| 11. |
- Giese, Markus, 1985, et al.
(author)
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An approximation of inner boundary conditions for wells intersecting highly conductive structures
- 2020
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In: Ground Water. - : Wiley. - 0017-467X. ; 58:4, s. 611-621
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Journal article (peer-reviewed)abstract
- Inner boundary conditions describe the interaction of groundwater wells with the surrounding aquifer during pumping and are associated with well‐skin damage that limits water production and water derived from wellbore storage. Pumping test evaluations of wells during immediate and early‐time flow require assignment of inner boundary conditions. Originally, these concepts were developed for vertical well screens, and later transferred to wellbores intersecting highly conductive structures, such as preferential flow zones in fractured and karstic systems. Conceptual models for pumping test analysis in complex bedrock geology are often simplified. Classic analytical solutions generally lump or ignore conditions that limit or enhance well productivity along the well screen at the onset of pumping. Numerical solutions can represent well drawdowns in complex geological settings, such as karst systems, more precisely than many analytical solutions by accounting for additional physical processes and avoiding assumptions and simplifications. Suitable numerical tools for flow simulations in karst are discrete pipe‐continuum models that account for various physical processes such as the transient hydraulics of wellbores intersecting highly conductive structures during pumping.
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| 12. |
- Giese, Markus, 1985, et al.
(author)
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Review: Saltwater intrusion in fractured crystalline bedrock
- 2021
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In: Hydrogeology Journal. - : Springer Science and Business Media LLC. - 1431-2174 .- 1435-0157. ; 29, s. 2313-2328
-
Research review (peer-reviewed)abstract
- During the past few years, the number of regional and national assessments of groundwater quality in regard to saltwater intrusion in coastal aquifers has increased steadily. However, most of the international literature on saltwater intrusion is focused on coastal plains with aquifers in unconsolidated material. Case studies, modelling approaches and parameter studies dealing with saltwater intrusion in those systems are abundant. While the hydrogeology of fractured rock has been intensively studied with both modelling approaches and parameter studies—mainly in relation to deep-laying fractured crystalline bedrock as potential waste repositories—case studies on saltwater intrusion in shallow fractured rocks are still an exception. This review summarizes the actual knowledge on saltwater intrusion in fractured crystalline rock. In combination with short overviews of the processes of saltwater intrusion, flow in fractured systems and the genesis of these systems, the review highlights the importance of the fracture systems and its specific characteristics. Fracture properties are a direct consequence of the geological history as well as the current situation of the coastal area. A holistic assessment of water quality in coastal areas hosting fractured crystalline bedrock therefore requires the combination of different approaches in order to investigate the impact of saltwater intrusion through the fractured system.
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| 13. |
- Giese, Markus, 1985, et al.
(author)
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Turbulent and Laminar Flow in Karst Conduits Under Unsteady Flow Conditions: Interpretation of Pumping Tests by Discrete Conduit-Continuum Modeling
- 2018
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In: Water Resources Research. - 0043-1397 .- 1944-7973. ; 54:3, s. 1918-1933
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Journal article (peer-reviewed)abstract
- Due to the duality in terms of (1) the groundwater flow field and (2) the discharge conditions, flow patterns of karst aquifer systems are complex. Estimated aquifer parameters may differ by several orders of magnitude from local (borehole) to regional (catchment) scale because of the large contrast in hydraulic parameters between matrix and conduit, their heterogeneity and anisotropy. One approach to deal with the scale effect problem in the estimation of hydraulic parameters of karst aquifers is the application of large-scale experiments such as long-term high-abstraction conduit pumping tests, stimulating measurable groundwater drawdown in both, the karst conduit system as well as the fractured matrix. The numerical discrete conduit-continuum modeling approach MODFLOW-2005 Conduit Flow Process Mode 1 (CFPM1) is employed to simulate laminar and nonlaminar conduit flow, induced by large-scale experiments, in combination with Darcian matrix flow. Effects of large-scale experiments were simulated for idealized settings. Subsequently, diagnostic plots and analyses of different fluxes are applied to interpret differences in the simulated conduit drawdown and general flow patterns. The main focus is set on the question to which extent different conduit flow regimes will affect the drawdown in conduit and matrix depending on the hydraulic properties of the conduit system, i.e., conduit diameter and relative roughness. In this context, CFPM1 is applied to investigate the importance of considering turbulent conditions for the simulation of karst conduit flow. This work quantifies the relative error that results from assuming laminar conduit flow for the interpretation of a synthetic large-scale pumping test in karst.
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| 14. |
- Haaf, Ezra, 1985, et al.
(author)
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Data-driven Estimation of Groundwater Level Time-Series Using Comparative Regional Analysis
- 2022
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Journal article (other academic/artistic)abstract
- A new method is presented to efficiently estimate daily groundwater level time series at unmonitored sites by linking groundwater dynamics to local hydrogeological system controls. The presented approach is based on the concept of comparative regional analysis, an approach widely used in surface water hydrology, but uncommon in hydrogeology. The method uses regression analysis to estimate cumulative frequency distributions of groundwater levels (groundwater head duration curves (HDC)) at unmonitored locations using physiographic and climatic site descriptors. The HDC is then used to construct a groundwater hydrograph using time series from distance-weighted neighboring monitored (donor) locations. For estimating times series at unmonitored sites, in essence, spatio-temporal interpolation, stepwise multiple linear regression, extreme gradient boosting, and nearest neighbors are compared. The methods were applied to ten-year daily groundwater level time series at 157 sites in alluvial unconfined aquifers in Southern Germany. Models of HDCs were physically plausible and showed that physiographic and climatic controls on groundwater level fluctuations are nonlinear and dynamic, varying in significance from “wet” to “dry” aquifer conditions. Extreme gradient boosting yielded a significantly higher predictive skill than nearest neighbor and multiple linear regression. However, donor site selection is of key importance. The study presents a novel approach for regionalization and infilling of groundwater level time series that also aids conceptual understanding of controls on groundwater dynamics, both central tasks for water resources managers.
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| 15. |
- Karmakar, Shyamal, et al.
(author)
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Numerical Benchmark Studies on Flow and Solute Transport in Geological Reservoirs
- 2022
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In: Water. - : MDPI AG. - 2073-4441. ; 14:8
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Journal article (peer-reviewed)abstract
- Predicting and characterising groundwater flow and solute transport in engineering and hydrogeological applications, such as dimensioning tracer experiments, rely primarily on numerical modelling techniques. During software selection for numerical modelling, the accuracy of the results, financial costs of the simulation software, and computational resources should be considered. This study evaluates numerical modelling approaches and outlines the advantages and disadvantages of several simulators in terms of predictability, temporal control, and computational efficiency conducted in a single user and single computational resource set-up. A set of well-established flow and transport modelling simulators, such as MODFLOW/MT3DMS, FEFLOW, COMSOL Multiphysics, and DuMu(X) were tested and compared. These numerical simulators are based on three numerical discretisation schemes, i.e., finite difference (FD), finite element (FE), and finite volume (FV). The influence of dispersivity, potentially an artefact of numerical modelling (numerical dispersion), was investigated in parametric studies, and results are compared with analytical solutions. At the same time, relative errors were assessed for a complex field scale example. This comparative study reveals that the FE-based simulators COMSOL and FEFLOW show higher accuracy for a specific range of dispersivities under forced gradient conditions than DuMu(X) and MODFLOW/MT3DMS. FEFLOW performs better than COMSOL in regard to computational time both in single-core and multi-core computing. Overall computational time is lowest for the FD-based simulator MODFLOW/MT3DMS while the number of mesh elements is low (here < 12,800 elements). However, for finer discretisation, FE software FEFLOW performs faster.
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| 16. |
- Lindgren, Amanda, et al.
(author)
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Acid sulfate soils and their impact on surface water quality on the Swedish west coast
- 2022
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In: Journal of Hydrology: Regional Studies. - : Elsevier BV. - 2214-5818. ; 40
-
Journal article (peer-reviewed)abstract
- Study region: Halland county, located on the Swedish west coast, with a special focus on the area around Ramsjö canal (Falkenberg municipality). Study focus: This study contributes to the understanding of the development of active acid sulfate soils and environmental consequences of leaching from these soils in an area without previous investigations. Samples taken from different soil profiles in an agricultural area and water samples from the agricultural drainage system were analysed in pH, electrical conductivity, and metal concentrations. To evaluate the on-site situation, the results of the soils sample analysis were compared to the national soil classification system and the water samples to areas in northern Europe affected by acid sulfate soils. New hydrological insights for the region: The results prove the existence of active acid sulfate soils in a coastal area in Halland. In the study region, acid sulfate soils were discovered in an agricultural area of drained wetland and peatland. Water samples taken from the drainage system and a man-made canal flowing into Kattegat Sea have low pH combined with high electrical conductivity and increased metal concentrations. This is a direct consequence of intensive leaching from the discovered acid sulfate soils in that area. Therefore, this is the first case study proofing the existence of acid sulfate soils and highlighting their environmental impact on surface water quality on the Swedish west coast (i.e. outside the Baltic Basin).
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| 17. |
- Nygren, Michelle, et al.
(author)
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Exploring groundwater drought responsiveness in lowland post‑glacial environments
- 2022
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In: Hydrogeology Journal. - : Springer Science and Business Media LLC. - 1431-2174 .- 1435-0157. ; 30, s. 1937-1961
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Journal article (peer-reviewed)abstract
- Groundwater drought response to meteorological forcing depends on initial hydrological conditions. This makes it difficult to characterise groundwater droughts and identify the drought vulnerability of aquifers. The objective is to increase the understanding of groundwater memory and response to meteorological forcing in lowland post-glacial environments. Eighty-one groundwater hydrographs are analysed, using the standardised groundwater level index (SGI) and the precipitation index. Memory and response times are assessed using auto- and cross-correlation functions. Response time is estimated by comparing two approaches: (1) the traditional use of the maximum cross-correlation, and (2) the alternative use of the cross-correlation slope. Results are interpreted for different hydrogeological settings. The analysis showed that sand aquifers have the longest memory and response times, particularly in confined settings where the memory could be over 4 years. Silts and tills have relatively short memories and response times, at less than 1 year, though median values for silt are higher than for unconfined sand aquifers. In this study, estimating response time using the correlation slope is superior at capturing the initial response time of groundwater to precipitation. However, the results showed that groundwater anomalies in lowland post-glacial environments are sometimes more influenced by climate teleconnections than concurrent forcing. This emphasises the need for a holistic approach for the characterisation and projection of groundwater drought, as it develops in simultaneous response to meteorological forcing at different timescales.
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| 18. |
- Nygren, Michelle, et al.
(author)
-
Recent trends in hydroclimate and groundwater levels in a region with seasonal frost cover
- 2021
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In: Journal of Hydrology. - : Elsevier BV. - 0022-1694. ; 602
-
Journal article (peer-reviewed)abstract
- In cold climate, the main annual recharge event is spring snowmelt. Projections of groundwater storage hence depend on winter hydroclimate. Winter recharge is a factor of rain, melt and ground frost. Increases in winter rain and melt have already been observed. However, due to complex processes involved in ground frost formation and its natural patchy coverage, the effect of ground frost on recharge is poorly understood. The objective of the study is to improve the understanding of the connection between groundwater storage and hydroclimate in Sweden and Finland, a region with temperate and cold climate, from a seasonal frost cover perspective. In the study, annual, frost-free season and frost season temperature and precipitation trends in the climate regions are compared between one global and two regional reanalysis datasets. Effective precipitation and wet day frequency trends between 1980 and 2010 are compared to groundwater level trends for the same period. Trends are calculated using the Mann-Kendall test and Sen’s slope. The results show that the region is generally experiencing warming and increased precipitation. These trends are strongest in the frost season. Further, the global dataset systematically underestimates median temperatures and precipitation compared to the regional datasets. Effective precipitation trends are generally weak, and indicate wetting in the frost season and drying in the frost-free season. Wet day frequency trends are decreasing significantly on the regional scale in Finland, but with nearly no trends in Sweden. Groundwater levels in southern Finland and southeastern Sweden are declining significantly, while groundwater levels in southwestern Sweden are significantly rising. The results indicate that groundwater trends in southern Finland are linked to wet day frequency and effective precipitation trends in the frost-free season, and that trends in the frost season have no impact on annual groundwater level trends. In southern Sweden, groundwater level trends appear linked to annual and frost-free season trends in effective precipitation, though insignificantly. These trends may be explained by century-scale trends in precipitation drought indices identified by previous studies.
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| 19. |
- Rudolph, Max Gustav, et al.
(author)
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A data-driven approach for modelling karst spring discharge using transfer function noise models
- 2023
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In: Environmental Earth Sciences. - 1866-6280 .- 1866-6299. ; 82
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Journal article (peer-reviewed)abstract
- Karst aquifers are important sources of fresh water on a global scale. The hydrological modelling of karst spring discharge, however, still poses a challenge. In this study we apply a transfer function noise (TFN) model in combination with a bucket-type recharge model to simulate karst spring discharge. The application of the noise model for the residual series has the advantage that it is more consistent with assumptions for optimization such as homoscedasticity and independence. In an earlier hydrological modeling study, named Karst Modeling Challenge (KMC; Jeannin et al., J Hydrol 600:126–508, 2021), several modelling approaches were compared for the Milandre Karst System in Switzerland. This serves as a benchmark and we apply the TFN model to KMC data, subsequently comparing the results to other models. Using different data-model-combinations, the most promising data-model-combination is identified in a three-step least-squares calibration. To quantify uncertainty, the Bayesian approach of Markov-chain Monte Carlo (MCMC) sampling is subsequently used with uniform priors for the previously identified best data-model combination. The MCMC maximum likelihood solution is used to simulate spring discharge for a previously unseen testing period, indicating a superior performance compared to all other models in the KMC. It is found that the model gives a physically feasible representation of the system, which is supported by field measurements. While the TFN model simulated rising limbs and flood recession especially well, medium and baseflow conditions were not represented as accurately. The TFN approach poses a well-performing data-driven alternative to other approaches that should be considered in future studies.
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