BG1.9

Overland flow (OF) and subsurface flow (SSF) are key processes that determine the streamflow response to precipitation, as well as water quality, but are affected by the land surface and soil characteristics. They can also modify the shape of our landscape. However, our understanding of the evolution of OF and SSF characteristics and the feedback mechanisms between hydrological, pedological, biological and geormorphological processes that affect OF and SSF during landscape evolution is still limited.

We used a space-for-time approach and studied 3 plots (4m x 6m each) on four different aged moraines (several decades to ~13.500 years) on the Sustenpass near the Steinglacier and in the karstic glacier foreland of the Griessfirn near Klausenpass (total of 24 plots) to determine how OF & SSF change during landscape evolution. We used artificial rainfall experiments with high rainfall intensities to determine runoff ratios, peak flow rates, timing and duration of OF & SSF. The addition of tracers (²H and NaCl) to the sprinkling water and sampling of soil water allowed us to determine the degree of mixing of the applied rainfall with water in the soil. Measurements during natural rainfall events helped to determine the impact of the rainfall volume and intensity on the runoff generation. In addition, the runoff samples and sensor-based turbidity measurements of OF provide an estimate of the erosion rates during extreme events. In order to link the differences in runoff generation with the pedological and biological characteristics of the slopes, vegetation cover, root density, soil texture, soil aggregate stability, and the saturated hydraulic conductivity (K_sat) were measured as well.

The results show that K_sat at both study areas decreases with moraine age and soil depth and is mainly driven by the increase in silt and clay content. Despite the high K_sat values, local OF occurs frequently on the youngest moraines due to the large rock and stone cover. Sediment flux and the related erosion rates are largest for the young moraines, since vegetation cover and soil aggregate stability are small. Soil and vegetation development change major OF and SSF characteristics during landscape development, such as the mixing processes and the pre-event water fraction in OF & SSF, which both increase for the older moraines. However, the rate of these changes during landscape evolution is controlled by the parent material. These results can be used to inform landscape evolution models and help us to understand processes within the critical zone during the first millennia of soil development.

How to cite: Maier, F. and van Meerveld, I.: Effects of soil and vegetation development on surface and subsurface hydrological properties and processes on moraines in the Swiss Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-830, https://doi.org/10.5194/egusphere-egu21-830, 2021.

Within the Critical Zone, the river water quality plays a key role for the related ecosystems. The impact of contaminants delivered to surface water from groundwater inputs are often neglected, while they can constitute the major loads of nutrients or pesticides in some specific river sections. In this study, we focus on a limited section of the Loire River in France, downstream Orleans city, where the increase of the river discharge cannot be attributed to the confluence of the small tributaries. Indeed, previous studies have pointed out the role of the groundwater discharge from the large Beauce aquifer located to the north of the river, mainly focusing on the quantitative aspects.

Based here on geochemical and isotopic tracers, we first confirm groundwater inputs to the Loire River and we clearly attributed those inputs to the Beauce carbonate aquifer using the relationship between ⁸⁷Sr/⁸⁶Sr and the Cl/Sr ratios. Secondly, the conservative tracers (Sr isotopes and Cl concentrations) allow assessing the groundwater contribution to the river to around 20% of the total discharge during low flow periods. This proportion is in full agreement with the previous studies based on heat budget method, where the river temperature is estimated with satellite thermal infrared images. Lalot et al. (2015) showed that the main groundwater discharge is concentrated along a 9 km transect just downstream of Orléans city with a discharge of 5.3 and 13.5 m³.s⁻¹ during summer and winter times, respectively. This is roughly in agreement with the calculations based on groundwater modelling (calculated groundwater discharge: 0.6 to 0.9 m³.s⁻¹.km⁻¹). Finally, we pointed out the quality impact of these groundwaters especially regarding nitrates. Groundwater impacts on surface water quality have recently been considered as a potential vector of surface water contamination but they are still weakly studied and quantified. Here, we show pics of nitrates concentrations that rapidly decrease in the Loire River (especially in low flow period) after the groundwaters inputs enriched in NO₃ coming from the highly anthropized Beauce aquifer because of intensive agriculture practices. The nitrate decrease in the river is probably due to a nitrate removal processes (plant/microbial uptake?). The impact of these inputs into the Loire but also into the small tributaries of the Loire River should be further investigated, especially regarding pesticides loads and fates, and their potential impact on the related ecosystems.

Lalot, E., Curie, F., Wawrzyniak, V., Baratelli, F., Schomburgk, S., Flipo, N., Piegay, H., Moatar, F., 2015. Quantification of the contribution of the Beauce groundwater aquifer to the discharge of the Loire River using thermal infrared satellite imaging. Hydrol. Earth Syst. Sci. 19, 4479–4492.

How to cite: Petelet-Giraud, E., Negrel, P., and Casanova, J.: Groundwater inputs towards surface water: quantification and impact on the river water quality using chemical and isotope fingerprints (87Sr/86Sr), example of the Loire River (France), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2968, https://doi.org/10.5194/egusphere-egu21-2968, 2021.

Collection of agricultural soil samples in Europe (0–20 cm, 33 countries, 5.6 million km²) during the GEMAS (GEochemical Mapping of Agricultural and grazing land Soil) continental-scale project allowed the study of geochemical behaviour of major elements during weathering (SiO₂, TiO₂, Al₂O₃, Fe₂O₃, MgO, CaO, Na₂O, K₂O, P₂O₅) using their total concentrations (XRF data). The chemical composition of soil represents to a large extent the primary mineralogy of the source bedrock, the effects of pre- and post-depositional weathering and element mobility, either by leaching or mineral sorting with the addition of formation of secondary products such as clays.

Bulk geochemistry is used to calculate a set of weathering indices such as chemical index of alteration CIA, reductive and oxidative mafic index of alteration MIA, the change in mass balance t (calculation relative to immobile Nb) for soil derived from silicate parent materials defined as granite, gneiss and schist at the European continental-scale. Silicate minerals of soil parent materials can be either very resistant to weathering or very soluble and export of elements in dissolved form and precipitation of secondary phases can occur at a large scale. Either way, they leave a strong chemical signature in derived soil, which can be quantified and classified with help of geochemical indices that are useful tools to evaluate chemical weathering trends. Weathering indices and gain-loss mass transfer coefficients were applied to agricultural soil to provide an insight into the weathering processes affecting three silicate parent rocks and their impact on soil development at the European scale. Distinct chemical composition and weathering patterns has been evidenced in silicate derived soil. The interpretation of geographical distribution of soil types with silicate substrate allows better understanding of soil nutritional status, metal enrichments, degradation mechanisms under various climate conditions.

How to cite: Negrel, P., Ladenberger, A., Reimann, C., Demetriades, A., Birke, M., and Sadeghi, M.: Major element geochemistry of European agricultural soil: weathering processes of silicate parent materials, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8412, https://doi.org/10.5194/egusphere-egu21-8412, 2021.

Groundwater is an important component of the Earth’s critical zone and within this ecosystem, microbial communities play an important role, interacting with, and contributing to, the geology and hydrology of these systems; understanding how microbial communities in this dynamic zone change over time is therefore crucial. However, subsurface aquatic environments are lacking high-resolution temporal data over long time periods. Based on 16S rRNA gene sequences collected monthly over a six-year period (n=230) in groundwater from fractured Triassic limestone-mudstone alternations of the Hainich Critical Zone Exploratory (central Germany), we, therefore, aimed to disentangle the temporal dynamics of bacterial communities. The bacterial communities in the shallow bedrock groundwater showed multi-annual cyclic dissimilarity patterns which corresponded to groundwater level fluctuation and, thus, recharge discharge periods. The impact of groundwater fluctuation and linked cross-stratal exchange on the groundwater microbial communities was associated with the recharge strength and local environmental selection strength. Sampling period was able to explain up to 29.5% of the variability in bacterial community composition (based on a 2-factorial PERMANOVA model). We observed an increase in dissimilarity over time (Mantel P > 0.001) indicating that the successive recharge events result in bacterial communities that are increasingly more dissimilar to the communities at the start of the sampling period. Most bacteria in the groundwater originated from the recharge-related sources (mean = 66.5%, SD = 15.1%) and specific bacterial taxa were identified as being either enriched or repressed during recharge events. Overall, we show that seasonal recharge patterns are important for shaping bacterial communities in shallow fractured-rock groundwater and act as drivers of cyclical patterns. Furthermore, the recharge events are successive shocks that perturbed the bacterial communities, leading to decreased similarity to the original state over time. These revelations highlight the importance of high temporal resolution research in the Critical Zone for investigating the complex interplay between surface/subsurface environmental dynamics and the biology of groundwater ecosystems.

How to cite: Hermans, S., Yan, L., Totsche, K. U., Lehmann, R., Herrmann, M., and Küsel, K.: Bacterial communities in shallow fractured-rock groundwater evolve over time, exhibiting cyclic patterns in response to multi-annual recharge events., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15245, https://doi.org/10.5194/egusphere-egu21-15245, 2021.

The intensity and occurrence of droughts is projected to increase due to climate change. Dried soils release high concentrations of dissolved organic matter (DOM) into subsurface waters when they are rewet, the so-called rewetting peak. To more accurately predict the role of rewetting of soils after drought on the carbon cycle in a changing climate, it is important to understand the processes behind this DOM release.

The DOM rewetting peak origin is disputed between soil organic matter (SOM) from breakdown of soil particles; accumulated root exudates; and microbial release due to a change in osmotic potential through osmolytes or cell bursting. To better understand the origin of the rewetting DOM peak, we took a rewetting series of soil water samples from different vegetation types between December 2018 and April 2019 for targeted and untargeted metabolomics. Initial results using untargeted ultrahigh-resolution mass spectrometry analysis revealed a clear temporal trend, indicating that vegetation-independent molecular changes occur following rewetting. An increase in O/C and a decrease in H/C over time was observed which is attributed to microbial decomposition, supported by a decrease in m/z over time. We also observed an increase in the content of lipidic compounds (R > 0.6) following rewetting. This indicates that cells do not burst upon rewetting and, over time, microbial activity increases, suggesting that the DOM rewetting peak is caused by a lack of decomposition, rather than a high production, of organic matter.

How to cite: Orme, A., Benk, S., Lange, M., Zerfaß, C., Pohnert, G., and Gleixner, G.: Molecular Changes in Dissolved Organic Matter After Soil Rewetting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14616, https://doi.org/10.5194/egusphere-egu21-14616, 2021.

Groundwater is a dilute, but biologically rich environment harbouring plenty microbial life [1]⁠. This microbiome requires influxes of dissolved organic matter (DOM) as energy source. Within the Hainich Critical Zone Observatory project [2,3]⁠, we have collected metabolomic information on groundwater DOM by liquid chromatography / mass spectrometry since late 2014, in an assembly of sampling wells across a 7 km transect. We analysed this long-term dataset with principal component analysis methods, inferring distance measures to translate individual sampling campaign information into long-term trajectories that can be compared against external influences (such as groundwater flow patterns).

Our data show that these sampling wells have fluctuating inter-well similarities in terms of the metabolome, giving evidence that some wells are mostly isolated entities while others may be receiving influxes from similar groundwater sources. We show that the groundwater hydraulic head fluctuations exert a key influence on similarity profiles, suggesting groundwater mixing. We emphasize that our inferred principal component measures may therefore serve as a basis to unravel the interactions and influences of groundwater flows, metabolome (biogeochemistry), and microbial life.

References

[1] Yan L, Herrmann M, Kampe B, Lehmann R, Totsche KU, Küsel K. Environmental selection shapes the formation of near-surface groundwater microbiomes, Water Res 2020, 170, 115341. DOI: 10.1016/j.watres.2019.115341.

[2] Küsel K, Totsche KU, Trumbore SE, Lehmann R, Steinhäuser C, Herrmann M. How Deep Can Surface Signals Be Traced in the Critical Zone? Merging Biodiversity with Biogeochemistry Research in a Central German Muschelkalk Landscape, Front Earth Sci 2016, 4. DOI: 10.3389/feart.2016.00032.

[3] Lehmann R, Totsche KU. Multi-directional flow dynamics shape groundwater quality in sloping bedrock strata, J Hydrol 2020, 580, 124291. DOI: 10.1016/j.jhydrol.2019.124291.

How to cite: Zerfaß, C. and Pohnert, G.: Understanding groundwater metabolome trajectories, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4526, https://doi.org/10.5194/egusphere-egu21-4526, 2021.

With surface systems changing rapidly on a global scale, it is important to understand how this will affect groundwater resources and ecosystems in the subsurface. The molecular composition of dissolved organic matter (DOM) integrates essential information on metabolic functioning and could therefore reveal changes of groundwater ecosystems in high detail. Here, we evaluate a 6-year time series of ultrahigh-resolution DOM composition analysis of groundwater from a hillslope well transect within the Hainich Critical Zone Exploratory, Germany. We predict ecosystem functionality by assigning molecular sum formulas to metabolic pathways via the KEGG database. Our data support hydrogeological characterizations of a compartmentalized fractured multi-storey aquifer system and reveal distinct metabolic functions that largely depend on the compartment’s relative surface-connectivity or isolation. We show that seasonal fluctuation of groundwater levels, coinciding with cross-stratal exchange can substantially impact the local inventory of functional metabolites in DOM. Furthermore, we find that extreme conditions of groundwater recharge following pronounced groundwater lowstand cause strong alterations of the functional metabolome in DOM even in aquifer compartments, which usually show minimal variation in DOM composition. Our findings suggest that bedrock groundwater ecosystems might be functionally vulnerable to hydrogeological extremes.

How to cite: Benk, S., Lehmann, R., Totsche, K. U., and Gleixner, G.: Using the molecular composition of dissolved organic matter in groundwater to assess the functional stability of subsurface ecosystems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16277, https://doi.org/10.5194/egusphere-egu21-16277, 2021.