Displays

Quantification of infiltration processes in the vadose zone of fractured-porous media and karst systems (epikarst), especially onset and magnitude of preferential flow, as well as the interaction between fast (fractures, macropores) and slow pathways (matrix), is still lacking a sound conceptualization.

This study presents results from laboratory experiments which were designed to delineate the control of network topology, fracture aperture, matrix imbibition and infiltration conditions on preferential flow dynamics. We create vertical 2-D fracture networks using a set of equally sized (Seeberger) sandstone blocks placed in between two transparent glass plates. Blocks are arranged to create an orthogonal network with vertical and horizontal fractures of constant aperture. Water is injected with a constant rate directly into the middle vertical fracture on the upper network boundary by a pump. Mass flux across the lower network boundary is measured by a scale to register first arrival. In addition, flow partitioning at intersections and advance of the wetting front were visually captured.

Two experiment series were carried out: (1) the effect of horizontal offset (2, 5, 10, 15, 20 and 24 mm) was studied for two different fracture apertures (1 and 3 mm), but constant infiltration rate (1.5 ml/min). (2) The fracture aperture was kept constant (1 mm) and infiltration rate was varied (0.75, 1.50 and 3.00 ml/min), as well as the offset. The first series demonstrates that greater offset is associated with pathway spreading and hence divergent behavior of the wetting front, as well as later arrival times. Pathway spreading increases the fracture-matrix interface area in total, thus preferential flow is slowed down more efficiently by the imbibition process. Less pathway spreading, and faster arrival times were observed for the larger aperture configuration (3 mm). Aperture (and infiltration rate) strongly controls flow modes. Whereas slug flow (liquid in contact with both fracture walls) is a dominant flow mode in the 1 mm aperture configuration due to capillary forces, it is not the prevailing mode in the 3 mm aperture configuration. The second series reveals faster arrival times for higher inflow rates (3.00 > 1.50 > 0.75 ml/min), as well as smaller differences between arrival times of different offsets as flow rate increases. 

To capture bulk infiltration dynamics, the results can help to parameterize analytical infiltration models such as the source-responsive dual domain model, which was developed by Nimmo (2010, VADOSE ZONE J) to capture preferential flow dynamics in soils.

How to cite: Rüdiger, F., Fehnker, H., Nimmo, J. R., and Kordilla, J.: Investigating preferential flow dynamics in idealized porous fracture networks via (quasi) 2-D lab experiments – Controls of fracture and matrix properties on flow behavior, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21318, https://doi.org/10.5194/egusphere-egu2020-21318, 2020.

The presence of fractures in consolidated media allows for rapid transport of aqueous contaminants through convoluted pathways and for diffusion into the rock matrix adjacent to the fracture, which significantly complicates our ability to make transport predictions. Despite the need to predict transport in fractures over substantive distances, very few tracer experiments have been conducted at large scale (>50m) due to experimental difficulty and cost associated with such experiments.  Even where these studies have been conducted, the results have often been difficult to model accurately without the use of extra fitting parameters. The objective of this study is to improve our understanding of key transport processes in complex large-scale fracture networks in carbonate rock by simulating the results of a tracer experiment conducted at a network scale. The tracer experiment used for this study was conducted previously by injecting a conservative dye tracer into an isolated 10 m section of a well and with breakthrough in six downstream observation wells open over a similar depth range. These observation points were located at distances of up to 245 m from the injection well. Measurement of the tracer breakthrough was conducted using a downhole fluorometer, allowing for observation of the full concentration profile in each well over time. To simulate the results, a DFN approach with a control-volume finite element model is used, which allows for irregular grid blocks and maintenance of the mass balance within the simulation domain. Because of the measurement of full concentration profiles, simulating transport inside the observation wells is also a focus of this study. In order to achieve a fit between the simulated and measured data, combinations of various fracture network geometries with aperture and matrix porosity heterogeneity are examined.

How to cite: Howroyd, M. and Novakowski, K.: Interpretation of a Network-Scale Tracer Experiment in Fractured Rock, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4200, https://doi.org/10.5194/egusphere-egu2020-4200, 2020.

Discharge curves of springs are the fingerprint of aquifers. In particular, the recession of flow after strong recharge events has been widely used for aquifer characterization. While an exponential decay is often found at long time scales, the short-term behavior is less unique and widely used in the context of characterizing karst systems. Several empirical and a few physically-based models describing the short-term recession behavior were proposed.

This study investigates the flow recession behavior of aquifers with preferential flow paths with a structure according to the concept of minimum energy dissipation.
Assuming a power-law relationship between hydraulic conductivity and porosity, the subsurface flow patterns used in our model are organized towards an optimal spatial distribution of these two parameters in a way that the total energy dissipation of the flow is minimized. This leads to two-dimensional dendritic network structures similar to river networks. Starting from a steady-state initial condition with a constant recharge rate we model the decrease of discharge over time, under the assumption of a linear storage behavior.
As expected the long-term flow recession can be approximated by an exponential function. At short times, however, our model predicts a power-law behavior with exponents ranging from 0.7 to 0.9. For the most realistic scenario, a quadratic relationship between hydraulic conductivity and porosity, the power-law exponent approximates 0.8 which corresponds well to what other studies have found for suitable recession events of karst springs.

How to cite: Strüven, J. and Hergarten, S.: Flow recession behavior of dendritic subsurface flow patterns, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9191, https://doi.org/10.5194/egusphere-egu2020-9191, 2020.

How to cite: Covington, M. and Gabrovšek, F.: Modeling the impacts of cave ventilation and CO2 dynamics on speleogenesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18738, https://doi.org/10.5194/egusphere-egu2020-18738, 2020.

In 2007, Langkawi Island in Malaysia was awarded UNESCO Global Geopark status, partly due to its diverse geological setting which includes tropical karst limestones. Whilst extensive studies of the geological setting at Langkawi Island have been carried out, the hydrogeochemistry and hydrogeology of the karst systems in this region have received little attention.

At the Penarak Nature Centre on Langkawi Island, the early stages of construction for an ecotourism resort within a limestone forest are taking place. At this site, water sampling and analyses were carried out, with a team of undergraduate science students, to characterise the baseline surface water and groundwater chemistry. Samples were collected from 10 locations at the limestone forest site. As no groundwater monitoring bores were available at the site, samples were instead collected from 4 identified groundwater seepage sites, in addition to 5 surface water sites and 1 seawater site. Field measurements of pH, EC, redox potential, dissolved oxygen and temperature were obtained. Samples from each site were also analysed for δ¹⁸O, δ²H and major ions.

Through a combination of geochemical plots and multivariate data analysis (Principal Components Analysis and Cluster Analysis), relationships between the water composition from different sites were identified. The water type for all of the groundwater seepage and surface water sites was Ca-HCO₃, indicating a high degree of groundwater-surface water interaction and rock dissolution. The multivariate data analysis results also provided an indication that groundwater sites located less than 300 meters apart may be flowing through separate karst conduits. One of the groundwater seepage sites, located close to construction, was identified as having higher concentrations of magnesium, calcium and sodium when compared with all of the other groundwater seepage and surface water sites. As there is little known about the karst hydrogeology and hydrochemistry on Langkawi Island, it is not clear whether the chemical differences identified at this construction site are occurring due to local heterogeneities in the karst system or due to anthropogenic impacts.

This study has been able to provide some baseline geochemical data and preliminary insights into groundwater and surface water interactions in a limestone forest at Langkawi Island. Recommendations for further research include a program of geophysical/hydrogeological mapping of the karst system and the establishment of a water monitoring network within the limestone forest.
 

How to cite: Reading, L.: An assessment of groundwater and surface water geochemistry in a limestone forest, Langkawi Island, Malaysia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4014, https://doi.org/10.5194/egusphere-egu2020-4014, 2020.

The Kläfferquellen as the largest springs in the north of the Hochschwab karst massif are an important source of drinking water for the city of Vienna. Due to the vulnerability of karst waters, parts of the Hochschwab Massive (80 km southwest of Vienna) are a spring water protection zone. A better understanding of the karst aquifer recharge is important for the karst water protection management. The upper zone of a karst aquifer, the epikarst plays an important role for the recharge and the regulation of water flow to the vadose zone below.

Hydrological monitoring in an Alpine vadose shaft (Furtowischacht) is the basis of these analyses. In order to quantify the water storage and flow in the upper vadose zone, a Thomson-weir was installed in a small canyon at 100 m below the entrance. Since 2016, electrical conductivity (EC), temperature (T) and water level have been measured at the weir at least every 10 minutes. The discharge shows extreme fluctuations between 0.003 to 19 l/s. Salt tracer experiments indicate tracer travel times between 10 min and 3 hours. The discharge behaviour after a precipitation event can be classified with hydrograph recession analysis in quick, intermediate and slow flow.  Snow melting events show intermediate and slow responses. The water storage within the epikarst can be inferred from the following observations: (1) Reactions of T and EC after the increase of discharge vary between 10 min and 5 h. (2) During summer rain events, EC increases (after a short decrease) and remains at an elevated level for longer period of time. (3) No drying up of the cave brook after long periods of no recharge, where baseflow shows a storage capacity. Single discharge events were successfully modelled with a bucket-type rainfall-runoff model thereby describing the water flow and storage in the upper vadose zone, respectively.

How to cite: Kaminsky, E.-F., Plan, L., and Wagner, T.: Characterization of the water flow regime in the upper vadose zone, Furtowischacht (Hochschwab, Austria), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18045, https://doi.org/10.5194/egusphere-egu2020-18045, 2020.

The Yucatan state, Mexico, encloses a large karstic aquifer, which is confined near the coast. There are places where this confinement is fractured creating springs that discharge to wetlands or shallow lagoons, which are important because of the freshwater ecosystems that develop around them (locally known as Petenes), and because they provide a thermohaline equilibrium in these systems. Previous studies reported that during spring low tide, the spring becomes a sink, reversing the flow in the Peten channel connecting the lagoon to the spring. Potentially, this inversion of the flow can cause the intrusion of brackish – and sometimes hypersaline - water from the lagoon into the aquifer, which can also have ecological implications. The flow reversal was observed in two sampling campaigns and it occurred a total of 16 times. In the last sampling campaign, it was confirmed that the spring becomes a sink, with measured water velocities of about 0.37 m/s at the channel end close to the spring. Preliminary results suggest that the flow reversal is controlled by the discharge from the aquifer, the confinement of the aquifer, the geometry of the lagoon and the inlet channel, as well as the sea tides. A simple 1D analytical solution was used to explain this phenomenon, which describes the hydrodynamic interaction between the lagoon and the aquifer by solving the heat equation, which can be used to simulate flow in both systems, a confined aquifer and a highly frictional lagoon. Therefore, the same solution was used to simulate the gradients observed between the confined aquifer and the lagoon. The solution was implemented as a Python library using Fourier series, and has the advantage that it can be used with more general boundary conditions and finite length systems. Finally, we present coefficients that can be used to determine under which conditions the flow reverses at the lagoon.

How to cite: Salles, P., Pacheco-Castro, R., Canul-Macario, C., and López, J.: An open water mechanism of seawater intrusion in the coastal Yucatan aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20411, https://doi.org/10.5194/egusphere-egu2020-20411, 2020.

Planinsko Polje is a typical example of structural polje located in South Slovenia that encounters regular flooding. The floods can occur several time per year and create a more than 10 km² large temporary lake. Even if the polje surroundings are densely populated, local people have known since centuries the average level reached by the majority of the floods and avoided constructing below it. However, the occurrence of severe events with maximum water levels far above common values is still possible. While such floods are unusual, they can last for months and drastically affect the life in the polje vicinity. As recent climatic projections expect an increase of extreme meteorological events in a near future, a resulting increase of severe floods in Planinsko Polje might be foreseen. This work aims to investigate the circumstances under which extreme flooding is occurring in Planinsko Polje. The method combines an analysis of historical flood records with a detailed description of five particular high-water events that happened during the last ten years. Then, different worst-case flood scenarios are proposed and tested with a discretized numerical model. The significance of maximum water levels and flood duration arising from modelling results is discussed in light with historical records of extreme floods. Results show that very-high flooding in Planinsko Polje is the consequence of a simultaneous reduction of the ponor drainage capacity with a combination of extreme meteorological events occurring within the polje recharge areas, while the aquifer downstream is already saturated with water. The method has practical applications for people living near Planinsko Polje and can be generalized to other poljes around the world.

How to cite: Mayaud, C., Gabrovšek, F., Blatnik, M., Kogovšek, B., Petrič, M., and Ravbar, N.: Assessment of different worst-case flood scenarios for Planinsko Polje (Slovenia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4926, https://doi.org/10.5194/egusphere-egu2020-4926, 2020.

Having shown potential for long-term monitoring of terrestrial water variation, satellite data from GRACE (Gravity Recovery and Climate Experiment) and its successor GRACE-FO (Follow-on) operating from 2002 could provide a cost-effective approach to water resource management in regions with scarce ground monitoring networks or in regions where representative in-situ monitoring is difficult to ensure, such as karstic areas. One such example is Dinaric karst, a large karstic aquifer system extending from Italy through Slovenia, Croatia, Bosnia-Herzegovina, Montenegro, Serbia to Albania. There, groundwater storage variation on a regional scale is difficult to infer from existing locally scattered data.

For that purpose, GRACE Level-3 gridded mass concentration terrestrial water storage (TWS) anomaly data was used. Gridded scale factors provided at 0.5° resolution based on land-hydrology models were considered as well. Spatial variability was analysed for the area of Dinaric karst and adjacent areas in Austria and Hungary owing to the resolution of the data. For preliminary validation, GRACE derived liquid water equivalent (LWE) thickness data was compared to data from available ground measurement points.

Based on the 2004-2009 average, the temporal data variability analysed for the period of March 2002 until September 2019 (containing 163 monthly data aggregates) showed variability of 17 cm to 83 cm with the average range amounting to 47 cm in the native GRACE resolution. According to the unscaled data, the variability is 29 cm to 54 cm with a mean of 43 cm. In both cases, higher amplitudes were observed at the southern parts of Dinaric karst. Weak negative temporal trend of water storage anomalies is present in all analysed land grid cells showing the difference of less than 10 cm during the entire measurement period, while the average monthly change in total water storage is around 4 cm.

How to cite: Vidmar, I. and Brenčič, M.: Remote sensing of water storage variation in Dinaric karst, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8089, https://doi.org/10.5194/egusphere-egu2020-8089, 2020.

Large scale modelling of fractured reservoirs is a persistent problem in representing fluid flow in the subsurface. Considering a geothermal energy prospect beneath the Drenthe Aa area, we demonstrate application of a recently developed approach to efficiently predict fracture network geometry across an area of several square kilometres.

Using a strain based method to mechanically model fracture nucleation and propagation, we generate a discretely modelled fracture network consisting of individual failure planes, opening parallel and perpendicular to the orientation of maximum and minimum strain. Fracture orientation, length and interactions vary following expected trends, forming a connected fracture network featuring population statistics and size distributions comparable to outcrop examples.

Modelled fracture networks appear visually similar to natural fracture networks with spatial variation in fracture clustering and the dominance of major and minor fracture trends.

Using a network topology approach, we demonstrate that the predicted fracture network shares greater geometric similarity with natural networks. Considering fluid flow through the model, we demonstrate that hydraulic conductivity and flow anisotropy are strongly dependent on the geometric connection of fracture sets.

Modelling fracture evolution mechanically allows improved representation of geometric aspects of fracture networks to which fluid flow is particularly sensitive. This method enables rapid generation of discretely modelled fractures over large areas and extraction of suitable summary statistics for reservoir simulation. Visual similarity of the output models improves our ability to compare between our model and natural analogues to consider model validation.

How to cite: Oldfield, S., Lüthje, M., Welch, M., and Smit, F.: Modelling large-scale fractured reservoirs efficiently for geothermal energy and groundwater flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9278, https://doi.org/10.5194/egusphere-egu2020-9278, 2020.

Recently, the technology of 3D-printing has been applied to literally look into flow and transport processes in fractures and fracture system using transparent material for printing. Preparing some own experiments with printed fractures it was noticed, though, that very little is known about dimensional accuracy of printed components. There are indications that fractures, as a rule of thumb, require an aperture of at least ten times of the printing resolution that is claimed by the manufacturer in order to be water conducting at all. Seemingly little attention has also been paid to the roughness of the printed fracture surfaces that would affect flow in an allegedly smooth plane fracture. Furthermore, the majority of resins  that are used for 3D-printing take up water which leads to swelling of the printed components. This has an up to now unknown influence on fracture aperture.

For these reasons a list of simple tests has been developed to check and compare the geometrical and physical properties of printed components right after production and under subsequent influence of water. Varied were printers, materials, sample orientations, sample geometries and postprocessing parameters like curing time. The dimensions of dry and wet samples were meticulously measured. Exemplarily on one sample, also the water uptake dynamics under the influence of water vapour under different degrees of saturation have been determined. A further test concerned the penetration depth of water.

The results form a little data base. Sample geometries and testing methods have been kept simple deliberately to allow for a meaningful comparison, not only for ourselves but also for other potential testing parties that might want to contribute data in exchange for the whole data base.

The major general conclusion that can be drawn from the tests is that contact time of 3D-prints with water should be minimised if dimensional accuracy is of importance. Tests concerning fracture flow in printed samples thus need to be performed as quickly as possible.

How to cite: Kröhn, K.-P. and Kröhn, M.: Producing fractures with a 3D-printer for flow experiments – the price to pay for the easy way out, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13702, https://doi.org/10.5194/egusphere-egu2020-13702, 2020.

Mid to high-altitude karst terrains in NW Romania are the sole sources of water for the numerous rural communities that dot the area. Due to the combination between the 1) specific hydrologic functioning of karst systems, 2) contamination from agricultural activities and 3) lack of waste water treatment plants, the water used for household purposes is subject to potentially high levels of pollution, with adverse effects on health and general well-being. Water flow through fractured systems is generally thought to be less effective in removing contaminants than flow through porous media, but a limited set of studies have shown that despite the lack of mechanical and chemical cleaning of water, microbial activity could lead to attenuation of pollution. To test this hypothesis, we have devised a two stage-approach in which we first targeted the hydrological functioning of karst systems and than analyzed the potential for microbial-mediated degradation of contaminants. We have investigated five hydrokarst systems (HKS, defined as systems with a point input given by either a ponor or cave and an outlet through springs or caves) in the Apuseni Mountains (Romania, East-Central Europe), by collecting monthly samples of water for three years and analyzing their physical, chemical and microbial characteristics. We present here the results of the stable isotope, physical and chemical analyses of water and discuss the sources of water feeding the HKSs and the transfer times and nature of flow, in order to make inferences on the connectivity between the different components of the HKSs. The stable isotope data indicates a rapid transfer of water through the conduits, with low underground retention times. Chemical and stable isotope data reveals a high degree of mixing between waters that enters the karst systems through pint inputs with diffuse infiltration. The different systems behave differently in terms of interaction with the host rock, in three (of the five) cases limestone being dissolved while in two cases no apparent dissolution takes place. On this hydrologic background, microbial analysis indicates an overall downstream decrease of microorganism diversity, potentially suggesting a natural attenuation of pollution.

Acknowledgments. This work was supported by a grant of Ministry of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4-ID-PCCF-2016-0016 (DARKFOOD), within PNCDI III. The research leading to these results has received funding from the EEA Grants 2014-2021, under Project contract no. 4/2019 (GROUNDWATERISK).

How to cite: Perşoiu, A. and Brad, T.: Dynamics of hydrological process in karst systems - stable isotope and geochemical approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15255, https://doi.org/10.5194/egusphere-egu2020-15255, 2020.

Three main aquifer systems developed on Kythira Island (Greece) include (Pagounis, 1981; Pagounis & Gertsos, 1984, Danamos, 1991; Koumantakis et al., 2006; Filis et al., 2019):

• The porous aquifer system in Neogene and Quaternary formations.
• The karst aquifer system in the carbonate formations of the Pindos and Tripolis Units.
• The aquifer system (both shallow and deep) in the fractured hard rocks mainly of the Phyllites – Quartzites Unit.

The main discharge of the aquifer systems takes place in coastal and submarine brackish springs around the island, except for its northern part where the Phyllites – Quartzites Unit outcrops and its central part where springs of small capacity discharge the carbonate formations of the Pindos Unit.

Precipitation is the direct recharge of the three aforementioned aquifer systems while indirectly lateral discharge occurs in places between adjacent and tangential aquifer systems and from the streams runoff as well.

In the area of Mylopotamos village four springs discharge the karst aquifer of the Pindos Unit within the channel of Kako Laghadi stream forming downstream the known “Neraida or Fonissa waterfall”. Moreover, along the dell of Kako Laghadi stream 22 watermills were built, among the plane trees and the ivy.

The most significant of the aforementioned springs is the Kamari spring (+282.28 meters a.s.l.) which emerge at the thrust fault between the overlying permeable carbonates and the underlying impermeable flysch formation of the Pindos Unit. The discharge of the Kamari spring presents annual fluctuation which varies from app. 45-50 m³/h (during winter) to total recession (during summer), due to restriction of the precipitation and the prolonged drought and overpumping of its recharge area mainly with boreholes.

The inactive municipal borehole of Mylopotamos village (+299.15 meters a.s.l.) is located app. 310 meters SSE of the Kamari spring within its recharge area (karst aquifer of the Pindos Unit). This borehole of a total depth of 40 meters penetrates carbonates of the Pindos Unit which thickness exceeds 100 meters in that area. Monthly measurements of the Kamari spring discharge and the water table head in the inactive borehole demonstrate clear and direct hydraulic correlation between them. The Kamari spring presents outflow only in the case when the water level head of the borehole exceeds +282.28 meters. This means that the water level head in the borehole should not exceed 16.87 meters from the earth surface. Taking into account all the aforementioned, the Kamari spring is designated as an overflow spring.

Finally, microbiological analysis from the Kamari spring showed qualitative degradation, due to human activities in the wider area (Pagounis, 1981; Filis et al., 2019).

How to cite: Karalemas, N., Filis, C., Skourtsos, E., Kranis, H., Lozios, S., Antoniou, V., Diakakis, M., Mavroulis, S., Danamos, G., Vassilakis, E., Mallinis, G., and Lekkas, E.: Karst springs in small islands: The Kamari spring (Mylopotamos) in Kythira Island, Greece, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21380, https://doi.org/10.5194/egusphere-egu2020-21380, 2020.

Fractured or karstified carbonate rocks constitute major drinking water resources all over the world. Nitrate is one of the major threads for drinking water suppliers in regions with intensive agrucultural use. Field scale observations in the Upper Muschelkalk aquifer in the area “Oberes Gäu”, SW-Germany, suggest that denitrification due to oxidation of Fe^(II) or sulfide-bearing minerals might be a relevant attentuation process at least in cases of extended mean residence times (> 5-40 years).

To identify reactive minerals within the rock matrix rock samples have been taken from major facies types within the Upper Muschelkalk from outcrops, drillings and carbonate quarries. Samples have been analysed in polished thin sections using transmitted & reflected light microscopy as well as electron microscopy including energy-dispersive elemental analysis.

Reactive Fe^(II) and sulfide-bearing minearls are pyrite and marcasite as well as saddle dolomites (Fe-bearing dolomites). Concentrations of these minerals depend on facies types and amount to several weight percent iron. Porosities range from very low values (<1 %) up to 25 %.

In combination with a hydrogeological characterization, these investigations allowed to delineate reactive zones within the fractured aquifer. Denitrification within these reactive zones depends on the amount and dissolution of these minerals as well as effective diffusion in the pore space.

How to cite: Rügner, H., Blendinger, E., Osenbrück, K., Jakus, N., and Grathwohl, P.: Denitrification potential and occurrence of reactive minerals in the fractured carbonate aquifer of the Upper Muschelkalk, SW-Germany, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21562, https://doi.org/10.5194/egusphere-egu2020-21562, 2020.

Karst aquifers are exceptionally vulnerable to pollution and may be impacted by multiple contamination sources. In rural and suburban areas, human wastewater effluent from on-site domestic wastewater treatment systems (DWTSs) and agricultural sources are the most significant threats to groundwater quality. It has been estimated that around 2.8 billion people worldwide rely on DWTSs for treating domestic wastewater. As karst groundwater is a major source of drinking water for at least one-quarter of the world’s population it makes protection and management of karst aquifers extremely important. These aquifer systems are highly complex and challenging to understand, especially with regards to the fate and transport of contaminants through such systems. Thus, significant knowledge gaps exist with respect to linking contaminants with the origins of pollution and quantifying different pollution impacts on groundwater quality in karst environments.

In this paper, a novel approach for investigation of the impact of contaminants from DWTS effluent on rural karstified aquifers using a range of source-specific tracers is proposed, as it is extremely difficult to distinguish between agricultural and DWTS effluent pollution using only traditional water quality parameters or any single environmental marker. Domestic wastewater is primarily discharged from toilets, washing machines, showers and dishwashers, but even after on-site wastewater treatment processes a large number of different contaminants, including source-specific ones, can still reach the groundwater and wider environment. One example are microplastic particles which are found with other solid materials in the wastewater effluent principally due to household washing and cleaning processes. Investigations of microplastic occurrences in groundwater systems are very rare but several karst springs in the west of Ireland have been sampled during this study for quantification and identification of microplastic particles using Fourier-transform infrared spectroscopy (FTIR). Many of these particles were successfully linked to human wastewater on the basis of their physical and chemical properties and/or adsorbed/absorbed pollutants. The overall numbers of microplastics and numbers of household-derived microplastic particles were linked to other well-known indicators of human contamination such as fluorescent whitening compounds (FWCs) and specific anion ratio signatures (Cl:Br). Our results show a significant correlation between microplastics and detected FWC signals at different karst springs over time, which suggests the majority of found microplastic particles to be from DWTS effluent. Notably, certain limitations were found and furthermore understood in terms of the capability of Cl:Br ratio method in determining human wastewater impacts on karst groundwater. Additionally, we have found that faecal sterol and stanol concentrations, as source-specific faecal markers, and their ratios can very successfully differentiate and quantify DWTS effluent pollution and agricultural faecal contamination at karst springs due to rapid and extensive transport of these contaminants particularly through the karst conduit networks. 

How to cite: Vucinic, L., O'Connell, D., Dubber, D., Behan, P., Crowley, Q., Coxon, C., and Gill, L.: The impact of on-site wastewater effluent on rural karstified aquifers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5113, https://doi.org/10.5194/egusphere-egu2020-5113, 2020.

Fractured rock aquifers are one of the most difficult aquifers to characterize due to complex geometry and fracture network. In Aravalli terranes of North Gujarat, communities depend on basement rock aquifers as the primary source of water supply. The hydrogeology of these aquifers is poorly understood and the drinking/irrigation wells are frequently placed in this area with little appreciation of the fracture systems. Increasing water demand puts stress to explore groundwater from less reliable sources of basement rocks and hence, makes it vital to identify potential hydrogeological zones. Lineament studies are commonly used for targeting groundwater bearing zones in hard rock terrane and very often ignore the other important structural settings viz. extension, transtension etc. For the present study, structural data pertaining faults and fractures have been mapped through fieldwork and Electrical resistivity imaging (ERT) technique. The key objective of the study is to correlate the structural features (extensional and transtensional settings) with geophysical profiles and to find out potential hydrogeological zones from where water can be explored economically. The study area comes under the Ambaji basin of Aravalli-Delhi fold belt which is a Proterozoic fold belt running 700-800 km in NE-SW direction and situated in NW India. The Aravalli-Delhi fold belt had undergone multiple phases of deformation. In this area, three major sets of fractures are present and are oriented largely in WNW-ESE, NE-SW, and NW-SE direction. The WNW-ESE fracture is dextral in nature which has interpreted from the displacement of fold limbs. Further, these are right lateral en-echelon normal faults where NE-SW extension has been taken place. There is another set of fracture i.e. NW-SE which is due to stretching of strike-slip fault. The ductile shear zones in the area are also parallel to the NW-SE fracture set. The shear zones are opened-up due to extension and formed potential aquifers. ERT has been carried out along and across the fractures to understand the subsurface fracture geometry. The ERT shows deep sited fractures and low resistivity values at the cross-section of WNW-ESE faults with the shear zone. This concludes a strong correlation between different structural settings with potential aquifers which could be used for pumping as well as artificial recharge sites for long term sustainability.

Keywords- Aravalli terrane, Aquifer, Extension, Fracture, ERT

How to cite: Pradhan, R. M. and Biswal, T. K.: Extension and transtension associated with strike-slip faults and its relation with aquifer potentials: a case study in Aravalli terrane (North Gujarat), India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18625, https://doi.org/10.5194/egusphere-egu2020-18625, 2020.

The waters of Treska river come from a fractured and karstified aquifer. A new highway route running over the aquifer has been designed for this area. Unfortunately, the construction of the new highway demands a series of mining blasts for the purpose of laying out the route.  Therefore, it was necessary to investigate whether the blasts could damage the aquifer and affect the water flow or its quality. To that effect, the capacity (flow) and the quality (purity, clarity of water, possible muddiness during and after trial mining) of water were monitored. A procedure had to be established for the purpose of investigating the influence of the blast waves on the rock medium in which the aquifer is embedded. The geological structure of the terrain along the route is represented by Triassic dolomite limestone that is cracked on the surface and karstified. Since our country doesn’t have any standards that cover the field of blasts and explosions, the Swiss standard (SWISS 640312:2013) was used.The main objective was  to evaluate the effects of blasting works on a cut in the zone of the Treska river spring, along the new highway route, namely subsection 2 of Kichevo – Ohrid motorway, in respect to the geological and hydrological characteristics of the terrain of the Treska river spring, define a methodology for performance of trial blasting, evaluate the effect of trial blasting, draw conclusions and provide  recommendations of measures and further activities.

Three trial blasts were performed on three trial fields according to the “far to close” principle and using  low destruction blasting technology with continuous monitoring of the velocity of blast waves as well as monitoring of the quantity and quality of the water (possible muddying) flowing from the spring.

The trial blasts were performed in sequences, on blasting fields with a radius of effect of 50 m and using quantities of explosives causing vibrations that are lower than the allowed ground vibration limit. According to the results obtained from the trial blasts, it can be concluded that the trial blasts in the vicinity of Treska river spring performed under good control and high quality monitoring of the blasting process will not have negative effect upon the spring flow regime. Also, the results obtained from the conducted geophysical surveys do not point to important changes of the physical-mechanical characteristics of the terrain structure after the trial blasts. In these zones, local disturbances as those during the referent measurements were defined. It is recommended to follow the same procedure for mining the rock medium during the construction works.

How to cite: Dojcinovski, D., Gjogjiev, I., Panovska, F., Panovski, A., Mitev, B., Gjorgjeska, I., and Stojmanovska, M.: Establishment of a Procedure for Blast Mining During Construction of New Highway in the Vicinity of Treska River Spring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18187, https://doi.org/10.5194/egusphere-egu2020-18187, 2020.

  Preferential flow is an important water infiltration phenomenon in karst regions because it can quickly transport surface water to deep soil and increases available water for underground root growth. The response of preferential flow to vegetation restoration requires urgent investigation due to the special soil structure of karst regions. In order to study the effect of vegetation restoration on water movement in karst regions, four kinds of ponded water infiltration experiments were carried out in Pinus Yunnanensis plantation forestland, secondary forestland, and natural grassland. A brilliant blue dyeing experiment was conducted to visualize the distribution of water infiltration in soil (a total of 150 stained images from vertical soil slices). Results showed that the average depth of matrix flow in natural grassland was approximately six times those in plantation and secondary forestlands. An increase in matrix flow will have a negative effect on the development of preferential flow. Water transported in preferential flow paths affects the distribution of nutrients and organic matter in the soil. However, preferential flow in grassland can promote the accumulation of available nutrients, and preferential flow in plantations can inhibit the loss of organic matter. Preferential flow in grasslands and forest plantations is less than that in native forests soils. The results of SEM showed that preferential flow increases the percolation of water in soils. The effect is that preferential flow can obstructs water uptake by the roots under low rainfall conditions, and decreases surface runoff before soil saturation under high rainfall conditions. In the process of nutrient element migration, preferential flow has a good contribution, which is conducive to the migration and accumulation of elements required for surface vegetation growth. The contribution of preferential flow needs to be considered in studies on vegetation restoration planning and land degradation. Reasonable allocation of plantation forests has a certain mitigation effect on soil erosion in Karst areas, and preferential flow under this special geomorphological type is worth studying. Preferential flow can transport nutrients to deeper soil for roots according to the data of this study. Therefore, plantation is feasible under karst landform conditions, but it is better to combine herbaceous plants in a plantation. The results could provide suggestions for the restoration of rocky desertification and the advantages or disadvantages of vegetation restoration engineering in karst areas.

How to cite: Kan, X. and Cheng, J.: Effects of Grass and Forests and the Infiltration Amount on Preferential Flow in Karst Regions of China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-47, https://doi.org/10.5194/egusphere-egu2020-47, 2020.

Numerical simulation is an effective tool for estimating the groundwater flow field in discretely fractured rocks (DFR). Unlike most numerical simulation methods that require the discretization of the model domain, boundary element method (BEM) is renowned of waiving the spatial discretization task but focusing on solving the integral form of the governing groundwater flow equation. However, for groundwater flow simulation in DFR, the solution obtained by BEM tends to have large error in the vicinity of fracture intersection. Therefore, a new numerical scheme, the green element method (GEM) is adopted in this study. GEM is built on the same mathematical background as BEM but turns the domain discretization back on as a necessary task. Using the second Green’s identity, GEM produces a general equation that applies to each grid block by integrating the governing equation. By making use of the singular characteristic of the Green’s function, GEM transforms the integral equation into a discretized system of equations with nodal head or nodal head gradient as unknowns. The cost of discretizing the model domain is compensated by the convenience of handling the heterogeneity of the medium. Conventional GEM manages the normal flux across a boundary segment by differentiating head values from 2 nodes in an individual grid block. This approximation overlooks the mechanism of normal flux as the exchange of fluid mass between grid blocks. To take this mechanism into consideration, a modified model of normal flux is proposed if the fracture plane is discretized into triangular elements. This model expresses the normal flux across a grid boundary segment in terms of the difference of head values in two grid blocks that are connected to this segment. For convenience, the head value at the centroid of a triangular element is used to calculate the normal flux. In other words, the unknowns of a triangular element are three nodal heads plus one centroidal head. Thus, the modified normal flux will be able to consider the interaction of all grid blocks that are connected to a target grid block. More importantly, the resulting global coefficient matrix is a square one and the system of equations is closed. The solution obtained from the closed system of equations will be exact but not a least-square approximated one. This modified GEM will be applied to simulate the steady state groundwater flow field in discretely fractured rocks.

How to cite: Liou, T.-S.: Numerical simulation of steady state groundwater flow field in discretely fractured rocks using the Green element method with the concept of multiple interacting normal flux, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12531, https://doi.org/10.5194/egusphere-egu2020-12531, 2020.

Due to the inhomogeneity of the carbonate rocks and discreteness of the karst water, delineation of the groundwater flow within karst area remains a challenging task as yet. Based on KunCheng tunnel of a water diversion project in KunMing, multi-scale groundwater flow models were set upto simulate the groundwater flow. Large scale model was used to obtain the boundary conditions and hydrogeological parameters, which were then assigned to the small scale model.The small scale model was generalized as an equivalent continuous medium, and two karst pipelines are established  by module River. After then,  the multi-scale numerical modelswere used to simulate the  groundwater seepage field and predict the recovery of groundwater after tunnel construction. The main results and conclusions are as follows.

(1)Black karst pipeline and white karst pipeline systems share one recharge source but have two independent discharge systems. The recharge source is the exposed karst rock in the northeast part of study area. Obstructed by aluminum clay rock of P₁d, groundwater discharge is divided into two parts during the runoff process.

(2)During the tunnel construction process, the water level at the exit of White karst pipeline reduced 9m in pipe model B₁ while reduced 10m in the solution fissure model B₂, both two models suggest that the tunnel construction will cause the drainage of White karst pipeline exit. The water level at the exit of black karst pipeline reduced 1m in pipe model B₁ while reduced 4m in the solution fissure model B₂.

(3)In model B₁, total water discharge during tunnel construction is 69876m³/d, in model B₂ , the total water discharge is 95817 m³/d  and  is much larger than model B₁ due to the quick groundwater transporting and exchange in karst pipeline..

(4)After the tunnel construction, exits of two pipelines and observation well see the water level recovery because of the formation sealing . The recovery trend is relatively rapid in the early stage, and slow in the later stage. It takes 8.5 years and 10 years for the exits of black and white pipelines and observation wells to reach the original water level, respectively. During the recovery process, groundwater exchange form was changing from pipe supplying aquifer to aquifer supplying pipe, which made model B₂ recovered faster than model B₁ in early stage, and vice versa.

Using large scale model combining with secondary scale model, and the module River to generalize karst pipeline can reflect the flow dynamic characteristics of karst pipeline effectively.

How to cite: Xu, M., Qi, J., Tang, Y., Li, X., and Guo, J.: Multi-Scale Numerical Model for Groundwater flow Simulation of a Karst Tunnel in Kunming,China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15112, https://doi.org/10.5194/egusphere-egu2020-15112, 2020.

Statistical analysis of development and distribution of fault fissures at 3720 m in the first mining region of Pulang Copper Mine, SW China was performed. Strongly heterogeneous hydro-geological numerical models influenced by fault fissures were set up. Effects of developmental distribution cracks on solute transport in 65° and 307°directions were simulated. Rock mass with strong permeability water-conducting fissure was found favorable for solute transport, water-blocking fault was found to inhibit solute transport. The direction of fracture development was found consistent with direction of groundwater flow, which would be beneficial for solute transport along water-flow direction. When direction of fracture development intersects with that of groundwater flow and the strong lens affected by fracture changes direction of groundwater flow, direction of solute transport is changed.

How to cite: Han, K., Zuo, R., He, Z., and Li, Q.: Impact of fault fracture development and distribution regularity on groundwater flow and solute transport in rock mass as exemplified at 3720 m of the first mining region of Pulang Copper Mine, SW China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12234, https://doi.org/10.5194/egusphere-egu2020-12234, 2020.

It has been recognized that karst aquifers are a unique hydrogeologic system because of strong heterogeneity originating from their complexity of the karst medium. Most karst aquifers have large ranges of flow distributions due to various types of porosity such as matrix, connected fractures, faults, and conduits developed from dissolution. In this study, we measured groundwater velocities in the Floridan aquifer within the springshed of Silver Springs, FL. Groundwater velocities were measured in situ using passive flux meters (PFMs) and karstic borehole dilution (KBHD). Based on these data and previous tracer tests, we estimated rock matrix and conduit velocities of 0.06 ± 0.02 m/day and 3.05 ± 8.1 m/day. These data were coupled with simple analytical solutions to identify the proportion of the aquifer that contributes most significantly to water flow into the spring with two different modeling scenarios: single domain and dual-domain including matrix and non-matrix zones. The dual-domain scenario suggest that matrix flow contribute approximately 4% of the total flow while non-matrix flow through conduits and fractures contribute approximately 96% of the total flow, even though the non-matrix zones account for only 5.2% of the total aquifer cross-sectional area, within approximately 3 km from the spring outlet (with the upstream capture zone representing approximately 1% of the springshed area (2300 km). The results offer field-measured hydrogeologic data that can be used for active resource management in the springshed and the simple modeling approach presented in this study might be applicable to other springs to estimate the portion of water flows and solute pathways to the spring outlet.

How to cite: Park, H. and Yang, M.: Hydrogeologic investigation of groundwater velocity for matrix and conduit in a karst aquifer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21175, https://doi.org/10.5194/egusphere-egu2020-21175, 2020.