Spatial Distribution of Natural Enrichments of Arsenic, Selenium, and Uranium in a Minerotrophic Peatland, Gola di Lago, Canton Ticino, Switzerland

Abstract

Gola di Lago is a small (ca. 3 ha), minerotrophic peatland in Canton Ticino, southern Switzerland. Chemical analyses of peat show remarkable concentrations of As, Se, and U. Coring at regular intervals (19 sites) revealed several zones of pronounced accumulation, with As concentrations up to 350 mg kg(-1) (2000 mg kg(-1) on a mineral matter basis). Both Fe and S are also enriched at this depth, suggesting that redox-related transformations have affected all three elements. High concentrations of Se (up to 28 mg kg(-1)) and U (up to 470 mg kg(-1)) were also detected, representing on a mineral matter basis 350 and 2900 mg kg(-1), respectively. An intermittent stream entering the peatland contained up to 400 microg of As L(-1), but the permanent stream leaving the mire contains <2 microg L(-1). A three-dimensional map of the spatial distribution of As shows that the main source of As is the intermittent stream and not the basal, mineral sediment underlying the peatland. Arsenic is highly enriched not only in shallow peat layers at the interface between the stream and peatland today but also in deeper peat layers in the center of the mire, at what must have been the stream-peat interface in the past. By sequential extraction of fresh peat samples, 100% of the As could be extracted from a shallow sample but only 19% from a sample taken from the deeper layers. In both cases, most of the As was associated with the organic matter fraction (73% and 57% respectively). Although this peatland is an effective geochemical trap for As in the stream waters, the mechanisms of removal remain unclear.

Natural wetland emissions of methylated trace elements

“…The concentrations of Se, S and As in the surface peat sampled in the 2012 campaign were 2.0±0.4 mg kg À 1 Se, 4500±700 mg kg À 1 S and 75 ± 35 mg kg À 1 As, on a dry weight basis. The detected concentrations are similar to previously reported concentrations of these elements in the surface peat 20,21 . Although As was previously shown to coincide with enrichment of S in Gola di Lago 21 (and explained by complexation with sulphide groups in organic matter in the form of thio-arsenates 20 ), in this study, S and As concentrations in the peat were not correlated.…”

“…Summarized, the recorded flux of volatile S (37 mg m À 2 day À 1 ) is within the same order of magnitude as previous field and laboratory experiments with soils but lower than the fluxes measured from sediments. The recorded fluxes of total As (0.16-0.54 mg m À 2 day À 1 ) are higher than the total As fluxes observed earlier in field and laboratory studies of pristine and amended soils, possibly due to local enrichments of As at the surface of the peat in Gola di Lago 20,21 . In contrast, the recorded fluxes of volatilized Se (0.11-0.12 mg m À 2 day À 1 ) are considerably lower than the total Se fluxes measured from Se-rich soils (both in the field and in laboratory studies).…”

“…Differences in the absolute amount and trends of volatilization of Se and As in 2010 and 2012 are potentially explained by spatial 21 and temporal heterogeneity in peat element concentration (for example, due to precipitation and surface water flow) during the two field campaigns ( Supplementary Fig. 1).…”

“…(Fig. 1), which is characterized by geogenic enrichments of As and Se at depth in the peat 20,21 . In the 2010 field campaign, Se and As volatilization fluxes were determined using gas-trapping experiments; in the 2012 field campaign these measurements were complemented by the determination of S volatilization, gaseous speciation analyses, and by qualitative and quantitative analyses of dissolved Se, As and S in the surface water.…”

Speciation of naturally-accumulated uranium in an organic-rich soil of an alpine region (Switzerland)

“…At neutral to alkaline pH conditions, U(VI) can be retained in soil by adsorption to soil minerals or precipitation as U(VI) minerals such as uranyl hydroxide, or calcium uranyl phosphate (e.g., uranophane). Further, uranyl is known to complex readily with organic molecules such as acetate, oxalate or humic acid (Haas and Northup, 2004), which might explain why high U concentrations have been reported in humic-rich environments such as peats and bogs (e.g., Read et al, 1993;Owen and Otton, 1995;Gonzalez et al, 2006). This correlation results from the fact that the humic and fulvic acids, which are the main components of aqueous organic matter (OM), are efficient at exchanging protons with metals thereby forming aqueous complexes (Stumm and Morgan, 1996).…”