Due to deposition of atmospheric acids in the northern parts of the Frankenwald, a mountain region in Northern Bavaria, forested watersheds are acidified. Preliminary qualitative investigations supported the hypothesis that alkalinity generating processes in fens contribute significantly to the neutralization of atmospheric acid deposition. It was hypothesized, that subsurface flow above basal layers of slate debris leaches Al, Fe and unbuffered mineral acids from the acidic soils to the spring fen. Incoming Fe and Al are precipitated as hydroxides releasing acidity. In the fen the acidity is consumed by biogeochemical processes, such as microbial reduction of sulfate and ferric iron. Also, sulfate adsorption to the Fe- and Al-hydroxides leads to an increase of alkalinity in spring waters.It was the object of this study to quantify the relative importance of redoxchemical processes for the neutralization of atmospheric acidification in a small watershed and to identify the relevant processes. Therefore water and mass balances for the whole catchment and for the spring fen were established.Our findings show that soils in the watershed are deeply acidified leading to acidic subsurface flow. However, in contrast to the hypothesis, water flow occurs mainly in the basal layer. Here, the groundwater flow can be divided into an acidic upper ground water with high contents of Al and sulfate, and an Al-free, Fe-rich deeper ground water of high alkalinity. Sulfate concentrations are lower in the deeper ground water and d34S measurements point to geogenic source of sulfate, which would be iron sulfide oxidation within the slate debris. In the spring fen the hydrochemically different ground waters are mixed. In the water year 1995 with high above-average discharge conditions the mass balance of the spring fen shows an unexpected alkalinity loss of about 1000 eqža-1. Fe2+ oxidation and precipitation of Fe- and Al-hydroxides account for 90 % of the alkalinity loss. Alkalinity generating processes in the spring fen such as sulfate reduction (~150 eqža-1) or sulfate adsorption to the Fe- and Al-hydroxides (less than ~20 eqža-1) were evident, but they had no significant effect on the mass and alkalinity budget during 1995. On the other hand the alkalinity gain of the terrestrial watershed is about 5700 eqža-1, which is due to weathering processes. Contrary to the hypothesis the spring fen itself is thus not a source of alkalinity. Rather alkalinity entering the fen is consumed, which lowers the alkalinity gain of the whole watershed. Even during low flow conditions from July - October 1994, no alkalinity gain could be measured in the water passing the fen.The hypothesis of a significant alkalinity generation in the fen must therefore be rejected for the investigation period. Because of the hydrogeological properties high water discharge leads to low retention times and high ion fluxes (for sulfate about 12000 eqža-1). In comparison to that biogeochemical alkalinity generating processes in the spring fen are insignificant. The chemistry of the water leaving the fen is controlled by the relative amounts of the two different groundwater discharges and by fast abiotic processes (Fe-, Al-hydroxide precipitation/solution and sorption/desorption of sulfate).
(final report 1998)
List of publications of this Project
Lindemann, J (1997): Quantifizierung biogeochemischer Eisen- und Sulfat-Umsetzungen in einem Quellmoor und deren Beitrag zur Säureneutralisierung in einem Einzugsgebiet des Frankenwaldes in Bayreuther Institut für Terrestrische Ökosystemforschung (BITÖK): Bayreuther Forum Ökologie, Selbstverlag, 51, 1-271 -- Details
Lindemann, J; Peiffer, S; Herrmann, R; Kaupenjohann, M; Zech, W (1995): Ökosysteminterne Neutralisierung atmosphärischer Säureeinträge durch redoxchemische Prozesse in Quellmooren (Frankenwald, NO-Bayern) in Bayer. Landesamt für Wasser and wirtschaft (eds.): Grundwasserversauerung durch atmosph. Deposition: Ursachen- Auswirkungen-Sanierungsstrategien, 3, 197-200