A Mineralogical and Geochemical Description of Potentially Acid-producing Gneisses from the Lillesand Area - Implications for Leaching Behaviour

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A Mineralogical and Geochemical Description of Potentially Acid-producing Gneisses from the

Lillesand Area

Implications for Leaching Behaviour

Adam Pearce

Master Thesis in Geosciences

Environmental Geology 60 credits

Department of Geosciences Faculty of Mathematics and Natural Sciences

UNIVERSITY OF OSLO

June 2018

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A Mineralogical and Geochemical Description of Potentially Acid-producing Gneisses from the Lillesand Area - Implications for Leaching Behaviour

http://www.duo.uio.no/

Trykk: Reprosentralen, Universitetet i Oslo

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Abstract

Sulphide-bearing gneisses of southern Norway have been observed to leach acidic waters with ecotoxic levels of metals when exposed to air and moisture following excavation for road and infrastructure projects. Traditionally, the sulphide mineral content (or % S) in these rocks has been used to estimate their acid producing potential. However, a number of other lithology related factors have been shown to influence the rate at which these minerals oxidise and the subsequent toxicity of the effluents. Through the use of column leaching experiments and detailed petrological and mineralogical analysis, the effects of mineralogy and texture, particle size and temperature on reaction rates and leaching behaviour has been investigated.

Leachates resulting from gneiss material having the fastest oxidation rates, lowest pH and fastest release rates of elements such as Al, Ni, Cd and Cu were from rock samples with a well-developed and rust-coloured weathering crust. Secondary sulphate and Fe-oxide minerals, typically dominating this weathered material have proven to play a significant role in the acid-forming processes despite making up only a small percentage of the total rock mass.

While overall sulphide mineral content was observed to influence ARD production, rock textural properties such as sulphide morphology and their spatial relationship to other minerals may also have influenced the observed oxidation rates. Anomalously low oxidation rates were, however, observed in one of the studied gneiss samples. This sample, characterised by a hard and crystalline texture containing significant quartz and small amounts of calcite, is believed to represent texturally controlled oxidation rates.

Smaller particle size ranges (0-4mm) were expected to produce faster oxidation rates than larger ranges (0-10mm), however, this was only observed for columns containing weathered crust material. This demonstrates a relatively higher reactivity of the finer-grained secondary minerals constituting the weathered crust. Reduced water flow through most columns with the smaller grain size range (0-4mm) was responsible for lower reaction rates as mineral dissolution became transport-controlled. In the coarser material, surface control is argued to prevail. While reaction rates of rock samples were faster in warmer temperatures, a threshold temperature where Al and heavy metal element release rates increased and accelerated was observed between 4 and 10°C. Results from this study show that although playing a major role, sulphide content alone cannot dictate a rock ARD potential. In addition, the integrated weathering history, inherent rock texture, primary mineralogy, sulphide morphology and grain

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size all influence the calculated oxidation rates resulting from the performed column experiments. Hopefully, these findings may contribute to a revision of the current sulphur- based guidelines for the classification of ARD potential in the gneissic rocks of the Lillesand area.

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Acknowledgements

The present work was carried out at the Norwegian Geotechnical Institute (NGI) and the Department of Geosciences at the University of Oslo (UiO) in the period between May 2017 and May 2018. The work was supervised by: Gijsbert Breedveld, professor II at the

Department of Geosciences, UiO and Technical Director (Environmental Engineering) at NGI; Andreas Olaus Harstad, senior geological advisor at NGI; and Erlend Sørmo, project advisor (Contaminants and Land Use) at NGI.

I would firstly like to thank my supervisors for giving me the opportunity to work on this interesting and meaningful project. After many years of working in the “dirty” oil industry, it is an honour to give something back to the environment and hopefully the results of this project can help Lillesand Kommune deal with the ongoing issue of their notorious gneisses. I am also very grateful that I was able work in the professional surrounds of NGI’s offices and laboratory. It has been interesting to experience how the environmental engineering group operates. I would also like to thank my supervisors for the professional and efficient advice and motivation they have given me over the past 12 months. Without them, I think this thesis would possibly look more like a high school essay.

The guys in the Technical Administration Group at the Department of Geosciences (UiO) have also been a huge help. Firstly, Thanusha Naidoo has offered her time and professional advice with regards to XRF and XRD on a countless number of occasions and I hope she didn’t get sick of my nagging and stupid questions. Mufak Said Naoroz and Magnus

Kristoffersen both have received my endless number of water samples with open arms and the girls in the dungeon (SEM lab), Siri Simonsen and Berit Løken Berg been amazing in

pointing out weird and wonderful minerals with the scanning electron microscope. I really do appreciate all the fantastic help and advice you have given me and I hope I haven’t been too much trouble for you all.

Thanks also goes to Per Hagelia from the Norwegian Roads Administration for sharing his findings and thoughts on the topic and to Ole Martin Aanonsen (ex Lillesand Kommune) for sharing information and showing us outcrops in the Lillesand region.

I’d also like to thanks my adopted Norwegian parents, Torbjørn and Marit for helping with picking the kids up from school/kindergarten and looking after them during times of need.

And finally to my wonderful wife, Ragnhild, who has been so patient with me going on this

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short “income-free” journey... you have been so supportive with my attempted career change and have sacrificed a lot. You understand that this means a lot to me so I express all my love and gratefulness to you. By the way, I know you won’t get past the first paragraph of the introduction but that’s ok;-)

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List of Figures

Figure 1 - 'Yellowboy' seen in a small creek in Lillesand ... 4

Figure 2 - pe-pH diagram for the Fe - O2 - H20 system ... 8

Figure 3 - Oxidation rates (log r) for sulphide minerals as a function of dissolved Fe³⁺ concentration in solution at pH2 (Rimstidt et al., 1993) ... 10

Figure 4 – The Lillesand Region of Southern Norway (sampling locations shown in red) ... 14

Figure 5 - Geological Map over the Kjevik - Lillesand - Kaldvell Area with sampling locations, 5, 7, 12 and E. Areas of rusted sulphidic gneisses as mapped by Statens vegvesen (2003) are marked with an ‘S’. (Adapted from Norges Geologiske Undersøkelse) ... 16

Figure 6 - ARD potential assessment used by the Swedish Road Authority. The left side of the triangle is S content in mg/kg, right side is the amount of rock mass and; the bottom is the pH attained from a leaching test. G1 is highest potential for ARD while G5 is the lowest (Swedish Transport Administration, 2015) ... 21

Figure 7 - Lillesand Sampling Location Outcrops. ... 24

Figure 8 – Handling of the four rock samples for analyses. This procedure was done for each rock type. ... 25

Figure 9 - XRD workflows used in this study ... 28

Figure 11 - Columns set up in stand ... 33

Figure 10 - Column set up ... 33

Figure 12 - Staining on Sample 5W (0-4mm) bottle ... 35

Figure 13 – Rock Sample 5 ... 40

Figure 14 - Rock Sample 12 ... 41

Figure 15 - Rock Sample 7 ... 41

Figure 16 - Rock Sample E ... 42

Figure 17 - Thin section pictures (cross polars) taken of weathered edge zone in sample 5- 1(top left); secondary minerals growing on weathered surface and in between grains in sample 5-2 (top right); Sericitisation occurring in plagioclase grains (bottom) ... 44

Figure 18 - Scanned thin sections 7-1 and 7-2. 7-1 is a more homogeneous rock while 7-2 displays hornblende-rich, sulphide mineral-free zones (left side) next to finer-grained zones with disseminated sulphide minerals (right side). Both sections are approximately 3cm long. ... 45 Figure 19 - Thin section pictures (cross polars) taken of 7-1 showing the more homogenous structure and euhedral quartz/plagioclase grains and lack of schistosity (top left); Hornblende

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and titanite grains in sample 7-2 (top right); magnetite grains (opaque), calcite (colourful grain in centre) and sericitisation occurring in a some plagioclase grains from thin section 7-4 (bottom) ... 46 Figure 20 - Thin section pictures (cross polars) taken of foliated zone in Sample 12-1 (left) and seriticisation occurring in a large plagioclase grain (right) ... 47 Figure 21 - Average major element content for each sample from XRF analysis (values are presented in wt %) ... 48 Figure 22 - Average trace content for each sample location from XRF analysis (N.B. 5W is a sample made mostly from weathered crust) ... 49 Figure 23 - XRD quantification results for the four subsamples from each sample location.

Only two weathered crust subsamples (5W) were analysed. ... 50 Figure 24 - Average mineral content as detected by XRD in Lillesand Samples ... 51 Figure 25 - Mineral content in weathered crust sample 5W (top) as detected by XRD (Fresh rock sample 5 is included at the bottom for comparison) ... 52 Figure 26 - Representative Sample 5 backscatter SEM image with main minerals highlighted.

More dense minerals appear brighter as seen by the white pyrrhotite grains ... 53 Figure 27 – Secondary electron SEM image from thin section 5-2 approx. 2.5cm in from weathered edge (as shown by red circle). Pyrrhotite grain relatively unaltered as is most biotite and plagioclases ... 55 Figure 28 - Backscatter SEM image from thin section 5-2 approx. 4mm in from weathered edge (as shown by red circle). Sulphide mineral alteration occurring with lightest colouring being less weathered pyrite/pyrrhotite and the darker whites being a secondary sulphate mineral which fills the dissolution voids ... 55 Figure 29 – Secondary electron SEM image from thin section 5-2 on weathered edge (as shown by red circle). Fe-oxide minerals plus schwertmannite and jarosite growing on and in between plagioclase and biotite grains. Inset shows small grains of plagioclase and pyrite in a”soup” of Fe-oxide minerals which has filled dissolution voids. N.B. Figure 17 shows this same image from a normal optimal microscope ... 56 Figure 30 – Secondary electron SEM image of the progressive weathering pattern in a

chemically weathered pyrrhotite grain 2-3mm from weathered surface in thin section 5-1. The original core is surrounded by a more oxygen -rich rim which contains less Fe and S than the core area. ... 56 Figure 31 – Backscatter images of chlorite forming in conjunction with an Fe-oxide mineral in the weathered zone of thin section 5-1 ... 57

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Figure 32 – Secondary electron SEM images of precipitate material from a sample taken from the top of column 5UW (0-4mm). Schwertmannite and jarosite forming on grain surfaces ... 57 Figure 33 – Backscatter SEM images of the yellow-brown Fe-oxide crust of a highly

weathered piece of rock sample 5 ... 58 Figure 34 - Backscatter SEM image of thin section 7-1 showing typical texture of sample 7. 58 Figure 35 - Backscatter SEM images of thin section 7-2. Left - zone with large grains of quartz, plagioclase, hornblende, biotite, calcite and titanite with no graphite nor sulphide minerals. Right – zone with smaller grain sizes and the presence of small pyrrhotite grains and graphite ... 59 Figure 36 - Backscatter SEM image of thin section 7-4 showing a central large magnetite grain surrounding by a quartz/plagioclase matrix with smaller grains of biotite, calcite,

titanite, zircon and K feldspar ... 60 Figure 37 - Backscatter SEM images of thin sections 7-2 and 7-4 showing element mapping of carbon (C) to signify the calcite grains ... 60 Figure 38 - Left - Suspected sericitisation seen on the weathered edge of thin section 7-1.

Right - The same weathered edge under SEM showing minimal weathering characteristics . 61 Figure 39 - Backscatter SEM image of thin section 12-2. Left - Muscovite and biotite grains align in a foliated zone with sericitisation occurring in minerals around the pyrrhotites; Right – a less foliated area in thin section 12-3 showing the interaction between pyrrhotite grains and matrix minerals ... 61 Figure 40 -The same weathered surface from thin section 12-1 under optical microscope (left) and SEM (right). The degree of sericitisation of plagioclase and biotite grains is less evident under SEM compared to optical microscope ... 62 Figure 41 - Backscatter SEM image of thin section 12-1 showing sericitisation of a large plagioclase grain. Muscovite appears in the weathered zone. The inset shows another

weathering product which is an unidentified clay mineral, possibly phlogopite or glauconite.

Bottom right is the same image from an optical microscope. ... 63 Figure 42 – Fe-oxide layer observed on weathered surface of a 12-4 rock sample ... 63 Figure 43 - White precipitate on the top of the 12 (0-10mm) column (left). Secondary electron SEM image of pickeringite from the same rock sample (right) ... 64 Figure 44 - Backscatter SEM image of rock sample EUW showing typical minerals observed ... 65 Figure 45 - UVD (Ultra Variable-Pressure Detector) SEM image of rock sample EUW

showing how jarosite grains are dispersed over grain surfaces... 65

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Figure 46 - Various secondary electron SEM images of sample EUW. Hexagonal jarosite grains form on surface(a) and often form clumps (b and c). Iron(III) oxide-hydroxide minerals also found in several places (d) ... 66 Figure 47 - Backscatter SEM image of sample EW showing jarosite grains sitting on a layer of a potassium poor sulphate mineral (possibly schwertmannite) ... 66 Figure 48 - Secondary electron SEM image of sample EW showing a Fe-oxide layer covering a quartz grain ... 67 Figure 49 - White precipitate on the top of the E (0-10mm) column (left). Secondary electron SEM image of possible alunite from the same rock sample (right) ... 68 Figure 50 - Grain Size Distribution Analyses for rock samples in 0-4mm (top) and 0-10mm (bottom) columns ... 69 Figure 51 - Cumulative Retained/Evaporated Water in the 12 Columns over the 40 Week Period ... 70 Figure 52 – Top: pH of water samples over time from all columns. Bottom: pH of water samples over time all columns minus the 7 (0-4mm) and 7 (0-10mm) columns ... 72 Figure 53 - Electrical Conductivity (E.C.) of water samples over time for all columns ... 73 Figure 54 - Sulphate concentrations in water samples from all columns over time. N.B. the highest value from the 5W (0-4mm) 10°C column (week 28) was measured with a dilution factor of 5000 which has added uncertainty. ... 74 Figure 55 – Na⁺, K⁺, Mg²⁺ and Ca²⁺ concentrations in water samples from all columns over time ... 74 Figure 56 - Trace element concentrations(Al, Fe, Cd, Ni, Cu, and U) in leachates from all columns over time ... 76 Figure 57 – The five 0-10mm columns: E, 12, 7, 5W, 5UW; showing varying degrees of iron colouring ... 77 Figure 58 - Colour variations in water samples from all columns after 16 weeks ... 78 Figure 59 - Colouring due to iron precipitation. Left: Differences over time between water samples in 4(left), 10(middle) and 20C (right). Right: Colour changes over time inside the columns ... 78 Figure 60 - Cumulative Release of Sulphate plotted against Cumulative Release of Leachate.

Plot on right is the zoomed in area on the main plot on the left. ... 79 Figure 61 – Steady State Sulphate Release Rates from all columns ... 79 Figure 62 - Steady State Release Rates for Al, Fe, Cd, Ni, Cu, and U from all columns ... 80

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Figure 63 - The Goldich Weathering Sequence from observations of their sequential

disappearance in soils (Goldich, 1938) ... 87

Figure 64 - Backscatter SEM image of a weathered zone 2-3mm from a weathered edge in thin section 12-2. Pyrrhotite grains show extensive oxidation and as a result, plagioclase and biotite have started to weather. Muscovite grain (46) shows minimal signs of weathering. ... 88

Figure 65 – Top: Thin Section 5-4 showing large pyrite grains. Bottom: Backscatter electron SEM image of thin section 5-4: a) large elongated pyrite grain with sutured edges. b) 2 phases of sulphide minerals adjacent to each other ... 90

Figure 66 - Backscatter SEM Image of thin section 12-2 showing a relatively intact pyrrhotite grain (left) and a highly altered pyrrhotite in direct contact with sphalerite ... 91

Figure 67 - Backscatter SEM images of weathered edges in thin section 7-1 (top) and 5-2 (bottom). The differences in the degree of pyrrhotite oxidation are partly due to the differing rock textures. ... 93

Figure 68 – Left: Total Al concentrations from the three 5W columns in 4, 10 and 20degC over the 40 week period; Right: Cumulative Al release plotted against cumulative leachate . 97 Figure 69 - Plots of leachate concentrations of sulphate, Al, Fe and trace elements against pH ... 100

Figure 70 - The relationship between E.C. (mS/cm) and leachate total Al concentrations ... 102

Figure 71 - Sulphide-bearing rocks in the Kristiansand-Lillesand and lower Birkenes region (1:50000)(Agder Naturmuseum og Botaniske Hage, 2009) ... 115

Figure 72 - Cartoon examples of ARD Index method for classification of a rocks ARD potential (Parbhakar-Fox et al., 2011) ... 116

Figure 73 - Aeromagnetic Survey over Lillesand Area (Source (Norges Geologiske Undersøkelse 2007) ... 116

Figure 74 - Possible muscovite grains (left) sieved out in sieving process ... 116

Figure 75 - Sample 12 Rock Samples ... 116

Figure 76 - - Sample 7 Rock Samples ... 116

Figure 77 Sample 5 Rock Samples ... 116

Figure 78 - Grain Size Distribution Comparisons of Sample E Columns ... 116

Figure 79 - Grain Size Distribution Comparisons of Sample 5W Columns ... 116

Figure 80 - Grain Size Distribution Comparisons of Sample 5UW Columns ... 116

Figure 81 - Grain Size Distribution Comparisons of Sample 7 Columns ... 116

Figure 82 - Grain Size Distribution Comparisons of Sample 12 Columns ... 116

Figure 83 - F and Cl concentrations in leachates from all columns over time ... 116

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Figure 84 - Cation %'s Balancing Sulphate Anionic Charge (N.B. due to exaggerated sulphate ion concentrations from ion chromatography measurements, the sulphate anionic charge was

reduced to match the total cation charge when E.B. errors were over 10%) ... 116

Figure 85 - Cation %'s Balancing Sulphate Anionic Charge over the 40 Week Period ... 116

Figure 86 - Zn, As, Cr, Mn, Ba and Pb concentrations in leachate coming from all columns. ... 116

Figure 87 - Linear regression equations, R2 values and steady state cumulative release rate values for Al, Fe, Cd and Ni ... 116

Figure 88 - Linear regression equations, R2 values and steady state cumulative release rate values for U and Cu ... 116

Figure 89 - XRD Diffractograms for subsamples 7-1, 7-2, 7-3, 7-4 ... 116

Figure 90 - XRD Diffractograms for subsamples 12-1, 12-2, 12-3, 12-4 ... 116

Figure 91 - XRD Diffractograms for subsamples E-1, E-2, E-3, E-4 ... 116

Figure 92 - XRD Diffractograms for subsamples 5-1, 5-2, 5-3 and 5-4 ... 116

Figure 93 - XRD Diffractograms for weathered crust subsamples 5W-1 and 5W-2 ... 116

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List of Tables

Table 1 – Current Guidelines for Disposal of Sulphide-bearing Waste Rock in Lillesand

(Lillesand Kommune, 2018) ... 20

Table 2 - Water and soil indicators of the potential for acid sulfate soils in inland aquatic ecosystems in Australia. (Source: Australian Government - Department of Agriculture and Water Resources (2011)) ... 22

Table 3 - Sampling Locations around Lillesand Area 5/5/2017 ... 23

Table 4 - Oxides and elements measured in XRF Analyses ... 27

Table 5 - Column specifications ... 34

Table 6 - Sample Mineralogy Overview ... 43

Table 7 - XRD averages from the four subsamples from each sampling location. N.B. Due to limits in detecting minerals of low quantities (<3%), there is more uncertainty with the quantity of certain minerals ... 51

Table 8 -D10, D30, D60, Cu and Cc values for each of the different rock types in the columns ... 68

Table 9 - Regression Equations, R2 and Calculated Steady State Release Rates of Sulphate for all 12 columns ... 79

Table 10 - Summary of mineralogical/textural characteristics of rock samples and their respective leachate chemistry ... 81

Table 11 – ARD Rock Characteristics ... 94

Table 12 - Summary of Water Chemistry Results for leachates in 20, 10 and 4C... 96

Table 13 - Results from Major Element XRF Analysis (values in wt % oxide) ... 116

Table 14 - Results from Trace Element XRF Analysis ... 116

Table 15 - pH and Temperature measurements from column leachates (temp in degree C) . 116 Table 16 - E.C. and Temperature measurements from column leachates (EC in mS/cm and temp in degree C) ... 116

Table 17 - Results from Ion Chromatography Analyses (plus AL, Fe, Mn, Zn and Ni for E.B. calculation - all values in mg/l; <LOD = Below Limit of Detection; NS = No Sample; E.B. = Electrical Balance); E.C. = Electrical Conductivity)) ... 116

Table 18 - Total Al and Fe concentrations in leachates from all columns (values in mg/l)... 116

Table 19 - Ni, Cd, Cu, U and Zn concentrations in leachates from all columns (values in μg/l) ... 116

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Table 20 - As, Cr, Mn, Ba and Pb concentrations in leachates from all columns (values in μg/l) ... 116

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1 Introduction

1.1 Problem Description

It is well known that sulphur-bearing rocks can be acid-generating and when exposed to the atmosphere and precipitation, the resulting effluents can have harmful effects on nearby aquatic and terrestrial ecosystems. The mining industry is the most common setting for these toxic effluents which are often called acid mine drainage (AMD), however, they can occur as a result of other types of human intervention or even in naturally in the environment. Sulphide minerals, such as pyrite and pyrrhotite, tend to occur in isolated concentrations and are therefore most often associated with metal ore and coal deposits. As it is also possible to find them disseminated in other common rock types as well as acid sulphate soils (ASS), they are often encountered during infrastructure projects such as those in the building and road construction industries (Åstebøl et al., 2011). In these settings, the preferred term for the acidic effluent is acid rock drainage (ARD). If the buffering capacity of the host rock is not sufficient, the acidic environment can result in the mobilisation of potentially toxic metal ions from the surrounding geology into aquatic ecosystems.

Parts of Norway and Sweden are challenged by the presence of Alum shale, a Cambrian to Ordovician black shale which is known for its ARD and radioactive potential and its subsequent threat to concrete structural integrity of buildings and human health (Andersson et al., 1985, Snäll, 1988). In Norway, strict regulations apply to the disposal of acid producing black shales (e.g. Alum Shale) and specially designed disposal sites have been developed (Norwegian Geotechnical Institute, 2015a, Norwegian Geotechnical Institute, 2015b). There are also cases where acidic gneisses such as the Stora LeMarstrand formation in south-west Sweden have caused severe acidification as a result of road projects, with measurements of pH 2-4 seen in nearby streams (Swedish Transport Administration, 2015). A similar rusty sulphide-bearing gneiss in southeastern Norway which has been extensively excavated during highway development has caused localised environmental problems due to ARD from a number of large waste rock deposits (Hindar and Nordstrom, 2015).

The Norwegian Public Road Administration (NPRA or Statens Vegvesen), the main road construction organisation in Norway, completed a four year research and development program in 2012 called Nordic Road Water (NORWAT) with the aim to minimise harm to aquatic environments during the building and operating of road networks (Norwegian Public Roads Administration, 2014). There has subsequently been extensive research completed on

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the mineralogy, geochemistry and environmental effects of Alum Shale in relation to highway building projects such as the upgrade of the state highway 4 (rv. 4)(Pabst et al., 2016, Engelstad, 2016, Fjermestad et al., 2017). Likewise, research was conducted into the problems associated with the sulphide-bearing gneisses in southeastern Norway (Hjulstad, 2015, Skipperud et al., 2016), however, the current guidelines for classifying potentially acid- producing rock are not entirely reliable.

There are regulations in Norway regarding soil masses which have been classified as contaminated. The Pollution Control Law states that any soil or rock mass which produces acidic effluent as a result of air/water exposure is considered contaminated and special disposal measures must be applied (Norwegian Ministry of Climate and Environment, 2009).

Much of the waste rock excavated in the Lillesand area is therefore regarded as contaminated and must go through expensive disposal processes. These processes may vary in complexity and cost depending on the ARD potential of the rock. It has been observed that classification and mitigation methods used on a “low ARD potential” waste rock deposit were inadequate as acidic, metal-rich leachates ran into nearby waterways (Hindar and Nordstrom, 2015).

Conversely, some sulphide-bearing rocks in the area may not produce acidic effluent and could unnecessarily be going through costly disposal treatments. A reliable classification method for ARD potential is therefore crucial for the sulphidic gneisses and results from this study may contribute to its establishment.

1.2 Objectives

As infrastructure projects and the subsequent excavation of sulphide-bearing rock masses continues in the Lillesand area, the overall aim of this study was to determine the main factors controlling production of environmentally harmful metal-rich effluents. Hopefully, the results of this study can contribute to the work of revising the current classification guidelines and further assess the environmental impact of sulphide-bearing rock disposal in the study area.

More specifically, the objectives of this project were to:

 resolve the mineralogical, geochemical and textural characteristics of ARD producing rocks in the study area.

 investigate the effect of temperature and particle size on the rates of ARD production.

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1.3 Scientific Approach

Natural geological systems are complex and as the rock mineralogy and other factors affecting the production of ARD can vary considerably from site to site, it can be difficult to predict the potential for the occurrence of ARD and the relevant costs involved (Blowes et al., 2003). This complexity also makes it difficult to link certain effects observed in the field to specific parameters. Laboratory studies are therefore preferred for this purpose as the experimental conditions can be much better controlled. Column leaching tests allow for 1) regular controlled leaching 2) easy collection of samples for analysis 3) calculation of acid- forming rates and metal release and 4) the prediction of overall water quality. Most column test procedures, however, do not include mineralogical analyses despite the fact that relationships between mineralogy, texture, microstructural features (i.e., fractures) and trace element deportment are critical in influencing leachate chemistry (Diehl et al., 2007).

This study utilised column leaching experiments to monitor how a number of factors influence the onset and extent of ARD from four different sulphur-bearing gneiss samples from outcrops in the Lillesand area of southeastern Norway. Mineralogical analyses were completed on all samples in the hope to find any links to the leachate chemistry. These included the adaptation of a rock textural classification system by Parbhakar-Fox et al. (2011) called the Acid Rock Drainage Index (ARDI). Two different grain size ranges were adopted for each rock sample as well as three different temperatures for one sample type. A final variable was the inclusion/exclusion of a deeply weathered crust. The different hypotheses are summarised below:

 Mineralogy –Rock types with higher sulphide mineral content are expected to produce more acidic effluent and have faster reaction rates than those with low sulphide content. Rock texture is also assumed to affect reaction rates.

 Particle size – Columns with smaller grain size ranges will potentially produce more acidic effluent and have faster reaction rates than larger grain size ranges due to larger surface areas available for reaction.

 Temperature – Columns in warmer temperatures will potentially produce more acidic effluent and have faster reaction rates than those in cooler temperatures.

 Weathering State – Columns with rock samples including a pre-existing weathered crust are expected to produce more acidic effluent and have faster reaction rates than those without due to the acidifying effect of secondary sulphate minerals in these crusts.

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2 Theoretical Background

2.1 Characteristics of ARD/AMD Environments

Acid rock drainage from sulphide-bearing rocks can occur naturally due to normal rock weathering processes, however, it can be exacerbated by large-scale earth excavations such as mining and other infrastructure projects. While ARD can vary in its chemical and visual characteristics, the main characteristics of the resulting effluent are generally similar.

Visually, waterways with acidic effluent often form a yellow-brown precipitation on rock surfaces, colloquially termed yellowboy (Figure 1). Its formation occurs when aqueous ferric iron ions (Fe³⁺) precipitate as iron(III) hydroxide, iron oxides or oxyhydroxides in circumneutral streams (Pennsylvania Department of Environmental Protection (PA-DEP), 1999). The main chemical characteristics are:

1. Low pH (Nordstrom, 2000),

2. high concentrations of dissolved heavy metals (Pabst et al., 2014) which are quite often above the acceptable limits for drinking water,

3. high concentrations of sulphate (SO42-) (Tsukamoto et al., 2004) and;

4. high concentrations of cations such as Ca²⁺, Mg²⁺ and Al³⁺ which come from the surrounding geology as a result of acidic dissolution.

Figure 1 - 'Yellowboy' seen in a small creek in Lillesand

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2.2 Oxidation of Sulphide Minerals

Sulphide minerals are inorganic compounds which are most valuable as metal ores. Their composition is expressed with the general chemical formula AmSn, in which A is a metal, S is sulfur, and m and n are integers. Some of the most common minerals found in ARD settings are pyrite (FeS2), pyrrhotite (Fe(1-x)S), sphalerite (ZnS), chalcopyrite (CuFeS2), galena (PbS) and arsenopyrite (FeAsS). The oxidation reactions involved with each mineral are described extensively in the literature (Blowes et al., 2003, Banks et al., 1997, Akcil and Koldas, 2006, Nordstrom et al., 2015).

As pyrite is the most common sulphide mineral in nature and prevalent in many ARD cases, its reaction equations and kinetics are presented below. Pyrite can be found as an accessory mineral in igneous, sedimentary and metamorphic rocks (Pellant, 2002). It has little economic value on its own but, as it is often found in association with valuable minerals such as galena, chalcopyrite and sphalerite, it often exists in mine waste. It is therefore the primary source of most ARD (Nordstrom et al., 2015). Pyrite has a chemical structure of one ferrous cation (Fe²⁺) and one S22- anion and while its ideal stoichiometric relationship is 1:2 for Fe:S, deviations exist in nature (Abraitis et al., 2004).

The oxidation of pyrite can occur in an oxygenated or anoxic environment if the mineral surface is exposed to an oxidant (generally O2 or Fe³⁺) and water. The process of oxidation of pyrite is quite complex but in a simple form it can be described as follows when oxygen is present (Stumm and Morgan, 1996):

2FeS2 + 2H20 + 7O2 → 2Fe²⁺ + 4SO42- + 4H⁺

pyrite + water + oxygen → ferrous iron + sulphate + protons (1) If the surrounding conditions are sufficiently oxidising and acidic, much of the ferrous iron (Fe²⁺) may then be oxidised to ferric iron (Fe³⁺):

4Fe²⁺ + 4H⁺ + O2 → 4Fe³⁺ + 2H2O (2)

The Fe³⁺ ions may then be used as electron acceptors which further enhance pyrite oxidation, especially if conditions become anoxic. This reaction releases more acid:

FeS2 + 14Fe³⁺ + 8H2O → 15Fe²⁺ + 2SO42- + 16H⁺ (3)

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Hydrolysis driven acid production may also occur. This reaction is only possible at low pH (<4) as Fe precipitates at higher pH values:

Fe³⁺ + 3H2O → Fe(OH)3 + 3H⁺ (4)

The overall reaction then yields:

4FeS2 + 14H2O + 15O2 → 4Fe(OH)3 + 8SO42- + 16H⁺ (logK (25°C) = +829.4) (5)

It can be seen from reaction 5 that ferric hydroxide precipitates with sulphate anions being released into solution. Four moles of H⁺ are generated for each mole of pyrite oxidised and it is these protons which can influence the release of heavy metals into solution. The large equilibrium constant value (K) for this reaction indicates that the reaction will proceed to the right meaning that it will continue until the reactants are depleted (Barnes and Romberger, 1968).

2.3 Factors Affecting Rate of Oxidation

As pyrite is the most common sulphide mineral in ARD cases, its rate of oxidation will be predominantly described below. The factors influencing the rate of oxidation of pyrite have been extensively researched (Evangelou and Zhang, 1995, McKibben and Barnes, 1986) and these include oxygen and Fe³⁺ concentrations, pH and temperature.

2.3.1 pH, Oxygen and Fe³⁺ concentrations

Oxidation of pyrite can occur through chemical, biological and electrochemical pathways involving surface interactions with O2 and Fe³⁺. Under the chemical pathway in acidic conditions, the major oxidant is Fe³⁺ (Singer and Stumm, 1970), however, at more neutral pH, the reaction is limited by the lower concentration of aqueous Fe³⁺ due to its low solubility at pH > 4. and oxygen becomes the predominant oxidant (Blowes et al., 2003). Although oxidation of pyrite by Fe³⁺ continues at a more neutral pH, the reaction needs dissolved oxygen to oxidise Fe²⁺ to Fe³⁺ (Evangelou and Zhang, 1995). Oxygen and Fe³⁺ can both attach chemically to the mineral surface, however, as Fe³⁺ displays a more efficient electron transfer, it results in faster oxidation (Luther, 1987).

Pyrite oxidation rates from oxygen and Fe³⁺ have been determined in a number of previous studies (Jerz and Rimstidt, 2004, Morth and Smith, 1966, Rogowski and Pionke, 1984, Nicholson et al., 1988). Williamson and Rimstidt (1994) compiled all of the pyrite oxidation

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rates with dissolved oxygen and formulated a rate law which is applicable for more than four orders of magnitude in O2 concentration and over a pH range of 2-10:

r_{𝐹𝑒𝑆2} = 10−8.19(±0.04). [𝑂₂]^0.5(±0.04)

[𝐻⁺]0.11(±0.01) (6)

where rFeS2 is the rate of pyrite dissolution in mol.m^-2.s^-1. This would suggest that with increased dissolved oxygen and, to a lesser degree, increased pH, the faster the reaction rate.

The same authors additionally determined a rate law applicable over six orders of magnitude of Fe²⁺ and Fe³⁺ concentration for the pH range of 0.5-3.0, under fixed concentrations of oxygen:

r_{𝐹𝑒𝑆2} = 10−6.07(±0.57).[𝐹𝑒³⁺]0.93(±0.07)

[𝐹𝑒²⁺]0.40(±0.06) (7)

where rFeS2 is the rate of pyrite dissolution in mol.m^-2.s^-1. It is clear that the higher the ratio of Fe³⁺ to Fe²⁺, the faster the rate. Consequently, the most critical factor in the oxidation of pyrite is possibly that Fe³⁺is a more effective oxidant than oxygen therefore the oxidation of Fe²⁺ to Fe³⁺,as seen in equation 2, is a very important process (Nordstrom et al., 2015).

Figure 2 is a pe-pH diagram for the Fe - O2 - H2O system where pe is the redox potential or the measure of the tendency of a chemical species to acquire electrons and thereby be reduced. It can be seen that at the Fe³⁺ species is stable in high pe (i.e. oxygen rich), low pH aqueous conditions typical of ARD settings. In more reduced acidic conditions (i.e. lower pe), Fe²⁺ is more stable. Similarly, as pH increases, Fe³⁺ will precipitate as Fe-oxides. The amount of oxygen available and pH are therefore important factors to the concentration of soluble Fe³⁺ and consequently the general oxidation rate of sulphide minerals.

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2.3.2 Temperature

As with most chemical reactions, the rate of pyrite oxidation is increased with increased temperature. Lu et al. (2005) found that release rates of Fe²⁺associated with the oxidation of pyrite increased with increasing temperature and Steger (1982) found that pyrrhotite oxidation rates doubled when temperature was increased from 28°C to 50°C with the process following Arrhenius behaviour, a formula for the dependence of reaction rates on temperature.

Schoonen et al. (2000) examined the effects of illumination on pyrite oxidation rates and found that its reaction rates were nearly doubled, however, some of this increase was attributed to light-induced heating of the pyrite surface.

2.3.3 Grain Size

Depending on the host rock lithology and excavation operations (blasting and crushing caused by heavy equipment, etc.), a wide range of particle sizes can be found at a waste rock site.

Particle size distribution is also an important key parameter for assessing ARD potential of any waste rock pile and this is due to the fact that the rate of acid generation is directly proportional to the surface area of acid-forming minerals exposed to weathering processes (Erguler et al., 2014).

Through column leaching experiments, Erguler et al. (2014) found that the time taken for leachate to become <pH 4, for some copper ore rocks containing mostly chalcopyrite and pyrite was 21-38 weeks for fine-grained samples (<0.625mm) and 43-65 weeks for coarser- grained samples (<3.35mm) with the range resulting from the different column dimensions

Figure 2 - pe-pH diagram for the Fe - O2 - H20 system

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used. Similarly, Parbhakar-Fox et al. (2013) also showed through column tests that a 4mm fraction material generated lower pH leachate, higher mass release of elements and sulphate for the majority of samples compared to a 10mm fraction material.

2.3.4 Mineralogy (Content and Texture)

Rock characteristics such as relative mineral content, sulphide morphology and sulphide mineral associations may play a role in sulphide mineral reaction kinetics.

Sulphide Mineral Content

Sulphide mineral content (and thus % S) in the rock is obviously a critical factor for its ARD potential. It has been shown through static and kinetic tests that rocks with higher S content produce effluents which were more acidic and contained higher sulfate (Lapakka, 1994, White and Jeffers, 1994). In guidelines for the prediction of ARD and metal leaching in British Columbia in Canada, rocks with < 0.3% S have been considered unlikely to oxidise at rates fast enough to result in acidification (Price, 1997) and this value was also adopted by Parbhakar-Fox et al. (2011). Similarly, regulations from Nova Scotia state that all sulphide bearing rock with >0.4 % S must go through a strict disposal process (NS Regulations, 1995).

Type of Sulphide Mineral

The type of sulphide mineral may also be an important factor in the rate of oxidation. From different oxidation rates measured from a number of studies (Morth and Smith, 1966, Jerz and Rimstidt, 2004, Rogowski and Pionke, 1984, Nicholson et al., 1988, Steger and Desjardins, 1978, Steger, 1982, Nicholson and Scharer, 1994, Kwong, 1995), it seems that when O2 is the main oxidant, the rate of pyrite oxidation is roughly an order of magnitude faster than pyrrhotite oxidation. This is despite Blowes et al. (2003) and Emmons (1917) stating that pyrrhotite reacts 20-100 times and 5 times faster than pyrite respectively. Rimstidt et al.

(1993) also compared oxidation rates of a number of different types of sulphide minerals under typical AMD conditions and found that pyrite, galena and arsenopyrite had the fastest rates which were around an order of magnitude faster than those for chalcopyrite and sphalerite (Figure 3). Parbhakar-Fox et al. (2013) also found from a series of column tests with varying ARD rock types that columns with abundant arsenopyrite were consistently the most acid-forming.

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Additionally, it has been shown that impurities in pyrite can influence its oxidation rate.

Kwong (1993) stated that the presence of Ni and Co in pyrite increases resistance to oxidation while the same author and others have stated that As impurities in pyrite increases reactivity (Blanchard et al., 2007, Lehner et al., 2007).

Neutralising Minerals

The buffering effect of carbonate minerals in sulphide-bearing rocks is commonly known.

Dogramaci et al. (2017) studied a large number of ground and surface water samples in a mining region of Western Australia and found that sulphate concentrations and pH levels were dependent on the carbonate content on the aquifer. The buffering effect of carbonates is also documented in studies on Alum shales of Norway (Erstad, 2017, Pabst et al., 2016).

Secondary Minerals

It is widely debated as to whether secondary minerals play a role in mitigating and/or causing acidity and ecotoxic metal mobility with a number of reviews done on the topic (Jambor et al., 2000, Nordstrom, 2000, Frau and Marescotti, 2011). Oxidation of sulphide minerals produces a variety of Fe-oxide precipitates and efflorescent salts depending on conditions. They form surface accumulations by evaporation and precipitation within ponded surface waters, at seeps or in groundwater discharges along fractures and cleavages (Hammarstrom et al., 2005). They contain minerals which can act as a sink for acidity and toxic metals (Carlson et al., 2002) but they can also release these metals when minerals breakdown (Stipp et al., 2002, Acero et al., 2006). It has been shown through dissolution experiments that efflorescent sulphate salts have a strong acid producing capacity (Carbone et al., 2013, Hammarstrom et al., 2005) and this is evident in mine sites with abundant quantities of these salts where negative pH has been

Figure 3 - Oxidation rates (log r) for sulphide minerals as a function of dissolved Fe³⁺ concentration in solution at pH2 (Rimstidt et al., 1993)

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recorded (Nordstrom and Alpers, 1999). On the other hand, it has been shown that minerals such as schwertmannite, an orange precipitate found in streams and lakes affected by ARD as well as in acid sulphate soils, can act as a sink for trace elements such as Cu, Cr, Ni and Zn.

Its progressive transformation to jarosite and then goethite, however, results in a depletion of some of these, in particular Zn and Ni (Carbone et al., 2013).

The stability of the various secondary minerals is therefore very important in an environmental context as they control metal distribution in the aqueous surface environment.

Ferrihydrite ((Fe³⁺)2O3.0.5H2O), for example, dominates at pH values >5.5, jarosite (KFe³⁺3(OH)6(SO4)2) dominates at pH 0.8-2.5 and schwertmannite (Fe8O8(OH)6(SO4)·nH2O) tends to dominate at intermediate pH levels (Bigham and Nordstrom, 2000). This was confirmed in a study by Murad and Rojík (2003) where precipitates under acidic (pH 3.7) conditions were predominantly schwertmannite and orange in colour. Following confluence with alkaline waters (pH 8.3) that had not been affected by mine drainage, the effluent pH rose to 7.3, the color changed abruptly to reddish-brown, and the principal precipitate mineral was ferrihydrite.

Additionally, it has been shown that primary sulphide mineral oxidation may be reduced after the formation of precipitate layers. Shengjia et al. (2013) observed that under a neutral pH with additional silica in solution, pyrite dissolution is reduced by up to 60% under various experimental conditions. This was due to the formation of an amorphous iron hydroxide passivating layer on the mineral surface which inhibited further pyrite oxidation.

Rock Texture

Pre-2011, ARD rock prediction and characterisation techniques did not include textural analyses. A few studies had focused on micro-scale textural evaluations but none considered mesotexture of acid forming phases and its control on acid formation. Parbhakar-Fox et al.

(2011) introduced the staged geochemistry-mineralogy-texture (GMT) approach as a means of addressing this, and includes the use of a novel textural evaluation scheme, the ARD Index (ARDI). The ARDI not only considers sulphide/neutralising mineral content and sulphide alteration but also sulphide mineral morphology and their spatial relationship to other minerals. It is done at micro- and meso-scale and the resulting classification score (between - 10 for acid neutralising to 50 for extremely acid-forming) determines the rocks acid-forming or neutralising potential (Appendix B).

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For sulphide morphology, highest ARD rankings were given for disseminated (meso-scale) and framboidal (seen in micro-scale) sulphide minerals while sulphidic veins (meso) and euhedral grains (micro) rank the lowest. With regards to sulphide mineral spatial relationships, higher ARD rankings are given when an acid-forming sulphide phase is in direct contact with another sulphide phase and lower rankings given if they are in contact with and inert or neutralising mineral. Finally, fresh unaltered sulphide minerals rank highest whereas sulphides replaced by secondary minerals as a result of extensive oxidation rank lowest.

Parbhakar-Fox et al. (2013) integrated these GMT and ARDI approaches on mine waste rock with the results of standard kinetic test procedures to identify the mineralogical changes that influence leachate chemistry over time. Results indicated that oxidation of pyrite was strongly influenced by their morphology, the presence of mineral micro-inclusions and galvanic interactions with other iron-sulphide minerals. More specifically, pyrite grains with irregular morphologies (i.e. anhedral) and those containing galena micro-inclusions weathered fastest.

However, pyrite was also observed to be galvanically protected by galena. Evidence of pyrite oxidation being affected by galvanic interactions is also reported by Chopard et al. (2017) where they found that pyrite oxidation was reduced when mixed in columns with other sulphide minerals such as chalcopyrite and sphalerite.

Weisener and Weber (2010) investigated the changes in pyrite mineralogical composition upon oxidation in relation to their different morphologies using laboratory-based kinetic tests.

They found that complete oxidation of 70-100% of framboidal pyrite in their samples occurred after 210 days while euhedral pyrite grains persisted and still contributed to the ongoing acidity after 390 days.

2.3.5 Microbiological Processes

It is well known that the oxidation of sulphide minerals such as pyrite and pyrrhotite is catalysed by the action of acidophilic sulphide-oxidising bacteria of which Acidothiobacillus ferrooxidans is the most studied (Fowler et al., 2001, Kocaman et al., 2016). By being a catalyst in the oxidation of Fe²⁺ to Fe³⁺(equation 2), this bacterium is therefore able to accelerate the chemical oxidation of iron sulphides (equation 3) which is typically quite slow.

As pH is even more reduced as a result of the reaction described by equation 3, the acidophilic bacteria are able thrive in an environment where most others cannot. While there is usually a mixed population of iron/sulphur-oxidising bacteria in ARD settings, the exact species and their activity levels is dependent on the conditions such as pH and temperature

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(Norris, 1990). The optimum temperature and pH for A. ferrooxidans and another acidophilic bacterium, Acidothiobacillus thiooxidans is 15-35°C and pH 2.0-2.5 (Blowes et al., 2003). It is therefore no surprise that in more neutral or alkaline waters where these bacteria do not thrive, that it is abiotic oxidation of dissolved Fe²⁺ to Fe³⁺which dominates (Banks et al., 1997). A. ferrooxidans is an aerobe which requires CO2 as a source of carbon for growth and it has been shown by Barron and Lueking (1990) that a CO2 concentration of 7-8% is optimal for its growth rate. They also found that that the concentration of FeSO4 has an influence on its growth with maximal growth rates in the presence of 2-3g Fe²⁺ per litre.

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3 Study Area

The study area was the Lillesand region in southern Norway. The municipality is bordered in the north by Birkenes municipality, to the east by Grimstad municipality and to the south by Kristiansand municipality. The E18 highway, which extends from Oslo in the north to

Kristiansand in the south, runs through Lillesand (Figure 4). Over the last 2-3 decades, redevelopment of this highway and other infrastructure projects involving excavation of large volumes of rock has occurred. The normal summer temperature in Lillesand is 15.4°C while in winter it is -1.1°C and the average yearly rainfall is 1230mm (Norwegian Meteorological Institute, 2017).

Figure 4 – The Lillesand Region of Southern Norway (sampling locations shown in red)

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3.1 Geological Setting

The geology of southern Norway comprises a variety of Precambrian rocks (1750 – 900ma) formed in different plate tectonic settings and deformed and metamorphosed during the Gothian and Sveconorwegian orogenies. The processes which occurred in the latter orogeny were responsible for the present appearance of the rocks with many large bodies of granite and gneiss of ages ranging from about 975 – 925ma (Ramberg et al., 2008). A major faulting zone extends NE-SW for about 200km from Porsgrunn to Kristiansand which separates the quartz-feldspar/granitised gneisses from the Bamble area in the south east from the non to slightly-granitised rocks of the Telemark area in the northwest. This granitisation increases towards the southwest and is almost complete around Kristiansand (Holtedahl, 1960).

Locally around the Lillesand area, there is a variation in rock types. The bedrock is from the Bamble Formation and is predominantly gneisses, rich in alkali-aluminium silicates with intrusions of amphibolite in the western parts of the watershed (Maijer and Padget, 1987). The sandstone and shale sediments originally deposited in the basin 1600-1700ma were later metamorphosed deep in the earth’s crust to granitic and augen gneisses as well as bodies of diabase and gabbro which transformed into amphibolite (Norges Geologiske Undersøkelse, 1996).

There has been a number of geological mapping surveys done in the area in particular in relation to the acid rock drainage issue, as well as an aeromagnetic survey in 2007 (Appendix C). Norges Geologiske Undersøkelse (2005) observed fine-grained, banded, granoblastic quartz-feldspar-biotite gneisses which dominated the geology along a northeast-southwest line passing 5km each side of Lillesand. The orange-pink colour from NGU’s geological map in Figure 5 represents this (amfibolitt, hornblendegneis, glimmergneis, stedvis migmatittisk).

Generally this gneiss is composed of more felsic (light coloured) than mafic (dark coloured) minerals, however, the relatives amounts alternate in layers so that banded features often result (Norges Geologiske Undersøkelse, 2005). Darker gneisses with higher quartz content are also observed in several places.

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In a survey done by Agder Naturmuseum og Botaniske Hage (2009), it is mentioned that sulphur in this area was sourced from volcanic plumes which then formed iron sulphide minerals in the pre-metamorphosed sediments. These minerals which are primarily pyrite and pyrrhotite are therefore disseminated in the quartz-feldspar-biotite gneiss but are also found along thin veins and spots of quartz and feldspar in this gneiss. Sulphide mineral content has been stated to be as high as nearly 10% in Storemyr, 5% in other localities such as Sørlandsparken in Kristiansand (Agder Naturmuseum og Botaniske Hage, 2009) but is most commonly under 1% (Norges Geologiske Undersøkelse, 2002). Geokart AS (2001) completed a geological survey on the sulphidic gneisses in the area and concluded that the sulphide minerals exist in 2 ways:

1. In layers linked to gneiss foliation (most likely dominated by pyrrhotite).

2. Along old fractures as pyrite (sulphide areas which have more or less weathered to iron hydroxide or iron oxyhydroxides).

NGU completed a geochemical (XRF) and petrographical study of ⁓20 samples over five different areas in Lillesand in 2002 and concluded that, due to generally low sulphide content in the rocks, they will most likely not contribute significantly to acidic runoff when exposed

Figure 5 - Geological Map over the Kjevik - Lillesand - Kaldvell Area with sampling locations, 5, 7, 12 and E. Areas of rusted sulphidic gneisses as mapped by Statens vegvesen (2003) are marked with an ‘S’. (Adapted from Norges Geologiske

Undersøkelse)

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to air or water (Norges Geologiske Undersøkelse, 2002). Statens vegvesen (2003) mapped areas where rusty yellow-brown weathered sulphidic gneisses exist (‘S’ markings on map in Figure 5) and stated that the main reason for leaching of sulphate is connected to the jarosite- iron oxy-hydroxide on the rusted weathered surfaces and in fractured zones. They continued to say that most weathered rocks were the quartz and muscovite-rich gneisses and that the contribution from primary pyrite and pyrrhotite was relatively small. The rusted surface is the result of chemical alteration of sulphide minerals (Norges Geologiske Undersøkelse, 1996), however, it is possible that the rust colouring can develop independently from the iron sulphide minerals, for example from biotite rich gneiss (Geokart AS, 2001). NGU also observed that the rusty weathered surface was easily washed off with brief periods of rain.

Statens vegvesen (2003) stated that there is generally little trace of sulphide or sulphate minerals in the rusted surfaces due to weathering after the last ice age (<10000 years ago).

They also state that these rusty weathered zones have been observed to be more intense deeper into the surface in some places which suggests it could be a much older weathering process. Furthermore, it has also been observed that muscovite-rich layers have both rusty weathering and sulphide minerals (Geokart AS, 2001).

3.2 Previous Work and Current State of Knowledge

Ever since the first environmental problems were observed in Lillesand, a number of geological surveys as well as hydro- and geochemistry studies have been carried out. Hindar and Lydersen (1994) studied Lake Langedalstjenn, a weakly acidified coastal lake which, in 1986, had a pH of 5.2 – 5.6 and sulphate and Al concentrations of 330 μeq.L^-1 and 10-20 μeq.L^-1 respectively. Heavy metals were also at or below ICP-MS detection limits. After the blasting of about 200000m³ of bedrock 200-300m upstream from the lake, it resulted in a pH of 4.4. They concluded that it was the subsequent oxidation of sulphide minerals as a result of the rock excavation which released an acidic effluent. Concentrations of cations (Mg²⁺ and Ca²⁺) and anions increased by a factor of 8 in the inlet stream and more than tripled in the lake. The most dramatic cation increase was dissolved inorganic aluminium species, Al³⁺

which were responsible for the death of brown trout in the lake.

Due to the threat of environmental harm from a proposed highway development, a geological survey by Norges Geologiske Undersøkelse (1996) was initiated where they mapped instances of rust-coloured outcrops as observed in the blasting area related to Lake Langedalstjenn. As planning for the development heightened, the Norwegian Institute for Water Research (NIVA) completed a report in 2003 which investigated the ecological status in streams, rivers,

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lakes and coastal waters, and the possible effects on water chemistry and biology due to acid runoff (Hindar and Roseth, 2003). In the same year, the Norwegian Public Roads Administration (Statens Vegvesen) also completed a report which summarised the problem and suggested possible mitigation measures (Statens vegvesen, 2003). It was in this report that the acidifying potential of weathered crust material was outlined. They completed abrasion pH tests of jarosite-rich samples which produced pH levels as low as 2.3. In a study by IFE (2002) using sulfur and lead isotopes as a tracer, results showed that fresh and undisturbed gneiss material with disseminated pyrrhotite contributed very little to the pollution which occurred in Lake Langedalstjenn. It was found that the largest contribution to the pollution came from muscovite gneiss with a weathered pyrrhotite as well as a pyrite on fractured surfaces. IFE's data also showed that yellow-brown weathering crusts gave very important contributions.

. A few years later, NIVA completed a report outlining proposed suggestions of countermeasures for dealing with the prospected large amounts of excavated sulphide-bearing rock from the highway development (Hindar and Iversen, 2006) as well as a report which measured pre-development water chemistry in 25 lakes and streams which may potentially be affected (Hindar, 2005). Norges Geologiske Undersøkelse (NGU) conducted a second geological survey in 2005 along the new E18 trajectory from Grimstad to Kristiansand (Norges Geologiske Undersøkelse, 2005). Their aim was to assemble existing data into a dynamic data format, make additional mapping, further develop field criteria to better locate and refine potentially acidifying rocks. They also created a classification system for sulphide rock weathering where areas with white micas and yellow jarosite were regarded as having the highest potential for ARD production.

The new highway was constructed in the period 2006 – 2009 and cuts partly through sulphide-bearing rock. During construction, the main focus in relation to classifying ARD potential rocks was on the content of intact iron-sulphide minerals. Rocks with an assumed sulphur content in the range of 1.25-5% were placed offsite in three deposits around the Lillesand area. COWI carried out extensive ΔT tests in an attempt to classify rocks according to their ARD potential (COWI, 2016). This method involved taking drill dust from various cores taken along the proposed route and combining it with the strong oxidising agent, hydrogen peroxide (H2O2), for a period of 25 minutes. As pyrite and pyrrhotite oxidation reactions release heat, the change in temperature (ΔT) was measured. If ΔT was over 1.2°C, the rock was classed as having over 1.25% sulphur content. The method was deemed to be

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effective, however, as jarosite and other sulphates do not contribute much heat, the method did not reflect the total sulphur content and misclassifications may have occurred (Hagelia, 2015).

While shell sands and liming products were added to the deposits, they were not effectively covered during the highway building operations so as to reduce contact with oxygen and water. ARD was subsequently produced and potential environmentally harmful effects were observed (Hindar et al., 2007). The resulting leachate in outlet streams was regularly measured at pH 4-5, contained sulphate levels up to 100 times higher than pre-construction levels and had environmentally harmful levels of labile Al and heavy metals (Cd, Co, Cu, Mn, Zn, Ni). Hindar and Nordstrom (2015) concluded that Al was mobilised when sulphate concentrations increased and pH dropped below 5.5 – 6. They also state that the lower 1.25%

S limit for disposable waste rock is much higher than a corresponding limit of 0.4% set by the Nova Scotia sulphide-bearing material regulations (NS Regulations, 1995) and may have resulted in acid-producing rock material outside the deposits. Added liming material to the waste rock deposits has balanced sulphate levels somewhat but due to the fast dissolution of CaCO3 compared to sulphide minerals, it is only really a short-term measure to stabilize pH and this reduce leaching. More effective countermeasures which reduce exposure to water and oxygen are required, such as capping with impermeable materials. Environment monitoring programs are continuing to this day.

After the highway development project was completed, a final geological survey was completed by the Agder Nature Museum (Appendix A) with assistance from geologist Ole Fridtjof Frigstad) to complete more extensive mapping of the sulphide-bearing rocks in Lillesand, Kristiansand and the southern part of Birkenes municipality area. A final map was produced showing areas of rocks with sulphur-rich minerals in red ( however, they have stated that it is most likely that there are rocks in these areas which have insignificant sulphide mineral content. It was also mentioned that much of the area mapped was unconsolidated soils or in other words, unexposed bedrock. This means that it is possible that rocks with significant sulphide mineral content could be excavated unexpectedly during a building process and would subsequently require assessment before disposal.

Lillesand municipality stated in Autumn 2017 that almost all development in the municipality was planned in areas likely to be found in sulphide-bearing rocks (Aanonsen, 2017). They subsequently began a so called “sulphide project” with the intention to establish a common

A Mineralogical and Geochemical Description of Potentially Acid-producing Gneisses from the Lillesand Area - Implications for Leaching Behaviour