Environ. Sci. Technol. 2007, 41, 1564-1569
Biogeochemistry of Organic and Inorganic Arsenic Species in a Forested Catchment in Germany
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.
JEN-HOW HUANG* AND EGBERT MATZNER Department of Soil Ecology, University of Bayreuth, D-95440 Bayreuth, Germany
The elevated concentrations of As in natural waters represent a major water quality and health problem for millions of people worldwide, for example, in Bangladesh, the United States, and China. The drinking water limit of As is today 10 µg L-1 (1), whereas some researcher suggest lower values (2). The toxicity and mobility of the large number of organic and inorganic species of As differ largely. Reduction of arsenate (As(V)) to arsenite (As(III)) represents generally an increase of the toxicity and mobility of As (3). Formation of organic As species leads generally to a reduction of As toxicity but to increasing mobility of As in soils, sediments, and natural waters (4, 5). Thus, the determination of Astotal alone, without speciation, is not able to evaluate the impact of As on the environment and public health. Forested catchments represent an important source of drinking water in many regions worldwide. Therefore, understanding the biogeochemical behavior of the different As species in forest soils and forested catchments is essential. At remote sites, atmospheric deposition is often the main As input. Arsenic enters the atmosphere through natural inputs, for example, from wind erosion and volcanic emission, and by anthropogenic sources, for example, from smelter operation and fossil fuel combustion. The estimated As deposition is 0.31 and 3.0-10 g ha-1 yr-1 at remote and rural areas, respectively (3). These values may be underestimations in case of forested sites because of neglection of dry deposition. The form of As in rainwater will vary with its source which is dominantly As(III)2O3 when derived from smelters, coal burning, and volcanic activities. Trace amounts of methylated As species may be found in the atmosphere because of release of volatile methylated As species by biomethylation. Reduced forms of As seem to undergo oxidation in the atmosphere (3). The biogeochemical behavior of individual inorganic and organic As species in the forest soils has been rarely studied. Lawson and Mason (6) found that only 50% retentions of atmospheric input in forested cachments (6, 7). The primary sorbents for As in soils include Fe, Al, and Mn oxides and clay minerals. Arsenic release from soils may thus be associated to reductive dissolution of Fe and Mn oxides (8). The sorption of As to organic matter was also reported (9). The high carbon contents and high microbial activities of the forest floor may benefit the As methylation. Ku ¨ hnelt and Go¨ssler (10) suggested that the formation of organic As species in soils can be caused by transfer of organic groups from the soil organic matter. Additionally, the presence of anaerobic microzones in the forest floor (11) may be relevant for the formation of methylated As species and for As reduction. The past studies, which were mostly based on incubation experiments in the laboratory, gave contrary results on As methylation in soils. Methylation of As in soils was evident under aerobic incubation of As contaminated soils (12, 13). Controversely, degradation of organic As species in soils was also found and methylation of As was thought to be negligible (14, 15). Thus, further investigations are required to clarify the relevance of As methylation in soils and especially in forest soils.
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
