Geological sources of boron and fluoride anomalies in Silurian–Ordovician aquifer system, Estonia

Abstract

The objective of this study was to examine the possible natural sources of fluorides and boron in SilurianOrdovician (S-O) aquifer system, as the anomaly of these elements has been distinguished in groundwater of western Estonia. Water-rock interactions, such as dissolution and leaching of the host rock, are considered to be the main source of high fluoride and boron concentrations in groundwater. Altogether 91 rock samples were analysed to determine if high F -and B levels in groundwater could be attributed to certain aquifer forming rock types. Fluorine and boron contents in limestones and dolomites vary from 100 to 500 mg/kg and 5 to 20 mg/kg, reaching up to 1,000 and 150 mg/kg in marlstones, respectively. K-bentonites, altered volcanic ash beds, are rich in fluorine (400-4,500 mg/kg) and boron (50-1,000 mg/kg). Thus, clay-rich sediments, providing ion-exchange and adsorption sites for F -and B, are the probable sources of both elements in S-O aquifer system in western Estonia.

Determination of boron and fluoride sources in groundwater: Batch dissolution of carbonate rocks in water

“…Furthermore, K-bentonites are rich in B (50-1000 mg/kg) and F (400-4500 mg/kg). Generally, an increase in clay content is followed by an increase in the B and F concentrations in carbonate rocks (Uppin and Karro, 2012). This can be seen in Fig.…”

“…A previous study focusing on the chemical composition of aquifer forming carbonate rocks (Uppin and Karro, 2012) showed that lower CaO/Al 2 O 3 ratios, representative of terrigenous material, were accompanied by elevated B and F concentrations in carbonate rocks. Furthermore, higher B and F concentrations in rocks were related to lower K 2 O/Al 2 O 3 ratios, common to clayey material (mainly illitic clay minerals) in carbonate rocks in Estonia.…”

“…Among the studied carbonate rocks, the proportion of illitic clay minerals (11.7-32.5%) is highest in the marlstones (Table 2). Estonian carbonate rocks containing apatite tend to be rich in F (Uppin and Karro, 2012). However, apatite is probably not a significant source of F -in the groundwater because of its low solubility (Desbarats, 2009).…”

“…Simple batch dissolution experiments were carried out on 20 different carbonate rock (limestone, dolomite, clayey limestone, clayey dolomite, and marlstone) samples selected from 91 rock samples previously analyzed by Uppin and Karro (2012 The mineral composition (quartz, calcite, dolomite, apatite, K-feldspar, albite, illite/illite-smectite, kaolinite, chlorite, pyrite, and biotite) of the rock samples were determined via using the Rietveld X-ray powder diffractometry (XRD) technique at Department of Geology, University of Tartu. Ground and homogenized unoriented powder samples were measured on a Bruker D8 diffractometer using Ni-filtered CuKa radiation over the range 2-70°2θ, with a scan step of 0.02°2θ and a count time of 2 s per step.…”

Deviation of Boron Concentration From Predictions Using Salinity in Coastal Environments

“…More B will tend to be released into groundwater from the adsorbed B and incorporated B if the groundwater has prolonged contact with the B‐rich clays in the aquifer, prior to it discharging into the coastal ocean. This is a possibility for river and groundwater discharging into the Baltic Sea (Karro & Uppin, ; Uppin & Karro, ), where a substantial difference in the B concentration from that predicted from the open ocean B/Cl ratio has been reported (Kuliński et al, ).…”