Efficiency of Drainage Practices for Improving Water Quality in Lithuania

Abstract

Abstract. Artificial drainage is a common agricultural practice in Lithuania. In this country, the total drained land area occupies 47% of the total land area and 87% of the agricultural land area. Therefore, this article presents recent research findings on agricultural drainage in Lithuania related to the practices designed to reduce nutrient, i.e., nitrogen (N) and phosphorus (P), losses from the soil via tile drainage and transport in open drains. Temporal changes in tile drainage flow over the last four decades are also discussed in this article. The results from experiments with controlled drainage practices in Lithuania showed promise. Compared to conventional drainage, controlled drainage reduced inorganic N by 42% to 77% and reduced total P by 34% to 72%. The reduced loads were the result of reduced drainage outflow. Moreover, research on the effects of additives in drainage trench backfills showed that woodchips, chopped straw, and lime additives mixed in the drainage trench backfill led to reductions in NO3-N concentrations of 78%, 69%, and 52%, respectively, in the drainage water. The addition of lime to drainage trench backfill reduced PO4-P concentrations in the drainage water by 39%, while woodchips and chopped straw increased the concentrations by 11% and 22%, respectively. It was determined that NO3-N in the drainage water was removed most effectively by woodchips and that PO4-P was removed most effectively by the addition of lime. The experiments with reactive filter materials used as in-ditch measures to remove phosphorus showed that the filter materials can be ranked as follows based on their P removal efficiencies: Polonite > slag > Filtralite-P > dolomite chips. Polonite had an advantage over the other tested materials due to its higher porosity, low sensitivity to clogging, and greater permeability. Laboratory-scale experiments using denitrification bioreactors filled with three types of woodchips (deciduous, coniferous, and mixed) showed no significant differences in NO3-N removal efficiency among the three materials. However, the tests showed that woodchip media are capable of achieving higher NO3-N removal rates due to higher flow rates. Therefore, better optimization and proper evaluation of the effects of hydraulic retention time are needed to improve the design of denitrifying woodchip bioreactors. Keywords: Agricultural drainage, Controlled drainage, Denitrifying bioreactors, Drainage trench backfills, In-ditch filters.