Insights into Riparian Zone Water Chemistry

Characterizing the spatio-temporal variability of water chemistry in the riparian zone is important for improving our understanding of the fundamental hydrological and biogeochemical processes that influence stream water quality. However, capturing this variability remains challenging due to the complexity of riparian environments, where dynamic surface water-groundwater exchanges, seasonal fluctuations in groundwater levels, subsurface heterogeneities, variability in flow paths and diverse land uses affect water chemistry.

In this study, we investigated small-scale variability in shallow groundwater chemistry within the riparian zones of three German headwater catchments located in the Black Forest, Sauerland, and Ore Mountains. These sites vary in land use, geology, climate, and other environmental attributes that potentially shape riparian water chemistry. Between summer and autumn 2022, we installed a total of 167 wells across nine riparian areas (three well fields per catchment). From 2023 to 2024, we conducted 10 snapshot sampling campaigns under a range of wetness conditions: three campaigns in the Black Forest and Sauerland, and four in the Ore Mountains. In total, we collected over 400 groundwater samples, which were analyzed for major cations, anions, and dissolved organic carbon.

Our comprehensive dataset showed pronounced variability in space and also between sampling times in all nine riparian areas. However, spatial variability often exceeded the temporal variability (i.e., the differences between the snapshot campaigns). The magnitude of both spatial and temporal variability  differed among individual ions. In particular, ions primarily linked with weathering processes (Na, Mg, Ca, Si) exhibited lower spatial and temporal variability compared to biogeochemically active solutes (e.g., NO₃^-, SO₄^2-, DOC). We also examined whether factors such as catchment wetness conditions, the well’s position relative to the stream, and groundwater levels at the time of sampling could explain variability of the individual ions. The results showed that catchment wetness conditions and well position relative to the stream did not consistently explain spatial variability across elements or sites, and groundwater levels at the time of sampling appeared to have an influence only in Sauerland. These findings highlight the complex interplay of factors driving the variability of riparian zone groundwater chemistry across seasons and study sites.