A catchment-oriented and cost-effective policy for water protection

Governmental programmes and international agreements to counteract eutrophication have largely not attained agreed objectives (e.g. reduction by half of the anthropogenic nitrogen load on Swedish coastal waters). Important components of such programmes are improved removal of nitrogen in municipal treatment plants and changed agricultural practices. In addition, increased N-removal during runoff, i.e. restoration of ponds and wetlands, is an important strategy. One explanation of the fact that the objectives have yet not been achieved might be that the most effective step to counteract diffuse pollution has not been fully implemented. It is therefore important to stress the potential of effective measures and find ways to fully implement them at the watershed level. It is important to avoid excessive applications of fertilizers because this leads to an exponential increase in leaching. Field experiments indicate that the use of winter crops or an undersown catch crop outside the main cropping season has reduced nitrate losses by up to 75% in single years, and by nearly 50% over successive years. In southern Sweden, the area of wetlands has been reduced considerably (more than 90%) by melioration activities. In a recent project, budget studies with restored ponds verified the importance of ponds and wetlands in nitrogen retention. Per unit area, increased nitrogen loading implied increased nitrogen retention, but often a decrease in the percent retained. Ponds with depths of 0.4-2.0 m and hydrological loads of 0.14-5.2 m(3) m(-2) day(-1) were created. One hundred and fifty to seven thousand kg N ha(-1) year(-1) was removed in ponds loaded by streams dominated by agricultural run off. A pond receiving pre-treated municipal wastewater removed 8000 kg N ha(-1) year(-1). The upper limit for N-removal is set by the hydrological conditions. Sedimentation of organic material must be favoured in order to obtain adequate conditions for denitrification. To achieve the governmental objective in nitrogen load reduction changed cultivation practices within the agricultural sector must be combined with restoration of ponds/wetlands. (C) 2000 Elsevier Science B.V. All rights reserved.