Modeling fate and transport of pesticides from dryland agriculture using SWAT model.

The aim of this study was to understand pesticide fate and transport from dryland agriculture in a major drinking water basin using SWAT and to identify critical source areas in the basin. Hydrological calibration results indicated satisfactory simulation of hydrologic processes within the catchment. Long term average observed sediment values (0.16 ton/ha) were compared with the annual average simulated SWAT outputs (0.22 ton/ha). Generally, the simulated concentrations were higher than the observed values, but the distribution pattern and trends were similar among the months. Average concentrations in water were 0.036 µg/L and 0.006 µg/L for fenpropimorph and chlorpyrifos, respectively. Transfer rates of pesticides from landscape to rivers showed that 0.36% of fenpropimorph and 0.19% of the applied amount of chlorpyrifos were exported to the river. Higher amount of fenpropimorph transport from land to the reach was attributed to its lower Koc (soil adsorption coefficient) value compared to chlorpyrifos. Higher amounts from HRUs were observed in the application month (April) and following month (May) for fenpropimorph, while the months after September showed higher amounts for chlorpyrifos. The specific HRUs (Hydrological Response Units) located in sub-basins 3, 5, 9 and 11 presented highest dissolved pesticide amounts, while HRUs in sub-basins 4 and 11 exhibited highest concentrations for adsorbed pesticides. Best management practices (BMPs) were recommended in critical subbasins to protect the watershed. Despite the limitations, the results demonstrate the potential contributions of modeling in terms of assessing pesticide loadings, critical zones and application timing.

Mitigation of organophosphate pesticide pollution in agricultural watersheds.

Adsorption and biodegradation processes for four organophosphate pesticides (chlorpyrifos, diazinon, fenthion, dichlorvos) in wetlands and agricultural drains in Meric-Ergene Basin, Turkey have been investigated. Koc (organic carbon normalized partition coefficient) values for all pesticides except diazinon were higher in more aromatic Pamuklu Drain sediments, indicating the possible influence of aromaticity on the extent of adsorption. The average half-lives of pesticides in Gala Lake sediments and Pamuklu agricultural drain sediments ranged from 2.25 to 69.31 days with chlorpyrifos exhibiting the slowest biotransformation rate and dichlorvos having the fastest biotransformation rate. The presence of humic substances and hydroperiod of wetlands have been identified as possible factors that affected the behavior of organophosphate pesticides in this study. The results from this study provide insight into the constructed wetland design offered for the mitigation of organophosphate pesticides in the basin.

Sources and transport of contaminants of emerging concern: A two-year study of occurrence and spatiotemporal variation in a mixed land use watershed.

The occurrence and spatiotemporal variation of 26 contaminants of emerging concern (CECs) were evaluated in 68 water samples in 2011-2012 in the Zumbro River watershed, Minnesota, U.S.A. Samples were collected across a range of seasonal/hydrological conditions from four stream sites that varied in associated land use and presence of an upstream wastewater treatment plant (WWTP). Selected CECs included human/veterinary pharmaceuticals, personal care products, pesticides, phytoestrogens, and commercial/industrial compounds. Detection frequencies and concentrations varied, with atrazine, metolachlor, acetaminophen, caffeine, DEET, and trimethoprim detected in more than 70% of samples, acetochlor, mecoprop, carbamazepine, and daidzein detected in 30%-50% of samples, and 4-nonylphenol, cotinine, sulfamethoxazole, erythromycin, tylosin, and carbaryl detected in 10%-30% of samples. The remaining target CECs were not detected in water samples. Three land use-associated trends were observed for the detected CECs. Carbamazepine, 4-nonylphenol, erythromycin, sulfamethoxazole, tylosin, and carbaryl profiles were WWTP-dominated, as demonstrated by more consistent loading and significantly greater concentrations downstream of the WWTP and during low-flow seasons. In contrast, acetaminophen, trimethoprim, DEET, caffeine, cotinine, and mecoprop patterns demonstrated both seasonally-variable non-WWTP-associated and continual WWTP-associated influences. Surface water studies of CECs often target areas near WWTPs. This study suggests that several CECs often characterized as effluent-associated have additional important sources such as septic systems or land-applied biosolids. Finally, agricultural herbicide (atrazine, acetochlor, and metolachlor) profiles were strongly influenced by agricultural land use and seasonal application-runoff, evident by significantly greater concentrations and loadings at upstream sites and in early summer when application and precipitation rates are greatest. Our results indicate that CEC monitoring studies should consider a range of land uses, seasonality, and transport pathways in relation to concentrations and loadings. This knowledge can augment CEC monitoring programs to result in more accurate source, occurrence, and ecological risk characterizations, more precisely targeted mitigation initiatives, and ultimately, enhanced environmental decision-making.

Phototransformation of pesticides in prairie potholes: effect of dissolved organic matter in triplet-induced oxidation.

Photochemical reactions involving a variety of photosensitizers contribute to the abiotic transformation of pesticides in prairie pothole lakes (PPLs). Despite the fact that triplet excited state dissolved organic matter (DOM) enhances phototransformation of pesticides by acting as a photosensitizer, it may also decrease the overall phototransformation rate through various mechanisms. In this study, the effect of DOM on the phototransformation of four commonly applied pesticides in four different PPL waters was investigated under simulated sunlight using photoexcited benzophenone-4-carboxylate as the oxidant with DOM serving as an anti-oxidant. For atrazine and mesotrione, a decrease in phototransformation rates was observed, while phototransformations of metolachlor and isoproturon were not affected by DOM inhibition. Phototransformation rates and the extent of inhibition/enhancement by DOM varied spatially and temporally across the wetlands studied. Characterization of DOM from the sites and different seasons suggested that the DOM type and variations in the DOM structure are important factors controlling phototransformation rates of pesticides in PPLs.

Sediment-water distribution of contaminants of emerging concern in a mixed use watershed.

This study evaluated the occurrence and distribution of 15 contaminants of emerging concern (CECs) in stream water and sediments in the Zumbro River watershed in Minnesota and compared these with sub-watershed land uses. Sixty pairs of sediment and water samples were collected across all seasons from four stream sites for over two years and analyzed for selected personal care products, pesticides, human and veterinary medications, and phytoestrogens. Spatial and temporal analyses indicate that pharmaceuticals and personal care products (urban/residential CECs) are significantly elevated in water and/or sediment at sites with greater population density (>100 people/km(2)) and percentage of developed land use (>8% of subwatershed area) than those with less population density and land area under development. Significant spatial variations of agricultural pesticides in water and sediment were detectable, even though all sites had a high percentage of agricultural land use. Seasonality in CEC concentration was observed in water but not in sediment, although sediment concentrations of three CECs did vary between years. Average measured non-equilibrium distribution coefficients exceeded equilibrium hydrophobic partitioning-based predictions for 5 of the 7 detected CECs by at least an order of magnitude. Agreement of measured and predicted distribution coefficients improved with increasing hydrophobicity and in-stream persistence. The more polar and degradable CECs showed greater variability in measured distributions across different sampling events. Our results confirm that CECs are present in urban and agricultural stream sediments, including those CECs that would typically be thought of as non-sorptive based on their log Kow values. These results and the observed patterns of sediment and water distributions augment existing information to improve prediction of CEC fate and transport, leading to more accurate assessments of exposure and risk to surface water ecosystems.

Identifying sources of emerging organic contaminants in a mixed use watershed using principal components analysis.

Principal components analysis (PCA) was used to identify sources of emerging organic contaminants in the Zumbro River watershed in Southeastern Minnesota. Two main principal components (PCs) were identified, which together explained more than 50% of the variance in the data. Principal Component 1 (PC1) was attributed to urban wastewater-derived sources, including municipal wastewater and residential septic tank effluents, while Principal Component 2 (PC2) was attributed to agricultural sources. The variances of the concentrations of cotinine, DEET and the prescription drugs carbamazepine, erythromycin and sulfamethoxazole were best explained by PC1, while the variances of the concentrations of the agricultural pesticides atrazine, metolachlor and acetochlor were best explained by PC2. Mixed use compounds carbaryl, iprodione and daidzein did not specifically group with either PC1 or PC2. Furthermore, despite the fact that caffeine and acetaminophen have been historically associated with human use, they could not be attributed to a single dominant land use category (e.g., urban/residential or agricultural). Contributions from septic systems did not clarify the source for these two compounds, suggesting that additional sources, such as runoff from biosolid-amended soils, may exist. Based on these results, PCA may be a useful way to broadly categorize the sources of new and previously uncharacterized emerging contaminants or may help to clarify transport pathways in a given area. Acetaminophen and caffeine were not ideal markers for urban/residential contamination sources in the study area and may need to be reconsidered as such in other areas as well.

Biotransformation of chlorpyrifos in riparian wetlands in agricultural watersheds: implications for wetland management.

Biodegradation of the organophosphate insecticide chlorpyrifos (O,O-diethyl O-(3,5,6-trichloropyridin-2-yl) phosphorothioate) in sediments from wetlands and agricultural drains in San Joaquin Valley, CA was investigated. Sediments were collected monthly, spiked with chlorpyrifos, and rates of chlorpyrifos degradation were measured using a standardized aerobic biodegradation assay. Phosphoesterase enzyme activities were measured and phosphotriesterase activity was related to observed biodegradation kinetics. First-order biodegradation rates varied between 0.02 and 0.69 day(-1), after accounting for abiotic losses. The average rate of abiotic chlorpyrifos hydrolysis was 0.02 d(-1) at pH 7.2 and 30 °C. Sediments from the site exhibiting the highest chlorpyrifos degradation capacity were incubated under anaerobic conditions to assess the effect of redox conditions on degradation rates. Half-lives were 5 and 92 days under aerobic and anaerobic conditions, respectively. There was a consistent decrease in observed biodegradation rates at one site due to permanently flooded conditions prevailing during one sampling year. These results suggest that wetland management strategies such as allowing a wet-dry cycle could enhance degradation rates. There was significant correlation between phosphotriesterase (PTE) activity and the chlorpyrifos biotransformation rates, with this relationship varying among sites. PTE activities may be useful as an indicator of biodegradation potential with reference to the previously established site-specific correlations.

Sizing mitigation wetlands in agricultural watersheds.

In this study, we investigated use of microcosms to supplement field studies for establishing the size of wetlands required to mitigate nitrate pollution in agricultural watersheds. Wetlands investigated in this study were located in San Joaquin Valley (California, USA) and demonstrated mean nitrate-nitrogen mass removal efficiencies ranging between 10 and 34%. Mean areal nitrate removal rates (J) ranged from 142 to 380 mg-N m(-2) d(-1). First-order rate constants determined from field data had a high variance, with confidence intervals greater than 57% of mean values. Sediments and rooted plants from one site were placed in a flow-through microcosm and measurements of nitrate removal kinetics were made and compared with field results. The apparent half-saturation constant (K(m)) and maximum removal rate (J(max)) for nitrate-nitrogen were 43.8 mg/L and 4.11 g m(-2) d(-1) in the microcosm. The first-order rate constant from the microcosm (10.4 cm d(-1)) was in close agreement with the value for the field site (11.9 cm d(-1)) and had a confidence interval of less than 16%. Using this improved first-order rate constant, it was determined that between 1.3 and 3.6% of the land in the watershed should be managed as mitigation wetland, with the area required dependent on the level of nitrate reduction desired and how closely the wetland design approximates plug-flow.