohibited due to its high toxicity to aquatic organisms. As rice field floodwater is ultimately released into nearby waterways, use of OXY with rice introduces the potential for its transport to the Sacramento River Basin where sensitive aquatic organisms may be harmed. Thus, it is imperative that registered uses be informed by a clear understanding of its transport and dissipation processes and potential for environmental impacts when it’s used as a rice herbicide. To that end, it is the objective of this work to elucidate the environmental fate and aquatic risk of OXY when used as an herbicide in California rice fields.Partitioning processes greatly influence overall fate by determining where a pesticide is found and which processes contribute most to dissipation. Thus, the soil-water partitioning behavior of OXY under simulated California rice conditions was characterized using a batch equilibrium method. Soil-water partitioning was investigated in two soils collected from Sacramento Valley rice fields, at rice field temperatures (15, 25, 35 ℃), and under various rice field salinity conditions. OXY showed high affinity for rice field soil (log[KF] 2.92–3.44) that was largely concentration independent (N 0.87–1.08) and correlated with soil organic carbon (log[Koc] 4.79–5.19) across all soil, temperature, and salinity treatments. Temperature enhanced binding affinity and bound OXY was poorly desorbed (9.3 to 27.0% desorption), exhibiting pronounced sorption hysteresis (HI > 0) in all treatments. These results indicate OXY is likely to concentrate in the sediment where it resists further dissipation, leading to persistence. Volatilization from rice field water is recognized as a significant dissipation route for recalcitrant herbicides. Thus, the air-water partitioning behavior of OXY was investigated through determination of Henry’s law constants (KH) at rice field temperatures. A screening approach for evaluating the feasibility of experimental determination of KH via gas-stripping method was developed and used to demonstrate that KH cannot feasibly be measured for OXY; it must be calculated. Thus, KH was calculated using four air-water partitioning models. Three (3) of the four models (EPI Suite, Kühne, and Two-Point Extrapolation) indicated that OXY is slightly volatile (KH 3.00E-07−1.00E-05 atm·m3·mol-1) at rice field temperatures (15−40 ℃), except at low temperatures (5−10 ℃) where it is nonvolatile (KH < 3.00E-07 atm·m3·mol-1). A single model (AQUAFAC-Sepassi) suggested OXY was substantially volatile (KH > 1.00E-05 atm·m3·mol-1) at all rice field temperatures; however, investigation revealed limitations in the ability of the model to predict key physical properties for OXY, suggesting less robust results. Thus, the preponderance of evidence suggests OXY is nonvolatile to slightly volatile in California rice fields. Partitioning and dissipation processes of OXY, in conjunction with anticipated use patterns in California rice fields, were then simulated using the Pesticides in Flooded Applications Model (PFAM). Estimated environmental concentrations (EECs) were calculated under two (2) California rice field soil conditions and one standard soil condition, with water holding periods of 30 days and 0 days (no holding period). OXY concentrated heavily in sediment (21-day Avg Benthic Sediment EEC: 220,747−411,000 µg/kg-oc) with limited presence in water (21-day Avg Water Column EEC: 2.97−31.4 µg/L). Dissipation was slow and severely limited by microbial metabolism (anaerobic) in the sediment, with an effective half-life of 610.6 days for all treatments. While dissipation in the water column was substantially faster (Cumulative Effective Half-life: 9.4−10.5 days), its limited availability in water rendered OXY less sensitive to water column dissipation pathways. Consequently, water holding period had little effect on rice field and release water concentrations. Overall, these results indicate OXY is likely to accumulate in soil over time, leading to chronic exposure conditions for aquatic life as it slowly releases into water. Risk to aquatic receptors was characterized using calculated EECs and in accordance with ecological risk assessment guidelines. Acute risk was generally low for water column animals (fish and invertebrates) and benthic invertebrates. However, chronic risk to freshwater fish (surrogates for aquatic-phase amphibians) under ultraviolet light conditions, chronic risk to benthic invertebrates, and risks to aquatic plants & algae exceeded risk thresholds under all conditions. California rice field soil conditions were associated with very low acute risk (RQ < 0.1) and less risk overall compared to standard conditions. All risk conclusions were unaffected by holding time, suggesting that water management needs of growers should be considered when stipulating water holding periods for OXY. However, environmental monitoring is suggested to address accumulation and persistence concerns when OXY is applied to California rice field soil.