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High resolution monitoring above and below the groundwater table uncovers small – scale hydrochemical gradients Niklas Gassen, Christian Griebler, Ulrike Werban, Nico Trauth, and Christine Stumpp Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03087 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017
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High resolution monitoring above and below the groundwater table uncovers small –
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scale hydrochemical gradients
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Gassen, N.1; Griebler, C. 1; Werban, U.2; Trauth, N.3, Stumpp, C. 1,*)
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for Environmental Health (GmbH), Ingolstaedter Landstr. 1, 85764 Neuherberg, Germany
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Research – UFZ, Permoserstr. 15, 04318 Leipzig, Germany
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Institute of Groundwater Ecology, Helmholtz Zentrum München, German Research Center
Department Monitoring and Exploration Technologies, Helmholtz Center for Environmental
Department of Hydrogeology, Helmholtz Center for Environmental Research – UFZ,
Permoserstr. 15, 04318 Leipzig, Germany
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*) Corresponding author:
[email protected]; tel. +49 89 31872084; fax: +49 89 31873361
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Abstract
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Hydrochemical solute concentrations in the shallow subsurface can be spatially highly
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variable within small scales, particularly at interfaces. However, most monitoring systems fail
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to capture these small scale variations. Within this study, we developed a high resolution
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multi-level well (HR-MLW) with which we monitored water across the interface of the
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unsaturated and saturated zone with a vertical resolution of 0.05-0.5 m. We installed three of
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these 4 m deep HR-MLWs in the riparian zone of a 3rd-order river and analyzed for 1
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hydrochemical parameters and stable water isotopes. The results showed three distinct vertical
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zones (unsaturated zone, upper saturated zone, lower saturated zone) within the alluvial
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aquifer. A 2 m thick layer influenced by river water (upper saturated zone) was not captured
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by existing monitoring wells with higher screen length. Hydrochemical data (isotopes, total
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ions) were consistent in all HR-MLWs and showed similar variation over time emphasizing
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the reliability of the installed monitoring system. Further, the depths zones were also reflected
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in the NO3-N concentrations; with high spatial variabilities between the 3 wells. The zonation
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was constant over time, with seasonal variability in the upper saturated zone due to the
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influence of river water. This study highlights the use of high resolution monitoring for
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identifying the spatial and temporal variability of hydrochemical parameters present in many
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aquifer systems. Possible applications range from riparian zones, agricultural field sites to
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contaminated site studies, wherever an improved understanding of biogeochemical turnover
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processes is necessary. Keywords: Multi-level well, capillary fringe, groundwater, riparian
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zone, nitrogen, heterogeneity
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Introduction
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Concentrations of nutrients and contaminants in both the unsaturated and the saturated zone of
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the shallow subsurface are highly variable in space and time. Temporally irregular and
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heterogeneous input pathways, e.g. due to preferential flow paths caused by sediment
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heterogeneities
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different residence times along various flow paths3,
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solute concentrations. In addition, reaction rates are dependent on multiple parameters on
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different scales or vary over time and along individual flow paths. Particularly,
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biogeochemical transformation processes take place at small scales and only where reactants
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, contributions from several sources (e.g., rainwater, river water) and 4
can lead to spatially heterogeneous
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and respective microorganisms meet 5. Here, interfaces between compartments are of
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particular interest because high reaction rates have often been observed where conditions
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change as flow paths meet
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limitations can be overcome and reactions rates are disproportionally high 8. Such an intra-
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compartment interface can be found between the unsaturated and the saturated zone9, 10, where
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often carbon and oxygen rich soil water meets groundwater with a different composition.
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These differences can lead to steep hydrochemical gradients across the interface at the
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unsaturated and saturated zone
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microbial activities pose a challenge when it comes to detailed understanding of
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ecohydrological processes, monitoring approaches and modelling of natural systems. Often,
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only integrated rates of nutrient cycling and transformation processes in catchments or sub-
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catchments can be inferred from available monitoring systems
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calculating overall budgets of elemental cycling, but lacks a detailed understanding of when
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and where processes take place. By simply integrating processes over time and space, we
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might miss important details, which may explain catchment responses to rainfall events13 and
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which can help to enhance or use specific ecosystem functions or turnover processes (e.g.
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nitrate removal) in catchments. A crucial step towards a higher understanding of
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spatiotemporal processes is to increase the resolution of existing monitoring systems
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which can be beneficial for contaminated site studies16 as well as for nitrate removal studies20.
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Existing high resolution monitoring systems have not focused on the unsaturated – saturated
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interface; simply because most systems target only one of both zones21, 22.
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In order to monitor solute concentrations across the groundwater table with a high resolution,
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a combined approach including available monitoring systems for the saturated and the
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unsaturated zone is urgently needed. To date, multi-level sampling in the saturated zone is
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. By bringing solutes of different origin together, substrate
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. The heterogeneous patterns of solute concentrations and
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. This is sufficient for
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often achieved with nested systems 23-25. However, when it is required to sample many depth,
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nested systems are limited as each depth requires a separate piezometer tube, increasing
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uncertainty arising from the lateral distance between the tubes
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natural water flow by the multiple tubes. It is apparent that custom made designs are
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necessary to get high resolution depth profiles at the centimeter scale. Few studies applied and
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adapted an earlier proposed system27, connecting small sampling ports via steel capillaries
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with the surface16,
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applied cheap and easy to construct multi-level samplers in the hyporheic zone26. However,
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all of these systems only allow sampling in the saturated zone. Water sampling in the
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unsaturated zone is more complex. Here, water has to be actively withdrawn and samples can
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be taken by the use of suction cup sampling
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different depths, either vertically from the surface or slanted in different angles in order to
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capture the vertical flow in the unsaturated zone
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unsaturated zone is limited by the rather large size of the suction cups which are necessary to
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extract the small amount of water available. Furthermore, installation of suction cups in
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greater depth requires drilling holes with the same diameter as the suction cup in order to
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avoid preferential flow along the drilling hole and to guarantee hydrologic contact between
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the suction cup and the soil. Therefore, it takes a lot of effort to permanently install such
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systems in vertical profiles for studying processes along flow paths.
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Only a few systems enabling water sampling in the unsaturated and saturated zone have been
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reported, which had maximum resolutions of 0.09 m 33, 34. An even higher resolution would be
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necessary, though, to capture hydrochemical gradients and processes which take place at the
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pore scale and which are limited by mass transfer processes 16; particular when it comes about
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identifying nonlinear, dynamic ecohydrological processes at interfaces like the unsaturated -
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and potential disturbance of
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, allowing maximum sampling resolutions of 0.03 m. Other studies
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. In most cases, suction cups are installed at
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. Depth specific resolution in the
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saturated zone18. To date, multi-level wells have been mainly designed to sample water from
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individual aquifer layers and it remains a challenge to sample water across the groundwater
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table at high resolution.
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Therefore, we introduce a novel sampling device and present first results from three high
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resolution
multi-level
observation
wells
(HR-MLW),
demonstrating
small-scale
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hydrochemical gradients across the water table at 0.05 m resolution. Our system provides
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impressive insights into small-scale variations of solutes and their temporal response at the
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interface of the unsaturated and saturated zone.
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Material and Methods
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High-resolution multi-level well construction and installation
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The HR-MLW extends over 4 m and contains 23 individual sampling ports, located in 0.05 m
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to 0.5 m intervals along the well casing (Figure 1). The highest spatial resolution of 0.05 m is
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given in the vicinity of the average groundwater level on site, which is hereafter referred to as
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the groundwater fluctuation zone. The lowest resolution (0.5 m) is given in the saturated zone.
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The HR-MLW was constructed from four 1 m long, 25/32 mm inner/outer diameter (ID/OD)
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high-density polyethylene (HDPE) tubes (Carl Hamm GmbH) with screw fittings for
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connecting the 1 m modules. Sampling ports were placed on the outside of the HDPE tubes
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and connected to the inside with stainless steel capillaries (Swagelok, 1 mm ID, 3 mm OD)
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and nylon hoses (1.6 mm ID, 2.8 mm OD) for glass frits and suction cups, respectively
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(Figure 2). Entry points of steel capillaries/nylon hoses were sealed with silicon glue in order
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to avoid hydraulic shortcuts between the ports. All sampling ports were connected to the
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surface through the inside of the HDPE tube. For the sampling ports in the unsaturated and the 5
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fluctuation zone, custom made ceramic suction cups (UMS Munich) with a length of 25 mm,
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a diameter of 12 mm and a mesh size of
