Effect of water-flow on nutrient mitigation from surface water in drainage ditches

Water quality is easily impacted by agricultural planting and drainage systems flowing into rivers. In this investigation, the effects of agricultural drainage ditches located in Sanyang wetland, Wenzhou, China, were studied for their ability to mitigate soluble inorganic nitrogen and phosphate under different sampling times and water-flow conditions. A significantly decreasing trend of NH4-N concentration in drainage ditches was observed with increasing distance under three water-flow velocities (0.03, 0.06 and 0.1 m(3)/h). The concentrations of NH4-N varied at different times during each day under 0.3 m(3)/h water-flow, with higher concentrations from 11:00 to 23:00 than at other times. There were significant correlations between on-site NH4+ and filtered NO2-N (r = 0.54236, alpha<0.001), and between on-site NH3 and filtered NO2-N (r = 0.52867, alpha<0.001). The different dynamic trend between filtered NO2-N and NO,-N may have involved the conversion of nitrate to ammonium and denitrification. No significant difference of PO43- concentration was observed over distances from 20 to 70 m on the 1st, 3rd, 5th or 7th day of retention under 0.03 m(3)/h water-flow conditions. These results can provide a better understanding of nutrient mitigation under different conditions of hydraulic retention time.