Geological Survey of Denmark and Greenland Bulletin 33, 2015, 33-36 33© 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 33–36. Open access: www.geus.dk/publications/bull Relationship between groundwater chemistry and the Precambrian basement rocks on eastern Bornholm, Denmark Peter Gravesen, Rasmus Jakobsen and Bertel Nilsson Bornholm is situated south of Sweden, in the Sorgenfrei– Tornquist Zone (Fig. 1). Th e Precambrian basement on northern and eastern Bornholm consists of diff erent types of granitic and gneissic rocks with pegmatites, aplites and dol- erite dykes (Callisen 1934). Th e age of the granite and gneiss is c. 1455 Ma (Waight et al. 2012). Th is study deals with the Østerlars–Svaneke area north of Paradisbakkerne and focuses on the geology and groundwa- ter chemistry of the groundwater aquifers in the Bornholm gneiss, Paradisbakke migmatite and Svaneke granite (Fig. 2). Th e geology and groundwater conditions in part of the study area were described by Gravesen et al. (2011, 2013, 2014). Miljøcenter Roskilde (2009) described the groundwater conditions at the Østerlars and Østermarie waterworks. Th e aim of this paper is to show the relations between rock com- position, mineral alteration and groundwater chemistry in the low-permeability rocks using existing data from outcrops and boreholes. Methods Data on geology and groundwater chemistry were obtained from the Jupiter database at the Geological Survey of Den- mark and Greenland. Information from more than 200 boreholes reaching basement rocks is included; in addition some data on the Quaternary sediments were available. Of the 200 boreholes, data on groundwater chemistry from 29 boreholes were available, in some boreholes as a series of analyses. Data on fractures were collected from outcrops in the area during fi eld work in 2011. Precambrian rock distribution and petrography Th ree main rock types are found in the area. Th e western- most type is the medium-grained, foliated Bornholm biotite gneiss which is usually dark grey, light grey or red grey; small areas with red grey granite or other colours also occur (Cal- lisen 1934; Platou 1970). Th e dark grey type consists of K- feldspar (35%), quartz (25%), plagioclase (25%), biotite (7%), hornblende (5%) and magnetite (3%) and minor amounts of apatite and traces of zircon, allanite, epidote and calcite (Micheelsen 1961). Light grey quartz gneiss and quartzitic types have a larger content of quartz and are oft en banded. Large crystals of apatite and plagioclase occur. Th e gneiss also contains skarn bodies with garnet and epidote or lenses with wollastonite, epidote and garnet. Fine- to medium-grained Paradisbakke migmatite oc- curs in a restricted area between the Bornholm gneiss and the Svaneke granite (Fig. 1) and consists of almost parallel, light grey granitic quartz-feldspar veins in a darker matrix. Th e migmatite consists of K-feldspar (35%), quartz (23%), plagioclase (25%), hornblende (8%), biotite (7%), magnetite (1%) and titanite (1%), zircon and traces of allanite; calcite and epidote occur in the darker part but are rare in the lighter part (Micheelsen 1961). Th e Svaneke granite consists of K-feldspar (36%), plagio- clase (26%) and quartz (25%) with biotite (7%), hornblende (2%) and contains minor amounts of magnetite and titanite, Palaeozoic sandstone Dolerite dyke Svaneke granite Paradisbakke migmatite Bornholm gneiss Fault 2 k m S v a n e ke Ø s t e r l a r s P a r a d i s b a k ke r n e 5 5 ° 6 ´ N 1 5 ° 2 ´ E L i s t e d N e x ø Ø s t e r m a r i e Å r s d a l e G r i s b y P r æ s t e b o q u a r r y S – T Z o n e B o r n h o l m Fig. 1. Geological map of part of eastern Bornholm, modified from Varv (1977). Inset: Bornholm’s location in the Sorgenfri–Tornquist Zone (S–T Zone). 3434 as well as traces of apatite, zircon, epidote, allanite, calcite and fl ourite (Micheelsen 1961). Many small inclusions rich in mafi c minerals also occur and locally the content of apatite and fl ourite in the rock appears to be higher. Th e granite is usually coarse-grained and greyish red but occasionally me- dium to coarse grained and yellow, yellow red or dark grey. Th e Svaneke granite can be divided into four types and a border facies bordering the gneiss and migmatite (Platou 1968, 1970). Th e border facies is commonly strongly line- ated but also has non-lineated parts and a varying content of dark minerals. Svaneke granite type I is light yellow and varies in grain size, and biotite is weakly altered to chlorite; this type occurs around Svaneke and Listed. Svaneke gran- ite type II covers an area from the north coast and inland between the border facies and type I and types III and IV. Th e rock is medium to coarse grained and among the dark minerals hornblende, biotite and sphene are dominant. Th e plagioclase and the dark minerals are weakly altered to un- altered. Svaneke granite type III covers a large area along the east coast from Grisby to the Palaeozoic sandstone north of Nexø and is exposed at several places. Th e rock is light yellow to light yellow red. Th e Svaneke granite type IV occurs as lenses in type III; seen for example at Årsdale. It is a dark red, coarse-grained rock and both the plagioclase and the dark minerals are strongly altered. In types III and IV the hornblende, pyroxene and biotite may be altered to green chlorite, and magnetite is altered to hematite. Especially in the coastal area around Årsdale and southwards towards Nexø the granite is strongly weathered and altered and contains 3–7% chlorite. Th e rock readily disintegrates to form coarse gravel (Årsdale gravel) and the fracture surfaces of the granite are covered by clayey material of green chlorite and yellow-brown limonite. Fracture systems Four fracture systems have been identifi ed in the area. Th e fracture systems are seen in quarries in the Paradisbakke migmatite along the northern rim of Paradisbakkerne, in coastal exposures of the Bornholm gneiss towards the north and in exposures of the Svaneke granite towards the east and the north (von Bubnoff 1942). Two vertical fracture sets with orientations NNE–SSW and ESE–WNW and two hori- zontal fracture sets are found. Th e 3D fracture network of crossing vertical, subvertical, horizontal and subhorizontal fractures forms the groundwater aquifers in the rocks. Th e vertical fractures are mainly transport paths for infi ltrat- ing water to the groundwater table, whereas the horizontal fractures form conduits for groundwater over long horizon- tal distances. Horizontal fractures are present at least up to 90 m depth below the ground surface; two other shallower sets of horizontal fractures are also found (Gravesen et al. 2014). Information from boreholes shows that groundwa- A B C D Fig. 2. The different basement rock types. A: Paradisbakke migmatite with pegmatite body, Præstebo quarry. B: Banded and folded Bornholm gneiss, west of Listed, C: Svaneke granite, east coast of Bornholm, D: Weathered and fractured Svaneke granite, east of Listed. Photographs: Merete Binderup. 35 ter is pumped up from both shallow and deep fractures, the deepest ones 144 m below ground surface. In the weathered rocks iron-bearing minerals are oxidised to yellow-brown clayey iron compounds and the dark miner- als are altered to clayey green chlorite that is found on the fracture surfaces. Clayey material found on fracture surfaces at depths up to 70 m probably have an impact on the chemis- try of the groundwater. Groundwater chemistry Th e number of chemical analyses of groundwater from wells in the study area is unfortunately rather small and some pa- rameters, especially from the Svaneke granite, appear impre- cise, e.g. pH values with only one digit. Measurements of Al concentrations are usually missing, which makes it impos- sible to assess the saturation state of Al-silicates. However, measured concentrations of major cations and measured al- kalinity (except for one outlier) appear reliable. Speciation and calculations of mineral saturations using the soft ware PHREEQC (Parkhurst & Appelo 2012) based on the pa- rameters available, with reservations for the approximate pH values indicates that groundwater in all samples is slightly su- persaturated with respect to calcite. A few analyses included measurements of sulphide and trace metal concentrations and the PHREEQC calculations indicate that concentra- tions of trace metals such as Pb and Zn are likely controlled by sulphide phases. Based on the major cations dissolved in the groundwa- ter, it appears that weathering of the diff erent granite types leads to diff erent water chemistry. In a ternary plot (Fig. 3) showing the relation between (charge equivalents) of Ca, Mg and Na+K in the water, the Bornholm gneiss is barely distinguishable from samples that come from aquifers in Quaternary sand and gravel, whereas water samples from the Paradisbakke migmatite and Svaneke granite are enriched in Mg and to some extent in Na. Na enrichment is most pro- nounced in samples collected close to the coast. High Na concentrations are correlated with high Cl concentrations, which indicates deposition of sea salts by dry and wet deposi- tion. Although also present in seawater, high Mg concentra- tions do not show any correlation with Cl concentrations, which indicates that Mg comes from weathering reactions. Th is is examined in more detail in Fig. 4, where Mg concen- tration versus alkalinity is plotted, which is a general indica- tor for the degree of weathering. As infi ltrating water rich in carbonic acid from the soil zone reacts with the rock, the car- bonic acid is used and the alkalinity in the water increases. A sample from the Svaneke granite with an unusually low alka- linity of 1.6 meq/l is considered an outlier and was removed from the dataset in Fig. 4. It should be noted that the groundwater may have passed through several rock types before reaching the borehole where it was sampled, and there are no reliable groundwa- ter ages so the relative weathering rates based on the water chemistry of the diff erent rocks implies that the bulk water samples from the boreholes represent comparable residence times of the water. Th ere are only three Paradisbakke migma- tite samples; they indicate both low and high rates of weath- ering. Two 14C datings have been made on water from the Paradisbakke migmatite, the ages are rather uncertain, but the oldest of these dates is from the borehole with the highest Mg concentration and alkalinity. Based on the low alkalinity of the water, samples from the Quaternary aquifers indicate the lowest rate of weathering, which could be expected. Of Fig. 4. Plot of Mg concentration versus alkalinity, which is considered an indicator for the degree of weathering. Bornholm gneiss Quaternary sand and gravel Paradisbakke migmatite Svaneke granite (near coast) Svaneke granite Mg% +K% Na Ca% 80 7070 80 9 0 7 0 7 0 7 0 7 0 8 0 8 0 9 0 Bornholm gneiss Quaternary sand and gravel Paradisbakke migmatite Svaneke granite type II (near coast) Svaneke granite types III + IV (near coast) Svaneke granite type II Svaneke granite types III + IV M g ( m m o l/ l) Alkalinity (meq/l) 1.2 1.0 0.8 0.6 0.4 0.2 0.0 4 5 6 7 8 9 10 Fig. 3. Ternary plot showing the relative distribution (on equivalent basis) of major cations (Mg, Ca and Na+K) in water samples from wells in differ- ent granite types. Part of the triangle was removed for clarity. 3636 the crystalline rocks, the Bornholm gneiss appears to have the lowest weathering rate. Two boreholes in the Bornholm gneiss have been sampled using two pumps, giving water from diff erent depths (Rasmussen et al. 2007). Th ese sam- ples show that the lowermost samples have much higher concentrations of fl uoride (1.1 and 1.9 mg/l compared to c. 0.6 and 0.4 mg/l for the mixed samples) and boron (34 and 47 μg/l compared to c. 20 μg/l for the mixed samples), pre- sumably released from either amphiboles or biotite, perhaps apatite in the case of fl uoride. Th e higher concentrations at depth indicate a longer residence time, but the PHREEQC calculations indicate that the water is close to saturation for apatite so using fl uoride concentrations to quantify residence time could be diffi cult. Th ere is no obvious relation between the diff erent types of Svaneke granite and the apparent weathering rates based on Mg and alkalinity concentrations in the groundwater. Still, weathering rates appear to increase from the inland Svaneke granite types III andIV over the near coastal Svaneke granite types III and IV to the Svaneke granite types I and II that show the highest Mg concentration and alkalinity, though types I and II visually appear to be the least weathered. Still the water chemistry indicates that the Svaneke granites weather faster than the other granite type, this could be re- lated to the many small, mafi c, mineral-rich inclusions found in the Svaneke granite or could refl ect a general, primary, tex- tural diff erence that leads to increased weathering rates. Conclusions In spite of the small diff erences seen in terms of bulk miner- alogy of the granitic and gnessic rocks and the subtypes there is a distinct diff erence in the observed groundwater chem- istry. Based on the major cations, especially Mg, it appears that weathering of the diff erent basement rock types leads to diff erent water chemistry. References Callisen, K. 1934: Das Grundgebirge von Bornholm. Danmarks Geologi- ske Undersøgelse II. Række 50, 266 pp. Gravesen, P., Binderup, M., Nilsson, B. & Pedersen, S.A.S. 2011: Geo- logical characterisation of potential disposal areas for radioactive waste from Risø, Denmark. 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Neues Jahrbuch für Mineralogie, Geologie und Paläontologie, Beilagen-Band 87, 277–396. Waight, T., Frei, D. & Storey, M. 2012: Geochronological constraints on granitic magmatism, deformation, cooling and uplift on Bornholm, Denmark. Bulletin of the Geological Society of Denmark 60, 23–46. Authors’ address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; E-mail: pg@geus.dk