Sorption behaviour of per- and polyfluoroalkyl substances (PFASs) as affected by the properties of coastal estuarine sediments.

The sorption of three perfluoroalkyl substances (PFASs), namely perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexane sulfonic acid (PFHxS), was determined in 19 coastal sediments. There are currently limited data on the sorption behaviour of these chemicals in marine or estuarine sediments and the properties controlling their sorption have not been well established. The median average PFOS Kd value (30.4 L/kg) was >8 times that for PFOA (3.3 L/kg) and PFHxS (2.8 L/kg). Highly significant (P < .001) linear relationships were found between values for sorption coefficients (Kd) for all three chemicals (PFOS, PFOA and PFHxS) to the estuarine sediments and organic carbon (OC) content with r2 values ranging from 0.87 to 0.91. The nature of the constituents of OC was determined by nuclear magnetic resonance (NMR) for a subset (10) of the sediments to assess whether the strong relationship between sorption and OC was due solely to an increasing amount of OC or to particular OC fractions. The NMR analysis could not provide strong evidence for one OC fraction type explaining the variation in sorption of the three PFASs. Further investigation using partial least squares of the whole spectra also did not show any particular OC components could explain the Kd variation. This data suggests that variation in sorption in these sediments was primarily due to the varying OC content and not its chemistry.

Sorption behaviour of per- and polyfluoroalkyl substances (PFASs) in tropical soils.

The sorption behaviour of three perfluoroalkyl substances (PFASs), namely perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexane sulfonic acid (PFHxS), was determined on 28 tropical soils. Tropical soils are often highly weathered, richer in sesquioxides than temperate soils and may contain variable charge minerals. There are little data on sorption of PFASs in tropical soils. The highest Kd values were found for PFOS with mean values ranging from 0 to 31.6 L/kg. The Kd values for PFOA and PFHxS ranged from 0 to 4.9 L/kg and from 0 to 5.6 L/kg, respectively. While these values are in the range of literature sorption data, the average Kd values for PFOS and PFOA from the literature were 3.7 times and 3.6 times higher, respectively, than those measured in this study. Stepwise regression analysis did explain some of the variance, but with different explanatory variables for the different PFASs. The main soil properties explaining sorption for PFOS and PFOA were oxalate-extractable Al and pH, and for PFHxS was pH.

Organic waste from sugar mills as a potential soil ameliorant to minimise herbicide runoff to the Great Barrier Reef.

We studied sorption potential for a range of herbicides using eleven waste materials (mill muds) containing organic matter (47.6 to 65.1%) produced by sugar mills and applied as soil conditioners by farmers. Sorption/desorption behaviour of five herbicides commonly used in sugarcane production (imazapic, atrazine, hexazinone, diuron and metribuzin) was studied on these mill muds, as is and after adding these to three soils at different rates (5-25%, dry weight basis). All mill muds had significant sorption capacity, especially for diuron, atrazine and metribuzin which was 6 to 26 times higher than the soil with 3.5% organic carbon (OC). Generally, sorption of the five herbicides assessed in all mill muds followed the order diuron > atrazine = metribuzin > hexazinone = imazapic. Eight out of 11 mill muds had similar sorption capacity for any given herbicides. Amending soils with selected mill muds significantly enhanced their sorption efficiency, depending on the rate of application especially in soil with low OC. Generally, application of mill muds at 5% w/w or 40 tons/ha increased sorption of studied herbicides by 2 to 10 folds. Soil amendment with mill muds also reduced the rate and extent of desorption of herbicides- especially mobile herbicides like metribuzin. Nearly 79% release of metribuzin was observed after three desorption steps in amended soil (at 5% w/w), whereas in unamended soil, 100% of metribuzin was released during first desorption step. The study demonstrates that wastes produced by sugar mills may have recycling use in enhancing the retention of mobile herbicides in soils with low OC content.

The role of surface charge and pH changes in tropical soils on sorption behaviour of per- and polyfluoroalkyl substances (PFASs).

This study investigated the effect of surface charge on the sorption of perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexane sulfonic acid (PFHxS) onto 7 tropical soils as a function of pH. The net surface charge became less negative with decreasing pH (from 7.5 to 3.5) in all soils. The rate of change in net surface charge varied from -0.6 to -2.8 (cmol/kg)/pH unit. The effect on sorption behaviour of PFASs was variable among soils. For two soils, the average sorption increased 54- and 45-fold for PFOS, 33- and 9-fold for PFOA, and 39- and 400-fold for PFHxS, across the pH range 7.5 to 3.5. Sorption in another sandier soil showed negligible change with decreasing pH. Sorption in the other soils did not change significantly until the pH decreased to approximately 5.5. The soils with high contents of sesquioxides (Fe and Al oxides) showed the most marked increase in sorption with decreasing pH. This study demonstrated that in addition to hydrophobic interactions with OC and other processes, electrostatic interactions are also important in the sorption process for these chemicals in soils. In acidic, variably charged tropical soils there is the possibility that any PFOS, PFOA or PFHxS sorbed to the soils may become desorbed if management practices (e.g. liming) raised soil pH.

A critical analysis of published data to discern the role of soil and sediment properties in determining sorption of per and polyfluoroalkyl substances (PFASs).

Widespread usage of per- and polyfluoroalkyl substances (PFASs) has caused major environmental contamination globally. The hydrophilic and hydrophobic properties of PFASs affect the sorption behaviour and suggest organic carbon may not be the only factor affecting sorption. We reviewed the quality of all data published in peer-reviewed literature on sorption of PFASs to critically evaluate the role organic carbon (OC) and other properties have in sorption of PFASs in soils or sediments. The largest data sets available were for perfluorooctanoic acid (PFOA, n = 147) and perfluorooctane sulfonic acid (PFOS, n = 178), and these analyses showed very weak correlations between sorption coefficient (Kd) and OC alone (R2 = 0.05-0.07). When only laboratory-derived Kd values of PFASs and OC were analysed, the R2 values increased for PFOA (R2 = 0.24, n = 42), PFOS (R2 = 0.38, n = 69), perfluorononanoic acid (PFNA, R2 = 0.77 n = 12), and perfluorodecanoic acid (PFDA, R2 = 0.78, n = 13). However, the relationships were heavily skewed by one or two high OC values. Similarly there was no significant relationship between Kd values and pH for PFOS (R2 = 0.06) and PFOA (R2 = 0.07), across a range of environmental pH values. Our analyses showed sorption behaviour of a range of PFASs could not be explained by a single soil or sediment property. Multiple regression models better explained the sorption behaviour of a number of PFASs. Regressions of OC and pH together explained a significant proportion of the variation in Kd values for 9 out of 14 PFASs and 8 of these regressions had ≥10 data points. This review highlighted that at least OC, pH and clay content are properties having significant effect on sorption. There is a clear need for more data and studies with thorough characterisation of soils or sediments to better understand their role in PFASs sorption. Current assessments based on OC alone are likely to be erroneous.

Field evaluation of two risk indicators for predicting likelihood of pesticide transport to surface water from two orchards.

Two pesticide risk indicators, Pesticide Impact Rating Index (PIRI) and Environmental Potential Risk Indicator for Pesticides (EPRIP), were used to determine the likelihood of off-site transport to surface water of pesticides used in a cherry (Prunus avium cultivars) and an apple (Malus domestica cultivars) orchard. The predictions of off-site transport of some of the pesticides were verified against actual pesticide concentrations in surface water continuously monitored over two years. To our knowledge, only one other study in the published literature has attempted this. Of the chemicals monitored there was good agreement between the predictions and the field measurements from the apple orchard, but less so for the cherry orchard. In both risk indicators the attenuation factor based on the width of the buffer strip over-estimated the effectiveness of the buffer strip. There was good agreement between the EPRIP and PIRI risk assessment except for ethephon which EPRIP rated a higher risk than PIRI and dithianon which EPRIP rated a lower risk than PIRI. A strong correlation was found between the field observations and the EPRIP predicted environmental concentrations for the majority of cases. This study showed that even simple risk indicators (e.g. PIRI and EPRIP) can be good predictors for a first tier risk assessment of pesticide transport to neighbouring water bodies.

Comparative environmental impact assessment of herbicides used on genetically modified and non-genetically modified herbicide-tolerant canola crops using two risk indicators.

Canola (Brassica napus L.) is the third largest field crop in Australia by area sown. Genetically modified (GM) and non-GM canola varieties released or being developed in Australia include Clearfield® (imidazolinone tolerant), TT (triazine tolerant), InVigor® (glufosinate-ammonium tolerant), Roundup Ready® - RR® (glyphosate tolerant) and Hyola® RT® (tolerant to both glyphosate and triazine). We used two risk assessment approaches - the Environmental Impact Quotient (EIQ) and the Pesticide Impact Rating Index (PIRI) - to compare the environmental risks associated with herbicides used in the canola varieties (GM and non-GM) that are currently grown or may be grown in the future. Risk assessments found that from an environmental impact viewpoint a number of herbicides used in the production of TT canola showed high relative risk in terms of mobility and ecotoxicity of herbicides. The EIQ field use rating values for atrazine and simazine in particular were high compared with those for glyphosate and trifluralin. Imazapic and imazapyr, which are only used in Clearfield® canola, had extremely low EIQ field use rating values, likely reflecting the very low application rates used for these chemicals (0.02 to 0.04kg/ha) compared with those used for atrazine and simazine (1.2 to 1.5kg/ha). The PIRI assessment showed that irrespective of the canola variety grown, trifluralin posed a high toxicity risk to fish (Rainbow trout, Oncorhynchus mykiss), algae and Daphnia sp. While the replacement of trifluralin with propyzamide had little effect on the mobility score, it greatly decreased the ecotoxicity score to fish, algae and Daphnia sp. due to the lower LC50 values for propyzamide compared with trifluralin. This study has shown that based on likelihood of off-site transport of herbicides in surface water and potential toxicity to non-target organisms, the GM canola varieties have no advantage over non-herbicide tolerant (non HT) or Clearfield® canola.

Banded applications are highly effective in minimising herbicide migration from furrow-irrigated sugar cane.

Runoff from farm fields is a common source of herbicide residues in surface waters in many agricultural industries around the world. In Queensland, Australia, the runoff of PSII inhibitor herbicides (in particular diuron and atrazine) is a major concern due to their potential impact on the Great Barrier Reef. This study compared the conventional practice of broadcast application of herbicides in sugarcane production across the whole field with the banded application of particular herbicides onto raised beds only using a shielded sprayer. This study found that the application of two moderately soluble herbicides, diuron and atrazine, to only the raised beds decreased the average total load of both herbicides moving off-site by >90% compared with the conventional treatment. This was despite the area being covered with the herbicides by the banded application being only 60% less than with the conventional treatment. The average total amount of atrazine in drainage water was 7.5% of the active ingredient applied in the conventional treatment compared with 1.8% of the active ingredient applied in the banded application treatment. Similarly, the average total amount of diuron in drainage water was 4.6% of that applied in the conventional treatment compared with 0.9% of that applied in the banded application treatment. This study demonstrates that the application of diuron and atrazine to raised beds only is a highly effective way of minimising migration of these herbicides in drainage water from furrow irrigated sugarcane.

Sorption of pesticides by a mineral sand mining by-product, neutralised used acid (NUA).

This study investigated the sorption-desorption behaviour of four pesticides by a by-product from mineral sand mining, commonly referred to as neutralised used acid (NUA). In batch studies the average amount of pesticide removed after 6h was 69% for atrazine, 89% for diuron, 61% for 2,4-D and 83% for chlorpyrifos. The lower sorption of 2,4-D to NUA compared with the other pesticides studied is most likely to be due to the high pH of the solutions (7.8 to 8.8) which would have resulted in 2,4-D being predominantly in an anionic form. The presence of other pesticides only significantly decreased the amount of 2,4-D sorbed from 59% to 34% when present in a mixture. Little (2 to 17%) diuron, chlorpyrifos, atrazine or 2,4-D were found to desorb from the NUA. The presence of nitrate or phosphate had minimal effect on the amount of diuron or atrazine sorbed to the NUA. However, all phosphate and nitrate treatments significantly (P<0.05) decreased the amount of 2,4-D sorbed (<50%) compared with when 2,4-D was present alone (65%). This study has shown that NUA has potential to be used as a sorbent for pesticides.

Spatial distribution of diuron sorption affinity as affected by soil, terrain and management practices in an intensively managed apple orchard.

We investigated how the sorption affinity of diuron (3'-(3,4-dichlorophenyl)-1,1-dimenthyl-urea), a moderately hydrophobic herbicide, is affected by soil properties, topography and management practices in an intensively managed orchard system. Soil-landscape analysis was carried out in an apple orchard which had a strong texture contrast soil and a landform with relief difference of 50 m. Diuron sorption (K(d)) affinity was successfully predicted (R(2)=0.79; p<0.001) using a mid-infrared - partial least squares model and calibrated against measured data using a conventional batch sorption technique. Soil and terrain properties explained 75% of the variance of diuron K(d) with TOC, pH(w), slope and WI as key variables. Mean diuron K(d) values were also significantly different (p<0.05) between alley and tree line and between the different management zones. Soil in the tree line generally had lower sorption capacity for diuron than soil in the alleys. Younger stands, which were found to have lower TOC than in the older stands, also had lower diuron K(d) values. In intensively managed orchards, sorption affinity of pesticides to soils was not only affected by soil properties and terrain attributes but also by management regime.

Faster degradation of herbicidally-active enantiomer of imidazolinones in soils.

Imidazolinones are chiral herbicides, comprised of two enantiomers with differential herbicidal activity. In this study, the selective degradation of enantiomers of the three imidazolinone herbicides, imazapyr, imazethapyr and imazaquin, was determined in a variety of soils selected to cover a broad range of physico-chemical characteristics. The R(+) enantiomer of all three herbicides, which has greater herbicidal activity (up to eight times), was found to degrade faster than the less active S(-) enantiomer. The enantiomer fraction (EF) was used as a descriptor of enantio-selectivity of the imidazolinone herbicides. The EF values increased with increasing incubation time for imidazolinones with a fast initial phase followed by a slower phase. While the enantio-selectivity was not significant in acidic soils (pH(w) 5.02 and 5.20), it was highly significant (P<0.001) in alkaline soils (pH(w) 7.6, 8.2 and 8.7). Significant positive correlations of EF values of imazapyr (P<0.001, R(2)=0.41), imazethapyr (P<0.002, R(2)=0.47) and imazaquin (P<0.001, R(2)=0.54) were found with the soil pH(w) ranging from 5.02 to 8.7. However, no correlation of EF was found with other soil properties. In addition to showing enantioselective degradation of the three herbicides in the soils studied, the study highlighted that for imidazolinones the herbicidally more active enantiomer can be preferably degraded by microorganisms.

Abiotic degradation (photodegradation and hydrolysis) of imidazolinone herbicides.

The abiotic degradation of the imidazolinone herbicides imazapyr, imazethapyr and imazaquin was investigated under controlled conditions. Hydrolysis, where it occurred, and photodegradation both followed first-order kinetics for all herbicides. There was no hydrolysis of any of the herbicides in buffer solutions at pH 3 or pH 7; however, slow hydrolysis occurred at pH 9. Estimated half-lives for the three herbicides in solution in the dark were 6.5, 9.2 and 9.6 months for imazaquin, imazethapyr and imazapyr, respectively. Degradation of the herbicides in the light was considerably more rapid than in the dark with half lives for the three herbicides of 1.8, 9.8 and 9.1 days for imazaquin, imazethapyr and imazapyr, respectively. The presence of humic acids in the solution reduced the rate of photodegradation for all three herbicides, with higher concentrations of humic acids generally having greater effect. Photodegradation of imazethapyr was the least sensitive to humic acids. The enantioselectivity of photodegradation was investigated using imazaquin, with photodegradation occurring at the same rate for both enantiomers. Abiotic degradation of imidazolinone herbicides on the soil surface only occurred in the presence of light. The rate of degradation for all herbicides was slower than in solution, with half-lives of 15.3, 24.6 and 30.9 days for imazaquin, imazethapyr and imazapyr, respectively. Abiotic degradation of these herbicides is likely to be slow in the environment and is only likely to occur in clear water or on the soil surface.

Dissipation of the herbicide benzofenap (Taipan 300) in a rice field ecosystem.

The fate of benzofenap [2-[4-(2,4-dicholoro-m-toluoyl)-1,3-dimethylpyrazol-5-yloxy]-4'-methylacetophenone] applied to flooded rice was studied at two locations in New South Wales (Australia). Solid-phase extraction (SPE) was compared with liquid-liquid extraction (LLE) for the determination of the commercial chemical in water samples. SPE performed well as compared to LLE (84 vs 80%) in irrigation waters. However, at the lower end of the concentration range (3 microg/L), LLE achieved higher recoveries than SPE (72 vs 59%). Rates of dissipation (DT50) from floodwaters and soils were measured. Dissipation of the herbicide from water and soil occurred fairly erratically in both mediums and can be best explained by a first-order decay process. The DT50 value for benzofenap was <1 day in irrigation water due to rapid deposition of the suspension concentrate formulation. The DT50 in surface soil was 44 days. The maximum measured concentration of benzofenap in a rice field floodwater was 39 microg/L, taking approximately 32 days to dissipate to <1 microg/L.

Field dissipation and environmental hazard assessment of clomazone, molinate, and thiobencarb in Australian rice culture.

The fates of clomazone [2-(2-chlorophenyl)methyl-4,4-dimethyl-3-isoxazolidinone], molinate (S-ethyl hexahydro-1-H-azepine-1-carbothioate), and thiobencarb {S-[(4-chlorophenyl)methyl]diethylcarbamothioate} applied to rice were studied at two locations in New South Wales (Australia). Rates of dissipation (DT50) from floodwaters and soils were measured. Dissipation of the three herbicides from water and soil can be best explained by a first-order decay process. DT(50) values for clomazone, molinate, and thiobencarb were 7.2, 5.1, and 3.5 days, respectively, in water and 14.6, 23.9, and >46 days, respectively, in surface soil. Maximum measured concentrations of clomazone, molinate, and thiobencarb in floodwaters were 202, 1042, and 148 microg/L, respectively, taking 18.4, 26.4, and 21.4 days to dissipate to concentrations set to protect aquatic ecosystems. A hazard assessment identified clomazone as presenting a low environmental hazard while molinate and thiobencarb presented a medium environmental hazard when used at registered field rates.

On-farm management practices to minimise off-site movement of pesticides from furrow irrigation.

Off-site movement of pesticides from furrow-irrigated agriculture has been a concern in the Ord River Irrigation Area, Western Australia. This paper reports on the effectiveness of incorporation of pesticides by cultivator or power harrows before irrigating, and spraying pesticides only onto beds to minimise off-site transport. Incorporation of pesticides by power harrows prior to irrigation was found to be more effective in decreasing the off-site transport of a more strongly sorbed pesticide, endosulfan. The average load of total endosulfan (alpha + beta + sulfate) decreased by 74% (P < 0.01) from 11.41 g ha(-1) from the conventionally treated bays to 2.96 g ha(-1) from the incorporated irrigation bays. The total average load of atrazine leaving the irrigation bays was decreased by 81% (P < 0.05) from 87.82 g ha(-1) under the conventional practice of spraying the whole field to 16.95 g ha(-1) by spraying the beds only. A reduction of 52% in total average load of metolachlor was observed following incorporation with power harrows, but this was not significant. Incorporation by cultivator or by power harrows decreased the total load of atrazine or metolachlor leaving the irrigation bays over the whole irrigation period, but these treatments were not shown to be statistically significant, which may have been due to the limited number of field replicates. Incorporation of strongly sorbed pesticides (e.g. endosulfan) prior to irrigation significantly decreased the off-site transport of these pesticides in a furrow irrigation system and may be a useful practice to minimise off-site transport of other similar pesticides. Minimising off-site transport of weakly sorbed pesticides (e.g. atrazine and metolachlor) from a furrow irrigation system is more difficult. The nature of furrow irrigation makes it highly conducive to pesticide transport, particularly of weakly sorbed pesticides, and further work is needed to develop strategies to minimise the movement of this group of pesticides to water bodies.

Sorption of pesticides in tropical and temperate soils from Australia and the Philippines.

The sorption behavior of diuron, imidacloprid, and thiacloprid was investigated using 22 soils collected in triplicate from temperate environments in Australia and tropical environments in Australia and the Philippines. Within the temperate environment in Australia, the soils were selected from a range of land uses. The average KOC values (L/kg) for imidacloprid were 326, 322, and 336; for thiacloprid, the values were 915, 743, and 842; and for diuron, the values were 579, 536, and 618 for the Ord (tropical), Mt. Lofty (temperate), and Philippines (tropical) soils, respectively. For all soils, the sorption coefficients decreased in the following order: thiacloprid > diuron > imidacloprid. There were no significant differences in sorption behavior between the tropical soils from the Philippines and the temperate soils from Australia. Sorption was also not significantly related with soil characteristics, namely, organic carbon (OC) content, clay content, and pH, for any of the three chemicals studied. When the data were sorted into separate land uses, the sorption of all three chemicals was highly correlated (P < 0.001) with OC for the rice soils from the Philippines. Sorption coefficients for all three chemicals were highly correlated with OC in temperate, native soils only when one extreme value was removed. The relationships between sorption of all three chemicals and OC in temperate, pasture soils were best described by a polynomial. Sorption coefficients for imidacloprid and thiacloprid determined in the temperate pasture soils remained fairly consistent as the OC content increased from 3.3 to 5.3%, indicating that, although the total OC in the pasture soils was increasing, the component of OC involved with sorption of these two compounds may have been remaining constant. This study demonstrated that the origin of the soils (i.e., temperate vs tropical) had no significant effect on the sorption behavior, but in some cases, land use significantly affected the sorption behavior of the three pesticides studied. The impact of land use on the nature of soil OC will be further investigated by NMR analysis.