| The volume of drainage from the rice fields of southern plain river network area of China is large.The substances of phosphorus and pesticides may transport to the ditches adjacent to paddy fields via surface runoff and shallow lateral seepage,or via the effect of wind drift,dry and wet deposition after fertilizer and pesticide applications,which would produce adverse environmental effects to the paddy and downstream water bodies,and pose aquatic ecological risks in these water bodies.This paper took the fields of agricultural water conservancy science experimental station of Jiangdu District,Yangzhou City,Jiangsu Province as the study area.The pulse processes of drainage and non-point source pollutants(phosphorus and pesticides)in paddy fields after fertilizer and pesticide applications were monitored with increased frequency.The process of drainage export from paddies was determined.The entire loss process of phosphorus,three insecticides frequently used for rice(chlorpyrifos,thiamethoxam and abamectin)and two fungicides frequently used for rice(tricyclazole and thifluzamide)were captured.The results indicated that:?.The drainage pulse feature of the paddies in the southern plain river network area of China can be summarized as:short duration(commonly less that 1 d),large flow rate(>3 L·s-1),frequent occurrence(could happen for more than 10 times in one month).The pulse process was significantly influenced by precipitation during the growing season and controlled drainage depth.The aforementioned drainage pulse feature would have a great effect on phosphorus export feature during the fertilization period and pesticide export feature during the pest-control irrigation period.?.The total phosphorus(TP)concentration of paddy surface water peaked at 0.02-0.79 d post fertilizations for tillering and head dressing.The TP concentration of paddy surface water post fertilization was merely 20%-50%of the peak concentration.The TP concentration fell down to the concentration level before fertilization within as short as 2 d after fertilization.The TP concentration peaked at approximately 1 d after basal fertilization,which was relatively later than fertilizations for tillering and head dressing.The variation process of TP concentration in paddy surface water shortly after fertilization could be classified into 3 stages:rapid rise stage,continuous high-level shock stage(lasting for as long as 0.5 d)and slow fall stage.Rainfall events could lead to rapid TP concentration peaks of surface water,while irrigation had less influence on TP concentration.The loss of phosphorus from the drainage of paddy field ditch after fertilization exhibited pulse export feature.The drift of fertilizer particles and lateral seepage during fertilization were probably significant source of phosphorus contamination in the ditch drainage.The TP concentration of the ditch drainage peaked at 0.11-1.2 d after fertilization,which was 19.4%-74.6%of the peak concentration of paddy surface water.The peak concentration of ditch drainage after fertilization can be 14.61 folds of the mean concentration measured during non-fertilization period.The concentration peak of the ditch drainage lasted for only 0.09-0.17 d,which was shorter than the peak of paddy surface water.The concentration of ditch drainage recovered to the level before fertilization within shorter than 1 d after fertilization.Based on increased-frequency monitoring,the calculated diurnal phosphorus load of the paddy drainage after fertilization was more than 6.19-20.55 g·hm-2;the loss rate was higher than 0.35%-1.75%.The calculated phosphorus load would be underestimated by 92.6%if the monitoring interval was as long as 1 d.The aforementioned results could provide decision basis for phosphorus loss reduction from paddies.The potential rainfall events happenning in the future 3 d should be avoided for fertilization.The key period for phosphorus loss control of paddies was 1-2 d after fertilization.It is recommended that controlled drainage be implemented during this period for preventing the non-point source phosphorus contamination in the downstream water bodies.?.With respect to temporal distribution feature of pesticides,the concentration of chlorpyrifos,abamectin,tricyclazole and thifluzamide of paddy surface water and drainage water peaked within 6 h after pesticide application.The duration of pesticide pulse export peak was between the peaks of drainage and pesticide concentration.The loss of pesticides(apart from thiamethoxam)after pest-control irrigation exhibited distinct pulse feature from perspectives of drainage flow rate,pesticide concentration,export load and loading rate.With respect to spatial distribution feature of pesticides,under adverse conditions(the wind direction was from the paddy to the field ditch,few aquatic plants were growing in the ditch water,the ditch water surface was not covered by plants),the chlorpyrifos concentration level ranking of paddy water bodies was:lower reach of the field ditch>middle reach of the field ditch>paddy surface water>paddy underground water.When the pesticides had been applied for more than 12 h,the concentration of drainage and paddy surface water tended to be similar with each other.The thifluzamide concentration of paddy underground water was higher than drainage and paddy surface water during the pest-control irrigation period,which was opposite to chlorpyrifos and tricyclazole.Thifluzamide has the highest leaching risk.As a typical representation,the transport process of the frequently used insecticide-chlorpyrifos was very complicated post application during the pest-control irrigation period in paddy fields.It was influenced by various factors including its physico-chemical properties,wind direction and wind speed,etc.The c-Q hysteresis phenomenon was observed for tricyclazole and thifluzamide,which have high leachability and were easily affected by hydrological conditions in the paddy fields.As for pesticides(e.g.chlorpyrifos)that are easy to volatilize(their transport behavior was easily affected by uncertain factors such as wind direction),the hysteresis phenomenon was not observed.The newly proposed kinetics model concerning pesticide export with drainage could well describe the entire rising and falling limbs of chlorpyrifos concentration,providing references for similar studies.Based on the these research results,comprehensive agricultural management practices were proposed aiming at the prevention and control of non-point source pesticide contamination of paddies.?.Based on the results obtained from the method of risk quotient,when chlorpyrifos and abamectin were applied with the recommended dose,chlorpyrifos posed higher ecological risks to aquatic organisms than abamectin.Chlorpyrifos contained in the ditch drainage water posed extremely high risks to eight of sixteen fish and high to extremely high risks to five shrimp and crabs.Abamectin posed low to median risks to sixteen of nineteen fish and low risks to 3 shrimp and crabs.The highest environmental exposure concentration of chlorpyrifos was higher than the safe concentration of most aquatic organisms.Based on six actual exposure scenarios obtained from the high-frequency monitoring of pesticides during the pest-control irrigation period that lasted for three consecutive years,the openGUTS model was applied to evaluate the ecological risks to seven representative native aquatic organisms in the Yangtze River Delta Region(e.g.Pelteobagrus fulvidraco).A method was proposed for classifying ecological risk grades based on the paremeter LP.The results indicated that the period of within 1d after pest-control irrigation was of not only high export risks,but also high aquatic ecological risks for chlorpyrifos.Chlorpyrifos posed quick killing risk to daphnia magna and macrobrachium nipponensis in most exposure scenarios.Chlorpyrifos posed similar ecological risks(median to extremely high risks)to prussian carp and pelteobagrus fulvidraco and low to high ecological risks to Yangtze River crab,bufo bufo gargarigans.The risks to cipangopaludina cathayensis was the lowest(extremely low to median risks).The model adequately considered the exposure and export process of pesticide pulse acquired by increased-frequency monitoring,making it possible to get accurate and detailed ecological risk assessment results.Scientific evidences were derived for the management and control of risks posed by pesticides during the pest-control irrigation period in paddies. |
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