Pools of sulfur in urban rubble soils

Abstract

Elevated concentrations of sulfate in groundwater are increasingly becoming a problem in several European cities. Building rubble from the World War II is assumed to be a major source of sulfate. This study characterizes pools of sulfur in rubble-composed technosols, and assesses their potential to release sulfate. Six urban soil profiles have been analyzed. Fractions of the main technogenic components in the skeleton fractions were determined by hand sorting approximately 100 kg of material. Total sulfur and water soluble sulfate were determined. Microplate-scale fluorometric assays were applied to measure the depth-dependent enzyme activity of arylsulfatase. The mineral composition of soil samples was analyzed using powder X-ray diffractometry. Binding forms of sulfur were determined using X-ray absorption near-edge structure spectroscopy. The maximum total content of sulfur is 4.6 g center dot kg(-1); that of readily soluble sulfur is 2.3 g center dot kg(-1). Both gypsum and traces of barite and ettringite were detected in some fine soil and component samples. Samples taken from deeper soil depths exhibited higher total sulfur and soluble sulfate contents. The depth profiles of sulfur and the activity of arylsulfatase suggest advanced leaching of inorganic sulfates from the upper horizons. Hence, sulfur is mainly organically bound in the topsoil. In the subsoil, however, sulfates make up about 90 % of total sulfur, approximately 30 % of which is readily soluble. The sulfur pool of rubble-composed soils differs completely from natural soils. This is particularly the case for subsoils, in which high contents of sulfur are readily soluble. This suggests that sulfate minerals such as gypsum predominate. Urbic technosols can therefore be assumed to be one of the main sources of sulfates in urban groundwater.