Analysis of Groundwater Level Changes and Land Subsidence In

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

Analysis of groundwater

level changes

and land subsidence

in Gothenburg, SW Sweden

Johanna Ljungdahl

ISSN 1400-3821 B845 Master of Science (120 credits) thesis Göteborg 2015

Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract

Changes of groundwater heads in confined aquifers, which are confined by thick clay layers, can lead to land subsidence and subsequently severe damages to building and infrastructures. The geology in Gothenburg is dominated by deep valleys, with thick overlaying clay layers, generating substantial geotechnical challenges. In order to minimise the risks of groundwater lowering and land subsidence, the Office of City Planning (SBK) in Gothenburg monitors the groundwater level and attempts to keep the levels stable. The aim of this study was to analyse the sources of groundwater level changes and land subsidence in Gothenburg. This has been done by analysing groundwater time series longer than 20 years, provided by SBK. Groundwater levels and groundwater drawdown were also compared with stratigraphic models of clay thickness and subsidence data, retrieved from PanGEO (http://www.pangeoproject.eu/). In addition, an attempt to locate infiltration areas connected to the lower aquifer, and estimate groundwater recharge was also done in this study. In general, the groundwater levels in Gothenburg show a rising trend, following the common trend of precipitation. In the central parts of the city the levels are obviously recovering from the deep drawdown in the 1970. Underground constructions and groundwater drainage have contributed to, in general, a lower groundwater level today, than before 1970. Groundwater levels responds to changes in precipitation with only a few months delay, indicating that the mechanism of recharge is fast and that the groundwater level variations are mainly driven by weather and climate. Analyzing the relation between groundwater level lowering, clay thickness and land subsidence shows that areas with high subsidence rate correlates to areas with high clay thickness and also to areas where the groundwater level have been lowered. By using a simple conceptual model a very rough estimation of groundwater recharge can be obtained. In this study the groundwater recharge was estimated to 8.5-38 mm/year, with the assumption that groundwater recharge take place where coarse material, connected to the lower aquifer, outcrops. Groundwater recharge in urban areas is influenced by several factors such as leaking pipes, underground constructions and drainage.

Key words: Gothenburg, Groundwater level lowering, Groundwater trends, Land subsidence, Aquifer, Conceptual model, Climate.

Sammanfattning Grundvattensänkning i slutna magasin, som överlagras av tjocka lerlager, kan orsaka marksättningar samt skada byggnader och annan infrastruktur. Göteborgs geologi, som delvis utgörs av djupa dalar och tjock lera, innebär stora geotekniska utmaningar. Grundvattennivåmätningar har utförts i Göteborg sedan 1970 och idag har Stadsbyggnadskontoret (SBK) i uppdrag att övervaka grundvattennivåerna i Göteborg. För att undvika grundvattennivåsänkningar som kan orsaka skada försöker man hålla nivåerna stabila. Syftet med denna studie har varit att analysera orsakerna till grundvattennivåförändringar och marksättningar i Göteborg. Detta har gjorts genom att analysera långa grundvattentidsserier, hämtade från grundvattendatabasen, upprättad av SBK. Grundvattennivåer och grundvattennivåsänkningar jämfördes också med stratigrafiska modeller över lerdjup och sättningsdata, hämtad från PanGEO (http://www.pangeoproject.eu/). Samt ett försök lokalisera infiltrationsområden som är sammankopplade med det undre magasinet och uppskatta grundvattenbildningen. Grundvattennivåerna i Göteborg visar generellt en stigande trend. I det undre magasinet i centrala delarna av Göteborg syns en tydlig återhämtningstrend efter en tidigare grundvattensänkning. På grund av undermarksanläggningar och dränering ligger grundvattenytan idag lägre än vad den gjorde 1970 då grundvattennivåmätningarna startade. Resultatet av att analysera förhållandet mellan grundvattennivåvariationer och nederbördsvariationer visar att grundvattenbildningen sker relativ snabbt, och att grundvattennivåvariationer i huvudsak sker på grund av väder och klimat. En jämförelse mellan grundvattensänkning, lerdjup och sättningshastighet konstaterar höga sättningshastigheter i områden där grundvattennivån sänkts samt där lerdjupet är stort. Genom att använda en enkel analytisk modell fås en ytterst grov uppskattning om storleken på grundvattenbildningen, i detta fall 8,5-38 mm/år, med antagandet att grundvattenbildningen sker i grövre material i närheten av bergssluttningar och där friktionsmaterial framträder i markytan. Grundvattenbildningen i urban miljö påverkas av flera faktorer så som läckande ledningar, undermarksanläggningar och dränering.

Nyckelord: Göteborg, Grundvattensänkning, Grundvattennivåförändring, Sättningar, Grundvattenmagasin, Grundvattendatabas, Analytisk modell, Klimat, PanGEO.

Table of Contents

1 Introduction ...... 1 1.1 General background and motivation ...... 1 1.2 Objective and aim ...... 1 1.3 Project outline ...... 2 2 Background ...... 3 2.1 Geology and hydrology in Gothenburg ...... 3 2.2 Geotechnical challenges in Gothenburg ...... 4 2.3 Future climate ...... 4 2.4 Description of the study areas ...... 5 3 Theoretical background – hydrology and soil deformation ...... 7 3.1 The hydrological cycle referred to an aquifer ...... 7 3.2 Subsurface processes ...... 8 3.3 Soil and aquifer properties and their effect on groundwater ...... 10 3.4 Confined and Unconfined aquifers ...... 13 3.5 Natural groundwater fluctuations ...... 14 3.6 Groundwater fluctuations cause by human activity ...... 15 3.7 Groundwater level monitoring ...... 15 3.8 Methods of quantifying groundwater recharge ...... 16 3.9 Impacts of urbanisation on groundwater ...... 17 3.10 Soil deformation and compaction ...... 18 3.11 Methods of quantifying land subsidence ...... 19 4 Methods and Data ...... 20 4.1 Data sources ...... 20 4.2 Methodology ...... 24 5 Result and Analysis ...... 29 5.1 Groundwater trends map ...... 29 5.2 Groundwater observations and time series analysis ...... 32 5.3 Potentiometric surface map ...... 39 5.4 Land subsidence ...... 40 5.1 Geological model ...... 42 5.2 Conceptual model and groundwater recharge ...... 50 6 Discussion ...... 53 6.1 Discussion of uncertainties and assumptions ...... 53

6.2 How have groundwater levels changed in Gothenburg since the beginning of systematic monitoring? ...... 54 6.3 Is it possible to separate the influence of urban development from influences of climate? 55 6.4 Analyse the spatial distribution of groundwater levels in Gothenburg ...... 56 6.5 Investigate the relation between land subsidence, clay thickness and groundwater lowering in Gothenburg ...... 56 6.6 Investigate the possibility to quantify groundwater recharge ...... 57 7 Conclusion ...... 58 8 Future studies ...... 59 9 Acknowledgement ...... 60 10 References ...... 61

1 Introduction

1.1 General background and motivation Urban development and new infrastructure will to some extent influence and change natural environments and natural processes e.g. subsidence due to external load from building, groundwater recharge due to drainage and impermeable soil cover. Groundwater fluctuations and land subsidence can cause damage to building and infrastructure, the total subsidence or settlement is difficult to estimate and has become a large infrastructural problem worldwide (Budhu & Adiyaman, 2010; Maxe & Thunholm, 2007). “Good-quality water” is one environmental goal in Sweden with focus on groundwater and states: “Groundwater level should be such that negative impacts on water, soil stability and wildlife in adjacent ecosystems do not occur” (Naturvårdsverket, 2012). The geology in Gothenburg is problematic in terms of soil stability and has caused several geotechnical problems such as groundwater lowering due to underground constructions and land subsidence. The Office of City Planning (SBK) monitors the groundwater level in Gothenburg in attempt to keep the groundwater level stable and in order to minimise risks of groundwater level lowering and in turns land subsidence. Groundwater is generally reduced in urban areas as a result of increased surface runoff from buildings, roads and drainage. However, due to leaking water supplies and leaking sewers, groundwater recharge may increase in some regions (Lerner, 1990, 2002). When planning for new infrastructure it is important to understand the processes which can lead to increased or decreased groundwater recharge. The groundwater situation in urban areas is complex and it is influenced by several dynamic and heterogenic parameters; such as climate, weather, land use, surface infrastructure, soil heterogeneity and artificial infiltration and pumping.

1.2 Objective and aim The aim of this study is to investigate relationships between changes in groundwater recharge, groundwater level changes and subsidence in Gothenburg. The focus will be on the impact that the climate variability has on changes of groundwater levels. More specifically this study aims at discussing and answering the following questions:

 How have groundwater levels changed in Gothenburg since the beginning of systematic monitoring?  Is it possible to separate the influence of urban development from influences of climatic variability?  Analyse the spatial distribution of groundwater levels in Gothenburg  Investigate the relation between land subsidence, groundwater level lowering and clay thickness in Gothenburg  Investigate the possibilities to quantify groundwater recharge in Gothenburg

1.3 Project outline The general approach of this thesis is to analyse the relation between groundwater level, weather and land subsidence. The focus areas are Gothenburg city quarters: Linné and Gårda. This study has been executed with guidance from Professor Roland Barthel at University of Gothenburg and Niklas Blomquist at SBK. The project is foremost based on data provided by SBK; groundwater observations, land subsidence measurements and stratigraphic data. A literature study has been performed to include knowledge and theoretical foundations to the approach to be used. The achieved results are an empirical model of the relation between groundwater level and precipitation, a conceptual model of the relation between land subsidence, groundwater level lowering and clay thickness and a conceptual model estimating groundwater recharge. Figure 1 shows a schematic workflow of this thesis. The green boxes illustrate the data sources that have been used, presented in more detailed under section 4.2. The orange boxes illustrate the achieved results which are presented under section 5. Parallel to this project another project with focus on land subsidence in Gothenburg has been carried out by Anna Albertsons and parts of her result is also included in this study, presented under section 4.1.3 (Albertsson, 2014).

Figure 1 Workflow scheme of this thesis. The green boxes illustrate what data that have been used and the orange boxes show the archived results.

Next chapter is describing the geology and the hydrological situation in Gothenburg, the geotechnical challenges and a more detailed description of the two investigation areas, Linné and Gårda, followed by a chapter with theoretical background to hydrology and soil deformation.

2 Background

2.1 Geology and hydrology in Gothenburg The geology of Gothenburg consists of bedrock, gneiss, and thick postglacial clay in the valleys of 50-100 m. Generally, on top of the bedrock underneath the clay is a layer of glacial deposit sand and till (Hultén, 1997). The typical soil stratification at the Swedish west coast is illustrated by Cato and Engdahl 1982 in Figure 2 (Persson, 2008). Göta Älv River has its outlet in vicinity of the Gothenburg harbour. The geology along Göta Älv River valley consists of 100 m of postglacial clay from marine settlement (Göransson, Bendz, & Larson, 2009). Gothenburg’s citizens is relying on fresh water from the river, and in order to minimize the risk of salt water intrusion up steams the water in Gothenburg’s harbour is kept at a stable level. The hydrological situation in Gothenburg is explained with two groundwater system, an upper aquifer and a lower aquifer, isolated from on another because of the clay in between. The upper aquifer is located in the artificial filling material, recharge occur from precipitation and leaking pipes. According to Norin et al (1999) up to 26 % of the produced fresh water in Gothenburg is lost by leaking water supplies. The lower aquifer is located in the glacial deposit and groundwater recharge occur close to the hill slopes (Hultén, 1997) (Norin, Hultén, & Svensson, 1999). The groundwater recharge to the lower aquifer is assumed to be <100mm/year (Trafikverket, 2013).

Figure 2 Schematic profile showing the soil stratification in western Sweden, modified from Cato and Engdahl (1982) (Persson, 2008).

2.2 Geotechnical challenges in Gothenburg The geology in Gothenburg is problematic in terms of soil stability and has caused several geotechnical problems such as land subsidence and groundwater fluctuation, which in turn damaged buildings and water pipes lines. Gothenburg city is built up during “four centuries” 1600, 1820, 1921 and 1970, see Figure 3. During these centuries many underground constructions were built, which resulted in groundwater extraction and drainage. Filling material has been a method in order to stabilise the ground and also to fill up land close to the Göta Älv River. The thickness of the filling material varies from 1 to 7 m see Figure 3, and has heterogenic composition; sand, gravel, wood, brick, glass. The filling material has contributed to subsidence due to external loading but also due to compaction of the material itself (Hultén, 1997). Subsidence in Gothenburg has also occurred due to lowering of groundwater, consolidation of clay and rotting of wooden piles, used as foundations (Alte, 1981). Reinforcement work and attempts to keep the groundwater level stable through groundwater monitoring and groundwater infiltration, are today methods trying to save old buildings and minimising the risk of subsidence due to groundwater lowering (Norin et al., 1999). Gothenburg city plans for further infrastructure development and expansion, e.g. the West Link Project, a train tunnel passing through Gothenburg city.

Figure 3 Urban expansion in Gothenburg during 1820-1921 digitalized from historical maps (Claesson & Höglund, 2014) and thickness of filling material, filling material above 3 m in close to Göta Älv River (Hultén, 1997). 2.3 Future climate In Sweden prediction of future climate are increased temperature by 3-4°C and increased precipitation by 10-30% during 2071-2100 compared with the reference period 1961-1990. As a result of a warmer climate and water expansion, the sea level will rise. The predicted sea level rise in Gothenburg region is estimated to be about 0.1-0.9 m, with the land rising of 3 mm/year included. The predictions of 100-years flood is also predict to increase by 5-10% until 2100 (SMHI, 2007). Increased sea level rise and more frequent storm events in Gothenburg will result in urban areas under water, which may be something to consider planning for new infrastructure (Göteborgs Stad, 2006).

2.4 Description of the study areas The investigation areas of this study are Gothenburg city quarters Linné and Gårda, see Figure 4. Linné has been chosen because it is an area where a lot of investigations have been carried out and where several observation wells with long groundwater time series are available. Gårda has been chosen because it is an area where new infrastructure is planned, e.g. the western link. The groundwater observations available in the Gårda region are few; therefore observations closest to the area will be used. In the following sections a general description of the two study areas will be given, with focus on its historical development and geological and geotechnical situation.

2.4.1 Linné Linné is located in the western part of central Gothenburg and has during several centuries been urbanized. The urban development of the area began in the 1880´s and ended in 1921. During infrastructure development of the area a river, Djupedalsbäcken, was excavated and filled up with 1- 3 m of filling material. Djupedalsbäcken was located close to where Linnégatan is today, see Appendix 1 (Aronsson, 1980). The general soil stratigraphy of Linné is filling material of 1,5-3 m, up to 70 m of clay and friction material 0,1-10 m on top of bedrock. Two aquifer systems is defined, one in the upper filling material and one in the lower friction material. Subsidence in Linné has occurred due to groundwater lowering, rotting of wooden foundation piles, consolidation of clay, compaction of clay due to external load and compaction of filling material. The subsidence rate have been measured to be 10-15 mm/year at some specific locations (Aronsson, 1980). Due to stabilisation problems in the area reinforcement work have been carried out, from 1930 and forward, in order to save buildings and also in order to prevent damages to buildings (Hummel, 2004). The groundwater level has in some parts of Linné been lowered 2-3 m during 1970 due to underground constructions. An attempt to increase the groundwater level again has been done by re-infiltrate the groundwater that leaked into a tunnel (Aronsson, 1991; Wassenius, 1993).

2.4.2 Gårda Gårda is located in the eastern part of central Gothenburg, dashed line in Figure 4. The urban development started in 1880, characterised as an industrial area. Today is Gårda both an industrial and an residual district (Strannelind, 1994). Mölndalsån River is passing through Gårda with its outlet in Göta Älv River. Mölndalsån River has a large catchment area and the water table in the river is regulated by dams in order to prevent flooding. The soil stratigraphy of Gårda is, as the rest of the city, consisting of filling material, clay and friction material on top of bedrock. The friction material in Gårda is 20-50 m compared to a few decimetres to maximum 10 m in the rest of the city (Banverket, 2006). The filling material along Mölndalsån River have been measured to 0-2.5 m, where loading from 0.5 m of filling material contributed to subsidence of 0.1 m (Wassenius & Jansa, 1979). An event arena, Scandinavium, close to Gårda showed during 1977-1978 subsidence of 8-10 mm/year and at some part of the building up to 20-30 mm/year (Svensson, 1991). The reasons behind the subsidence is said to be groundwater level lowering due to underground constructions, consolidation of clay and compaction of the filling material. In 1969 the groundwater level in the lower aquifer in specific parts of Gårda dropped about 7 m because of underground constructions (Aronsson, 1985). In the end of 1976 groundwater infiltration

5 was used in order to raise the groundwater level again but the groundwater level have not been fully recovered (Svensson, 2008).

Figure 4 The geology of Gothenburg showed by a soil cover map provided by SGU. The locations of the groundwater observation wells are marked in dark blue. The locations of the study areas are marked with a solid rectangle, Linné, and with a dashed rectangle, Gårda.

3 Theoretical background – hydrology and soil deformation

3.1 The hydrological cycle referred to an aquifer The hydrological cycle is defined by several processes such as; precipitation, evaporation, transpiration, discharge and storage, where solar energy is the primary driving force. The water balance of an aquifer is the relation between inflow, outflow and groundwater storage, see Eq. 11. Fluctuations in groundwater level are the result of changes in recharge and/or discharge (C. Svensson, 1984).

푃 + 푄푖푛 = 퐸 + ∆푀푠 + ∆푀푔 + 푄표푢푡 Eq. 1

Where P = Precipitation E = Evapotranspiration

Qin = Recharge to surface- or groundwater Qout = Discharge from surface- or groundwater ∆Mg = Change in Groundwater Storage ∆Ms = Change in Surface water Storage

3.1.1 Precipitation In humid and semi-humid climates, precipitation is usually the primary source of groundwater recharge. In order to analyse changes in precipitation long and continuous measurements are needed. Distribution of the measuring stations is also of importance due to local and geographic variations in precipitation. The Swedish Meteorological and Hydrological Institute, SMHI, has around 600 stations for precipitation observations. Several factor effect the data accuracy such as observation technique, wind and evaporation (Knutsson & Morfeldt, 2002).

3.1.2 Evapotranspiration Evapotranspiration is the total vaporization from open water, evaporation, and from plants, transpiration. The distinction between evaporation and transpiration is difficult to determine therefore referred to as evapotranspiration. Temperature is the controlling factor, hence increased evapotranspiration during summer months. Potential evapotranspiration refers to the evapotranspiration that would occur if there was unlimited available water. Actual evapotranspiration is depending on the available water. Water may be limited during some periods and therefore is the actual evapotranspiration lower than the potential. In humid climate the yearly precipitation is higher than the evapotranspiration. Evapotranspiration is an important parameter of the hydrological cycle hence the accuracy of observation is of great importance (Knutsson & Morfeldt, 2002). Vaporization depends on several factors; temperature, plant-type, soil parameters and groundwater level, thereby it can be difficult to measure (Olin, 1994). In Gothenburg region there are water shortage during the summer months, where evaporation is higher than precipitation, and water excess during autumn and winter months, see Figure 5.

Figure 5 Water balance for Gothenburg region, southwestern Sweden. The data are calculated from daily values from 1961-2012, precipitation from SMHI and evaporation from Climate research unit (CRU).

3.1.3 Groundwater recharge Groundwater recharge is the water entering the saturated zone. The precipitation that does not vaporize is referred to as the net precipitation. Groundwater recharge occurs from precipitation that infiltrates through the unsaturated zone, from surface body water and from artificial activity. The net precipitation is low or negative during summer months hence groundwater recharge limited. During the autumn evapotranspiration is low and precipitation high, high net precipitation. During winter months the precipitation may fall as snow, and accumulates in the snow package which minimizes recharge until the temperature rises and snowmelt occur. Precipitation and evaporation differ between geographic locations hence the groundwater fluctuations and groundwater regime, illustrated with a figure in Appendix 2 (Knutsson & Fagerlind, 1977). Methods of how groundwater recharge can be quantified will be explained in section 3.7. Variation in groundwater recharge is depending on precipitation, evapotranspiration, land surface, soil moisture and discharge. The total discharge is a sum of surface runoff, interflow and baseflow and will be described in more detail in next chapter. The subsurface processes are important, in order to understand the hydrological system and the groundwater flow pattern.

3.2 Subsurface processes

3.2.1 The unsaturated zone, vadose zone The vadose zone or unsaturated zone, is between the land surface and the water table, see Figure 8. The flow of water in the unsaturated zone is dependent upon several soil factors; porosity, specific yield and permeability. Soil properties affect the infiltration rate and how much rainfall that will infiltrate and become groundwater (Wilson, 1971). The infiltration rate at soil surface decreases with time. When the water from a rainfall exceeds the soils infiltration capacity at the surface, soil moisture content is 100 %, surface runoff occur. During heavily rainfall events more rainfall will go into surface runoff because the maximum infiltration capacity for the soil is reached quicker, see Figure 6.

Figure 6 The relationship between infiltration capacity, rainfall and surface runoff (Fetter, 2001).

The soil moisture zone defines the soil zone, intermediate zone and capillary fringe zone. The water content in the soil zone is reached by the vegetation root systems. The intermediate zone is in between the soil and capillary fringe zone, the amount of water content in this zone is defined by the field capacity of the soil (the water content in the soil profile after excess water has drained away). The capillary fringe zone is above the water table and the water content in this zone is often high and saturated because of water rising from the saturated zone due to capillary forces. The capillary fringe zone is irregular and its height is defined by the grain size, whereas the capillary effect is stronger in a fine grained soil. When the soil is saturated beyond its field capacity, percolation of water downwards to the saturated zone starts, see Figure 7 (Fetter, 2001). Interception is known as the water stored in plants or in lakes, before it ends up as groundwater or evaporates. Water that infiltrates into the ground has a vertical flow direction. When water meets a soil layer with lower hydraulic conductivity, e.g. clay, a horizontal flow will occur, which is known as interflow, runoff in the unsaturated zone, see Figure 8.

Figure 7 The passage of water thorough the unsaturated zone during a rainfall event (Fetter, 2001).

3.2.2 The saturated zone, groundwater zone The groundwater zone starts where the soil is fully saturated. The groundwater flow and the groundwater table are dependent on the hydraulic conductivity, transmissivity and storativity of the soil material. Groundwater recharge takes place at the water table, below the capillary fringe zone.

As a result of groundwater recharge the groundwater level will raise and a change in the groundwater storage occur. The groundwater storage is a balance between inflow and outflow; as inflows occur through groundwater recharge, outflow referred to as baseflow will occur, see Figure 8 (Fetter, 2001; Wilson, 1971).

Figure 8 Flow chart of the hydrologic cycle modified from Bergström (1993). 3.3 Soil and aquifer properties and their effect on groundwater The flow of water in soil is dependent upon; porosity, specific yield, permeability and degree of saturation. These parameters affect how much of the rainfall that infiltrates and become groundwater. Heterogeneity and anisotropy of the soil material is present in most environments and effects the infiltration.

3.3.1 Porosity The porosity of a material is the percentage of voids within a sample and defined by n;

푉푣표푖푑 푛 = Eq. 2 푉푡표푡푎푙

Where n = Porosity

Vvoid = Volume of void space Vtotal = Total volume of material

The ability of groundwater movement and the size of groundwater storage are depending on porosity of the material. Porosity is affected by alignment, particle size and packing and sorting of material e.g. a poorly sorted soil will have low porosity or for example a soil that has been used for agricultural farming may also have lower porosity.

3.3.2 Specific yield Specific yield or effective porosity is the volume of water that can be drained from a soil sample. A drained soil sample will still contain water, as water is attached onto the soil particles due to, surface tension, cohesion or adhesion, called the capillary effect. Some water can also be trapped within pore spaces if passageways between pores are missing called specific retention. The specific yield, Sy, is the volume of all connected pores;

푉푑푟푎푖푛푒푑 푆푦 = Eq. 3 푉푡표푡푎푙

Where Sy = Specific yield (effective porosity)

Vdrained = Volume of connected void space Vtotal = Total volume of material

Specific yield is influenced by grain size. A soil with smaller grain size has a larger total surface area; therefore clay compared to sand will hold more water due to stronger capillary forces. Effective porosity will always be less than porosity, n. The relation between grain size, porosity, specific yield and specific retention is shown in Figure 9.

Figure 9 The relation between grain size, porosity, specific yield and specific retention (Davis & Dewiest, 1966).

3.3.3 Permeability and hydraulic conductivity Permeability indicates how easily a fluid will flow though a media. The interconnection of pores spaces is the dominant factor deciding permeability. Pore spaces in clay are not well connected hence low permeability. Layers of low permeability limit infiltration and can work as an aquitard which allows no flow. Permeability is defined in darcys, m2, see Table 1. The Permeability in rocks depends upon fractures and porosity and the interconnection in between the fractures. Generally, coarse sedimentary rocks can obtain high permeability as a result of high porosity whereas crystalline rocks have low permeability due to low porosity (Fetter, 2001). Hydraulic conductivity, K, describes the flow of water in the saturated zone, the hydraulic conductivity for different material is seen in Table 1. It is defined by the porosity and the degree of saturation of the material as well as the fluid properties such as density and viscosity. Hydraulic conductivity is a coefficient which is proportional to flow of water expressed in m/s or cm/s, Darcy’s law;

푑ℎ 푉 = 퐾 Eq. 4 푑푙

Where V = Flow rate, specific discharge [m/s] K = hydraulic conductivity [m/s] Dh/dl = hydraulic gradient

Table 1 The permeability and hydraulic conductivity of unconsolidated sediments (fetter)

Material Permeability Hydraulic conductivity (darcys, m2) (cm/s) Clay 10-6-10-3 10-9-10-6 Silt, sandy silt, clayey sand, till 10-3-10-1 10-6-10-4 Silty sand, fine sand 10-2-1 10-5-10-3 Well-sorted sand, glacial outwash 1-102 10-3-10-1 Well-sorted gravel 10-103 10-2-1 Bedrock 0-1 0-1

3.3.4 Transmissivity Transmissivity addresses how much water that can be transmitted through the thickness of an aquifer. Transmissivity is a result of the aquifer thickness and the hydraulic conductivity (Wilson, 1971).

푇 = 푏푘 Eq. 5

Where T = Transmissivity b = Aquifer thickness (saturated) K = Hydraulic conductivity

3.3.5 Storativity The effect on how much water that can be stored or subtracted from a saturated unit by the compaction/expansion of aquifer and the compression/expansion of water compressibility as water level per unit change is defined as specific storage.

푆푠 = 𝜌푤𝑔(훼 + 푛훽) Eq. 6

Where Ss = Specific storage 3 Ρw = density of water (1000 kg/m ) ɡ = acceleration of gravity (9.8 m/s2) α= aquifer compressibility n = porosity β = compressibility of water (4.4 * 10-10 m2/N)

Subtractions of water in confined aquifer are not effecting the saturation of the aquifer. Thereby the storativity is defined as the aquifer thickness, b, times the specific storage.

푆 = 푏푆푠 Eq. 7

As the water level changes in an unconfined aquifer the storage is a function of the specific yield, the thickness and the specific storage of the aquifer and therefore defined as;

푆 = 푆푦 + 푏푆푠 Eq. 8

3.4 Confined and Unconfined aquifers An aquifer is a geological setting of high permeable material, which can store a lot of water. There are generally two states of aquifers; confined and unconfined, see Figure 10. A confined aquifer is sealed from above of a layer with low permeability and low hydraulic conductivity e. g clay. Confined aquifers do not get groundwater recharge from above, this due to the stratigraphic layers. A layer with low permeability on top of a layer with high permeability is called an aquitard. If the aquitard is of different sealing capacity the aquifer will be classified as a leaking aquifer. The water pressure of the confined aquifer is referred to as the potentiometric surface. An unconfined aquifer is not sealed by any layer with lower hydraulic conductivity. Groundwater recharge to an unconfined aquifer is located right above the aquifer. In unconfined aquifers the water pressure level is on the same location as the water level (Freeze & Cherry, 1979).

Figure 10 Two types of aquifers; confined and unconfined aquifer (Freeze & Cherry, 1979).

Topography and terrain will affect the water and groundwater flow. In rural environments the groundwater table generally follows the terrain profile. High terrain areas are defined as infiltration areas and low terrain as discharge areas. When water infiltrates at the soil surface it is described as a vertical downward flow which changes towards a vertical upward flow at the discharge area, see Figure 11. A steep topography increases the horizontal flow, surface runoff, hence decreases the groundwater recharge. A catchment is defined as the area within the watershed divides (Fetter, 2001; Freeze & Cherry, 1979; Knutsson & Morfeldt, 2002).

Figure 11 Groundwater flow direction in rural environments. The high topographical location with vertical downward groundwater flow indicates infiltration areas. The vertical upward groundwater flow indicates the discharge areas (Fetter, 2001). 3.5 Natural groundwater fluctuations Groundwater level fluctuations are mainly caused by seasonal weather variations which lead to changes in recharge and discharge. The characteristics of groundwater level fluctuations depend on geological and topographical parameters. A small aquifer with low storativity will obtain high variations, and larger aquifers with high storativity obtains, in general, less variations in groundwater level fluctuations (Knutsson & Fagerlind, 1977). Groundwater fluctuations in different geological settings in Sweden are shown in Figure 12. Large aquifers, consisting of well sorted material, with high specific yield would in general, obtain less groundwater level fluctuations than an aquifer with poorly sorted material with low specific yield or a fractured aquifer with low hydraulic conductivity. In Swedish environments the groundwater level is often shallow, which give a quick response between fluctuations in precipitation and groundwater level fluctuations. Fluctuations in the same aquifer can differ between locations in the topography and terrain profile; larger fluctuation will be obtained closer to the infiltration area (Knutsson & Morfeldt, 2002). Correspondence between rainfall and recharge decreases with depth to water table whereas short distance give fast response (Wu, Zhang, & Yang, 1996).

Figure 12 Groundwater fluctuations in different geological settings (Knutsson & Fagerlind, 1977).

Gravitational effects and changes in air pressure also influence the groundwater level. These are short term variations and are analysed at a daily basis. If the measuring interval becomes irregular or not frequent short term variations is difficult to analyse. Long term variations, annual variations, depends on the seasonal variation of precipitation, temperature and evapotranspiration (Wedel, 1978). Tidal effects of the global water also affect the groundwater level in both confined and unconfined aquifers. Nearby river systems and their water levels will affect the water pressure in the aquifers hence the groundwater table. In cities, river systems are often regulated by dams to avoid flooding and damages to constructions.

3.6 Groundwater fluctuations cause by human activity Artificial factors may change the natural groundwater fluctuations. As areas become urbanized the pathways for precipitation and groundwater recharge may change. Impermeable soil cover increases the surface runoff and allows less water to become groundwater. Artificial factors such as groundwater infiltration, groundwater drainage and underground constructions may both rise and lower the groundwater level, which will be explained in more detail under in section 3.9.

3.7 Groundwater level monitoring In Sweden the Swedish Geological Survey, SGU, is responsible for the national monitoring of groundwater. Groundwater monitoring is important in order to control the groundwater level, as it is known that both increased and decreased groundwater levels can damage buildings and harm natural ecosystems (Maxe & Thunholm, 2007; Naturvårdsverket, 2012). Which measuring frequency and which measuring period that is needed is depending on project, but also a function of costs. Svensson (1984) compared the difference between continuously measurements and increased interval between measurements. The result showed less groundwater fluctuations with less frequent measuring, Appendix 3. The natural groundwater fluctuation differs between aquifer which gives a

15 request of what measuring interval is needed in order to measure the natural variations (Svensson, 1984). Smaller groundwater aquifers fluctuate more than large and therefore, more frequent measurement is needed (Lundmark, 2001). Lundmark (2001) states that in order to analyse the groundwater trend a time series of at least two years is needed. According to the Water committee in Sweden “vattenförvaltningsförordningen” groundwater observations should be once per months in pore aquifers, twice per months in fracture aquifers and twice per months in groundwater aquifers where the groundwater extraction is larger than the groundwater recharge (Maxe & Thunholm, 2007). Measuring errors is difficult to exclude, different types of errors can be divided into accidental errors; measuring, data gathering or data processing, systematic errors; changes of measuring reference, or specific errors; artificial pumping, increased withdrawal or dry wells (Svensson, 1984).

3.8 Methods of quantifying groundwater recharge Groundwater recharge is a function of different controlling factors; climate, topography, soil and vegetation. Three major types of groundwater infiltrations is assumed and referred to as direct recharge; recharge from precipitation over large areas, focused recharge; concentrated recharge such as streams and lakes, and indirect recharge; recharge from different pathways beneath features such as rivers and lakes (De Vries & Simmers, 2002). In humid regions direct recharge dominates, the water table is often shallow and the aquifer saturated, therefore, the recharge rate is mostly dependent upon storativity and transmissivity of the aquifer. In arid regions groundwater recharge is limited by evapotranspiration which is a major part of the water balance. In arid regions focused recharge and deep water tables is common (Scanlon, Healy, & Cook, 2002). There are different approches to quantify groundwater recharge, in this study the focus will be on the direct model approch and the usage of infilitration coefficents. The direct model approch is based on the water balance equation:

푅 = 푃 − 퐸푇 ∓ 푀 Eq. 9

Where R = Groundwater recharge P = Precipitation E = Evapotranspiration M = Storage

Groundwater storage is often neglected when calculating the water balance for a long time period and that the total discharge may be equal to the groundwater recharge, assuming no runoff (Knutsson & Morfeldt, 2002). The net precipitation, calculated as the precipitation minus evapotranspiration, gives a rough assumption of the actual groundwater recharge. In Sweden the net precipitation varies from <200->600 mm/year, Appendix 4. In swedish environment surface runoff is a result of high intensity rainfall, fully saturated soil surface, rock outcrops and snow melt (SGU, 2013). By analysing different controlling parameters; infiltration capacities for soils, land use and slope, groundwater recharge can be quantified using infiltration coefficients, calculated as a constant part of the net precipitation. Soil type, different amount of water will infiltrate depending on infiltration capacity of the soil. Infiltration in fine grained soils is limited due to its very low hydraulic conductivity. Slope, catchments with great slopes has increased surface runoff compared to catchments with lower slopes. Land use, as areas become urbanized it gets covered with

16 impermeable soil cover such as asphalt and concrete foundation, which will increase the surface runoff (Fetter, 2001). The water balance equation using infiltration coefficient is described as the effective precipitation times the infiltration coefficient, C.

푅 = (푃 − 퐸푇) ∗ 퐶 Eq. 10

Water balances are not a very accurate method to estimate groundwater recharge where several parameters are difficult to quantify (Lerner, 2002). The difficulty to analyse groundwater recharge in urban areas is based on the difficulty to obtain accurate estimation of all ongoing paramters; surface runoff, draingae and artifical extraction and infilitration. Barkels and Parra (2010) used infiltration coefficients in order to analyse the groundwater leakage into a tunnel in Stockholm, eastern Sweden. In order to estimate the potential groundwater recharge over the area they used infiltration coefficient evaluated by Von Brömssen in 1968, see Table 2. Depending on soil types and net precipitation they calculated the groundwater recharge to be 89 mm/year using infiltration coefficients. They discussed that using infiltration coefficient may not be suitable because it may underestimate the actual groundwater recharge. The infiltration coefficient for clay thicker than 150 cm is defined as insignificant. Wedel (1978) studied groundwater recharge and groundwater flow in soils and in bedrock, a few miles north of Gothenburg, western Sweden. The soil stratigraphy is clay of 0-20 m, friction material of 0-1.8 m on top of bedrock. The result showed a heterogenic hydraulic situation, hydraulic conductivity of the soil increased with depth to K<1*10-9m/s. The hydraulic conductivity of the friction material was 1*10-4m/s and the hydraulic conductivity of the bedrock was between K 1*10-8 - >7*10-3 m/s. The bedrock was assumed to be impermeable where the topographic elevation of the bedrock surface was low. The result showed that the permeability and the hydraulic conductivity of the geological settings would allow all rainwater to infiltrate. Blom (2013) describes groundwater recharge as to be a constant part of the net precipitation determined by experience, see Table 2.

Table 2 Groundwater infiltration coefficients determined by Von Brömssen (1968) and Blom (2013).

Soil material Anders Blom Von Brömssen 1968 2013 Clay>150 cm 0.21 Clay<150 cm 0 Gravel 1.0 0.40 Sand 0.8 0.39 Till 0.5 0.23 Bedrock 0.1-0.2 0.26

3.9 Impacts of urbanisation on groundwater Groundwater is generally reduced in urban areas due to impermeable pavement e.g. asphaltic and concrete foundations. Increased surface runoff in urban areas is an effect of the reduced infiltration (Lerner, 1990). Buildings, roads and infrastructure foundations and drainage network will change the pathway for precipitation. According to Hultén (1997) the impermeable layer in an urban area is assumed to be up to 50 %, and even more in central urban areas, as for an example in

Stockholm the groundwater recharge is estimated to be around 40 mm/year due to impermeable pavements (SGU, 2013). Infiltration may also increase in urban areas due to leaking water supply, underground pipes and tanks, which can be about 100-300mm/year (Lerner, 2002). In Birmingham, United Kingdom, the leaking was is estimated to 25 % of the public water supply and about 26 % in Gothenburg (Lerner & Barrett, 1996; Norin et al., 1999). Water from leaking pipelines exists but if it actually contributes to groundwater recharge is difficult to quantify because it most likely occur both inflow and outflow to the water supply systems (Lerner, 1990, 2002). Groundwater drainage is a common process in urban areas. Underground constructions can cause groundwater drainage during long period due to sealing difficulty. Storm water drainage systems are dimensions to take in count the heavy rain events, which allow less water to become groundwater. As an area becomes urbanized, more water and surface water will end up in the storm water pipes, though the dimensions of the systems is important on order to prevent flooding (Holmstrand, 1980). In order to dimension the storm water drainage systems in urban asreas, a so called Rational Method can be used. The method is used in order to estimate how much rainfall that becomes surface runoff basedon the total area, the runoff coefficient and the intensity and duration of the rainfall event. Stepenson, D (1981) used runoff coefficients in order to estimate how much water from the rainfall that would become storm water, the runoff coefficents is used as a constant part of the net precipitation, see runoff coefficient in Appendix 5 (Hultén, 1997). Groundwater extraction, e.g. pumping can be used in order to minimise the effect from groundwater during infrastructure constructions, artificial infiltration is often used as a method in order to raise the groundwater level back to its initial level e.g. in Gothenburg groundwater infiltration have been used in order to prevent wooden foundation piles from rotten (Boutelje, 1981).

Figure 13 Urban effects on groundwater recharge (Lerner, 1990). 3.10 Soil deformation and compaction Land subsidence or compression of soil is a result of increasing effective stress. Increased stress is a result of reduced pore water pressure in the soil, which occurs through; external load, increased total stress or/and groundwater drawdown. The weight of the overlying material over an aquifer is the total stress, vertical downward. The total stress, 𝜎0, is balanced by the water pressure in the

18 aquifer, 푢, and by the effective stress developed from the aquifer soil skeleton, vertical upwards, see Eq. 11. During groundwater extraction, removal of water pressure, the effective stress, developed by the soil skeleton, will increase which results in compaction of the soil and the grain matrix (Sällfors, 2001).

′ 𝜎0 = 𝜎0 + 푢 Eq. 11

Where 𝜎0 = total stress 𝜎’0 = effective stress u = pore water pressure

How much the soil will deform or compact is also dependent on how much stress the soil skeleton has been exposed to before. Higher effective stress than total stress occurs in an overconsolidated soil, a soil that has been subjected to a now removed load. When the total stress is equal to the ′ effective stress 𝜎0 = 𝜎0, it is a normally consolidated soil. Though the total stress can also be equal to the effective stress when consolidation occurs, pore overpressure in the soil. Secondary consolidation may also occur in soils even without the effects of external load or removal of pore pressure. The subsidence that occurs as a result of groundwater extraction can be calculated using Hooke’s law, see Eq. 12 (Sällfors, 2001). 𝜎′ ∆푆 = ∆퐻 ∗ 0 Eq. 12 푀

Where: ∆S = Subsidence M = Mean deformation module of the soil ∆H = Clay thickness

𝜎’0 = The effective stress (generated by the removal of water)

Consolidation is a slow and long going process especially in fine grained soils such as clay. Urban development allows external load from buildings on the ground and typically groundwater level lowering. Land subsidence caused by groundwater extraction is very common in cities with fine grained materials. Hansbo (1981) analysed land subsidence that was caused by groundwater leakage into a tunnel. The subsidence was measured to 0.3-0.5 m. The groundwater head dropped about 10 m and the cone of depression could be measured 50 m away from the tunnel. Sun, Grandstaff, and Shagam (1999) also discussed subsidence caused by groundwater level lowering. In New Jersy, USA, the subsidence within the next 20 years is assumed to be 3 cm. They also discussed the environmental problem with land subsidence combined with an average global sea level rise of 2 mm/year.

3.11 Methods of quantifying land subsidence Land subsidence can be measured with different methods. Common methods are Global Positioning Systems, GPS, satellite measuring and manually levelling measuring of subsidence. Persistent Scatter Interferometry, PSI, is a new and widespread analysis where land subsidence is measured from satellite images. The satellite measures distances to different objects e.g buildings and roads. This technique has both advantages and disadvantages. The advantages are the possibility to cover large areas and get information from areas where manually measurements are not available. As the satellite measures the distance to an object with an angle this allows measuring the directions of the object as well as the vertical movement (Engdahl & Jelinek, 2013). Therefore, the method is not exactly comparable to the actually land subsidence because it also measures the movement of the object itself (Crossetto, Monserrat, Iglesias, & Crippa, 2010).

4 Methods and Data In this chapter the datasets and each specific method will be presented. The working hypotheses are explained under each method.

4.1 Data sources

4.1.1 Groundwater level data The groundwater observation time series are obtained from SBK. The groundwater level database contains observations from over 400 monitoring wells around Gothenburg. The observation wells in vicinity of Gothenburg city are shown in Figure 4. The groundwater observations start starts after 1970 except a few which starts a few years before 1970. The measuring periods varies from a few months to decades and the measuring interval differs from well to well. Today, the groundwater level is still monitored in 140 observation wells in Gothenburg region. The groundwater situation in Gothenburg has become more stable after the 1990. Figure 19 shows three groundwater observation time series from the groundwater dataset, with an almost stable groundwater level after 1990.

4.1.2 Climate data Gothenburg has an average annual precipitation of about 840 mm, see Figure 14. The moving average is used in order to see the long term trend. Precipitation shows an increasing trend during period 1961-2013. The data were obtained from SMHI and measured at a daily basis. The data are from two measuring stations within Gothenburg region, Säve and Gothenburg measuring station. The average annual evapotranspiration in Gothenburg is 563 mm, see Figure 15. The data set is obtained from the Climate Research Unit, CRU.

Figure 14 Mean monthly precipitation in Gothenburg region. The data are obtained from SMHI's measuring station in Gothenburg. Data between 1976-1995 and 1998-1999 is added from Säve measuring station. MovAve 10 is the moving average of 10 months used in order to see the long term trend.

Figure 15 Mean monthly evaporation in Gothenburg region. The data are obtained from The Climate Research Unit, CRU. MovAve 10 is the moving average of 10 months used in order to see the long term trend.

4.1.3 Subsidence data The land subsidence to be used in this project is subsidence data from SBK’s subsidence database and data from a satellite project PanGEO. The subsidence database from SBK consists of levelling data in central parts of Gothenburg city. The total levelling points are 5362. A few observations starts 1886 but most of them are regularly measured from 1970 to 2013. The data processing of the database is carried out by Albertsson (2014), where the measuring data was plotted against time in order to calculate the subsidence rate and to exclude time series with a positive trend and those with a root mean square less than 0.75. The result of her analysis is presented in Figure 16. The satellite data were obtained from a satellite project, PanGEO carried out by Treuropa (http://www.pangeoproject.eu/). The measuring interval is close to monthly from 1992 to 2000. No data are available from October 1993 to May 1995. Totally, 69 pictures have been used in their data processing and 184 600 data points. The cell size is 20*4 m and one measuring location possible within one cell. The accuracy of measuring location is ± 2 m in north-south direction and ± 7 m in east-west direction. The accuracy of the total subsidence rate about 1 mm/year according to Colombo (2014) . The PanGEO subsidence data are shown in Figure 17.

Figure 16 Land subsidence measurements from SBK, measuring period 1970 - 2012. An area in central Gothenburg shows a subsidence rate of about 10 mm/year and an area in southwest region shows subsidence rate of over 5 mm/year. The data processing is done by Albertsson (2014).

Figure 17 Land subsidence measurements from satellite project, PanGEO.

4.1.4 Stratigraphic data The stratigraphic data depth to bedrock and clay thickness is obtained from SBK, from the Geological Survey of Sweden, SGU and from old geotechnical investigation found at SBK’s geological archive. Information from a soil cover map made by Lars-Gunnar Hellgren whom worked at SBK has been used. The map contains data such as; thickness of filling material, clay thickness and depth to bedrock, the information is gathered from geotechnical investigations, see Figure 18. The clay thickness is primary gathered from the report about the western link made by Banverket (2006) the map is presented in Appendix 6. Elevation data and soil cover data provided by SGU.

Figure 18 Soil cover map over central parts of Gothenburg city made by Lars-Gunnar Hellgren, SBK. The map shows depth to bedrock, clay thickness, thickness of friction material and thickness of filling material. 4.2 Methodology

4.2.1 Groundwater trends In order analyse how the groundwater levels have changed in Gothenburg since the beginning of systematic monitoring, groundwater trend maps were constructed for each aquifer; the lower aquifer, the upper aquifer and the bedrock aquifer. The long term trend of the groundwater level situation was analysed and groundwater time series longer than 20 years was used. The analysis is based on a visual evaluation of the trends, totally 208 observations were analysed. The groundwater time series are first divided into different categories decided by the long term trend; Falling trend, Rising trend and No trend, see Figure 19. Later the time series were subdivided into different reoccurring features, observed in the groundwater observations; “Initial value”, “Variance” and “Plateau” in the beginning or “Plateau” in the end. The “Initial value” reflects groundwater levels at a high level followed by a drop in the groundwater table. The “Variance” is shown by a change in amplitude in the groundwater time series. “Plateau” refers to a stable groundwater table in the beginning or in the end of the time series, see Figure 19. In very few time series, where measurements started before 1970 it could be observed that the groundwater level started at a higher level that after the 1970. It is assumed that this “Initial value” may correspond to an “undisturbed” situation that existed before constructions of buildings and tunnels, which according to Fetter (2001) is associated to the lowering of groundwater levels.

Figure 19 Three examples of how the groundwater time series have been divided into different classifications. GW 494 is classified as Rising, Initial (red circle) and Plateau at the end, GW 442 is classified as Falling, Plateau at start and Plateau at end, GW 744 is classified as No trend and Variance.

4.2.2 Groundwater observations and time series analysis In order to analyse if it is possible to separate the influence of urban development from influences of climate, an empirical model showing the relation between groundwater level and precipitation was performed using a basic time series analysis. The areas of interests are Linné and Gårda and groundwater observation, longer than 20 years, located in the lower aquifer close to these areas was analysed, see Figure 20. The groundwater level time series were analysed with respect to their response to weather variations, the mean monthly precipitation. The basic time series analysis was done by normalising the groundwater time series and the precipitation time series. Normalization of the time series allows the mean to be zero and the standard deviation to be one, which makes the time series more comparable. The response time between the fluctuations in precipitation and fluctuations in groundwater level was visually analysed. The groundwater level data which is clearly influenced by artificial factors is removed from the analysis.

In order to see how the groundwater level response to precipitation on a yearly basis, the difference in groundwater level, from one year to the next, was plotted agents the deviation value from the mean precipitation. That will almost remove the long term trend and make it possible to analyse the correlation between precipitation and the groundwater level variations.

Figure 20 Location of analysed observation wells in Linné and Gårda respectively. The dashed lines show the location for the different profiles. The soil cover map is obtained from SGU.

4.2.3 Potentiometric surface map In order to locate the infiltrations areas for the lower aquifer a potentiometric surface map where created. Data to be used are average values from the latest level recordings from all wells, the data may differ in time as the measuring period between each groundwater observation differ. Using ArcGIS, the level data were transformed into a grid surface by using an interpolation method Topo- to-Raster, based on iterative finite difference interpolation technique. Topo-to-raster provides a result with respect to a connected drainage structure, a drainage flow from high to low values. The boundary conditions are set to data location and the extent of the lower aquifer, where the bedrock outcrops.

4.2.4 Land subsidence The two different subsidence dataset, PanGEO dataset and the data set from SBK, have been compared in order to see if the subsidence rates corresponds. Albertsson (2014) did the data processing of the subsidence dataset obtained from SBK, her result presented under section 4.1.3. The PanGEO dataset were also used in the geological model where the relation between land subsidence, groundwater level lowering and clay thickness were analysed, explained in detail under section 4.2.5. The PanGEO dataset were converted into a surface using an interpolation method based on iterative finite difference interpolation technique, Topo-to-Raster. To get a more detailed result the grid size of 5 m was used. The subsidence data obtained from The Office of City Planning were not used in the geological model because of the limited extent of data.

4.2.5 Geological model Stratigraphic cross sections were created in order to analyse the extent of the friction material, the lower aquifer, and the relation between land subsidence, groundwater level lowering and clay thickness. The cross sections were created by using elevation data, soil cover data and stratigraphic data from SGU’s database, the hand-drawn map created by Lars-Gunnar Hellgren and data from old geotechnical investigations obtained from the Office of City Planning. Four cross sections were created for Linné and Gårda respectively. The locations of the cross sections are decided by the extent of the data and where most data are available, see Figure 20. Interpolation of the bedrock data was done using kriging, an interpolation method often used in geology, creating a surface from data points. The used grid cell size is 5 m and the boundary is set to the area of interest, the central parts of Gothenburg. Interpolation of the clay thickness was done using Topo-to-Raster interpolation method, because the data set is a combination of contour lines and point data. The grid size is 5 m and the boundary conditions are decided by the data distribution. In order to get the cross section in meter above sea level each interpolated layer will be extracted from the elevation data surface. Analysing the relation between land subsidence, groundwater level lowering and clay thickness was done by combining the geological cross sections with the land subsidence data from PanGEO and groundwater time series data from nearby observation wells.

4.2.6 Conceptual model of groundwater recharge A conceptual model was constructed in order to investigate the possibility to quantify groundwater recharge. The groundwater recharge in the lower aquifer was estimated by using two methods based on a water balance equation and infiltration coefficients, the two methods are explained in more detail below. These methods only give a first rough idea about the magnitude of recharge. Many of the parameters needed to do a more accurate calculation are not known, and it was not possible to determine them given the scope and the timeframe of this thesis. The parameters used were climate data; yearly average precipitation and yearly average potential evaporation, soil stratigraphy, infiltration coefficients and percentage of different soil types, analysed from a local catchment area close to Gårda. The local catchment area was defined using elevation data and watershed delineation tools in ArcGIS. The areas of the different soil types, within the catchment, were calculated using a soil cover map from SGU, where soils of the same character, such as glacial clay and postglacial clay was merged into one soil type.

4.2.6.1 Method 1 The first method to calculate the groundwater recharge was by using a very simple water balance equation, see Eq. 13.

푅 = 푃 − 퐸푇 Eq. 13

Where R =Groundwater recharge [mm/year] P = Precipitation ET = Evapotranspiration

The result is the net precipitation and represent the amount of water that potentially can become groundwater recharge. Multiplying the effective precipitation with the total area gives the potential groundwater recharge in m3/year. The used parameter values are shown in Table 3.

Table 3 Parameter values calculating the groundwater recharge by using method 1.

Precipitation Evapotranspiration 833 mm/year 563 mm/year

4.2.6.2 Method 2 The second method to calculate groundwater recharge was by using infiltration coefficients for different soil types. The infiltration coefficient is how much of the net precipitation that will infiltrate into the ground and become groundwater, see Eq. 11.

퐴푖 푅 = ∑ ∗ (푃 − 퐸푇) ∗ 퐶 퐴 푖 Eq. 14 푖→푛

Where R = Groundwater recharge [mm/year] A = Total area

Ai = Area of specific soil type P = Precipitation ET = Evaporation

Ci = Infiltration coefficient

The infiltration coefficient was set to a maximum and minimum in order to give a wider range, the coefficients are mainly referred to values found in the literature. The values are from Von Brömsens study in 1968 (obtained from Barkels and Parra (2010) study), and values from Blom (2013) and some values are estimated by own assumptions see Table 4. The infiltration coefficient used for bedrock is chosen with respect to Wedel (1978) whom confirming that bedrock may have the same permeability and hydraulic conductivity as till. Wedel’s and von Brömssen estimations of infiltration in bedrock are conformable. The area of clay thinner than 1.5 m is estimated to be 3% in this study with respect to the deep valleys in Gothenburg.

Table 4 Parameter values calculating the groundwater recharge, using method 2.

Soil type Infiltration coefficient Min Max Sand 0.39 0.8 Bedrock 0 0.26 Clay 0 0.001 Clay<150 cm 0.001 0.21 Till 0.23 0.8 Precipitation [mm/year] 833 Evapotranspiration [mm/year] 563

5 Result and Analysis

5.1 Groundwater trends map In order to see how the groundwater levels have changed in Gothenburg since the beginning of systematic monitoring groundwater observations longer than 20 years were classified into different long term trends; Falling, Rings and No trend, and subdivided into a number of reoccurring features; Initial, Variance, Plateau at end and Plateau at beginning, more information under section 4.2.1. The results of the classification are shown in Figure 21, Figure 22 and Figure 23 for the lower, upper and bedrock aquifer respectively. The result of how all groundwater trends have been classified is labelled in a table in Appendix 7.

5.1.1 Lower aquifer The result of groundwater trends in the lower aquifer is shown in Figure 21. The groundwater trends in the lower aquifer can be divided into two regions where more observations with falling trends is observed in the western parts of Gothenburg, which may be a result of an area where underground constructions contributes to groundwater drainage. And in general, more groundwater observations with rising trends are concentrated towards the eastern parts of the city, areas less affected by groundwater drainage. Though, this evaluation can be diffuse as groundwater observations with no trend occur in all regions. The reoccurring feature, “Initial value” marked in red, in Figure 21, indicates a significant drop in the groundwater level which is a result of groundwater extraction and probably related to underground constructions. The observations with “Plateau at the end” are obtained in several of the groundwater observations, assuming that the groundwater level today is at a stable groundwater level. The plateau at the end, also, corresponds well with the explanation about that after 1990 the city attempt to keep the groundwater level at a stable level. Very few groundwater time series have a stable groundwater level in the beginning of the measuring period. The “Variance” is observed in some groundwater observations, showing a change in groundwater level fluctuations, that according to Svensson (1984) is associated to a change in measuring interval, where a more frequent measuring interval give rise to larger amplitude compared with less frequented measurements.

Figure 21 Groundwater trends in the lower aquifer.

5.1.2 Upper aquifer Groundwater trends in the upper aquifer are shown in Figure 22. The groundwater trends are rising and falling; rising with “plateau at end”, rising with “initial value” and falling with “variance” and one time series shows no trend. The groundwater observations are few and specific trends in specific areas cannot be distinguished.

Figure 22 Groundwater trends in the upper aquifer.

5.1.3 Bedrock aquifer The result of the groundwater trend analysis in the bedrock aquifer is shown in Figure 23. Three wells locate close to each other in the central part of Gothenburg show falling, rising and no trend. The groundwater observation with a falling trend also show “plateau at end”. The groundwater observations longer than 20 year in the bedrock aquifer are few and specific trend in specific areas cannot be distinguished.

Figure 23 Groundwater trends in the bedrock aquifer. 5.2 Groundwater observations and time series analysis The relation between groundwater level and precipitation was analysed in order to investigate if it is possible to separate the influences of urban development from influences of climate. The locations of the analyses observation wells are shown in Figure 20. The groundwater level and the precipitation was first plotted in the same graph in order to observed which part of the groundwater time series that should be removed, that part influenced by artificial factors, see Figure 24 for Linné and Figure 26 for Gårda. The moving average of 10 has been used in both time series in order to see the long term trend. The next step in the analysis was to normalise both time series; mean is zero and the standard deviation is one. By normalize the time series they are much easier comparable with each other. The normalised time series are shown in

Figure 25 for Linné and in Figure 27 for Gårda. The result and analysis of both study areas are described in detail below.

5.2.1 Linné The analysed groundwater time series in Linné are shown in Figure 24. Precipitation from 1961- 2013 is marked with green and the groundwater time series in blue, for different wells. The results of the normalisation are shown in Figure 25, where parts of the time series influenced by artificial factors is removed, marked with grey. The result from the time series analysis shows that groundwater levels responds quick to changes in precipitation, the lag time is just a few months. The correlation between precipitation and groundwater level is partly very good, generally, when precipitation decreases the groundwater level is low. It may be seen from the removed groundwater time series that the groundwater level have been lowered or influenced by artificial impacts during the same period, most probably influenced by underground constructions. Observation well GW431, GW433 and GW435 shows a very high groundwater level around 1975, which may be a result of groundwater infiltration. Observation well GW1194, GW1196, GW1517, GW441, GW442 and GW969 shows a rapid decrease in groundwater level around 1984 indicating groundwater extraction, probably result from underground constructions. In all those wells the groundwater trend seems to be falling after 1984, probably due to groundwater drainage. Though, the groundwater level in observation well GW1194 and GW1196 shows that the groundwater level today is nearly the same as the initial level. Observation well GW252 shows a falling trend and no good correlation with precipitation one explanation may be that it is disconnected from its infiltration area.

Table 5 Analysed groundwater observation time series in Linné

Observation First Last well observation observation GW1194 1980 2008 GW1196 1970 2008 GW1517 1982 2008 GW1518 1984 2008 GW252 1968 2008 GW254 1968 2000 GW431 1968 2008 GW433 1971 2013 GW435 1973 2013 GW441 1974 2008 GW442 1974 2013 GW457 1974 2008 GW969 1974 2013

Figure 24 Mean monthly precipitation 1961-2013 and the whole observation time series for the analysed observation wells in Linné. A groundwater level drop can be seen in several observations around 1984.

Figure 25 Normalised groundwater time series compared to precipitation.35 The removed groundwater time series is marked in grey. The response of groundwater levels to changes in precipitation is quick with only a few months delay. 5.2.2 Gårda Groundwater observation wells with series longer than 20 years were scarce in Gårda. Therefore only four time series were analysed. Figure 26 shows the precipitation from 1961- 2013 in green and the whole observation time series in blue for the different wells. Figure 27 shows the result of the normalisation, where parts of the groundwater time series, affected by artificial factors have been removed, marked in grey. The measuring periods of the groundwater observations are labelled in Table 6. Two groundwater wells GW492 and GW494 are closely located and affected by the same groundwater extraction in 1970. The groundwater level has dropped about 2 m, most likely due to underground constructions in the nearby area. The groundwater level in observation well GW492 shows approximately the same level as the initial level, before the groundwater level drop, which indicating full recovery of the groundwater level at the specific location. The observation well is closely located to a hill slope where sand outcrops at the surface, if the sand is connected to the lower aquifer the observation well is closely located to the infiltration area which may explain the full recovery, see Figure 20. The groundwater level in observation well GW494 is 0.7 m below the initial groundwater level, which indicated limited or reduced groundwater infiltration to the location. Possible explanation to areas where no full recovery has occurred can be groundwater drainage because of underground constructions. No groundwater level drop is seen in observation well GW1044 or GW973 which may be because of the first observation was after 1970. If the groundwater level is lowered than the initial is still to be concluded. In general, the groundwater observations show a stable groundwater level toward the end of the monitoring periods. The response time between precipitation and groundwater level fluctuations is very quick, just a few months delay. Quick response time may indicate close distance to infiltration area and/or fast mechanisms of recharge.

Table 6 Analysed groundwater observation time series in Gårda.

Observation First Last well observation observation GW1044 1979 2013 GW492 1967 2003 GW494 1967 2013 GW973 1974 2013

Figure 26 Precipitation from 1961-2013 and the whole observation time series for observation well GW1044, GW492, GW494 and GW973. A significant drop in 1970 is seen observation well GW492 and GW494.

Figure 27 Normalized precipitation and normalized groundwater observations. The removed part of the groundwater time series is marked in grey. Groundwater level responds to changes in precipitation with only a few months delay.

5.2.3 Groundwater level response to precipitation on a yearly basis The relation between groundwater level and precipitation, on a yearly basis, was analysed in four wells. The correlation for observation well GW1517 and GW1194 is shown in Figure 28. The correlations for observation well GW1196 and GW443 are shown in Appendix 8. This analysis allows elimination of long terms effects, such as pumping, and shows the variability driven by climate mainly. Figure 28 shows precipitation above average and the difference in groundwater level from one year to another. The result shows that the groundwater level is high during wet years and low during dry years. Exactly how good the groundwater level and precipitation correlates can be seen in the scatterplots, shown in the same figure. The regression coefficient R2 shows values of 0.34 for GW1517 and 0,119 for observation well GW1194, a poor correlation. If there was a perfect match, regression coefficient, R22 = 1, it would be possible to use the result on order to estimate how much the groundwater level would rise with increased precipitation. In this case, a very roughly estimation would allow an increased groundwater level of 0, 12 m with an increased precipitation of 30 % above average, Table 7. More analysis would be needed in order to validate these estimations.

Table 7 Analysed groundwater observation time series on a yearly basis.

Observation well Y=Kx+m R2 GW1517 0.0005x - 0.0122 0.3442 GW1194 0.0003x + 0.0113 0.119 GW1196 0.0007x - 0.0239 0.1475 GW443 0.0004x - 0.0092 0.0555 Average, x 0,000475 Average precipitation 843 mm/year 1961-2013 Increased precipitation 30 % 1095 mm/year Precipitation above average 252 mm/year Increased groundwater level 0.12 meter

Figure 28 Correlation between precipitation and groundwater level on a yearly basis for observation well GW 1517 and GW1194. 5.3 Potentiometric surface map The potentiometric surface map was created in order to analyse the spatial distribution of the groundwater level in the lower aquifer and to locate potential infiltration areas. The result of the potentiometric surface map is shown in Figure 29. The groundwater level is increasing towards the bedrock area in the southwest, >20 meter above sea level and decreasing towards the Göta Älv River, >1 meter above sea level. The groundwater flow is vertical the equipotential lines and areas with high levels and steep gradients indicate potential infiltration areas where more water entering the system. By comparing the groundwater situation in southwest, Linné, to northeast, Gårda, higher groundwater levels and steeper gradients are observed in southwest. The areas with high groundwater level and steep gradient seem generally to be located close to bedrock outcrops. Steep gradient can be a result of low transmissivity, but it can also be explained by more input of water to the groundwater system, which gives a suggestion of potential infiltration areas. It is generally assumed that coarse material is located close to hill slopes where water can infiltrates to the lower aquifer. A location in northwest, close to a bedrock outcrop, shows a depression in the groundwater level, which may be a result of groundwater drainage. The pumping stations, where groundwater extraction occur, and the infiltration stations, where water is infiltrated to the lower aquifer, may influence the potentiometric surface but to which extent have not been further analysed in this study.

Figure 29 A potentiometric surface map of the lower aquifer. Areas with high groundwater levels and steep gradients are observed close to the bedrock outcrops in southwest, indicating possible infiltration areas. 5.4 Land subsidence The result from the manual subsidence observations, data processing done by Albertsson (2014), is shown in Figure 16. The spatial distribution of the manual observations is poor and therefore not interpolated. The interpolation of the PanGEO data are shown in Figure 30. And a closer view over the subsidence rate in Linné and Gårda together with the contour lines of clay thickness is shown in Figure 33 and Figure 34. The PanGEO data shows generally high subsidence rate along the valleys and along Göta Älv River. The manually observations is located where subsidence have been observed, by damages to buildings. Where the SBK subsidence data intersects with data from PanGEO it is generaly seen that the subsidence rate from PanGEO is higher, which is probably due to the different types of measuring techniques. In Gårda, high subsidence rate is observed close to Mölndalsån, > 5mm/year, Figure 33. It is difficult to see a clear pattern between subsidence rate and clay thickness, which may be due the measuring technique of the PanGEO data that allows measuring of not only vertical movement. The measurement of manually observations is limited in this region.

In Linné areas with subsidence of > 5 mm/year are observed. A general trend seems to be that the subsidence rate increases with increased clay thickness. For example in the northern parts of Linné the clay thickness is 70 m and the subsidence rate 5mm/year, see Figure 34. Land subsidence occurs due to several joint factors: natural soil compaction, soil compaction due to external load, soil compaction due to water extraction, thickness of the clay and thickness and content of filling materials. The heterogeneity within each parameter may also influence the total subsidence rate.

Figure 30 Interpolated land subsidence data obtained from PanGEO. Highest subsidence rate is found in areas close to the Göta Älv River and along the valleys.

5.1 Geological model The geological model was to be created in order to see the extent of the friction material underneath the clay, the extent of the lower aquifer. In order to analyse the relation between land subsidence and groundwater level lowering and clay thickness, eight 2D cross sections was created and presented in next subsection. The result of the interpolated surface of depth to bedrock and clay thickness is shown in Figure 31 and Figure 32. The maximum clay thickness is 95 m. Depth to bedrock is increasing toward the Göta Älv River and the observed maximum is 120 m. A closer view of all interpolated surfaces over Linné and Gårda can be in Figure 33 and Figure 34.

Figure 31 Interpolated depth to bedrock surface, stratigraphic data were obtained from SGU and from SBK. The maximum depth is observed to 120 m.

Figure 32 The interpolated clay thickness, stratigraphic data were obtained from Banverket (2006) and from SBK.

5.1.1 Cross sections Four cross sections were created for Gårda and Linné respectively; the locations are seen in Figure 33 and Figure 34 as well as the interpolated surfaces of the depth to bedrock, clay thickness, the subsidence rate and the potentiometric surface. Cross section 1 and 2 are shown in Figure 35 and Figure 36. The profiles runs in west-east direction, vertical to the Mölndalsån River which is located about 300 m into the profiles. Cross section 5 and 8 in Linné are shown in Figure 37 and Figure 38 respectively. Profile 5 runs in west-east direction, vertical to Linnégatan that is located 450 m into the profile. Profile 8 runs almost parallel to Linnégatan. The result shows that land subsidence increases with increased clay thickness and that the subsidence rate is generally higher in areas where the groundwater level has been lowered. Cross section 3, 4, 6 and 7 is figured in Appendix 9.

Figure 33 The interpolated surfaces over Gårda, a) depth to bedrock, b) clay thickness, c) subsidence rate and contour lines of clay thickness, d) the potentiometric surface.

Figure 34 interpolated surfaces over Linné, a) depth to bedrock, b) clay thickness, c) subsidence rate and contour lines of clay thickness, d) the potentiometric surface.

5.1.1.1 Gårda The stratigraphy of Gårda shows clay thickness of 30 m followed by friction material up to 20 m on top of the bedrock. Where the friction material outcrops at the surface, or where coarser material close to the hill slopes are connected to the friction material underneath the clay can be interpreted as potential infiltration areas to the lower aquifer. The extent of the friction material allows interpretations of one connected aquifer, with the thickness decreasing towards north direction. Profile1: It is a vague correlation between clay thickness and subsidence rate. The subsidence rate increases toward the middle of the valley where clay thickness increases. The highest subsidence rate is found 470 m into the profile 8 mm/year. The potentiometric surface, the groundwater pressure of the lower aquifer, is shallow, with higher level toward the hill slopes. The friction material is interpreted to outcrops at the surface about 150 m into the profile, which could be one of the potential infiltration areas to the lower aquifer. No wells are located close to profile 1. Profile 2: The thickness of the friction material in profile 2, see Figure 36, is less than in profile 1, which interprets that the thickness of the friction material is decreasing in north direction. The subsidence rate shows a coherent pattern with the clay thickness, with a deviation 450 m into the profile, which could be a result of an object with good foundation. The subsidence rate is about 5 mm/year over the whole profile. Observation well GW492 is located close to profile 2, the groundwater level dropped about 2 meter in 1970 but with no correlated subsidence observed.

Figure 35 Profile 1 runs in west-east direction, vertical to Mölndalsån River. The clay thickness is at maximum 30 m and the subsidence rate is about 5mm/year close to the river. The potentiometric surface is higher towards the bedrock outcrop in east. The friction material underneath the clay is interpreted to outcrop about 150 m into the profile, which could be the potential infiltration area to the lower aquifer. No observation well is located close to the profile, and therefore no correlation between groundwater level lowering and land subsidence is observed.

Figure 36 Profile 2 runs in west-east direction, vertical to Mölndalsån River. In east, close to the bedrock hill slope, coarser material is interpreted to be connected with the friction material underneath the clay. The Subsidence rate varies from 0-5 5mm/year. Observation well GW492 is close located to the bedrock outcrop in west. In 1970 the groundwater level dropped 2 m, but no subsidence is observed in the area close to the observation well. The potentiometric surface is increasing towards the bedrock outcrop in west, indicating a potential infiltration area to the lower aquifer.

5.1.1.2 Linné In Linné the friction material underneath the clay is thin 0 - 10 m. The accuracy of the stratigraphic data are low because of few data point, and it may be unclear if the lower aquifer is continuous or if it is divided into several small aquifers. Profile 5 runs in west-north direction and the average clay thickness is about 20 m, see Figure 37. The accuracy of friction material is low and if the friction material is connected throughout the whole profile is unknown. High subsidence rate is concentred to the central parts of the profile, which is the location of Linnégatan. Three observation wells, GW969, GW441 and GW457, are located close to the profile. In GW969 the groundwater level have dropped about 1,5-2 m and corresponds to an area with subsidence rate of <5mm/year. In GW441 the groundwater level has dropped about 2 m which corresponds to subsidence of 10 mm/year. GW452 do not show a groundwater level lowering and the area do not show any land subsidence. The potentiometric surface increases towards the hill slopes and the steeper gradient may indicate an infiltration area. Profile 8 runs parallel to Linnégatan from Linnéplatsen in south to Järntorget in north, see Figure 38. The extent of the friction material is difficult to analyse and it is unclear if it is one connected aquifer. Close to Linnéplatsen, observation well GW441, the subsidence rate is > 10mm/year, the clay thickness about 25 m and the groundwater level has dropped about 2m. In the middle of the profile observation wells GW441, GW435 and GW431 are closely located and if the groundwater level have been lowered here is difficult to analyse, the depth to bedrock is just a few meter and the subsidence rate is low. In the end of the profile the subsidence rate is high above 5 mm/year, no groundwater level lowering is observed, though the clay thickness is about 70m.

Figure 37 Profile 5 runs in west-east direction. Coarser material is located close to the bedrock outcrop in west and interpreted as connected to the friction material underneath the clay, which could be a potential infiltration area to the lower aquifer. The potentiometric surface is increasing toward the will slopes in west and in east. In observation well GW969 the groundwater level has dropped between 1,5-2 m and the subsidence rate is >5mm/year. Close to observation well GW441 where the groundwater level have dropped 2 m the subsidence rate is close to 10 mm/year. In the area around observation well457 no change in the groundwater level have been observed, as well as no land subsidence. It is unclear if the lower aquifer is connected across the profile or not.

Figure 38 Profile 8 runs along Linnégatan from Linnéplatsen on south to Järntorget in north. The subsidence rate close to observation well GW441, where the groundwater level has dropped 2 m, is measured to be about 10 mm/year. In north where the clay thickness is high, about 70 m, the subsidence rate is measured to be >5 mm/year. Close to the observation wells GW435, GW443 and GW431 the clay thickness is thin and the subsidence rate is low.

5.2 Conceptual model and groundwater recharge The conceptual model was constructed in order to estimate the groundwater recharge to the lower aquifer in Gothenburg. The groundwater recharge was estimated by using two different methods. The result of each method is explained below and illustrated in Figure 39 and Figure 40 respectively. The local catchment area in vicinity of Gårda was calculated to be 0.9 km2. The different soil types within the area are 23 % bedrock, 6 % sand, 65 % clay, 3 % till and 3 % is estimated to be clay thinner than 1,5 m. The extent of the catchment and the different soil types is figured in appendix 10.

5.2.1 Method 1 The groundwater recharge was estimated by using Eq. 13 and parameter values in Table 3 presented in section 4.2.6.1. The result is illustrated in Figure 39. The groundwater recharge is estimated to be 270 mm/year and the total groundwater recharge over the local catchment would be 243 000 m3/year, 0.0077 m3/s. This method assumes all water to infiltrate and the total volume of water is large. The lower aquifer is relatively thin and if all water would infiltrate is unlikely. Also, clay which is the dominating soil type in this area, and has low permeability and hydraulic conductivity and does not allow much water would infiltrate and to recharge groundwater. Therefore this method exaggerates the actual groundwater recharge which is also discussed by Barkels and Parra (2010).

Figure 39 Conceptual model illustrating groundwater recharge over a local catchment area in Gårda. The groundwater recharge is 270 mm/year using method 1, assuming no surface runoff.

5.2.2 Method 2 The groundwater recharge was estimated by using Eq. 11 and parameter values presented in section 4.2.6.2. The result is illustrated in Figure 40. By using infiltration coefficient the groundwater recharge is estimated to be 8.5-38 mm/year and the total groundwater recharge over the local catchment would be 7 650-34 200 m3/year, see Table 8.

In the cross section, where the lower aquifer is interpreted to outcrop at 120 m into the profile, it allows direct and quick response between precipitation and groundwater recharge. Coarser material, close to the hill slopes, assumed to be connected to the lower aquifer can also be interpreted as potential infiltration areas. The groundwater recharge through different soil types is estimated to 6.5 -13 mm/year in sand, 0-0.2 mm/year in clay, 0.01-2 mm/year in clay <1.5 m, 2-6 mm/year in till and 0-16 mm/year in the bedrock. The area with clay < 1.5 m was estimated to be 3 % and if the area is underestimated it will have a large effect on the result. The infiltration coefficient used for clay <1.5 m is comparable to infiltration coefficient used for till. The groundwater recharge through the bedrock is mainly dependent on the hydraulic conductivity in the fractures, which have not been further analysed in this thesis. But as shown in the groundwater time series analysis the groundwater level fluctuations response very quickly to precipitation, and even if the hydraulic conductivity in the bedrock is high the transport time within the fracture zone toward the lower aquifer may be long. Thereby an assumption can be drawn that infiltration from the bedrock to the lower aquifer is limited. Infrastructure development would influence the groundwater recharge, with increased and decreased groundwater recharge. More water would be available due to leaking water supply systems. Though, the available water would also decrease as a result of more surface runoff and more water that ends up in storm water drainage systems. Urban expansion would have greatest effects on groundwater in areas with highest groundwater recharge, where the friction material outcrops at the surface. If the usage of water balance equations is a suitable method to use in urban areas is to be discusses. The surface runoff and groundwater recharge may be more dependent on the rain fall event, the intensity and/or the duration of the rainfall, than a constant part of the net precipitation. The heterogeneity within each parameter is difficult to address and influence the accuracy of the result.

Table 8 Percentage of the different soil types of the local catchment area, used infiltration coefficients for different soil types and the groundwater recharge as percentage of the specific area.

Groundwater recharge Area Area Infiltration Infiltration related to the Soil type Km2 % coefficient mm/year percentage of the area mm/year Min Max Min Max Min Max Sand 0.058 6.2 0.39 0.8 105.3 216.1 6.5 13 Bedrock 0.217 23.2 0 0.26 0 70.2 0 16 Clay 0.601 64.2 0 0.001 0 0.3 0 0.2 Clay<150 0.032 3.4 0.001 0.21 0.3 56.7 0.01 2 cm Till 0.028 3 0,23 0.8 62.1 216.1 2 6 Total 0.936 100 8.4 38.3 m3/year 7 650 34 200 m3/s 0.00024 0.0011

Figure 40 The conceptual model illustrating groundwater recharge over a local catchment area in Gårda. The groundwater recharge is 8,5-38 mm/year using method 2.

6 Discussion In the following sections the questions raised in the introduction will be discussed. Limitations of the analysis performed will be explained and the errors of assumptions discussed.

6.1 Discussion of uncertainties and assumptions Before going into the detailed discussion of the individual results, some of the common sources of potential errors and uncertainties are discussed.

6.1.1 Meteorological data The accuracy of meteorological data and measuring of meteorological data are affected by several parameters e.g. precipitation can varies between locations and evaporation depends on temperature, wind and soil etc. The precipitation data obtained from SMHI is from two different measuring stations, data between 1961-2013 are mainly from Gothenburg’s measuring station except data between 1976-1995 which are from Säve measuring station. The evaporation data are obtained from CRU and the reliability of the source is unknown to the author.

6.1.2 Groundwater observations In order to provide the groundwater trend map, the groundwater observations longer than 20 year were analysed visually, and may include uncertain interpretation. The classification criteria used in the trend analysis where excluding groundwater time series that showed falling trend with “initial value”, and resulted in that features as “initial value” were only observed in groundwater time series with rising trends. Measuring errors within the groundwater time series can not be excluded and are a potential source of error in the groundwater observations and time series analysis.

6.1.3 Potentiometric surface map The potentiometric surface map was created by using the mean value of the latest part of the groundwater level observations. The measuring period in different groundwater observation wells varies from a few years to decades, resulting in groundwater levels from different time periods and the potentiometric surface map may not represent the actual groundwater level of today.

6.1.4 Land subsidence data The accuracy of the subsidence data obtained from SBK is unknown to the author but measurement errors of unknown magnitude can not be excluded. The accuracy of the PanGEO data set is difficult to analyse because of the measuring technique that measures distances to objects, with an angle, and allows measuring of the object itself e.g. movement of a building within itself. Satellite data may not only measure the vertical movement of the ground, and can not be directly compared to manual levelling data. The accuracy of the PanGEO data are according to Colombo (2014) about ±1 mm/year.

6.1.5 Geological model This thesis was developed assuming that stratigraphic models or cross sections showing different stratigraphic units in Gothenburg was already available. But that was not the case, information on stratigraphy is abundant, yet there seems to be no complication of the huge amount of data retrieved from individual studies. The stratigraphic models created in this study are limited to few locations with accurate data. The data of the clay depth used from Banverket (2006) was partly interpreted and it seems that the clay thickness at some locations have been exaggerated and the extent and location of the friction material has therefore been very difficult to analyse.

6.1.6 Conceptual model of groundwater recharge The conceptual model of groundwater recharge was constructed with a lot of assumptions. The groundwater recharge was first estimated using a water balance equation, assuming no runoff. This contributed to a large amount of water and because of a very small groundwater system it is not likely that all that water would become groundwater. Using infiltration coefficient, assuming infiltration as a constant part of the net precipitation may not either be very accurate. The calculated catchment area, the “local” catchment, may not represent the soil cover of the actual catchment. It is important to point out that the infiltration is probably more dependent upon the duration and the intensity of the rainfall event. The surface runoff is influenced by the slope, geology and heterogenic parameters, which have not been analysed in this study. The water that actually contributes to groundwater recharge is also dependent on connected pathways from the surface e.g. outcropping of coarser material, connected to the lower aquifer, pathways between bedrock and the lower aquifer and also, in urban areas, where a lot of boreholes punctures the clay and generates pathways for the water to enter and exit the groundwater system.

6.2 How have groundwater levels changed in Gothenburg since the beginning of systematic monitoring? The analysed groundwater time series were restricted to series longer than 20 years in order to analyse the long term trend. The groundwater trend in the lower aquifer showed generally more falling groundwater trends in the western parts of Gothenburg city, which may be a result of underground constructions contributing to groundwater drainage. In the eastern parts of the city the long term trend is generally raising, which may be a result of fewer artificial influences. This assumption may be diffuse as observations with no trend occur in all regions. In a small number groundwater level series with measurements starting before 1970, it could be observed that the groundwater level started from a much higher level than what is observed after the 1970. These have been classified as an “initial” groundwater level, which may correspond to the undisturbed groundwater level before 1970, which have after 1970 been lowered because of underground constructions. Most groundwater time series in the lower aquifer show a stable groundwater level towards the end of the measuring period, which corresponds to the efforts made after 1990 to keep groundwater levels at a stable level. At locations where it is clear that the groundwater level has been lowered by artificial causes, such as groundwater extractions, most groundwater observations show a recovery period, where the groundwater level rises again. The recovery time is a few years on average but seems to vary between different locations. In some locations it seems as the groundwater level never fully returns to its initial groundwater level. Groundwater levels with a rising trend can be a result of increased precipitation as well as recovery from earlier groundwater extractions. Groundwater level with a significant rise can be a result of artificial infiltration which has been used in order to raise the groundwater level back to its initial level. The effects of artificial pumping, infiltration and underground constructions is most likely present in several locations and may affect the recovery time as well as the groundwater level and the groundwater trend. The specific locations of underground constructions are not presented in this thesis due to reasons of confidentiality. A changing climate with increased precipitation and with heavier rainfall events will most likely lead to more surface runoff and increased risks of flooding. If the groundwater level in the lower

54 aquifer will rise as a result of increased precipitation will be controlled by the extent of groundwater drainage.

6.3 Is it possible to separate the influence of urban development from influences of climate? The groundwater level in the lower aquifer show relatively quick responses to fluctuations in precipitation, the response time is in the range of a few months. There is a clear coherent pattern between fluctuations in precipitation and fluctuations in groundwater levels. As the groundwater level in the lower aquifer follows the common trend of precipitation it is possible to separate the influence of artificial factor such as groundwater extraction or drainage, seen as deviation from the fluctuations in precipitation. The quick response time between precipitation and groundwater level indicate fast infiltration to the lower aquifer, or that the location of the observation wells is close to the infiltration area. The precipitation in the Gothenburg region has increased since 1961. More precipitation is likely to result in more groundwater, but the intensity and the duration of the rainfall event controls the rate of groundwater recharge. And an intense rainfall event may instead only allow more surface runoff which is also discussed by Fetter (2001). If the groundwater level is to increase, follow the common trend of precipitation, is difficult to analyse as the groundwater level also depend on drainage of underground constructions, pipelines and storm water systems. Several groundwater observations show a rising trend, influenced by increased precipitation but maybe most likely the rising trend is reflecting the recovery period from a previous groundwater extraction e.g. observation well GW1194 in Figure 24. Groundwater levels lower than the initial groundwater level may indicate limited infiltration to the aquifer or drainage due to neighbouring underground constructions. Groundwater observations in proximity to constructions sites show less correlation between groundwater level fluctuations and fluctuations in precipitation e.g. observation well GW252 in Figure 25. Groundwater recharge is often thought to decrease in urban areas, as a result of impermeable soil cover, more surface runoff and drainage. But that is not always the case; groundwater recharge may also increase in urban areas due to leaking water supply systems, contributing to more groundwater recharge, also been discussed by Lerner (1990). However, it is most likely that in humid regions with a shallow water table, the drainage into water pipes and into storm water drainage systems is comparable to the leakage, how much water that actually contributes to groundwater is difficult to quantify. In order to separate the influences of urban development from influences of climate the natural groundwater fluctuation need to be known, and the groundwater time series need to be long enough. In the analysis used in this study the coherent pattern between precipitation and groundwater levels was mainly seen after the 1990, which may be due to a disturbed groundwater situation before 1990. In order to confirm if groundwater level lowering is natural occurring or result of artificial factors a groundwater time series longer than six months is required according to Lundmark (2001).

6.4 Analyse the spatial distribution of groundwater levels in Gothenburg The created groundwater contour map of the lower aquifer in Gothenburg shows high groundwater level close to the bedrock outcrops in southwest region. High groundwater levels and steep gradients indicate areas where more water is entering the groundwater system, potential infiltration areas. The groundwater situation in the northeast does not show steep gradients or high groundwater levels, assuming less infiltration. Important to mention is the extent and locations of the observation wells where more observation wells are located in the southwest compared to northeast and may influence the result of the potentiometric surface map. The used groundwater levels used in the interpolation of the potentiometric surface represent groundwater levels from not exactly the same period so the created potentiometric surface may not represent the groundwater level of today. Also to notice is the artificial influences on groundwater levels, infiltration and extraction, which influence the accuracy of the potentiometric groundwater map and the interpretations of the infiltration areas. The response time between precipitation and groundwater level variations is quick, inferring fast groundwater infiltration to the lower aquifer. The potential infiltration areas may be close to hill slopes or/and where coarser material connected to the lower aquifer outcrops In order to improve the groundwater contour map over Gothenburg city, groundwater level data from a recent measuring period would be needed and also increase the numbers of observation wells in northeast region.

6.5 Investigate the relation between land subsidence, clay thickness and groundwater lowering in Gothenburg The relation between land subsidence, clay thickness and groundwater level lowering has been illustrated by eight stratigraphic cross sections. The cross sections show areas with high subsidence rate that coincide with areas of high clay thickness or/and where the groundwater level have been lowered. Previous studies conducted in Gothenburg have confirmed that lowering of the groundwater level as well as the usage of filling material have contributed to land subsidence e.g. subsidence in Gårda studied by Svensson (1991). The land subsidence data from the satellite project PanGEO shows in general higher subsidence rates than the manually measurements obtained from SBK. The main reason for that is probably that the satellite data measures both the vertical and the horizontal movement of the object an example is settlement of the building in itself, which cannot directly be converted to the land subsidence. The uncertainties about the measuring technique are also discussed by Crosetto et al. (2010). The accuracy of the stratigraphic data varies, where the thickness of the clay has been exaggerated and resulted in areas where the clay is thicker than the actual depth to bedrock. This has contributed to difficulty when analysing the extent and thickness of the friction material, the lower aquifer. The relation between subsidence and groundwater level lowering has been difficult to analyse. The total subsidence rate is more likely a result of different processes; compaction of filling material and consolidation of clay due to external load and groundwater level lowering. New infrastructure development, external load on the ground, impermeable soil cover and groundwater drainage due to underground construction will most likely lead to additional and continued subsidence in Gothenburg region.

6.6 Investigate the possibility to quantify groundwater recharge The groundwater recharge in Gårda, have been estimated using a water balance equation and infiltration coefficients. The groundwater recharge has been estimated to be 270 mm/year using the water balance equation and assuming no runoff. By using infiltration coefficient the groundwater recharge was estimated to be 8.5-38 mm/year. By using infiltration coefficients the groundwater recharge is assumed to be concentrated to areas where coarser material outcrops at the surface and are connected to the lower aquifer. The groundwater recharge through the bedrock is dependent on the permeability and the hydraulic conductivity of the fractures, which have not been further analysed in this project. If all water infiltrates, or how much water that contributes to the surface runoff is still to be defined. Surface runoff in rural environments is mostly dependent on the intensity and the duration of the rainfall event, discussed by Wilson (1971). In urban areas, surface runoff is mostly dependent on the extent of impermeable soil covers and the groundwater drainage caused by underground constructions and storm water drainage systems. This conceptual model gives a very rough estimate of the actual groundwater recharge. Groundwater recharge in urban areas is a complex system and influence by several parameters. Constructing a better geological model, analysing the potential infiltration areas and quantify the infrastructure parameters would give a better understanding of the groundwater situation in Gothenburg. In order to address the question how groundwater is influenced by urban development, several parameters need to be known and analysed together. These parameters are of great importance but they may also be difficult to quantify; how much groundwater gets drained by underground constructions, how much surface runoff occurs due to impermeable soil cover, how much artificial extraction and infiltration occur and how much water from leaking pipes contributes to groundwater recharge?

7 Conclusion Groundwater in urban area is dependent upon several factors and thereby a complex system. Groundwater monitoring is an important method to control and analyse the effects of infrastructure development on the groundwater level. In terms of water quality, ecosystems and soil stability it is important to keep groundwater at a stable level. Both rising and falling groundwater level may harm the environment and infrastructure. The hydrological situation of Gothenburg addresses two aquifers; one unconfined aquifer in the filling material and one confined aquifer in the friction material underneath the clay. The lower confined aquifer is thin and most likely separated into several small aquifers. The storativity of the aquifer is low and groundwater extraction or limited recharge will allow large changes in the groundwater level over large areas. This study has concluded that:

 In general, the groundwater levels in Gothenburg area seems to be rising, following the common trend of precipitation. In the central parts of the city the groundwater levels are obviously recovering from the deep drawdown in the 1970.  The mechanism of recharge seems to be very fast, shown by a short delay between precipitation and groundwater level response.  The groundwater infiltration areas seem to be close to bedrock outcrop and/or where coarser material, connected to the lower aquifer outcrops, shown by high groundwater level and steep groundwater gradients.  Areas where the groundwater level has been lowered shows high subsidence rate.  Areas with thick clay shows high subsidence rate.  Quantifying groundwater recharge using a conceptual model and infiltration coefficients gives a rough estimation.

8 Future studies The fact that groundwater in urban area is a complex system is well known. Understanding of the system is of great importance in order to avoid damages to infrastructure and ecosystems. In the beginning of this study the final goal was to achieve a model predicating groundwater level changes in an urban area as a result of different processes, such as climate change and urban development. This instead may be a recommendation for what can be done in future studies Figure 41 illustrated the workflow of this thesis and, in blue boxes, recommendations of what can be done in the future. This study has focused on the long term behaviour of the groundwater fluctuation in the lower aquifer and it would be interesting to involve shorter groundwater time series in the analyses. Further enhancement can be analysing groundwater time series in the bedrock aquifer and in the upper aquifer. Creating a more detailed geological model would benefit for future studies and may also increase the understanding of the groundwater situation in Gothenburg. The analysis of groundwater recharge patterns in relation to the spatial distribution of groundwater levels and the response of groundwater level to precipitation leaves some open questions concerning the generally applied models of groundwater recharge in Gothenburg. It may be recommended that these are checked and verified using appropriate methods, including groundwater models or/and tracer based methods.

Figure 41 Illustration of the workflow of this thesis and with recommendations of future studies, blue boxes.

9 Acknowledgement I would like to thank my supervisor Roland Barthel, thanks for great support during this project. And also a great thank to my supervisor at the Office of City Planning, Niklas Blomquist, for providing data and for making this thesis possible. Thanks Lars Over-Lång at SGU for helpful information about groundwater in Gothenburg. Thanks to Davide Colombo at treuropa for helpful discussions about Pangeo data. Great thanks to Ezra Haaf and to my opponent, Marléne Gustavsson, for good comments and critical reading of this report. And at last a grateful acknowledgment to all friends at University of Gothenburg and at Uppsala University.

10 References Albertsson, Anna. (2014). Ground subsidence and its associations to geological properties and groundwater level changes in Göteborg. Alte, Bo. (1981). Att mäta sättningar på sikt. Statens råd för byggnadsforskning, 51-55. Aronsson, Inge. (1980). RTAB Geotekniskt utlåtande för planerande nybyggnader. Geoteknisk utredning från stadsbyggnadskontorets geoarkiv. Aronsson, Inge. (1985). Gårda Göteborg. RTAB. Aronsson, Inge. (1991). Haga 8:3 Utredning av erfodeliga åtgärden avseende grundläggning. RTAB. Banverket. (2006). Västlänken en tågtunnel under Göteborg, underlagsrapport Grundvatten. Barkels, David, & Parra, Alejandra Silva. (2010). ANALYS ÖVER INLÄCKAGE AV GRUNDVATTEN TILL FÖRBIFART STOCKHOLM FÖR DELTUNNEL UNDER LOVÖ. Bergström, Sten. (1993). Sveriges hydrologi: grundläggande hydrologiska förhållanden: Sveriges meteorologiska och hydrologiska institut (SMHI). Blom, Anders. (2013). Grundvattenbildning - Ett sätt. Boutelje, Julius. (1981). Ruttnande grund. Statens råd för byggnadsforskning. Budhu, Muniram, & Adiyaman, Ibrahim Bahadir. (2010). Mechanics of land subsidence due to groundwater pumping. International Journal for Numerical and Analytical Methods in Geomechanics, 34(14), 1459-1478. Claesson, A, & Höglund, M. (2014). Göteborgs stadsexplandering år 1820-1920. Poster. Colombo, David. (2014). Limitations of the PanGeo data. E-mail correspondance 24 march 2014. Crossetto, M, Monserrat, O, Iglesias, R, & Crippa, B. (2010). Persistent Scatterer Interferometry: Potential, Limits and Initial C- and X-ban Comparison. Pothogrammetric Engineering and Remote Sensing, 76(9), 1061-1069. Davis, Stanley N, & Dewiest, Roger J. (1966). Hydrogeology. De Vries, Jacobus J, & Simmers, Ian. (2002). Groundwater recharge: an overview of processes and challenges. Hydrogeology Journal, 10(1), 5-17. Engdahl, Mats, & Jelinek, Cecilia. (2013). Geohazard decription for Göteborg Version 1. Fetter, Charles Willard. (2001). Applied hydrogeology (Vol. 3): Prentice Hall Upper Saddle River, NJ. Freeze, R Allen, & Cherry, John A. (1979). Groundwater, 1979. Printice-Hall, New Jersey. Göransson, Gunnel I., Bendz, David, & Larson, P. Magnus. (2009). Combining landslide and contaminant risk: a preliminary assessment. Journal of soils and sediments, 9(1), 33-45. Göteborgs Stad. (2006). Extrema vädersituationer - Hur väl rustat är Göteborg? , Rapport 1. Hansbo, Sven. (1981). Det farliga droppet - om tunnelbygge i otätt berg. Statens råd för byggnadsforskning, 14-16. Holmstrand, Olov. (1980). Lokalt omhändertagande av dagvatten-Sammanfattning av forskning om dagvatten infiltration vid CTH 1976-79: Chalmers University of Technology. Hultén, Anna-Maria. (1997). Grundvatten i urban miljö: grundvattnets nivåvariationer i de övre marklagren i Göteborg: Chalmers University of Technology. Hummel, Jens. (2004). Skansberget hydrogeologisk utredning. Ramböll. Knutsson, Gert, & Fagerlind, Torbjörn. (1977). Grundvattentillgångar i Sverige: SGU. Knutsson, Gert, & Morfeldt, Carl-Olof. (2002). Grundvatten: teori & tillämpning: Svensk byggtjänst. Lerner, David N. (1990). Groundwater recharge in urban areas. Atmospheric Environment. Part B. Urban Atmosphere, 24(1), 29-33. Lerner, David N. (2002). Identifying and quantifying urban recharge: a review. Hydrogeology Journal, 10(1), 143-152. Lerner, David N, & Barrett, Mike H. (1996). Urban groundwater issues in the United Kingdom. Hydrogeology Journal, 4(1), 80-89. Lundmark, Annika. (2001). Analys av grundvattennivåer vid undermarksbyggande i urban miljö. Examensarbete. Institutionen för Mark-och Vattenteknik, KTH.

Maxe, Lena, & Thunholm, Bo. (2007). Områden där grundvattennivån är av särskild betydelse för vattenkvalitet, markstabilitet eller ekosystem. SGU-rapport. http://www. sgu. se/dokument/service_sgu_publ/SGU-rapport_2007-20. pdf Hämtad, 24, 2012. Naturvårdsverket. (2012). Miljömål. Retrieved 2012-06-07 from http://miljömål.se/sv/Miljomalen/9-grundvatten-av-god-kvalitet/Preciseringar-av- grundvatten-av-god-kvalitet/ Norin, M, Hultén, AM, & Svensson, C. (1999). Groundwater studies conducted in Göteborg, Sweden. INTERNATIONAL CONTRIBUTIONS TO HYDROGEOLOGY, 21, 209-216. Olin, Malin. (1994). Modelling of Groundwater Recharge and Aquifer Charachteristics [sic] from Level Recordings in Aquifers. Geologiska institutionen, Chalmers tekniska högskola och Göteborgs universitet. Persson, H. (2008). Estimation of Pore Pressure Levels in Slope Stability Calculations: Analyses and Modelling of Groundwater Level Fluctuations in Confined Aquifers Along the Swedish West Coast: Chalmers University of Technology. Scanlon, Bridget R, Healy, Richard W, & Cook, Peter G. (2002). Choosing appropriate techniques for quantifying groundwater recharge. Hydrogeology Journal, 10(1), 18-39. SGU. (2013). Bedömningsgrunder för grundvatten. rapport 2013:01. SMHI. (2007). Västkusten - Temperaturförändring. Strannelind, M. (1994). Mölndalsåns dalgång: M. Strannelind. Sun, H, Grandstaff, D, & Shagam, R. (1999). Land subsidence due to groundwater withdrawal: potential damage of subsidence and sea level rise in southern New Jersey, USA. Environmental Geology, 37(4), 290-296. Svensson, Chester. (1984). Analys och användning av grundvattennivåobservationer: Analysis and use of ground-water level observations. Svensson, Lennart. (1991). Scandinavium heden 34:17. J&W. Svensson, Lennart. (2008). Gårda 18:25 PM Geoteknik och Grundläggning. WSP. Sällfors, Göran. (2001). Geoteknik. Göteborg, Chalmers Technical University. Trafikverket. (2013). PM Hydrogeologiska förutsättningar. Wassenius, Jan. (1993). Geoteknisk rapport. Scandiaconsult, Göteborgs stads bostad ab. Wassenius, Jan, & Jansa, P O. (1979). Göteborg Liseberg - Geoteknisk undersökning. Allmäna ingenjörsbyrån AB. Wedel, PO. (1978). Grundvattenbildning, samspelet mellan jordlager och berggrund-Exemplifierat med ett försöksområde i Angered: Chalmers University of Technology. Wilson, Eric Montgomery. (1971). Engineering hydrology Engineering hydrology: Macmillan. Wu, Jinquan, Zhang, Renduo, & Yang, Jinzhong. (1996). Analysis of rainfall-recharge relationships. Journal of hydrology, 177(1), 143-160.

Appendix 1

Djupedalsbäcken

Djupedalsbäcken along Linnégatan that have been excavated when the area became urbanized (Aronsson, 1980).

Appendix 2

Groundwater regime in Sweden

The figure below shows the different groundwater regimes in Sweden in different geographical location. The temperature allows precipitation to vaporize during summer months and the precipitation to accumulated in some regions during the winter months (Knutsson & Fagerlind, 1977).

Appendix 3

Measuring interval

Analysis of different measuring intervals shows that less frequent measuring will exclude the short- term fluctuations (C. Svensson, 1984).

Appendix 4

Effective precipitation in Sweden

The effective precipitation in Sweden, variations from <200->600 mm/year. The effective precipitation gives a rough estimation of the potential groundwater recharge (SGU, 2013).

Appendix 5

Runoff coefficients

Runoff coefficients in urban and rural environments (Hultén, 1997) .

Runoff coefficients Area location Urban runoff City 0,70-0,90 Suburb 0,50-0,70 Development area 0,30-0,70 Park 0,10-0,30 Soil type Rural runoff Soil without vegetation 0,4 Soil with vegetation 0,35 Agriculture 0,3 Forest 0,18

Appendix 6

Clay thickness

The primary data used for the Caly thickness have mainly been obtained from Banverket’s report “Järnvägsutredning Västlänken” (Banverket, 2009) .

Appendix 7

Groundwater trend analysis

The table below shows the analysed groundwater time series in the lower aquifer, the upper aquifer and in the bedrock aquifer. The coordinates for each groundwater observation well is given in Sweref99. If the groundwater time series is marked with “clear/unclear” it indicates that the long term trend can clearly bee seen or not. If the groundwater time series is marked “weird” it is referred to something that could be a measuring error in the observation time series, for example if only one measuring point is deviating from the trend. In the “remark” column it is written if the groundwater time series shows a fluctuation pattern coherent to fluctuation patterns seen in precipitation

variations.

Weird

Aquifer

Remark

Variance

t beginning or end orbeginning t

Initial value

Clear Clear /unclear

Observation wellObservation

Rising /falling /no /no trend Rising /falling Plateau a

GW118 6392293,27 145443,4 Bedrock F Pb c GW1435 6397410,6 147403,41 Bedrock F Pe c GW1932 6397601,24 147292,96 Bedrock N u GW796 6406717,16 149064,86 Bedrock N c GW1933 6397583 147336,17 Bedrock R c GW41 6395787,54 143610,64 Bedrock R c GW1196 6397286,22 147245,13 Lower F Pe c GW130 6395935,57 144581,28 Lower F Pe c w GW1424 6396308,19 148219,1 Lower F Pe c GW1430 6396956,43 147613,75 Lower F Pe c w GW1437 6408034,37 151720,24 Lower F Pe c w GW148 6396394,82 145335,99 Lower F Pe u GW1496 6407131,8 153921,73 Lower F Pe u w GW1511 6396972,35 146160,91 Lower F Pe c GW1512 6397095,76 146218,18 Lower F c GW1513 6396923,83 146211,42 Lower F Pe c GW1514 6396734,64 146132,48 Lower F Pe c GW1518 6397257,63 146913,84 Lower F Pe u GW1523A 6407468,43 150945,62 Lower F Pe c w

GW1584 6397209,19 147443,34 Lower F Pe c GW160 6396253,95 145128,55 Lower F w

GW164 6396164,15 145011,63 Lower F Pe u GW168 6396095,04 145216,92 Lower F Pe c GW169 6396220,86 145428,22 Lower F Pe c GW1741 6397816,11 150179,67 Lower F Pe c w GW1793 6407959,56 151707,84 Lower F c GW18 6392368,75 142798,54 Lower F c GW186 6395369,87 146785,5 Lower F c GW201 6398305,725 144081,541 Lower F Pe c GW244 6397277,61 146165,04 Lower F c Climate GW246 6397037,9 145815,58 Lower F Pe c GW249 6396859,34 145869,75 Lower V F Pe c GW252 6397554,21 146954,87 Lower F c GW254 6397459,64 146912,9 Lower F Pe c GW255 6396736,53 146142,5 Lower F Pe c GW32 6392087,69 142911,72 Lower F c GW40 6395829,6 143461,26 Lower F Pe c GW441 6397016,11 147158,36 Lower F Pb c GW442 6396848,31 147137,64 Lower F Pb c GW451 6397294,29 147725,22 Lower F Pe c GW56 6397795,04 143564,3 Lower F Pe c GW588 6408151,21 148046,73 Lower F Pe c w GW969 6397048,36 146939,69 Lower F Pe c GW973 6397128,86 149227,57 Lower F Pe u GWSK4 6397650,74 147292,87 Lower F Pb u GW1001 6408817,26 149038,82 Lower N c GW1028 6403152,1 147850,38 Lower N u GW1044 6397733,17 149101,77 Lower N u GW1111 6396698,08 144922,08 Lower N u w GW113 6392533,96 145593,85 Lower N c GW1209 6401419,85 147657,71 Lower N c GW1212 6396740,69 150404,8 Lower N u GW1223 6396416,67 150511,36 Lower N c GW124 6403857,86 148078,33 Lower N u GW146 6396347,85 145624,52 Lower N u GW1509 6396785,7 145931 Lower N c GW1581 6397409,31 147529,4 Lower N u GW1583 6397254,68 147492,81 Lower N u GW162 6396093,36 144893,9 Lower N u GW1627 6405213,85 151579,64 Lower N c GW1628 6405197,7 151594,48 Lower N u GW1655 6401291,92 151471,38 Lower N u GW167 6396158,57 145165,57 Lower N c GW1672 6409093,35 145101,57 Lower N c w

GW1679 6397802,06 151311,19 Lower N u GW1709B 6398368,65 147984,27 Lower N u GW174 6396069,02 145606,67 Lower N u GW1758 6400950,49 144121,46 Lower N c Climate GW1927 6397643,04 147338,85 Lower N u GW1929 6397633,62 147360,47 Lower N u w GW194 6398573,57 143999,28 Lower N c w GW203 6398332,55 144195,82 Lower N u GW217 6397557,27 144039,25 Lower N c Climate GW218 6397450,59 144094,78 Lower N c GW225 6398798,06 145121,61 Lower N u w GW235 6397059,86 145154,74 Lower N u w GW262 6402388,3 144228,47 Lower N c Climate GW285 6396146,51 147893,45 Lower N u GW302 6395428,67 147583,1 Lower N c GW315 6395042,89 147851,14 Lower N u GW325 6395154,66 147194,29 Lower N u GW332 6396207,02 148737,13 Lower N c GW362 6398173,74 147975,26 Lower N u GW378 6397983,1 147745,3 Lower N u w GW44 6395489,4 143719,23 Lower N u GW457 6397094,04 147749,17 Lower N u GW624 6414920,72 149084,29 Lower N u GW677 6399052,95 150416,19 Lower V R c GW759 6407221,97 148791,87 Lower N c GW773 6407149,87 148726,34 Lower N u GW795 6406710,6 148977,97 Lower N c w GW83 6390239,95 146248,35 Lower N u GW843 6402409,73 148177,77 Lower N u GW889 6410144,98 152198,99 Lower N u GW988 6406119,45 149137,11 Lower N u GW990 6406046,5 149075,76 Lower N c GW991 6406021,14 149054,11 Lower N u S01A 6397235 149361,37 Lower N u w SGU Lower N c Varberg 13:2 SGU Lower N c Kungsbac ka 52:9 GW1021 6407681,53 149873,92 Lower R Pe u GW107 6392812,29 145169,89 Lower R Pe u GW1114 6396673,35 144895,83 Lower R Pb u w GW116 6392258,46 144966,56 Lower R Pb c

GW1194 6397376,16 147446,05 Lower R Pe c GW1203 6402024,42 147407,9 Lower R P c GW1214 6396648,5 150333,73 Lower I R Pe u GW122 6396633,01 144345,4 Lower V R Pe c GW1227 6395833,14 151383,97 Lower I R c GW1255 6396510,75 145731,19 Lower R Pe c GW127 6396216,6 144385,15 Lower R c Climate GW1432 6397190,58 147697,16 Lower R Pb c GW1433 6397284,16 147446,11 Lower R Pe c Climate GW144 6396435,08 145310,4 Lower V R Pe u GW1517 6397333,62 146914,61 Lower I R Pe c GW1525 6401254,19 151501,42 Lower R Pe u GW1536 6406593,55 154033,13 Lower R Pe c GW154 6396337,98 145514,42 Lower R u w GW1552 6408943,61 158269,48 Lower R Pe c GW1558 6409182,88 158573,39 Lower R Pe u GW1572 6402569,78 152948,52 Lower R Pe u GW1580 6397469,68 147493,02 Lower R Pe u GW1582 6397362,99 147462,92 Lower R Pe u w GW171 6396193,61 145549,94 Lower R w GW1726A 6398227,79 148164,82 Lower R c GW1759 6400310 144167,89 Lower R Pe c GW184 6394553,22 146047,06 Lower R Pe c GW1924 6397683,34 147295,31 Lower R Pe c GW1926A 6397657,08 147356,94 Lower R c GW193 6394903,77 146890,72 Lower I R Pe u GW214 6397941,51 144054,1 Lower R Pe c GW247 6396943,42 145861,61 Lower R c GW270 6404666,43 146942,92 Lower R c w GW278 6396228,43 147176,89 Lower R c GW326 6395114,56 147203,88 Lower R u GW333 6395947,93 148961,73 Lower I R c GW352 6398201,45 147614,54 Lower I R Pe c GW385 6397911,09 147454,55 Lower I R Pe c GW388 6397874,23 147538,17 Lower I R Pe c GW415 6397615,8 147384,51 Lower I R Pe c GW416 6397624,41 147422,6 Lower I R Pe c GW431 6397454,5 147120,85 Lower R Pe u GW433 6397388,42 147129,18 Lower R u w GW435 6397324,28 147143,02 Lower I R Pe u w GW439 6397217,49 147316,42 Lower R Pb c GW465 6397946,24 148315,93 Lower I R Pe c GW477 6397769,44 148004,11 Lower R Pe c

GW485 6397475,6 148572,11 Lower R c GW487 6396909,48 148387,3 Lower R u w GW492 6398042,62 149253,95 Lower I R c GW494 6397975,11 149011,25 Lower I R Pe c GW53 6397858,35 143533,95 Lower R Pb c GW617 6412010,73 148492,37 Lower I R Pe c GW680 6399098,05 151513,84 Lower R Pe c GW693 6400633,69 151980,63 Lower R Pe c GW700 6400552,74 152434,44 Lower R Pe c GW705 6400443,39 151924,38 Lower R c GW717 6400042,64 152055,52 Lower I R c GW721 6399871,37 152566,98 Lower R Pe c GW733 6399909,19 151921,48 Lower R Pe u w GW744 6399770,95 152102,59 Lower V N c GW745 6399750,13 151346,84 Lower I R Pe c GW763 6399573,75 151111,16 Lower I R Pe c GW768 6399474,65 151286,51 Lower R c w GW79 6390376,96 146001,92 Lower R c Climate GW894 6409863,93 152679,9 Lower R Pe c GW905 6408939,48 153183,95 Lower R c GW940 6408928,5 153039,93 Lower R Pe c GW975 6401270,46 144645,69 Lower R u w GW141 6396532,91 145716,41 Lower w GW1428 6396314,83 147797,46 Lower w GW1436 6397571,04 147361,05 Lower w GW1441 6407936,84 151777,61 Lower w GW1520 6397304,68 147103,32 Lower w GW1537 6406581,31 154070,76 Lower w GW1554 6408942,67 158395,89 Lower w GW1626 6405262,14 151551,14 Lower w GW1678 6397804,53 151290,66 Lower w GW173 6396092,55 145458,9 Lower w GW1844 6405701,3 151653,53 Lower w GW36 6395260,65 142711,51 Lower w GW414 6397578,01 147364,12 Lower w GW430 6397462,08 147064,93 Lower w GW432 6397427,71 147100,58 Lower w GW250 6396872,57 145943,89 Lower N c GW319 6395213,64 147195,9 Lower N u GW1027 6403256,98 148988,98 Upper F c GW129 6395913,08 144562,16 Upper F Pe c GW468 6397898,21 148124,44 Upper V F Pe c GW1666 6388738,75 146682,48 Upper N c Climate

GW1709 6398368,67 147982,27 Upper N c GW593 6406809,89 147697,59 Upper N c GW601 6411090,89 149156,94 Upper N c SGU Upper N u Lerum 54:10 GW1226 6395904,11 151210,44 Upper R Pe c GW156 6395917,97 144542,1 Upper R w GW1719 6398217,641 147887,712 Upper R u w GW1726 6398230,8 148163,85 Upper R Pe c GW493 6398003,44 148979,54 Upper I R Pe c GW77 6390775,37 146652,78 Upper R Pe c GW772 6399276,32 152381,66 Upper R u GW1846 6397979,71 148952,29 Upper w

Appendix 8

Comparison between precipitation and groundwater level in well 239 and 445

Regression coefficient, R2, for observation well 239 is 0,1475 and for observation well 445 R2 = 0,0555.

Appendix 9

Cross section showing the relation between, groundwater level, land subsidence and clay thickness

Profile 3 and 4 is located in the Gårda region and profile 6 and 7 is located in the Linné region.

Appendix 10

Local catchment of Mölndalsån River

The figure below shows the result of the GIS analysis of the local catchment area to Gårda. The calculated soil type areas are used as parameter values in the conceptual model calculating the groundwater recharge. The local catchment is made up of 23% bedrock, 6% clay (postglacial clay and glacial clay), 3% till and 6% of sand.