Groundwater management of acid sulfate soils using controlled drainage, by-pass flow prevention, and subsurface irrigation on a boreal farmland

Abstract

Sulfide-bearing anoxic sediments are found in coastal regions around the world including Australia and the Baltic. Upon lowering of the groundwater by drainage, they are oxidized and form acid sulfate soils (pH < 4) that mobilize plenty of potentially toxic metals into watercourses with serious environmental consequences. Being highly valued for their excellent crop yields, there is an urgent need to find management solutions that minimize the oxidation. In this study, possibilities to manage the groundwater with controlled subsurface drainage (CD) and subsurface irrigation (CDI), which included a vertical plastic sheet to prevent by-pass flow into the main drain, was examined on a Boreal farmland in Western Finland. During a 3-year study, the groundwater in the reference field (Ref) with conventional subsurface drainage pipes at 1.1–1.4 m depth typically dropped down to almost 2 m in the end of summers (September) due to evapotranspiration exceeding precipitation. CD delayed the groundwater drop, shortening the time of oxidation. In CDI system, the groundwater could be kept at c. 1 m or shallower throughout the summers, thereby preventing oxidation of critical sulfide horizons in the lower subsoil. Differences in total discharge and soil geochemistry features were small during the course of the study period. Salt accumulation seemed to be a small risk for crop growth but the capillary rise of acidity to the surface horizon may be increased in CDI, possibly increasing the need for surface liming. A “floating groundwater antenna,” indicating groundwater fluctuations, proved to be an easy and reliable tool to farmers for proper management of controlled drainage.