Biogeochemistry of organic and inorganic arsenic species in a forested catchment in Germany

Little is known about the fate and behavior of diffuse inputs of arsenic (As) species in forested catchments which often are the sources of drinking water. The objective of this study was to investigate the mobility and transformation of different As species in forest ecosystems to assess the environmental risk related to the diffuse pollution of As. We determined concentrations and fluxes in precipitation, litterfall, soil solutions (Oa horizon and 20- and 90-cm depth), and runoff of organic and inorganic As species and Astotal in a forest ecosystem in NE-Bavaria, Germany. The concentrations of Astotal were mostly <1 μg As L-1 in aqueous samples and were highest in forest floor percolates (7.6 μg As L-1). In litterfall, the concentrations of As species never exceeded 0.1 μg As g-1. Arsenate and arsenite were the prevalent As species in all samples. Organic As species, comprising monomethylarsonic acid, dimethylarsinic acid, trimethylarsine oxide, arsenobetaine, and three unidentified organic As species, were mostly found in throughfall reaching up to 45% of Astotal. The total deposition of Astotal (calculated as throughfall + litterfall) was 5.6 g As ha-1 yr-1 with 16% contribution of litterfall. The annual Astotal fluxes were 30 g As ha-1 yr-1 for forest floor percolates, 8.0 g As ha-1 yr-1 at 20-cm soil depth, and 1.4 g As ha-1 yr-1 at 90-cm soil depth. The annual runoff of Astotal from the catchment amounted to 3.8 g As ha-1 yr-1. The annual fluxes of total organic As species was highest in total deposition (1.1 g As ha-1 yr-1) and decreased largely with depth in the soil profile. The annual runoff of total organic As species was only 0.08 g As ha-1 yr-1. Significant correlations in soil solutions and runoff were found between Astotal and dissolved organic C and Fe. Correlations between Astotal concentrations in runoff and water fluxes were seasonally dependent and with a steeper slope in the growing season than in the dormant season. The elevated concentrations of organic As species in throughfall indicate microbial methylation of As in the phyllosphere, but no evidence for methylation in the soil was found. The mass balance of the catchment points out the strong retention and probable degradation or oxidation of organic As species and arsenite but also to mobilizable pools of Astotal and arsenate. The forest floor is presently a source, whereas the mineral soil is a sink for Astotal and arsenate. The As concentrations in runoff seem to be controlled by As mobilization from forest floor and riparian wetland soils during heavy rain events and superficial flow. The risk for excessment of the drinking water threshold concentrations of As in runoff and soil solutions is considered low at our site.