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Groundwater Recharge in Crystalline Bedrock: Processes, Estimation, and Modelling
Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
2004 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Grundvattenbildning i kristallin berggrund : Processer, uppskattning och modellering (Swedish)
Abstract [sv]

Kunskap om grundvattenbildningen är nödvändig för att man ska kunna förutsäga konsekvenserna av grundvattenuttag och underjordsbyggande. Grundvattenbildningen i berggrunden är dock svår att uppskatta. Syftet med avhandlingen var att öka förståelsen av grundvattenbildningen i kristallin berggrund, att undersöka hur grundvattenbildningen ska uppskattas samt att utveckla nya modeller för att beskriva grundvattenbildningen. Studien grundades på tre angreppssätt: grundvattendatering med freoner (CFC), geohydrauliska observationer och matematisk modellering.

Koncentrationerna av CFC-11 och CFC-113 befanns vara låga i det undersökta berggrundvattnet, vilket i kombination med låga syrgashalter tyder på anaerob nedbrytning. Koncentrationerna av CFC-12 och tritium överensstämde ganska väl, vilket betyder att den skenbara åldern kan vara den sanna åldern. Resultaten tyder på att CFC-datering inte är pålitlig i skogsmiljöer med finkornigt jordtäcke.

Vid nederbörd observerades ett snabbt gensvar i den hydrauliska potentialen i den studerade bergakviferen, trots det 10 m mäktiga moräntäcket. En avsevärd del av de observerade potentialvariationerna befanns vara belastningseffekter, som inte innebar någon magasinsförändring eller något vattenflöde. Berggrundens belastningseffektivitet uppskattades, ur potentialens svar på lufttrycksförändringar, till 0,95. Ytbelastningen beräknades från mätningar av lufttryck, vatten i jordtäcket och snö. Omkring 20 % av årstidsvariationen hos den hydrauliska potentialen uppskattades bero på enbart belastningsförändringar. En enkel begreppsmässig modell kunde användas för att simulera den observerade hydrauliska potentialen. För att beskriva enskilda grundvattenbildningstillfällen på bästa sätt var det nödvändigt att ta hänsyn till effekten av ytbelastningen.

Numeriska experiment gjordes med en modell av en jord–bergprofil. När berget modellerades som ett heterogent kontinuum bildades omättade zoner i berget vid stora hydrauliska gradienter. Fenomenet uppträdde i områden där låggenomsläppliga zoner låg uppströms höggenomsläppliga zoner, och ledde till minskad hydraulisk konduktivitet i berget.

Abstract [en]

Knowledge about the groundwater recharge is essential for the prediction of impacts of groundwater withdrawal and underground construction. Recharge in the bedrock is, however, difficult to estimate. The objectives of this thesis were to increase the understanding of groundwater recharge in crystalline bedrock, to investigate how the recharge could be estimated, and to develop new models to describe the recharge. The study was based on three approaches: groundwater dating using chlorofluorocarbons (CFCs), geohydraulic field measurements, and mathematical modelling.

Low concentrations of CFC-11 and CFC-113 were found in the bedrock groundwater, which in combination with low dissolved-oxygen levels indicated anaerobe degradation. The CFC-12 and tritium concentrations agreed fairly well, which means that apparent ages could be true ages. The results suggest that CFC dating may not be reliable at forested, humid sites covered by fine-grained soil.

A quick response in hydraulic head to precipitation was observed in the studied bedrock, despite the 10-m thick till cover. A substantial portion of observed head variations was found to be loading effects, involving no storage changes or water flow. The loading efficiency of the bedrock was estimated, from the air-pressure response, to be 0.95. The surface loading was calculated from measurements of air pressure, water in the soil, and snow. About 20% of the seasonal variation of the hydraulic head was estimated to be related to loading changes only. A simple conceptual model could be used to simulate the observed hydraulic heads. The loading effect had to be included to properly describe individual recharge events.

Numerical experiments were performed with a soil–bedrock profile. When the rock was modelled as a heterogeneous continuum, unsaturated zones developed at high hydraulic gradients. The phenomenon appeared in areas where low-conductive zones were located upstream of high-conductive zones, decreasing the effective hydraulic conductivity of the material.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2004. , p. 48
Series
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1104-232X ; 1019
Keywords [en]
Hydrology, groundwater recharge, bedrock, crystalline rocks, tracers, CFC, surface loading, loading efficiency, barometric effect, heterogeneity, stochastic modelling
Keywords [sv]
Hydrologi
National Category
Oceanography, Hydrology and Water Resources
Identifiers
URN: urn:nbn:se:uu:diva-4573ISBN: 91-554-6051-8 (print)OAI: oai:DiVA.org:uu-4573DiVA, id: diva2:165174
Public defence
2004-10-22, Hambergsalen, Geocentrum, Villav. 16, Uppsala, 10:00
Opponent
Supervisors
Available from: 2004-09-27 Created: 2004-09-27 Last updated: 2018-01-13Bibliographically approved
List of papers
1. Accuracy of CFC groundwater dating in a crystalline bedrock aquifer: Data from a site in southern Sweden
Open this publication in new window or tab >>Accuracy of CFC groundwater dating in a crystalline bedrock aquifer: Data from a site in southern Sweden
2004 In: Hydrogeology Journal, ISSN 1431-2174, Vol. 12, no 2, p. 171-183Article in journal (Refereed) Published
Identifiers
urn:nbn:se:uu:diva-92186 (URN)
Available from: 2004-09-27 Created: 2004-09-27Bibliographically approved
2. Role of rock heterogeneity on lateral diversion of water flow at the soil–rock interface
Open this publication in new window or tab >>Role of rock heterogeneity on lateral diversion of water flow at the soil–rock interface
2004 In: Vadose Zone Journal, ISSN 1539-1663, Vol. 3, no 3, p. 786-795Article in journal (Refereed) Published
Abstract
Identifiers
urn:nbn:se:uu:diva-92187 (URN)
Available from: 2004-09-27 Created: 2004-09-27 Last updated: 2017-09-21Bibliographically approved
3. Surface loading effects on groundwater pressure in a crystalline bedrock aquifer
Open this publication in new window or tab >>Surface loading effects on groundwater pressure in a crystalline bedrock aquifer
In: Hydrogeology Journal, ISSN 1431-2174Article in journal (Refereed) Submitted
Identifiers
urn:nbn:se:uu:diva-92188 (URN)
Available from: 2004-09-27 Created: 2004-09-27Bibliographically approved
4. Groundwater recharge in a hard rock aquifer: A conceptual model including surface-loading effects
Open this publication in new window or tab >>Groundwater recharge in a hard rock aquifer: A conceptual model including surface-loading effects
2006 (English)In: Journal of Hydrology, ISSN 0022-1694, E-ISSN 1879-2707, Vol. 330, no 3-4, p. 389-401Article in journal (Refereed) Published
Abstract [en]

The groundwater level in a fractured rock aquifer in Sweden was found to respond quickly to rainfall, although the bedrock was covered by 10-m-thick tilt soil. A considerable portion of the response was caused by surface loading, i.e., by the weight increase of the soil due to the addition of water from precipitation, whereas the rest reflected recharge. The hypothesis that the bedrock aquifer was recharged by vertical flow from groundwater in the overlying soil was tested with a simple recharge model, in which the bedrock-groundwater levels were simulated from the soil-groundwater and estimated surface-loading variation. The model had three parameters: the ratio between the equivalent vertical hydraulic conductivity governing the recharge and the storage coefficient of the bedrock reservoir, the recession coefficient for the bedrock-groundwater level, and the bedrock-groundwater level at which the outflow ceases. The model could be reasonably well calibrated and validated to head observations in one of two boreholes. The fit to the seasonal variation was similar when calibrating the model with or without surface loading, but surface loading had to be included to property simulate individual recharge events. The relative temporal variation in the fluxes could be determined by the calibration. The variation in the recharge was from -10% to +25% in relation to the mean flux. The variation in the discharge was only 1%. By applying a storage coefficient of the reservoir of 5 x 10(-4), the simulated mean recharge was about 20 mm yr(-1). The results support the hypothesis that the bedrock-groundwater at the site is fed by local recharge from the overlying soil aquifer.

Keywords
ground water, recharge, bedrock, theoretical models, surface loading, Sweden
National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-92189 (URN)10.1016/j.jhydrol.2006.03.032 (DOI)000242023700001 ()
Available from: 2004-09-27 Created: 2004-09-27 Last updated: 2017-12-14Bibliographically approved

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