Stream water quality affected by interacting hydrological and biogeochemical processes in a riparian wetland

Riparian wetlands as both hydrological and biogeochemical hot spots often have a major impact on the release of solutes from headwater catchments. Numerous studies give some evidence of a rather complex interplay of hydrological and biogeochemical processes that is still poorly understood. This study seeks to address this challenge using a multivariate solute concentration data set from a small riparian headwater wetland. First, a non-linear variant of the Principal Component Analysis (Isomap) was performed in a preceding study to identify prevailing biogeochemical processes controlling water chemistry. Second, the scores of the components of the stream draining the wetland were subjected to a cluster analysis to identify typical biogeochemical patterns for different biogeochemical and hydrological boundary conditions. Four different clusters could be identified, which roughly followed a seasonal pattern, although modified by hydrological boundary conditions in the short-term. During the first three months of the year, Cluster 3 prevailed, indicating a discharge of rather unaltered shallow to mean depth groundwater. Cluster 3 was increasingly replaced by Cluster 2 and then subsequently by Cluster 1, indicating increasingly anoxic conditions, increasing denitrification and desulphurization, and increasing decomposition of organic carbon reflecting increasing biological activity and increasing water residence time within the wetland. However, stream water during stormflow after extended periods of low groundwater level in the second half of the growth season exhibited a very distinct pattern, represented by the fourth cluster. It indicated strong oxic conditions causing enhanced oxidation of sulphides, a corresponding decrease in pH values, and a substantial increase in the concentration of alkaline earth ions, manganese and in electric conductivity during the dry period. It is concluded that temporal variations in stream water chemistry clearly reflected the intensity of biological activity in the wetland, interacting with water table dynamics. Our results provide strong evidence for major effects of single extreme events like drought periods which are expected to become more frequent because of climate change.