The Hydrogeochemistry and Water Balance of Åknes Rock Slope
Frida Liv Biørn-Hansen
Thesis submitted for the degree of Master of science in Geosciences
Physical Geography, Hydrology and Geomatics 60 Credits
Department of Geosciences
Faculty of Mathematics and Natural Sciences
UNIVERSITY OF OSLO
June 2019The
Slope
Frida Liv Biørn-Hansen
Master Thesis in Geosciences Department of Geosciences
Faculty of Mathematics and Natural Sciences June 2019
UNIVERSITY OF OSLO
c 2019 Frida Liv Biørn-Hansen
The Hydrogeochemistry and Water Balance of Åknes Rock Slope
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Abstract
Norway is challenged by a high number of unstable rock slopes, of which seven are under constant surveillance. Åknes, Synnulfsfjorden in Møre og Romsdal, is the largest moni- tored instability. A collapse would cause a tsunami with disastrous consequences for the county. Groundwater oscillations have a large effect on the slope’s movements. Therefore, a project was initiated in 2017 by the Norwegian Water Resources and Energy Directorate to assess drainage as a stabilizing measure. As part of the project, this study’s objectives are to describe the hydrogeological conditions, the hydrogeochemistry and estimate the water balance. Data collection was carried out between June-September, 2018. The field work consisted of discharge and field parameters (electrical conductivity, pH and temper- ature) measurements, and collection of water and rock samples.
The catchment area above the backscarp should be considered as two independent seg- ments, C1 in the west and C2 in the east. From C1, the water enters the backscarp and flows westwards through sub-surface fracture systems. The input from C2 enters the rock slope as surface runoff, infiltrates and migrates southwards. Evapotranspiration has been estimated by four methods (Tamm, Thornthwaite, Hargreaves, XGEO), which delimited the 2017/2018 water input to 86-91% of the total precipitation and snowmelt (2338.51 - 2460.97 [mm]). The flow rate measurements revealed large temporal variations in groundwater discharge. Therefore, the values could not be generalized, and the annual groundwater discharge was not possible to estimate. Short-term water balances could be estimated for the field campaigns, and revealed that July experienced a decrease in groundwater storage, and September an increase. Rainfall, snowmelt and evapotranspi- ration were included in the water balance calculations, while soil retention time, field capacity and the groundwater’s residence time were excluded.
Findings indicate that Åknes consists of two separate hydrological systems. The eastern part is fed by C2, and is characterized by perched aquifers. The springs are diluted and ephemeral. C1 feeds the western part, which is fully saturated. In the middle horizon, the springs have heightened mineral concentrations and are perennial. The lower horizon
ii
receives the bulk of the water input, and is likely fed by both C1 and C2. These springs have the highest mineral concentration and field parameters.
In general, the water’s ion concentration and field parameter values increase downslope, and from East to West. The upper spring horizon and the Eastern stream have the shortest residence times, followed by the middle and the lower spring horizon respectively. All the springs across Åknes display temporal variations in flow rate and chemical composition.
A likely explanation is that they are fed by two fracture systems; a shallow fast-response and a deep slow-response set.
First and foremost I want to thank my main supervisor, Clara Sena. Thank you for being so understanding throughout the year, and your valuable feedback. I also want o thank my co-supervisor, Alvar Braathen.
Thank you to Norges Vassdrags- og Energidirektorat for facilitating my field work, espe- cially Gustav Pless, the project coordinator.
A big thank you to my field assistents: Katrine Louise Kølleskov Gunnulfsen, Stig Runar Steinsland Ringstad, Ina Cecilie Storteig and Halvor Rønneberg Bruun. You were so kind to help me, under the muddy, wet and cold conditions.
I am very grateful to Mufak Said Naoroz and Magnus Kristoffersen for performing the chemical analyses of my many samples. I am also very thankful to Thanusha Naidoo, for spending two full days in the XRF-lab with me.
I want to thank my fellow students at room 210, you have made this year a good one.
You have provided me with comments, feedback, breaks from the tedious work of writing a thesis, and have acted as rubber ducks throughout the year.
Finally, a big thank you to family and friends whom have supported me during this year.
Frida Liv Biørn-Hansen
v
vi
Abstract iii
Acknowledgements v
List of Figures xvii
List of Tables xviii
1 Introduction 1
2 Study site 6
2.1 Regional geology . . . 6
2.2 The geology of Åknes . . . 7
2.2.1 Quarternary geology . . . 9
2.2.2 Movement and avalanche scenarios . . . 9
2.3 Hydrogeology of Åknes . . . 12
2.3.1 The hydrological conditions . . . 12
2.3.2 Groundwater wells and time series . . . 13
2.3.3 Groundwater flow . . . 15
2.3.4 Groundwater borehole field parameter profiles . . . 17
3 Theory 21 3.1 Hydrogeology . . . 21
3.1.1 The hydrological cycle . . . 21
3.1.2 Definitions of hydrogeological terms . . . 22
3.1.3 Groundwater regions . . . 24
3.1.4 Groundwater temperature . . . 24
3.1.5 Sub-surface flow . . . 25
3.2 Hydrogeochemistry . . . 28
3.2.1 Groundwater chemistry . . . 29
3.2.2 Solute transport . . . 30 vii
CONTENTS viii
3.2.3 Reactions in the unsaturated zone . . . 31
3.2.4 Electrical conductivity and pH . . . 31
3.3 Rock avalanches . . . 32
3.3.1 Avalanche mechanisms . . . 32
3.3.2 The effect of water on stability . . . 32
3.4 Meteorology . . . 34
4 Data collection and analysis methods 37 4.1 Field work . . . 37
4.1.1 Groundwater and stream discharge . . . 38
4.1.2 Water temperature, pH and electrical conductivity (EC) . . . 38
4.1.3 Water sampling procedure . . . 39
4.2 Methods of analysis . . . 41
4.2.1 Alkalinity analysis . . . 42
4.2.2 Ion chromatography . . . 43
4.2.3 Quadrapole inductively coupled plasma mass-spectrometry (Q-ICP- MS) . . . 44
4.2.4 Seal Auto-analyzer . . . 44
4.2.5 X-Ray Fluorescence (XRF) . . . 45
4.3 Meteorological data . . . 45
4.4 Estimation of water balance . . . 48
4.4.1 Catchment areas . . . 49
4.4.2 Evapotranspiration . . . 50
4.5 Groundwater time series analysis . . . 54
4.6 Statistical analysis . . . 55
4.7 Previously obtained data . . . 55
5 Results 57 5.1 Field observations . . . 57
5.2 Climate at Åknes . . . 61
5.3 The 2017/2018 water balance . . . 61
5.3.1 Meteorological data . . . 62
5.3.2 Groundwater discharge measurements . . . 64
5.3.3 The water budget and input estimations . . . 70
5.4 Groundwater borehole time series . . . 74
5.5 Field parameters . . . 78
5.6 XRF and hydrochemistry of the water samples . . . 82
5.6.1 XRF . . . 82
5.6.2 Rainwater and Instevatnet . . . 84
5.6.3 Concentration ratios . . . 86
5.6.4 Sodium-chloride ratios, silica, strontium and bromine . . . 88
5.6.5 Correlation between dissolved components . . . 91
6 Discussion 93 6.1 Field parameters: EC, pH and temperature . . . 95
6.1.1 The 2007 continuous monitoring . . . 97
6.2 Hydrogeochemistry . . . 99
6.2.1 Groundwater spring samples . . . 100
6.2.2 Bivariate analysis . . . 106
6.2.3 Spring S28 - A polluted spring? . . . 106
6.3 Hydrogeology . . . 107
6.3.1 Groundwater time series . . . 107
6.3.2 Hydraulic fractures and potential rainwater infiltration . . . 109
6.3.3 Conditions above the backscarp . . . 110
6.3.4 Eastern part . . . 112
6.3.5 Central part . . . 114
6.3.6 Western part . . . 116
6.3.7 Hydrogeological profiles . . . 118
6.4 The 2017/2018 water balance and input estimates . . . 118
6.4.1 Evapotranspiration . . . 119
6.4.2 The 2017/2018 water input estimates . . . 120
6.4.3 Water balance calculations . . . 121
6.5 Drainage considerations . . . 123
6.6 Method applicability . . . 126
CONTENTS x
7 Conclusions and Future Work 129
7.1 Future work . . . 131
Bibliography 137
APPENDICES 146
A Prior XRD analyses 147
B 2007 continuous electrical conductivity and temperature monitoring 149
C Boreholes at Åknes 153
D Climate at Åknes 154
E Evapotranspiration and water input calculations 155
F XGEO simulation routines 160
F.1 Temperature and precipitation simulation routines . . . 160 F.2 Snowmelt simulation routine . . . 160 F.3 Evapotranspiration simulation routine . . . 161
1.1 Southern Møre og Romsdal is located within the red square in the right map. Ålesund, Stranda and Åknes marked to the left. . . 1 1.2 Risk matrix for scenario A and B (named scenario 1 and 3 in Figure 2.2).
The squares mark the scenario, and the crosses indicate the uncertainty.
Modified by author from ngu (2016). . . 2 2.1 Western Gneiss Region (WGR), Åknes marked in red. Modified by author
from (Ramberg et al., 2008). . . 7 2.2 Åknes rock slope, with boreholes, springs and spring horizons. . . 9 2.3 Vegetation cover and avalanche debris in the upper spring horizon. . . 10 2.4 Figure 1) and 2) are from borehole KH-02-18, figure 3) from KH-01-17.
The sediment layer is both thick and unsorted, and is alternately covered by vegetation or scree material. . . 10 2.5 (a) Sub-domain 1-4, and (b) Traverse A-A’ (Ganerød et al., 2008). . . 11 2.6 Groundwater oscillations 01.10.2015-30.09.2018 (ftp, 2018). . . 14 2.7 Equipotential lines drawn by Frei (2008), and average water table [m.a.s.l.]
(ftp, 2018). . . 14 2.8 Conceptual model of pressure and flow system in KH-01-06, modified by
author from (Storrø and Gaut, 2008). . . 17 2.9 Field parameters profile for KH-03-06 (Elvebakk, 2008a). The blue line
marks the 2006 survey, the red line the 2007 survey. The horizontal lines mark the water table. . . 19 2.10 Field parameters profiles for a) KH-01-06, and b) KH-02-06 (Elvebakk,
2008a). The horizontal lines mark the water table. . . 20 3.1 Regional hydrological characteristics in the Nordic countries. Both figures
are modified by author. Figure (a) from Kirkhusmo and Sønsterud (1988), and figure (b) based on Kirkhusmo and Sønsterud (1988), from Hilmo et al.
(1998). . . 25 xi
LIST OF FIGURES xii
3.2 A: Primary porosity, unconsolidated sand / gravel. B: Primary porosity is reduced by cementation a/o the presence of clay and silt. C: Secondary porosity, bedrock rendered porous by fractures. D: Secondary porosity, consolidated fractured rock rendered more porous by dissolution. Figure modified by author after (MacDonals et al., 2005). . . 26 3.3 Hydraulic, pressure and gravitational head. . . 27 3.4 (a) Boreholes in fractured bedrock, MacDonals et al. (2005). (b) The bore-
hole flow (Cook, 2003), a) with packers, b) radially, c) linearly. . . 29 4.1 (a) Example of the impermeable cloth fitted to the stream bed using rocks
as weights, in spring LS1. (b) Example of discharge measurement, with the help of a cloth and the 1 [L] bottle at LE. . . 39 4.2 Examples of pH and EC measurements in stream S21, in June (a) and
September (b) respectively. In (a), a bottle is used to insert the probes, and works as a flow-through cell where stream water enters and leaves the bottle with a laminar flow, whereas in (b) the probes are directly inserted into the flowing stream. . . 40 4.3 The fault rock was collected above the backscarp by Stig Runar Steinsland
Ringstad, the concrete core from KH-01-17 and the gneiss and granite cores at KH-02-18. . . 45 4.4 The precipitation and temperature data from Åknes (CautusWeb, 2019),
compared to the simulated data from XGEO (www.xgeo.no, 2018a). . . 47 4.5 A conceptual model of the water balance calculation approach. . . 48 4.6 1) The western part of the backscarp, and 2), the eastern part, seen from
spring S23. . . 50 4.7 Catchment C1 and C2 above the backscarp, and streams mapped by Frei
(2008) and in project. Instevatnet is a potential groundwater contributor to Åknes rock slope. . . 50 5.1 Vegetation cover above the backscarp. . . 59 5.2 Instevatnet. White arrows mark inflow routes, black the main outlet, and
red dots the sampling sites. . . 60
5.3 Temperature and precipitation trends at Åknes since 1958, based on data from www.xgeo.no (2018a). . . 61 5.4 Monthly temperature and precipitation for the hydrological year 2017/2018
and the standard normal period, 1961-1990. Clear deviations from the normal period are observed for the year 2017/2018. . . 62 5.5 Precipitation (right axis) and snowmelt (left axis) at Åknes 900 [m.a.s.l.]
2017/2018. . . 63 5.6 (a) Annual and, (b) monthly PET. . . 64 5.7 CMB/BMB calculations for a) 7-days cumulative AET [mm], and b) 2-days
cumulative AET [mm]. Compared with data from www.xgeo.no (2018a),
and error bars indicate the range in results from the CMB/BMB-computations. 65 5.8 June discharge measurements. Flow rate is on the left axis, rainfall on the
right. . . 66 5.9 Discharge measured in the July / August field campaign. Flow rate is on
the left axis, rainfall on the right. . . 66 5.10 Discharge in the lower spring horizon throughout July / August. Flow rate
is on the left axis, rainfall on the right. . . 67 5.11 Discharge in the middle spring horizon throughout July / August. Flow
rate is on the left axis, rainfall on the right. . . 67 5.12 Flow rate measured in the Eastern stream in August. Flow rate is on the
left axis, rainfall on the right. . . 67 5.13 September discharge. Flow rate is on the left axis, rainfall on the right.. . . 68 5.14 Discharge in the lower spring horizon in September. Flow rate is on the
left axis, rainfall on the right. . . 68 5.15 Discharge in the middle spring horizon in September. Flow rate is on the
left axis, rainfall on the right. . . 69 5.16 Discharge measured in the Eastern stream in September. Flow rate is on
the left axis, rainfall on the right. . . 69 5.17 Discharge measured in the upper spring horizon in September. Flow rate
is on the left axis, rainfall on the right. . . 70 5.18 Discharge measured above the backscarp in September. Flow rate is on the
left axis, rainfall on the right. . . 70
LIST OF FIGURES xiv
5.19 Annual input from C1 + C2. P is precipitation, SM is snowmelt, both in [mm]. . . 71 5.20 In a), monthly input from catchment C1, and b), monthly input from
catchment C2. . . 72 5.21 CMB/BMB calculations for a) 7-days cumulative input [mm], and b), 2-
days cumulative input [mm]. Compared with data from www.xgeo.no (2018a). Error bars indicate the range in results from the CMB/BMB computations. . . 73 5.22 In a), weather and groundwater fluctuations during the 2017/2018 (ftp,
2018). The dotted line at 0 in the groundwater plot marks the 2017/2018 average water table for all the wells. In b), weather, discharge sum and groundwater fluctuations during summer 2018. In both plots, the temper- ature is the air temperature. . . 76 5.23 EC of springs and stream water, measured throughout the field periods. . . 79 5.24 pH of springs and stream water, measured throughout the field periods. . . 80 5.25 Temperature of springs and stream water, measured throughout the field
periods. . . 81 5.26 Results from XRF-analysis of the rock and concrete samples. . . 83 5.27 Monthly average NaCl-ratios from 2017 for three stations along the West
coast, along with a 2018 rain sample and 2019 snow sample from Åknes. . 84 5.28 Comparison of the 2018 rain sample and the 2019 snow sample. . . 84 5.29 a), Instevatnet sampled in July campaign, and b) samples from September.
Inste 1 is sampling site 1, Inste 2 is sampling site 2 and Inste 3 is sampling site 3 (figure 5.2). . . 85 5.30 Piper plot of the selected springs across Åknes. . . 86 5.31 The ratios between average ion concentration from seven selected springs
across Åknes. In (a) major elements, in (b) minor elements. . . 88 5.32 NaCl-ratios for all water samples. . . 89 5.33 a), Silica and b), strontium concentrations [ppm] of selected streams across
the rock slope on September 4th/5th and 13th/14th. . . 90 5.34 Bromine concentrations [ppm]. Sample 22 is rainwater. See appendix for
the sample legend. . . 91
5.35 Elements with a high Pearson correlation, and their linear regression lines.
Concentrations are in [mmol/L]. . . 92 5.36 Elements with a high Spearman correlation, and their linear regression
lines. Concentrations are in [mmol/L]. . . 92
6.1 2D representation of Åknes, with schematically located boreholes and springs.
Dashed line indicates unknown flow paths, and the X points to a potential unknown aquifer. Note; figure is not to scale. . . 94 6.2 Average concentrations of dissolved major components. These are average
values for each sampling site from all field campaigns. Locations in figure 6.1: S21 in ABS, S26 in UH, UE in ES,S30-31 in MH, LS1-LS3 in LH. . . . 99 6.3 The hydrogeological model of the the eastern profile. Blue arrows are
hydraulic fractures, and grey arrows are dry fractures. The top arrow represents precipitation. Both axis are in [m]. . . 108 6.4 The hydrogeological model of the the western profile. Blue arrows are
hydraulic fractures, and grey arrows are dry fractures. The top arrow represents precipitation. Both axis are in [m]. . . 108 6.5 Conceptual discrete fracture network model of bedrock fracture connectiv-
ity, above the backscarp, in quartz dioritic gneiss. Dark red fractures have high connectivity, and green low (Bruun, 2019). . . 111 6.6 Conceptual model of water flow in the scree covered central part of Åknes,
surrounding KH-01-06. Precipitation infiltrates the scree material, and reaches the bedrock surface. The hydraulic fractures run parallel to the surface topography, and the pressure differences between them cause up- and down flow within the borehole. Modified by author from (Storrø and Gaut, 2008). . . 114 6.7 Updated map with equipotential lines [m.a.s.l.] of Åknes. Blue lines mark
the equipotential of what is assumed to be fully saturated aquifers, and green lines what is assumed to be perched aquifers. Groundwater levels are all time averages. . . 119
LIST OF FIGURES xvi
6.8 An updated conceptual water balance computation model. Black terms were included in the calculations, whereas future studies should attempt to the incorporate the grey terms as they affect the water balance calculations.
ABS = above the backscarp. . . 123 6.9 A proposed drainage solution from Moen (2008), which involves the whole
unstable rock slope. Brown lines: drainage adits, orange lines: drain holes. 125 B.1 Spring S30. The green line indicates the date of the sea water injection. . . 149 B.2 Spring LS5. The green line indicates the date of the sea water injection. . . 150 B.3 Spring LS1. The green line indicates the date of the sea water injection. . . 151 B.4 A detailed look at groundwater and air temperature, precipitation and EC
of spring SN3a (LS1). . . 152 F.1 Conceptual model of HBV’s snow routine used in xgeo simulation of snow
data. Figure from www.xgeo.no (2018b), later modified by author. . . 161
2.1 Hydraulic fractures at depth [m] and their strike / dip (Elvebakk, 2008a,b) 16 4.1 Dates for water sampling and pH, EC and temperature measurements
throughout the 2018 field work campaigns. . . 41 4.2 Pearson correlation between simulated climate data from www.xgeo.no
(2018a) and the Åknes meteorological station. T stands for temperature [◦C] and P for precipitation [mm]. . . 47 5.1 Sum D is the measured discharge for 5 days (August), 11 days (July), and
10 days (September). P+SM is the cumulative precipitation and snowmelt during the discharge measurement period: raw means that evapotranspi- ration has not been subtracted, while ET indicates the evapotranspiration method whose results have been subtracted from the P+SM. % is the per- centage that must have contributed to discharge. ∆S is the resulting change in storage. All values in [mm]. . . 74 5.2 Sum D is the calculated discharge for the full month. P+SM is the cu-
mulative precipitation and snowmelt during the discharge measurement period: raw means that evapotranspiration has not been subtracted, while ET indicates the evapotranspiration method whose results have been sub- tracted from the P+SM. % is the percentage that must have contributed to discharge. ∆S is the resulting change in storage. All values in [mm]. . . 75 5.3 Groundwater level statistics for three boreholes. KH-01-12, KH-02-06 and
KH-03-06 are based on data from 2015-2018 (ftp, 2018). Average, max and min in [m.a.s.l.]. . . 77 5.4 Seasonal groundwater level statistics for three boreholes at Åknes, during
2017/2018 (ftp, 2018). Average, max and min are in [m.a.s.l.]. . . 77 5.5 Correlation between boreholes KH-02-06, KH-03-06 and KH-01-12 based
on time series from 2015-2018 (ftp, 2018). Pearson correlation is above the diagonal, Spearman correlation below. . . 78 5.6 Average ion concentration ratios during 2018. Input is S21 and US44. . . . 87
xvii
LIST OF TABLES xviii
6.1 Average electrical conductivity, pH and temperature, from all measure- ments between June-September. . . 95 A.1 XRD-results in % from Grøneng, Nilsen, and Sandvern (2009). All samples
are collected from what is assumed to be the toe zone at 180 [m.a.s.l.]. . . 147 A.2 XRD-results in % from Langeland (2013). Sample 1-2 are from a shear
zone near the Western gully. Sample 3 is from the clay zone in KH-01-12. . 148 C.1 Boreholes and their acronyms. The coordinates are in UTM 32N, elevation
(Z) in [masl]. . . 153 C.2 Borehole, diameter [mm], dip [◦], depth [m] and elevation (Z) [masl]. . . 153 D.1 Climatic deviation [%] of the hydrological year 2017/2018 from the stan-
dard normal period. T = temperature [◦C], P = precipitation [mm]. The maximum (max) and minimum (min) are the highest and lowest values within the normal period. . . 154 E.1 Annual net input [mm] from catchments C1 and C2, sorted by ET estima-
tion method. . . 155 E.2 Monthly net water input [mm] from catchment C1. PET=potential evap-
otranspiration, P=precipitation, SM=snowmelt. . . 156 E.3 Monthly net water input [mm] from C2. PET=potential evapotranspira-
tion, P=precipitation, SM=snowmelt. . . 157 E.4 CMB computations. Spring, date of sampling with corresponding Cl con-
centration [ppm], days [x], rain [mm], estimated AET [mm] and the result- ing recharge [mm/ x days]. . . 158 E.5 BMB computations. Spring, date of sampling with corresponding Br con-
centration [ppm], days [x], rain [mm], estimated AET [mm] and the result- ing recharge [mm/ x days]. . . 159 E.6 XGEO computations. Spring, date of sampling with corresponding, days
[x], rain [mm], AET [mm] and the resulting recharge [mm/ x days]. . . 159
Norway, a country known for its deep fjords and alpine landscape, has always been prone to landslides and rock avalanches. There are currently seven rock slopes under continuous monitoring by the Norwegian Water Resources and Energy Directorate (nve). One of these is Åknes; a highly unstable rock slope located at the western side of Sunnylfsfjorden in Stranda, Møre og Romsdal county (figure 1.1).
Figure 1.1: Southern Møre og Romsdal is located within the red square in the right map.
Ålesund, Stranda and Åknes marked to the left.
Åknes is part of the Western Gneiss Region, and is characterized by an igneous and meta- morphic bedrock. As a result of the multiple orogenesis and the physical strain caused by the ice ages, the region is heavily fractured. Åknes is no exception, and fractures have been observed as deep as 200 [m]. Movements have been measured down to 62 [m]
below the ground surface. All findings imply that the movement is related to the waters migration through the slope. The main infiltration zone is the backscarp. From there, the water moves through the fracture systems and reappears as springs across the slope.
The springs are clustered in limited regions, and there are large variations in discharge.
Blikra et al. (2006) states, ’Rock avalanches and related tsunamis represent one of the 1
2
most serious natural hazards in Norway (...)’. If Åknes fails, it may result in a tsunami that can potentially affect 10 municipalities and several villages in the Storfjord region (Eidsvig and Harbitz, 2005). In addition comes the more than hundred of thousands tourists that visit the region every year. The two most likely avalanche scenarios are shown in figure 1.2 (ngu, 2016). The scenarios are both above medium risk class and can potentially result in loss of lives. The consequences would be catastrophic for either one of them.
Figure 1.2: Risk matrix for scenario A and B (named scenario 1 and 3 in Figure 2.2).
The squares mark the scenario, and the crosses indicate the uncertainty. Modified by author from ngu (2016).
Background for thesis
Geological mapping of Møre og Romsdal has identified several rock-slope failures since the last glaciation, approximately 10.000 years ago (Blikra et al., 2006). Only in Storfjor- den, 59 rock-slope failures more than 0.5 [million m3] have been established (Blikra et al., 2005). At Åknes itself, three historic avalanches have been mapped (Kveldsvik et al., 2005).
Åknes has become an object of public interest object due to the severe consequences if it should collapse. Therefore, in 2017 a project was initiated to assess the possibility of
stabilizing the slope through drainage. This method comprises installing horizontal drain holes, and potentially drainage adits, to lower the groundwater table. The project is a collaboration between Norges Vassdrags- og Energidirektorat (NVE), Norges Geotekniske Institutt (NGI), Universitetet i Oslo (UiO), Norges Teknisk-Naturvitenskapelige Univer- sitet (NTNU), and Schlumberger.
At UiO, three master theses on Åknes are currently being undertaken as part of this project. Two of them focuses on the area above the backscarp. Stig Runar Steinsland Ringstad’s thesis includes an assessment of the stability of this area. Halvor Rønneberg Bruun explores the water bearing capacity of the fractures. This thesis focuses on the hydrogeochemistry and water balance of the unstable area.
Aim of thesis
The aim of this thesis is threefold. The first objective is to provide a description of the hydrogeochemistry of Åknes. Included in this are chemical analyses of water and rock samples collected during the 2018 field campaigns. At the same days as the sampling was performed, the field parameters (pH, temperature and electrical conductivity) were measured. These combined provide an overall overview of the hydrogeochemistry of the rock slope.
The second objective is to estimate the water balance. Based on the available meteoro- logical data (snowmelt, temperature and precipitation), it was possible to estimate the evapotranspiration. Through field observations and Digital Elevation Models (DEM), two catchment areas were drawn above the backscarp. Seen together, it was thus possible to calculate the total input into Åknes. Groundwater discharge was measured in field, which gives an indication of the amount of water that passes through the rock slope. The measurements show which parts of Åknes receives the largest amount of water, and the reaction time of the aquifers to rainfall and snowmelt events.
The third objective is to develop a conceptual model of the hydrogeological conditions of Åknes rock slope. The proposed models are based on results from both objective one and two, in addition to a study of groundwater borehole time series and fracture mapping.
4
Thesis outline
In the next chapter (2) Åknes as a study site will be presented. Chapter 3 explains the theoretical framework for this thesis. The field work and analysis methods are described in chapter 4, and chapter 5 provides a summary of the field observations. Chapter 6 presents the results, which are further discussed in chapter 7. Lastly, in chapter 8, comes the conclusion and suggestions for future work.
2 Study site
This chapter will present Åknes and its geological development towards the unstable rock slope seen today. It begins in section 1 with a description of the rock slopes geology, and a summary of the estimated avalanche scenarios. Section 2 focuses on the hydrogeological conditions.
2.1 Regional geology
Åknes is located in the western Gneiss Region (WGR) that dominates the south-western part of Norway (figure 2.1) (Austrheim et al., 2003). The region is part of the Baltic shield, which encompasses Norway, Sweden, Karelia and Kola peninsula. Precambrian bedrock is the main constituent of the Baltic shield, which originated and was shaped through multiple orogeny events between 3500-1500 m.a. (Gaál and Gorbatschev, 1987a). The bedrock in the WGR dates from the Proterozoic. It consists of granite that was forged and reworked during the Gothic orogeny approximately 1700-1500 m.a. (Austrheim et al., 2003). The bedrock was later strongly affected and exposed to metamorphic processes during the Caledonian orogeny 450-350 m.a. (Austrheim et al., 2003), (Ganerød et al., 2008).
6
Figure 2.1: Western Gneiss Region (WGR), Åknes marked in red. Modified by author from (Ramberg et al., 2008).
2.2 The geology of Åknes
Åknes shares the geological characteristics of the western Gneiss Region. The rock slope (figure 2.2) faces southwards, has a width of 800 [m] and spans 900 [m] downslope (Gan- erød et al., 2008). Structurally it is confined by an extensional 500 [m] long backscarp located between 900-700 [ma.s.l.], from west to east. The scarp is best defined in the west, where it is approximately 20-30 [m] wide. It closes eastwards to approximately 1 [m], and appears more as a cliff at the eastern end of the instability (figure 4.6). A steep crevasse, a NNW-SSE trending strike slip fault, confines the rock slope to the west. The crevasse is filled with materials from past avalanches, and a perennial stream. The eastern boundary
2.2. THE GEOLOGY OF ÅKNES 8
consists of a 35◦-40◦ dipping fault. The Eastern stream runs approximately along and parallel to it. At 150 [m.a.s.l.] there is compressional toe zone, with an adjacent spring horizon.
Based on surface mapping, and logging of boreholes and drill cores, three fracture sys- tems have been detected. Two are steeply dipping fractures with N-S and E-W strikes, the third runs parallel to the bedrock’s foliation. The fracture frequency is higher close to the surface, and at its greatest in the uppermost 60 [m] of the slope. The drill cores, (Ganerød et al., 2007) (Ganerød, 2013), reveal a very heterogeneous bedrock. Pegmatite, biotittic-, granittic-, amphibolic- and diorittic gneiss are all present, in addition to biotite schist layers within the gneisses. These layers can be as much as 20 [cm] thick, and coin- cide with high fracture frequencies (Ganerød et al., 2008).
The foliation below 600 [m.a.s.l.] dips with 30-35◦towards SE, and is oriented parallel to the topography. Although the foliation has a similar strike above 600 [m.a.s.l.], there are more folds, and consequently a more complex situation. Central Åknes is character- ized by sub-horizontal to gently dipping foliation towards the fjord (Ganerød et al., 2008), (Jaboyedoff et al., 2011). This area has the highest frequency of foliation parallel fractures.
The marine limit is at approximately 90 [m.a.s.l.] (ngu, 2019). It can be seen as a roughly 1-2 [m] wide flat path running from east to west, right below the lower spring horizon.
Whether or not there are marine sediments below this limit is currently unknown.
Figure 2.2: Åknes rock slope, with boreholes, springs and spring horizons.
2.2.1 Quarternary geology
Åknes rock slope is covered by material from past avalanches: from the backscarp, the area above teh backscarp, and smaller cliffs in the rock slope. In the central part of Åknes, the scree is seen as large rocks an boulders (figure 2.4). Meanwhile, the upper spring horizon is characterized by vegetation and sporadic avalanche debris (figure 2.3). In the unstable part of Åknes, there are also thick layers of unsorted material. The material is most likely till, and/or a result of in situ weathering. Examples can be seen in figure 2.4. These profiles have been bared during the digging of borehole platforms, and the sediment layers might be thicker than what is seen in the open.
2.2.2 Movement and avalanche scenarios
According to (Blikra et al., 2013), the slope-parallel foliation and weak biotite-rich layers control the large-scale displacement dynamics. Surface displacement is monitored contin- uously at Åknes, and this have confirmed the division of the rockslide into the subgroups (figure 2.5a).
2.2. THE GEOLOGY OF ÅKNES 10
Figure 2.3: Vegetation cover and avalanche debris in the upper spring horizon.
Figure 2.4: Figure 1) and 2) are from borehole KH-02-18, figure 3) from KH-01-17.
The sediment layer is both thick and unsorted, and is alternately covered by vegetation or scree material.
The subgroups move in different directions, at different velocities. They overthrust each other, and surface at different levels (Blikra, 2008). Whereas the two uppermost parts are characterized by extensional movements, the lower one shows compression. Sub-domain 1 and 2 have the highest velocities, and the direction of movement for subdomain 1 is S-SE, and for S-SW for subdomain 2 (Ganerød et al., 2008). Velocities as high as 8 [cm/year]
have been registered in sub-domain 1 and 2 Blikra (2012).
(a) Sub-domains (b) Traverse A-A’
Figure 2.5: (a) Sub-domain 1-4, and (b) Traverse A-A’ (Ganerød et al., 2008).
Moving on to the potential collapse of Åknes: the principal uncertainty is the depth of the sliding plane(s). Blikra (2012) and Hole et al. (2011) have identified three possible avalanche scenarios. The scenarios and their annual probability are,
1. The whole mountainside: 54 mill. [m3] Annual probability: [1/1000 - 1/5000]
2. The northern parts, subdomain 2 and 3: 18 mill [m3] Annual probability: >1/1000
3. The north-western flank, subdomain 1: 6-11 mill [m3] Annual probability: >1/1000
However, Nordvik et al. (2009) assumes a shallower sliding plane, and decreases the vol- umes,
1. The whole mountainside: 43 mill. [m3]
2.3. HYDROGEOLOGY OF ÅKNES 12
2. The northern parts, subdomain 2 and 3: 14.5 mill [m3] 3. The north-western flank, subdomain 1: 2.5 mill [m3]
That being said, the instability is expanding. Kristensen et al. (2013) found that the upper graben structure is evolving eastwards, and that this area’s movements (subdomain 2) are increasing. In addition to this, Bruun and Ringstad (2018-2019) have examined the area above the backscarp in great detail. The resulting conclusion is that this area is also unstable, and needs more careful monitoring.
2.3 Hydrogeology of Åknes
Åknes is a fractured rock aquifer, and without comprehensive knowledge on the slope’s geology the groundwater’s circulation might appear random. The groundwater flow fol- lows preferential flow paths governed by the fracture sets, and it accumulates wherever the aquifer’s geometry allows. With this perspective, Moen’s (2008) claim that "The ground- water is in general both irregularly and illogically distributed within the rock mass"
(translated by author), is not out of place.
The hydrogeological conditions of any site are related to and dependent on the area’s ge- ology. This is especially true at Åknes, who’s on-going deformation continuously modifies the hydrogeological conditions. In the following section there will be references to studies as far back as 2007. Their observations and conclusions might not be valid in 2018.
2.3.1 The hydrological conditions
Åknes, and the surrounding area, experiences mild winters, cool summers and high levels of precipitation. It currently falls within subgroup cbf in Koppens climate classification (Kottek et al., 2006). Both Frei (2008) and Ganerød et al. (2008) conclude that the slope is fed by precipitation and snowmelt both in the rockslide and the catchment area.
However, neither of them rejects the possibility that the slope might also be fed by lakes further east of Åknes.
According to Frei (2008), Åknes can be divided into areas of surface water infiltration and areas of groundwater discharge. The main infiltration zone lies above the back scarp, whereas three main discharge zones are identified as the upper, middle and lower spring horizons (figure 2.2, detailed maps digital appendix ’DA4.pdf’ ). The upper spring hori-
zon comprises springs S23, S24 and S26. The middle spring horizon consists of S29-S31, S34 and S37. The lower springs are LS1-7. Infiltration zones are in the downstream part of streams S21-S26, S31, S41-S44 (photos of each sampling spot are in digital ap- pendix DA3). Further on, Frei (2008) defined the equipotential lines at Åknes (figure 2.7).
Frei (2008) attempted an estimation of the water balance at Åknes. By assuming an average annual groundwater discharge of 9.8 [l/s] throughout 2005-2006 , the conclusion was that [35-73]% of the precipitation contributed to flow. Neither evapotranspiration, sublimation nor snowmelt were included in this calculation.
2.3.2 Groundwater wells and time series
Per 01.10.2018, 12 boreholes have been drilled at Åknes, located in map 2.2. Most of them are, or have been, equipped with monitoring equipment. This allows for monitoring of groundwater levels, which provides valuable insight into the sub-surface hydrological conditions. There are currently four name systems in use at Åknes, and a full list of acronyms is found in C.1.
The temporal water level fluctuations in the boreholes seem to harmonize (figure 2.6).
Both short-term precipitation-induced and seasonal variations are apparent (Ganerød et al., 2008), although abrupt shifts can occur, as seen for KH-01-12 in late 2017. Ac- cording to Blikra (2012) the water level can increase by as much as 7 [m] during snowmelt.
The depth to the water table varies across the slope (figure 2.7). It is deepest close to the backscarp, and shallower in the central-west. The highest water table is in borehole KH-01-18, near spring S28. Thöeny (2008) found clear indications of unsaturated con- ditions at depth in KH-02-06 and KH-01-05, based on a complete loss of drilling fluid pressure approximately 190 [m] below surface. KH-02-06 was later sealed, to enable fur- ther monitoring. Blikra et al. (2013) reports that the water table lies just beneath the fastest moving sliding plane. However, the boreholes are both deep and open. This might lower the water table locally, and the true level might be higher, and in contact with the sliding plane, in the surrounding area (Blikra, 2012).
The slope’s movements and groundwater flow are interdependent (Grøneng et al., 2010).
2.3. HYDROGEOLOGY OF ÅKNES 14
Figure 2.6: Groundwater oscillations 01.10.2015-30.09.2018 (ftp, 2018).
Figure 2.7: Equipotential lines drawn by Frei (2008), and average water table [m.a.s.l.]
(ftp, 2018).
Blikra et al. (2013) reported an increase in deformation along a sliding plane in late March 2011. One week later there was an abrupt increase in the water level of borehole KH-01-
06. The groundwater temperature dropped 1◦C, which indicates snowmelt input. After the peak, the water table sank approximately 2 [m] before it stabilized. Nevertheless, the displacement continued for a long time afterwards. This chain of events was likely started by an internal change in the hydrogeological conditions. It began with a change in the fractures’ openings, and the increased local hydraulic conductivity allowed for more water flow. The subsequent heightened water table led to a general increase in the displacement.
2.3.3 Groundwater flow
The groundwater flow at Åknes is turbulent. According to the tracer experiments Frei (2008) conducted, the groundwater flow seems to focus towards the middle and lower spring horizon. There appear to be flow paths that cross each other, although at dif- ferent depths. Another interesting finding was that not all the water that infiltrates the backscarp reappears at one of the spring horizons. This indicates the presence of deeper groundwater flow paths that re-emerge beneath sea level. Frei (2008) also discovered that KH-01-06 and KH-02-06 are not related, although their variations seem to harmonize.
All of Frei’s 2008 tracers were diluted, to a greater or lesser extent. This coincides with Storrø and Gaut (2008) experience with a sea water tracer test. Their experiment con- sisted in injecting sea water with a known EC into the backscarp, and mounting EC sensors in three springs. They did not manage to trace it, and concluded that the most likely reason was dilution. That implies a sea water / fresh water ratio equal to 1:350.
Considering the results from Frei (2008), a dilution ratio equal to 1:13 000 was computed.
This indicates that 4050 [m3] was available as ’dilution’ water within the flow path (Storrø and Gaut, 2008).
Åknes is heavily fractured, and these fractures serve as preferential flow paths for the groundwater. Thöeny (2008); Elvebakk (2008a,b) have found that there is a complex system of inflow and outflow through fractures at different depths in the boreholes. A few selected boreholes have been studied with televiewers, which has enabled mapping of these hydraulic fractures (Elvebakk, 2008a,b). These are listed in table 2.1. All but one of the fractures have a strike approximately NE-SW. Each of them run more or less foliation parallel.
2.3. HYDROGEOLOGY OF ÅKNES 16
Table 2.1: Hydraulic fractures at depth [m] and their strike / dip (Elvebakk, 2008a,b)
Borehole Depth In/Out Strike/Dip KH-03-06 43.06 Out N134 / 21.70 KH-03-06 42.64 Out N163 / 52.5 KH-01-12 62.70 Out N164 / 30.3 KH-01-12 72.54 In N152 / 28.4 KH-01-12 72.57 In N145 / 39.8 KH-01-12 74.83 In N083 / 29.7
As part of his master thesis, Thöeny (2008) measured groundwater flow in six boreholes, among them KH-01-06. This revealed upwards and downwards flow between different sections. A likely interpretation is that there are pressure differences between fractures.
Based on those findings, Storrø and Gaut (2008) drew a conceptual model of the flow sys- tem as it appeared in borehole KH-01-06. The driving forces are differences in hydraulic pressure gradients. The data indicated that the fractures pressure level differs with depth.
Between 2005-2006 there were several wells open for groundwater flow. Yet there were no observed leveling of the pressure differences between fractures/fracture systems at various depths. This implies pressure differences so great that not even free flow through the boreholes can equalize it. Consequently, these hydraulic fractures must be hydraulically disconnected, from each other and from the surface. It then follows that these fractures must run approximately parallel to the topography (Storrø and Gaut, 2008). These ob- servations hold for the central part of Åknes, where no shear zones resurfaces.
This last conclusion is supported by the video logging of KH-03-06, conducted by Elvebakk (2008a). In the log it is apparent that nearly all the fractures are foliation fractures with a strike/dip equal to that of the topography. The observed pressure differences are therefore likely a result of the infiltrated amount of water, and the elevation at which the water enters the fracture systems. Continuing with this line of reason, a possible explanation of the pressure differences is that the fractures with observed high pressure originate in the backscarp. The ones with lower pressure receive water from higher up in the terrain, but a smaller amount. Consequently the whole area above the backscarp must be viewed as an infiltration zone (Storrø and Gaut, 2008).
Figure 2.8: Conceptual model of pressure and flow system in KH-01-06, modified by author from (Storrø and Gaut, 2008).
2.3.4 Groundwater borehole field parameter profiles
Three boreholes (KH-01-06, KH-02-06, KH-03-06) have had their pH, EC and tempera- ture measured and reported in Elvebakk (2008a) (figures 2.10a, 2.10b and 2.9). Elvebakk (2008a) have not commented on the weather conditions before/during these survey. It is not known precisely how long after the drilling these profiles were measured, nor the composition of the drilling mud, antifreeze etc. that were used.
Starting at KH-03-06, the measurements were done twice: in October 2006 (blue line), and in June 2007 (red line). The 2006 survey was conducted without a plastic casing, the 2007 survey with a plastic casing. Elvebakk (2008a) debates if the differences were due to different weather conditions or the mountains deformation (flow patterns within the borehole had changed during this period). However, both profiles display an increase in temperature with depth. The EC is notably higher in 2006 than in 2007, however, in 2007 there is a much larger jump at approximately 165 [mbgs]. With regards to the pH, it is quite similar, with two large deviations at roughly 50 and 165 [mbgs].
In KH-01-06 the pH and EC reach approximately constant values 65 [mbgs]. This lasts
2.3. HYDROGEOLOGY OF ÅKNES 18
until 120 [mbgs], where both parameters show a sudden drop/rise, respectively. After this, both rise towards the bottom of the borehole. Temperature increases steadily, with only a small change at 120 [mbgs]. Moving on to KH-03-06, all three field parameters stabilize just below the water table. Afterwards all the parameters experience slow, but steady, increases with depth.
Figure 2.9: Field parameters profile for KH-03-06 (Elvebakk, 2008a). The blue line marks the 2006 survey, the red line the 2007 survey. The horizontal lines mark the water table.
2.3. HYDROGEOLOGY OF ÅKNES 20
(a) (b)
Figure 2.10: Field parameters profiles for a) KH-01-06, and b) KH-02-06 (Elvebakk, 2008a). The horizontal lines mark the water table.
Hydrogeology is an interdisciplinary science, comprising geology, hydrology, geophysics and geochemistry. This chapter offers an overview of this science, and the theoretical foundation for the thesis. It includes an introduction to hydrogeology in section 1. Section 2 is on hydrogeochemistry. In section 3 the basic principles of rock avalanches, and water’s effect on stability are presented. Lastly, section 4 is devoted to meteorology.
3.1 Hydrogeology
This section begins with an introduction to groundwater, and the definition of some fundamental terms. Then groundwater regions are established, prior to an explanation of the principles of sub-surface flow.
3.1.1 The hydrological cycle
The hydrological cycle is the water’s continuous circulation, largely driven by solar radi- ation. If one can set a starting point, it would be the ocean. From there, the sea water evaporates and enters the atmosphere (Hiscock and Bense, 2014). If the air is supersatu- rated and there are sufficient aerosols, the water condenses to form droplets and clouds.
The residence time is typically a few days, and precipitation occurs when the droplets are sufficiently large to overcome gravity (Wallace and Hobbs, 2006). In continuation, the water enters the bio- and lithosphere as dew or precipitation. The precipitation can be either snow, rain, sleet, or a mix of them all. From this point, the water can either sublimate or evaporate directly from the lakes, surface streams, the ground surface or the vegetation’s leaf. If it is taken up by the plants the water will transpire. In continental areas this recycling often repeats itself a number of times (Dingman, 2015). Precipitation can also join the cryosphere, and be stored in one of the worlds glaciers or snow covers.
That being said, parts of the water will eventually enter the soil storage. From there it can either evaporate, or percolate further down to the groundwater. The residence time varies from aquifer to aquifer. Finally the water continues towards the ocean (Hiscock and Bense, 2014), and the cycle repeats itself.
The water is stored temporally within glaciers, streams, aquifers etc. The rate of change 21
3.1. HYDROGEOLOGY 22
in these storages can be described by the water balance equation 3.1,
P −ET −Sr−GWr = ∆S (3.1)
where P [L/T] is precipitation, ET [L/T] is the evapotranspiration, Sr [L/T] is surface runoff, Gr [L/T] is groundwater runoff and ∆S [L/T] is the change in storage (Hiscock and Bense, 2014).
3.1.2 Definitions of hydrogeological terms
Groundwater is water present in the saturated zone of the ground, which is under a pres- sure equal to or higher than the atmospheric. In this zone all pores and fractures are filled with water. Within the unsaturated zone however, the pores will contain a mixture of both air and water. Due to capillary forces, the pressure is lower than the atmospheric in the unsaturated zone. The water table is the fluctuating boundary between the saturated and the unsaturated zones, i.e. where the pressure equals the atmospheric. This bound- ary will vary on a daily and a seasonal scale, in response to snowmelt, rainfall, pumping and the aquifer’s storage capacity (Dingman, 2015).
Aquifers are geological formations with a sufficiently high porosity to enable water stor- age, and a permeability which allows it to transmit significant quantities of water. There are two main types of aquifers: confined and unconfined. A confined aquifer is delimited by impermeable boundaries. It is fed mainly through a recharge zone where the aquifer crops out, or leakage through a leaky confining layer. An unconfined aquifer does not have these impermeable boundaries, and is recharged primarily by precipitation or lateral groundwater flow. In addition, an aquifer situated atop a non-permeable layer that hin- ders further percolation, allows for the presence of an unsaturated zone below the aquifer.
In this case it is classified as a perched aquifer. Aquifers can be further divided into homogeneous, heterogeneous, isotropic and anisotropic aquifers. If the hydraulic conduc- tivity and storativity is equal at any point within an aquifer it is defined as homogeneous, and if not it is defined as heterogeneous. If the hydraulic conductivity and storativity properties are the same in every direction, the aquifer is termed isotropic as opposed to anisotropic. (Fetter, 2001). In a porous homogeneous aquifer, groundwater flows through gaps between grains, micro- and macro pores. In a heterogeneous aquifers, the flow is through preferential flow paths. Fractured aquifers are characterized by a pronounced heterogeneity and anisotropy due to a spatial variability in hydraulic conductivity, flow
rate and velocity. Yet these are two extremes cases, and most aquifers can be considered as a type in between (Cook, 2003).
A hydrological year spans from October 1st one year to September 30th the next year.
Hence it starts approximately at the beginning of the snowfall, and ends at the end of the snowmelt season (USGS, 2017) in regions with a snow season.
The catchment area can be defined as the area that appears, topographically, to contribute with water that passes through a stream’s cross section at a given point. The catchment’s geological characteristics, land use and climate are the main controlling factors of the magnitude, timing and quality of the stream- and groundwater outflow. Depending on the area’s characteristics, the groundwater catchment may or may not coincide with the surface water’s. A catchment’s time of concentration is the time it takes for the water to travel from the furthest point to the spring’s outlet (Dingman, 2015; Hiscock and Bense, 2014).
Flow is water in motion due to gradients in either pressure or gravity. In a stream it can be separated into inter flow (flow in the soil horizon), base (slow) flow and event (fast) flow. The base flow is the constant groundwater contribution, whereas event flows are sudden increases caused by abrupt recharge events. Flow can be either laminar or turbulent based on the ratio of the inertial to the viscous forces, known as Reynolds num- ber,Re. Sub-surface groundwater flow with Re <1 are dominated by viscous forces, and are therefore laminar. In this case, the water molecules move slowly, and along parallel streamlines. In turbulent flow the inertial force dominates, and the flow is rapid and chaotic (Dingman, 2015). Flow is defined as steady-state when its characteristics (magni- tude or direction of velocity) do not change in time. In transient flow they do change, as can the potentiometric conditions in response to changes in groundwater storage (Hiscock and Bense, 2014). Whether or not a system should be deemed steady-state depends on the timescale under considerations. Still, per definition, a system with cyclic variations in re- and discharge is transient.
Water flowing over a sloping surface is called overland flow. This happens when then
3.1. HYDROGEOLOGY 24
surface is saturated either from above (when the rainfall rate exceeds the infiltration capacity), or from below (as the water table rises), named Hortonian and Dunne flow, respectively. Depression storage occurs when overland flow accumulates due to irregular topography (Dingman, 2015).
3.1.3 Groundwater regions
Kirkhusmo and Sønsterud (1988) attempted to classify groundwater regions based on water level fluctuations throughout the year. They present three main types of hydro- graphs for aquifers fed by precipitation and snowmelt (figure 3.1a). These hydrographs are generalized, and phase offsets can occur. In lowlands two minimums / maximums in groundwater level are observed, before and after the spring snowmelt and the heightened precipitation during fall. In coastal areas the maximum occurs during winter and then decreases throughout the year. The minimum occurs during fall. Lastly, in mountain re- gions the minimum occurs prior to the spring snowmelt, and the maximum shortly after.
Local maximums are often present during fall. Besides the climatic factors, important components controlling the water table fluctuations are 1) the geological setting, 2) the residence time in the unsaturated zone, and 3), the potential presence, type and thickness of a top sediment layer.
3.1.4 Groundwater temperature
The groundwater temperature depends mainly on its surroundings and depth. Fourier’s law, (equation 3.2 (Hiscock and Bense, 2014)) tells us that if there is a thermal gradient, then the Earth must be conducting heat from its interior to its exterior.
Q=−κedT
dz (3.2)
where Q [W] is heat flow density, κ [mKW ] is the effective thermal conductivity, T [C ◦] is temperature and z [m] is depth. Compared to air, hard rock has a higher heat capacity1, and a lower thermal conductivity2. The rock’s actual values depend on its density, effec- tive porosity and degree of saturation (Cho et al., 2009). Mountains keep a fairly stable temperature throughout the year. Nevertheless, there is an upper transition layer, which can be a couple of meters thick, that is directly affected by the air temperature. This layer will experience greater fluctuations. The geothermal gradient differs considerably
1The relationship between the amount of heat a medium receives and the increase in temperature it experiences
2A mediums ability to conduct heat
between locations, yet typical values are in the range 2-3.5◦C per 100 [m] (Banks, 2012).
The groundwater temperature in Norway is relatively stable, and at 10-15 m below sur- face it approximates the mean yearly air temperature (Kirkhusmo and Sønsterud, 1988).
Average measured groundwater temperature in Norway (figure 3.1b). The highest tem- peratures are found along the eastern coast of Norway and the western coast of Sweden.
Further north, the coastal and continental groundwater temperatures decrease. Central Norway has even lower temperature, down to 2-3◦C. In Møre og Romsdal it is approxi- mately 5-6◦C. In shallow sediments, it is common to observe a delay in the variations in the groundwater temperature relative to the air temperature. In accordance with this, the lowest temperatures are measured during summer, and the highest during winter.
Empirically the temperature is seen to increase by 1◦ per 100 m (Banks, 2012).
(a) Hydrological regions (b) Groundwater temperature◦ C
Figure 3.1: Regional hydrological characteristics in the Nordic countries. Both figures are modified by author. Figure (a) from Kirkhusmo and Sønsterud (1988), and figure (b) based on Kirkhusmo and Sønsterud (1988), from Hilmo et al. (1998).
3.1.5 Sub-surface flow
Porosity is the ratio of void volume to total volume, of the material in question. Perme- ability depends on the porosity, and the degree to which the pores are interconnected.
Permeability describes a medium’s capacity to transmit any given fluid, whereas the hy-
3.1. HYDROGEOLOGY 26
draulic conductivity also includes its physical properties. There are two main types of porosity: primary and secondary (figure 3.2). Primary porosity is formed at the same time as the rock, and is thus an inherent characteristic. For most igneous and metamorphic rocks this is negligible. The secondary porosity comes at a later stage, caused by chemical and mechanical weathering. It includes fissures, fractures and more (Hiscock and Bense, 2014), (Fetter, 2001). Especially the upper meters of an outcropping rock may attain a considerable secondary porosity (figure 3.4a).
Figure 3.2: A: Primary porosity, unconsolidated sand / gravel. B: Primary porosity is reduced by cementation a/o the presence of clay and silt. C: Secondary porosity, bedrock rendered porous by fractures. D: Secondary porosity, consolidated fractured rock rendered more porous by dissolution. Figure modified by author after (MacDonals et al., 2005).
Groundwater possesses mechanical energy in the form of kinetic energy, gravitational potential energy and the energy of the fluid pressure. These three components are defined in equation 3.3a, wherem [kg] is the mass of water,v [m/s] is the water velocity,g [m/s2] is the gravitational acceleration, ρ [kg/m3] is the water density, z [m] is the gravitational head andP [m] is the pressure head (figure 3.3). Considering a unit mass so thatm =ρ, the equation can be transformed into the Bernoulli equation 3.3b. In general, the velocity component is ignored due to its low value in groundwater flow compared to the other factors. After removing this term one can define the total hydraulic head (figure 3.3), h=
P
gρ, as energy per mass unit per gravitational acceleration (equation 3.4a). Groundwater flow is a consequence of energy differences, and a hydraulic gradient must exist for it to take place. The hydraulic gradient, I [-], is defined in equation 3.4b. dh is the change in the hydraulic potential, i.e. the change in head, between two points at a distance dl.
Figure 3.3: Hydraulic, pressure and gravitational head.
Etotal = 1
2ρv2+ρgz+P (3.3a) EBernoulli= v2
2 +z+P
gρ (3.3b)
h=z+hp (3.4a) I = dh
dl (3.4b)
In 1856 Henry Darcy formulated the empirical law governing groundwater flow in a sat- urated material, equation 3.5a (Darcy, 1856). In continuation Darcy derived equation 3.5b, which describes the linear Darcian velocity,
Q=−KAdh
dl (3.5a)
v = Q
Ane (3.5b)
where Q [L3/T] is the discharge, K [L/T] the hydraulic conductivity, ne3 [-] is the effec- tive porosity andA[L2] the cross-sectional area. These equations apply in situations with laminar flow and can be used at macroscopic scales, in materials with continuous and smoothly varying properties. Hence they break down at higher velocities and in hetero- geneous systems where dispersion and diffusion takes place, (Fetter, 2001). Experiments have shown that Darcy’s law is only applicable when Re<=[1, 10].
Fractured rocks are examples of highly heterogeneous systems, with sudden variations in groundwater velocity. To describe the flow rates, Darcy’s original equation had to be modified. The relation between flow and the hydraulic gradient for individual fractures, under laminar flow conditions, is considered to be governed by the ’cubic law’ (equation 3.6). It is thus assumed that flow rate will increase with the fracture aperture (Hiscock and Bense, 2014).
Qf =−2 3
wρgb3 µ
dh
dl (3.6)
3The porosity available for fluid flow
3.2. HYDROGEOCHEMISTRY 28
Qf [m3/s] is the flow within the fracture,w [m] is the fracture width,ρ[kg/m3] is fluid den- sity,µis fluid viscosity,b [m] is fracture height andg the gravitational acceleration [m/s2].
In crystalline bedrock of low permeability, the water table reflects a subdued version of the natural topography. Groundwater commonly emerges as springs, which can be thought of as locations where the water table intersects ground level (Banks and Robins, 2002).
By comparing the dip / strike of the sub-surface fractures, the springs and terrain profiles one can link specific fractures to springs.
Groundwater wells in fractured media
Drilling groundwater wells in fractured bedrock is a difficult task, illustrated in figure 3.4a. The challenge lies in hitting the hydraulic fractures4. Wells drilled between frac- tures might be dry, whereas nearby wells may have a water table. Nevertheless, the water level measured in these boreholes is in some way artificial. When a borehole penetrates a confined aquifer, the high pressure will cause the water table to rise to well above the aquifer. This water table is called the potentiometric surface. It is an imaginary surface, and the potential height of the water table if the aquifer was under atmospheric pressure (Fetter, 2001), (Dingman, 2015). In fractured aquifers it is not given that all fractures beneath the potentiometric surface conduct water. Therefore, it might actually be that the drilling activates deeper fractures, and lowers the water table.
With cameras, televiewers, electrical conductivity profiles and more, it is possible to map the hydraulic fractures in a borehole. Packers, inflatable rubber sealing sleeves, can be placed beneath these fractures (figure 3.4b). This makes it possible to monitor actual water levels at various depths (Cook, 2003). The water can flow towards the well in two ways; either radially as during a pumping test, or linearly if the fractures has a given hydraulic gradient or the well intersects a vertical fracture (Cook, 2003).
3.2 Hydrogeochemistry
To start with, groundwater chemistry is presented, in addition to solute transport mech- anisms. This is followed by definitions of electrical conductivity and pH.
4Fractures that allows for water flow
(a) (b)
Figure 3.4: (a) Boreholes in fractured bedrock, MacDonals et al. (2005). (b) The borehole flow (Cook, 2003), a) with packers, b) radially, c) linearly.
3.2.1 Groundwater chemistry
The composition of an aqueous solution is dependent on the initial composition of the recharge water, the partial pressure of the gaseous phase, the mineral composition, the pH, the solution’s oxidation potential and the potential presence of a biotic assemblage.
Chemical reactions in groundwater are normally reversible, and in equilibrium with its hydrochemical environment. The law of mass action describes how a reaction strives for equilibrium. It states that in an aqueous solution a reaction will go both ways simulta- neously (equation 3.7a). If the environment is at equilibrium, the rates are equal. If in imbalance, the reaction will proceed in one direction. As an example one can consider the dissolution of NaCl. If a solution is undersaturated then more salt will dissolve, if it is supersaturated then the salt will crystallize (equation 3.7b) (Fetter, 2001).
cC+dDxX+yY (3.7a)
N aClN a++Cl− (3.7b)
Appelo and Postma (2016) write that precipitation has a chemical composition dependent on its origin, and its travel through the atmosphere. As it interacts with sediments, veg- etation and bedrock, the composition is altered before it percolates to the groundwater table. Water is an effective solute, and consequently materials in contact with water can dissolve. Thus, groundwater consists of more substances than just pure H2O-molecules.
According to Brattli (2009) there are four dominant factors that affect the chemical com-
3.2. HYDROGEOCHEMISTRY 30
position of groundwater,
1. The chemical composition of the infiltration water.
2. The bedrocks geochemistry.
3. The mineral’s surface area. As the surface area increases the dissolution grows more and more efficient.
4. Residence time in the aquifer. The concentration of solutes will increase with time.
3.2.2 Solute transport
Molecules are transported in a fluid by diffusion, advection and dispersion. Diffusion is a result of concentration gradients that are leveled out by random Brownian movements, and can be considered a statistical process. The rate at which solutes spread is decided by the ion mobility; a function of the friction an ion encounters in the fluid, that depends on both the ion size and the fluids viscosity. Advection is the transport of the solute by the fluid flow. As such, the solute movement will follow the average linear velocity v.
Yet the apparent solute advection can be lower, if the solute reacts with the medium it flows through. Whether advection or diffusion is the most efficient transport mechanisms is ruled by the permeability of the medium, the travel distance and the time period considered. Dispersion is the spreading of a concentration front caused by obstacles such as sediment grains, low-conductive bodies or fractures. The transport mechanisms can be related through the ARD equation 3.8; Advection, Reaction and Dispersion (Appelo and Postma, 2016), (Fetter, 2001).
δc
δt =−vδc δx − δq
δt +DLδ2c
δx2 (3.8)
wherev [L/T] is the waters average linear velocity,cis the solute concentration [mol/kgw], t [T] is time, x [L] is the travel distance and DL [L2/T] the longitudinal dispersion coef- ficient. DL equals De+αLv, where DE [-] is the effective diffusion coefficient and αe [L]
is the dispersivity.
Ion retardation compared to the fluid migration is a consequence of sorption and mineral precipitation. It is related to the specific characteristics of each ion. The mass-balance of
