Humus samples as an indicator of long-term anthropogenic input – A case study from southern Finland

Abstract

Humus has been recognized as one of the best sampling media for mapping regional environmental contamination. However, because of the complex and incompletely understood geochemistry of humus, reflecting to differing extents the underlying geology, the biospheric activity, and airborne deposition, its application is still not widespread.This study was conducted within the framework of a comparison between regional humus and topsoil geochemical baseline data in order to analyze the element concentrations in humus and to find possible associations with the underlying geology and long-term atmospheric deposition. The analyzed samples formed part of a geochemical mapping program carried out by the Geological Survey of Finland (GTK) from 2002, and humus samples collected over an area of 37,000km2 in southwestern Finland are elaborated here for the first time. Altogether, 816 sample pairs (humus and topsoil samples) were selected for statistical analysis and analysis of correlations between the element concentrations in humus and minerogenic topsoil and between the element and organic carbon contents.Statistical graphs constructed for 31 elements revealed that most of the elements have a completely different distribution of concentrations in humus and topsoil samples. The concentrations of some elements in humus, such as K, Mg, Fe, and Al, are controlled by the content of mineral matter, which is derived from soil dust (<0.002mm). Other elements, such as As, Bi, Cd, Co, Cu, Mo, Ni, Pb, Rb, Th, V, and Zn, showed evident outliers in humus samples with a probable anthropogenic origin. In order to explain these anomalous high values in humus, the geographic distributions of these elements in humus and topsoil were analyzed and then compared with deposition data obtained from national moss surveys.According to the results presented here, the humus element concentrations are more strongly affected by atmospheric inputs than by the lithogenic contribution, which is opposite to the case for the topsoil samples. The topsoil element distribution maps clearly show a strong relationship with the bedrock geology, such as the relationship of the Ni distribution with the Ni-Co-Cu metallogenic area of the Tampere Region, while the element distributions in humus show a strong correlation with the anthropogenic impact of known deposition from local pollution sources. No long-range transport was clearly identified, but point sources, such as the Pori-Harjavalta smelters, and more densely populated and industrialized areas, such as the city of Tampere and the coastline from the city of Porvoo to the capital region of Helsinki, cause locally high anomalies in humus concentrations, even many years after the reduction of industrial inputs. Further humus samples should be collected with a sufficient sampling density close to city centers and industrialized areas in order to improve knowledge of element concentrations in humus and anthropogenic inputs to the topmost layer of the ground surface, as well as potential risks to the environment and human health.