Reliable quanti®cation of suspended sediment (SS) and particulate phosphorus (PP) transport, and identi®cation of the various delivery pathways at the catchment level, is an important and necessary aid to appropriate catchment management. In this study we measured storm event, seasonal and annual losses of SS and PP from a Danish arable catchment, Gelbñk Stream, using a multisampling strategy. SS losses for the study years May 1993±April 1994 and May 1994± April 1995 ranged from 71 to 88 kg ha À1 , while PP losses ranged from 0 . 32 to 0 . 36 kg P ha À1 . In both cases losses mainly occurred during infrequent storm events. In comparison with intensive storm sampling, infrequent (fortnightly) sampling underestimated annual transport during the two study years by À24 and À3317, respectively, for SS, and by À8Á6 and À1517, respectively, for PP. Reliable estimation of the transport of sediment and sediment-associated nutrients and other substances thus necessitates the use of an intensive monitoring approach. Turbidimeters proved to be a good substitute for direct measurement of SS, especially during storm events, although careful calibration is needed at the seasonal and storm event levels.Experience shows that in arti®cially drained and geologically complex catchments such as Gelbñk, simultaneous comparative monitoring of dierent sources (e.g. subsurface drainage water) is an important means of reliably discriminating between the various diuse sources of sediment and phosphorus. Subsurface drainage water was found to account for 11±15% of the annual SS export from the catchment; the corresponding ®gure for PP being 11±18%. Surface runo was only a source of SS and PP during the ®rst study year, when it accounted for 19% of SS and 7% of PP catchment export. Stream bank/bed erosion must therefore have been the major diuse source of SS and PP in both study years. The study also revealed that analysis of the trace element content (e.g. 137 Cs, 210 Pb) of the SS transported in subsurface drainage water and stream water during storm events is a useful means of discriminating between diuse losses of SS delivered from topsoil and subsoil compartments.

Movement of particles by water through the soil can be a significant pathway for P transport to surface waters in certain soil types. Our objective was to describe and quantify particulate matter (PM), particulate phosphorus (PP) and dissolved phosphorus (DP) transport tile drains during controlled plot experiments. The results were compared to corresponding studies of natural storm events in the tile‐drained catchment as a whole. Six rain simulations (irrigation 15.3–37 mm) were carried out at two 25 m2 plots on a loamy soil. Tracer chloride concentration in the drainage water peaked within 1 h of the onset of irrigation, thus indicating rapid macropore flow to the drains. PM, PP, and DP concentrations were highest in the initial drainage flow: 63 to 334 mg PM L−1, 0.177 to 0.876 mg PP L−1, and 0.042 to 0.103 mg DP L−1, respectively. Particulate matter and PP loss rates measured for the rapid drainage flow response were in the same range in the plot experiments as for nine precipitation events in the tile‐drained catchment (13.3 ha): 171 to 630 g PM ha−1 mm−1 vs. 141 to 892 g PM ha−1 mm−1, and 0.57 to 1.75 g PP ha−1 mm−1 vs. 0.71 to 5.92 g PP ha−1 mm−1, respectively. Tracer analysis using 137Cs revealed that the PM in the drainage water was derived from the topsoil.

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Pesticide leaching is an important process with respect to contamination risk to the aquatic environment. The risk of leaching was thus evaluated for glyphosate (N-phosphonomethyl-glycine) and its degradation product AMPA (amino-methylphosphonic acid) under field conditions at one sandy and two loamy sites. Over a 2-yr period, tile-drainage water, ground water, and soil water were sampled and analyzed for pesticides. At a sandy site, the strong soil sorption capacity and lack of macropores seemed to prevent leaching of both glyphosate and AMPA. At one loamy site, which received low precipitation with little intensity, the residence time within the root zone seemed sufficient to prevent leaching of glyphosate, probably due to degradation and sorption. Minor leaching of AMPA was observed at this site, although the concentration was generally low, being on the order of 0.05 microg L(-1) or less. At another loamy site, however, glyphosate and AMPA leached from the root zone into the tile drains (1 m below ground surface [BGS]) in average concentrations exceeding 0.1 microg L(-1), which is the EU threshold value for drinking water. The leaching of glyphosate was mainly governed by pronounced macropore flow occurring within the first months after application. AMPA was frequently detected more than 1.5 yr after application, thus indicating a minor release and limited degradation capacity within the soil. Leaching has so far been confined to the depth of the tile drains, and the pesticides have rarely been detected in monitoring screens located at lower depths. This study suggests that as both glyphosate and AMPA can leach through structured soils, they thereby pose a potential risk to the aquatic environment.

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