Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14599号 / 工博第3067号 / 新制||工||1456(附属図書館) / 26951 / UT51-2009-D311 / 京都大学大学院工学研究科都市環境工学専攻 / (主査)教授 大西 有三, 教授 間瀬 肇, 准教授 西山 哲 / 学位規則第4条第1項該当

Populations residing on and near the world’s coasts have become increasingly dependent on coastal groundwater for their supply of freshwater. Under the conditions of predicted climate changes, the expected rise in global sea level can adversely affect the quality and quantity of freshwater resources in coastal areas as a result of saltwater intrusion. In this study, a suite of two- and three-dimensional variable-density groundwater flow models of major coastal landforms (e.g. deltas, estuaries and small islands) has been constructed to assess the effects of sea level rise (SLR), using different SLR rates of 0.5 m, 1m and 1.5 m over the next 90 years, from 2010-2100. The model results indicate that in natural coastal systems the extent of saltwater intrusion is significantly controlled by the stratigraphy of the depositional environments. Among deltaic aquifers, wave-dominated deltas are more prone to saltwater intrusion than river- and tide-dominated deltas. In case of a partially mixed, microtidal estuary, SLR can cause extensive porewater salinity increases, especially within estuarine sand deposits. Simulations of atoll and barrier islands reveal that carbonate atoll islands with high conductivity units, are severely affected by SLR, resulting in significant reduction of the volume of freshwater lens. In contrast, migrating sandy barrier islands could retain their freshwater resources with rising sea level under conditions of increased recharge, assuming the barriers can migrate in response to SLR. The freshwater lens of barrier island aquifers would reduce in size due to increased evapotranspiration caused by change in vegetation pattern. When examined for anthropogenic impacts of groundwater withdrawal through pumping, all the coastal aquifers show evidence of saltwater intrusion, with varying degrees of impact. Wave-dominated deltas are more affected by groundwater withdrawal than river- and tide-dominated deltaic aquifers. Saltwater intrusion in atoll islands is further enhanced by pumping withdrawal. It is evident from the results of the simulations that, the potential effects on coastal aquifers of groundwater withdrawals for potable water can easily exceed the adverse effects of SLR in terms of salinity increase.

In the era of intense pressures on water resources, the loss of groundwater by increased seawater intrusion (SWI), driven by climate, sea level and landscape changes, may be critical for many people living in commonly populous coastal regions. Analytical solutions have been derived here for interface flow in coastal aquifers, which allow for simple quantification of SWI under extended conditions from previously available such solutions and are suitable for first-order regional vulnerability assessment and mapping of the implications of climate- and landscape-driven change scenarios and related comparisons across various coastal world regions. Specifically, the derived solutions can account for the hydraulically significant aquifer bed slope in quantifying the toe location of a fresh-seawater sharp interface in the present assessments of vulnerability and safe exploitation of regional coastal groundwater.  Results show high nonlinearity of SWI responses to hydro-climatic and groundwater pumping changes on the landside and sea level rise on the marine side, implying thresholds, or tipping points, which, if crossed, may lead abruptly to major SWI of the aquifer. Critical limits of coastal groundwater change and exploitation have been identified and quantified in direct relation to prevailing local-regional conditions and stresses, defining a safe operating space for the human use of coastal groundwater. Generally, to control SWI, coastal aquifer management should focus on adequate fresh groundwater discharge to the sea, rather than on maintaining a certain hydraulic head at some aquifer location. First-order vulnerability assessments for regional Mediterranean aquifers of the Nile Delta Aquifer, the Israel Coastal Aquifer  and the Cyprus Akrotiri Aquifer show that in particular the first is seriously threatened by advancing seawater. Safe operating spaces determined for the latter two show that the current pumping schemes are not sustainable under declining recharge. / <p>The thesis was founded by two research programmes: NEO private-academic sector partnership and Ekoklim, a strategic governmental funding through Stockholm University</p><p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 3: Manuscript.</p><p> </p>

An areal numerical simulation has been carried out to investigate effects of below-sea-level (BSL) excavation in the raw material site of a cement factory in Turkey. A finite element model (585 nodes with 534 elements) is formed to solve for the head distribution in the quarry site upon quarry operation planned to be implemented in the near future. The model is calibrated to the field conditions and appropriate boundary conditions and the physical parameters are obtained to be used in future prediction studies. After a successful calibration the model is run to estimate the water levels and the discharge rates required during below sea level quarry operations. Above sea level (ASL) and below sea level (BSL) operations are simulated and water level contour maps are obtained both for above sea level (ASL) production for the 2000-2030 period, and for each BSL (-10m, -20m, -30m) production periods, which would totally take 13 years. Estimation show that the proposed model runs properly and it calculates the water levels and discharge rates accurately for probable future quarry operations. It is clear that quarry operations would not create a serious problem in terms of water discharge from the quarry site.

Philosophiae Doctor - PhD / The study addressed the groundwater assessment and sustainable management of the coastal alluvial aquifers in Namib Desert, the Omaruru River Delta Aquifer (Omdel Aquifer) was used as a case study. Sustainable utilization of groundwater in parts of hyper-arid Sub-Saharan Africa, like the Namib Desert, is always a challenge due to lack of resources and data. Understanding of hydrogeological characteristics of the Omaruru Delta Aquifer System is a pre-requisite for the management of groundwater supply in the Central Namib area (Namib Desert). For the Omdel Aquifer in the Omaruru catchment, Namibia, issues to investigate include the lack of information on the geology and hydrogeological setting, the hydraulic properties and geometry of the aquifer at the inflow and outflow sections, groundwater recharge conditions upstream of the aquifer, and the impact of artificial recharge. Omdel Aquifer occurs in a desert environment with less than 20 mm of rainfall per annum, it’s regarded to receive no direct groundwater recharge from rainfall, only from occasional (inconsistent) flooding of the Omaruru River, due to periodic thunderstorms in the upstream catchment. Since the Omdel Aquifer does not receive direct recharge from rainfall, an artificial recharge scheme was implemented to augment the water supply. One of the objectives of the study is to integrate artificial recharge with hydrogeological understanding of the Omdel Aquifer to establish a conceptual framework for assessment of groundwater recharge and discharge, water chemistry and balanced water supply.

>Magister Scientiae - MSc / This study is part of the current investigation of the Elandsfontein aquifer to assist with the management of the system and to ensure the protection of the associated Langebaan Lagoon RAMSAR listed site. The Elandsfontein aquifer unit is situated adjacent to the Langebaan Road aquifer in the Lower Berg River Region. The aquifer unit is bordered by the Langebaan Lagoon (west), possible boundary toward the Langebaan Road aquifer (north), the Groen River bedrock high (southeast) and the Darling batholith (south).

Water resources in hard rock terrain are difficult to characterize due to heterogeneity and anisotropy in the fracture network, low porosities and limited recharge volumes available during the summer season. Three methods were developed and evaluated in order to assist in water supply planning. A groundwater resources potential index was estimated using multivariate statistics, where physical and geological variables were classified using Analysis of Variance and Fisher's Least Significant Difference tests according to their effect on hydraulic properties. Principal component analysis was used to assign weights to the different classed variables. Classes and weights were used to produce an index referred to as groundwater resources potential (GRP), which correlated significantly with well data. Nearly 80% of the wells with less than median specific capacity values also had GRP values at those locations of zero or lower. Non-stationary variance was observed in specific capacity sub-samples taken from the Geological Survey of Sweden's well archive, despite homogeneous geology and topography. Spatial statistical analyses showed that spatial correlations were weak in well archive samples, implying that regional approximations based on sparse point data are highly error prone. Kinematic porosity estimated using superficial fracture measurements correlated significantly with well archive data. However, low correlation coefficients indicated that well data is likely not a suitable method for predicting water supply characteristics. This approach is an efficient method which shows promise in preliminary estimations of groundwater storage in heterogenic terrains. A groundwater balance model which describes seasonal groundwater storage changes was created in order to better approximate the groundwater situation often found in Swedish urbanized and semi-urbanized hard rock terrains. The model was based on a water budget approach at the pixel scale, and allows for approximation of well extraction which is not uniformly distributed in space. The model showed that in specific regions groundwater extraction may lead to severe decreases in groundwater level, where these impacts may not otherwise be expected. Dry season modelling with 10% increased evapotranspiration showed that in several areas groundwater reservoir depletion may be influenced by more than 50%. / <p>QC 20140331</p>

Vatten är en livsviktig resurs för allt liv. I Sverige kommer hälften av allt dricksvatten från grundvatten. I kustnära områden begränsas grundvattentillgången av omgivningens lagringsförmåga och begränsad tillrinningsyta och det leder till att tillgången på vatten ett växande problem. Det är därför viktigt att förstå alla parametrar som påverkar grundvattnet, både för nutida och framtida brukare. Förändringen av grundvatten i geologiskt blandad miljö är svårt att kartlägga på grund av lagerföljder, materialens heterogenitet, låg kinematisk porositet och okunskap om strömningen mellan olika lager. Grundvattennivåerna skiftar dessutom med klimatet, både över året och över längre tid. Även människans påverkan är av stor betydelse. Rapportens syfte är att se hur dessa parametrar påverkar grundvattenytan i förhållande till varandra. För att undersöka dessa parametrar har tre olika geografiska, kustnära områden jämförts: Stor, Blidö, och Insjön, Rådmansö, i Norrtälje kommun samt Klintemåla i Oskarshamns kommun. För att sätta vattenanvändningen i ett sammanhang har uppgifterna relaterats till vattenexploateringsindexet (WEI). Uppgifter har hämtats från SGU, SMHI och Lantmäteriets arkiv. I två fall har kompletterande fältundersökningar genomförts. Informationen har bearbetats i Excel och programmet GWBal. I dessa områden råder lokal akut eller total vattenbrist under vissa sommarmånader. Förekomsten av vatten är en fråga om lokal tillgång och en generellt god situation i Sverige döljer stora regionala och lokala obalanser. Vad fritidsboende gör spelar större roll än permanentboende då det är under sommarsäsongen problemen är störst. Mindre reservoarer är känsligare när parametrar för klimat, vattenförbrukning och kemisk sammansättning förändras. De klimattendenser som nu uppvisas leder till konklusionen att grundvattentillgången i områdena under den kritiska perioden kommer att minska. / Water is a vital resource for all human life. Half of all drinking water in Sweden comes from groundwater and in some coastal areas, the access to water is a growing problem. It is therefore crucial to properly understand all parameters that affect the access to water, both for present and future users. The change in groundwater level in geologically varied environments is hard to get a complete picture of. Stratification, heterogeneity of soil and rock, low kinematic porosity and flow between the layers are partially unknown parameters and therefore highly uncertain. Groundwater levels also change with the climate, both over the year and over longer period of time. The impact of humans is of great importance. The objective is to see how these parameters effects groundwater level in relation to each other. To analyze these factors, three different geographic areas in Sweden will be compared: Stor, Blidö, and Insjön, Rådmansö, in Norrtälje kommun and Klintemåla in Oskarshamns kommun. To put the water use and potential stress in context, the results will be related to the Water Exploitation Index (WEI). Data was obtained from the archives of SGU, SMHI and Lantmäteriet. In two cases, the information was supplemented with field investigation. The information was processed in Excel and the program GWBal. Overall, the local shortage of water in these areas is urgent during some summer months. The water supply is a local resource and the generally good situation in Sweden conceals major regional and local imbalances. The part time residents have a much greater impact since the problem is at its peak during summer. Changes in the climate, withdrawal of water and chemical composition have greater impact on small reservoirs then on big ones. With today’s trend in climate, the conclusion is that the availability of groundwater in the areas will decrease.

Yes / Field measurements and numerical simulations demonstrate the existence of an upper saline plume in tidally dominated beaches. The effect of tides on the saltwater-freshwater mixing occurring at both the upper saline plume and lower salt wedge is well understood. However, it is poorly understood whether the tidal driven force acts equally on the mixing behaviours of above two regions and what factors control the mixing fluctuation features. In this study, variable-density, saturated-unsaturated, transient groundwater flow and solute transport numerical models are proposed and performed for saltwater-freshwater mixing subject to tidal forcing on a sloping beach. A range of tidal amplitude, fresh groundwater flux, hydraulic conductivity, beach slope and dispersivity anisotropy are simulated. Based on time sequential salinity data, the gross mixing features are quantified by computing the spatial moments in three different aspects, namely, the centre point, length and width, and the volume (or area in a two-dimensional case). Simulated salinity distribution varies significantly at saltwater-freshwater interfaces. Mixing characteristics of the upper saline plume greatly differ from those in the salt wedge for both the transient and quasi-steady state. The mixing of the upper saline plume largely inherits the fluctuation characteristics of the sea tide in both the transverse and longitudinal directions when the quasi-steady state is reached. On the other hand, the mixing in the salt wedge is relatively steady and shows little fluctuation. The normalized mixing width and length, mixing volume and the fluctuation amplitude of the mass centre in the upper saline plume are, in general, one-magnitude-order larger than those in the salt wedge region. In the longitudinal direction, tidal amplitude, fresh groundwater flux, hydraulic conductivity and beach slope are significant control factors of fluctuation amplitude. In the transverse direction, tidal amplitude and beach slope are the main control parameters. Very small dispersivity anisotropy (e.g., α_L⁄α_T <5) could greatly suppress mixing fluctuation in the longitudinal direction. This work underlines the close connection between the sea tides and the upper saline plume in the aspect of mixing, thereby enhancing understanding of the interplay between tidal oscillations and mixing mechanisms in tidally dominated sloping beach systems. / Shenzhen Key Laboratory for Coastal Ocean Dynamics and Environment (No. ZDSY20130402163735964), National High Technology Research & Development Program of China (No. 2012AA09A409).

The mixing zone developed at freshwater-seawater interface is one of the most important features in complex coastal hydrogeologic systems, which controls subsurface flow and reactive transport dynamics. Freshwater-seawater mixing-zone development is influenced by many physical and chemical processes, such as characteristics of geologic formation, hydrodynamic fluctuations of groundwater and seawater levels, fluid-rock interactions, and others. Wide mixing zones have been found in many coastal aquifers all over the world. However, the mechanisms responsible for wide mixing zones are not well understood. In this thesis, two hypotheses were proposed to explain wide mixing zones in coastal aquifers: (1) kinetic mass transfer coupled with transient conditions, which create the movement of the mixing zone, may widen mixing zones; and (2) aquifer stratification may widen the mixing zone. The hypotheses were tested by both multiscale numerical simulations and laboratory experiments. Numerical simulations were based on a variable-density groundwater model by varying mass transfer parameters, including immobile porosity, mobile porosity, and mass transfer coefficient, and the hydraulic conductivity contrast between aquifer layers. Laboratory experiments were conducted in a quasi-two-dimensional tank, where real beach sands were installed and foodstuff dyes were used to visualize the development of freshwater-seawater mixing zone. Major conclusions included (1) the mixing zone can be significantly widened when the mass transfer timescale and the period of transient boundary is comparable due to the nonequilibrium mass transfer effects; and (2) a thick mixing zone occurs in low-permeability layer when it overlays upon a fast flow layer. These results not only improve the understanding of the dynamics of mixing-zone development and its associated geochemical processes in coastal aquifers, but also identify hydrogeologic conditions for the model of sharp-interface approximation to be valid. In addition to better understanding the mechanisms and dynamics of mixing zone, this thesis also investigates cost-effective management of coastal groundwater resources. To protect and conserve limited water recourses in coastal regions, interest in aquifer storage and recovery (ASR) has been growing in recent years. ASR is a promising strategy for water resources management and has been widely used in many contaminated and saline aquifers. However, its performance may be significantly constrained by mass transfer effects due to the mobilization of solutes initially residing in immobile domains. Better understanding of kinetic mass transfer effects on ASR is needed in order to aid the decision-making process. A numerical model is developed to simulate ASR performance by combining the convergent and divergent dispersion models with a first-order mass transfer model. By analyzing the concentration history at the pumping well, we obtain simple and effective relationships for investigating ASR efficiency under various mass transfer parameters, including capacity ratio and mass transfer timescales, and operational parameters. Based on such relationships, one can conveniently determine whether a site with mass transfer limitations is appropriate or not for ASR and how many ASR cycles are required for achieving a positive recovery efficiency (RE).