es7b03087_si_001.pdf (115.95 kB)
High Resolution Monitoring Above and Below the Groundwater Table Uncovers Small-Scale Hydrochemical Gradients
journal contribution
posted on 2017-11-13, 00:00 authored by N. Gassen, C. Griebler, U. Werban, N. Trauth, C. StumppHydrochemical solute concentrations
in the shallow subsurface can
be spatially highly variable within small scales, particularly at
interfaces. However, most monitoring systems fail to capture these
small scale variations. Within this study, we developed a high resolution
multilevel well (HR-MLW) with which we monitored water across the
interface of the unsaturated and saturated zone with a vertical resolution
of 0.05–0.5 m. We installed three of these 4 m deep HR-MLWs
in the riparian zone of a third-order river and analyzed for hydrochemical
parameters and stable water isotopes. The results showed three distinct
vertical zones (unsaturated zone, upper saturated zone, lower saturated
zone) within the alluvial aquifer. A 2 m thick layer influenced by
river water (upper saturated zone) was not captured by existing monitoring
wells with higher screen length. Hydrochemical data (isotopes, total
ions) were consistent in all HR-MLWs and showed similar variation
over time emphasizing the reliability of the installed monitoring
system. Further, the depths zones were also reflected in the NO3–N concentrations; with high spatial variabilities
between the three wells. The zonation was constant over time, with
seasonal variability in the upper saturated zone due to the influence
of river water. This study highlights the use of high resolution monitoring
for identifying the spatial and temporal variability of hydrochemical
parameters present in many aquifer systems. Possible applications
range from riparian zones, agricultural field sites to contaminated
site studies, wherever an improved understanding of biogeochemical
turnover processes is necessary. Keywords: Multilevel well, capillary
fringe, groundwater, riparian zone, nitrogen, heterogeneity