Field study of phosphorous transport and retention in drainage reaches

Phosphorous (P) transport and retention is investigated here based on data from a 3-year field study of five drainage ditch reaches at Färingsö, west of Stockholm, Sweden. Results show that P retention rate in the drainage reaches greatly depends on and decreases with increasing water flow and watercourse depth. This supports previous simulation indications and implies that changing drainage reach depth and flow conditions may considerably change P loading to downstream waters. In deeper drainage reaches with tile-drained fields in their catchment areas, which are common in Nordic lowlands, particulate P amounted to about 80% of total P, with about half of this fraction being bio-available. As much as 70% of that particulate P was retained in a reach section that was flow- and depth-restricted by heavy vegetation. Retained P in sediments was found to be mostly bound to Fe and/or Al. Some of that PAl–Fe fraction may after dissolution become more permanently retained by re-binding to Ca.