Impact of redox and transport processes in a riparian wetland on stream water quality in the Fichtelgebirge region, southern Germany

Abstract

Abstract:Biologically mediated redox processes in the riparian zone, like denitrification, can have substantially beneficial impacts on stream water quality. The extent of these effects, however, depends greatly on the hydrological boundary conditions. The impact of hydrological processes on a wetland's nitrogen sink capacity was investigated in a forested riparian fen which is drained by a first-order perennial stream. Here, we analysed the frequency distributions and time-series of pH and nitrogen, silica, organic carbon and oxygen concentrations in throughfall, soil solution, groundwater and stream water, and the groundwater levels and stream discharges from a 3-year period. During baseflow conditions, the stream was fed by discharging shallow, anoxic groundwater and by deep, oxic groundwater. Whereas the latter delivered considerable amounts of nitrogen (¾0Ð37 mg l 1 ) to the stream, the former was almost entirely depleted of nitrogen. During stormflow, near-surface runoff in the upper 30 cm soil layer bypassed the denitrifying zone and added significant amounts to the nitrogen load of the stream. Nitrate-nitrogen was close to 100% of deep groundwater and stream-water nitrogen concentration. Stream-water baseflow concentrations of nitrate, dissolved carbon and silica were about 1Ð6 mg l 1 , 4 mg l 1 and 7Ð5 mg l 1 respectively, and >3 mg l 1 , >10 mg l 1 and <4 mg l 1 respectively during discharge peaks. In addition to that macroscale bypassing effect, there was evidence for a corresponding microscale effect: Shallow groundwater sampled by soil suction cups indicated complete denitrification and lacked any seasonal signal of solute concentration, which was in contrast to piezometer samples from the same depth. Moreover, mean solute concentration in the piezometer samples resembled more that of suction-cup samples from shallower depth than that of the same depth. We conclude that the soil solution cups sampled to a large extent the immobile soil-water fraction. In contrast, the mobile fraction that was sampled by the piezometers exhibited substantially shorter residence time, thus being less exposed to denitrification, but predominating discharge of that layer to the stream. Consequently, assessing the nitrogen budget based on suction-cup data tended to overestimate the nitrogen consumption in the riparian wetland. These effects are likely to become more important with the increased frequency and intensity of rainstorms that are expected due to climate change.

Colloid-associated export of arsenic in stream water during stormflow events

“…The catchment area covers 4.2 km 2 at an elevation between 695 and 877 m.a.s.l. (Lischeid et al, 2007). The mean annual air temperature is 5°C and the mean annual precipitation is approximately 1150 mm (Huang and Matzner, 2007b).…”

“…The regolith overlying the granitic bedrock is 40 m thick and consists of sand, loam, gravel, and granite boulders (Lischeid et al, 2007). The catchment is drained by a dense network of natural streams and artificial ditches.…”

DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

“…This may also be caused by a simple dilution effect of wetland pore water with groundwater, though. This could not be ruled out based on the available data, as δ 18 O and δ 2 H of shallow wetland groundwater and deep groundwater were not sufficiently different and also chloride could not be used (not shown), but a major contribution of water originating from wetland areas during storm events was already stated by Lischeid et al (2007). Nevertheless, one could expect a formation of a relatively iron enriched flocculation or precipitation product, as has been observed at the site in form of iron rich colloids (Neubauer et al, 2012, unpublished).…”

Grasping the heterogeneity of kettle hole water quality in Northeast Germany

“…Maassen & Balla (2010) found that small-scale heterogeneous patterns of groundwater infiltration and exfiltration had substantial impacts on microbial processes and phosphorus turnover in a small stream in Brandenburg. Lischeid et al (2007) observed a complex interplay between hydrological and biochemical processes in a small wetland area, where deep oxic groundwater and reduced shallow groundwater get mixed. Here, residence time of water in the uppermost soil layers seemed to exert a major impact on quality of the discharging water.…”