307 FLUORIDE LEVELS IN THE SILURIAN-ORDOVICIAN AQUIFER SYSTEM OF WESTERN ESTONIA Enn Karro, a Alar Rosentau Tartu, Estonia 307 SUMMARY: Groundwater from the Silurian-Ordovician aquifer system of western Estonia was analyzed for various ionic components including fluoride. High fluoride (F) concentrations of natural origin (up to 6.1 mg/l) are common in these ground waters. In general, high F levels are associated with Na-Cl-HCO 3 chemical type waters having low concentrations of Ca 2+. The amount of F increases with increasing ph, and the highest F concentrations are detected in deep (150 200 m) wells in which the groundwater has a long residence time in the host rock. Keywords: Carbonate rocks; Groundwater fluoride; Silurian-Ordovician aquifer; Western Estonia. INTRODUCTION Fluorides in the environment are of public and scientific interest because of their effects on health. Unlike many other elements, a large portion of fluoride (F) is ingested from drinking water. In recent years, the putative anti-caries effects of F in drinking water have been increasingly questioned. 1 However, it is well known that elevated F in drinking water can cause dental and skeletal fluorosis. 2-5 Dental fluorosis caused by excess F in drinking water has been recorded in Estonia, 6 where World Health Organization guidelines have been adopted to recommend that the F concentration in potable water should not exceed 1.5 mg/l. 7 Drinking water supplies in Estonia are derived mainly from groundwater, especially in rural areas, where such water is used directly without purification. The Silurian-Ordovician aquifer system is an important potable water source in northern and western Estonia and on islands of the West-Estonian Archipelago. It consists of diverse limestone and dolomite with clayey interlayers. The upper portion of the water-bearing rocks are 30-m thick and are intensively fractured and cavernous. 8 The aquifer system has a characteristic Ca-Mg-HCO 3 and Mg-Ca- HCO 3 water type with total dissolved solids (TDS) mainly below 0.6 g/l in its upper 30 50 m thick portion. In coastal areas and at greater depths, the content of Cl and Na + in groundwater increases, and Na-Mg-Ca-Cl-HCO 3 type water with TDS between 0.3 1.5 g/l is widespread. 9 This pilot study was initiated to obtain information on the occurrence and geochemical behaviour of high fluoride levels in western Estonian groundwater and their relation to the depth of water abstraction. MATERIALS AND METHODS Fifteen drinking water supply wells trapping the Silurian-Ordovician aquifer system in western Estonia were sampled in 2002 for major ions as well as F and boron (B). The wells range in depth between 60 and 191 m. The in situ ph of the groundwater was also measured. Water samples were taken only after pumping two to three well volumes and stabilizing field parameters (temperature, ph, Eh). a For Correspondence: Dr Enn Karro, Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia. E-mail: enn.karro@ut.ee
308 All samples for ion analysis were filtered and acidified with 1% nitric acid to stabilize the concentrations of trace elements. The levels of Ca 2+, Mg 2+, K +, Na +, Cl, SO 4 2, HCO 3, F, and B were determined in the laboratory of Tartu Environmental Research Ltd. Data processing and visualization were performed by MapInfo Professional 6.0. RESULTS AND DISCUSSION Hydrochemical mapping in the Geological Survey of Estonia: 1997-1998 10,11 revealed that the concentrations of F were mostly below 1.5 mg/l in the Silurian-Ordovician aquifer system (Figure 1). 308 Figure 1. Location of the study area (A) in Estonia. The F contents (mg/l) in the Silurian-Ordovician aquifer system analysed in 1997-1998 10,11 are marked. Waters in eastern Estonia are characterised by low F, whereas high F (1.99 3.79 mg/l) is found in the western part of the country. Results of the present study confirm that extensive F-risk areas with occasionally high F concentrations of natural origin (up to 6.1 mg/l) are common in western Estonia (Figure 2, Table). Only two of sampled wells meet the national drinking water standard that F not exceed 1.5 mg/l. Ground waters with high F content are generally Na-HCO 3 -type waters 2 that are relatively low in Ca 2+. Several authors 12,13 have shown that in waters with high F concentrations, the amount of F is proportional to the HCO 3 concentration and the ph. This same trend can be observed in the Estonian Silurian-Ordovician aquifer system; elevated F contents are associated with ph values over 7.6 (r = 0.72) (Figure 3).
309 309 Figure 2. Map of western Estonia showing sampled wells where F concentrations (mg/l, given in the Table) were determined. Table. Chemical parameters (mg/l) and ph values in sampled wells Well a Depth (m) ph F B Ca 2+ Mg 2+ Na + K + Cl HCO 3 SO 4 2 1 85 7.98 5.7 0.9 24.1 15.6 95 8.7 54 230 51 2 105 8.00 3.1 1.0 24.5 15.8 140 8.2 150 190 55 3 100 8.14 3.7 1.4 24.1 13.6 210 8.4 260 200 45 4 170 8.36 4.4 1.5 14.8 5.1 170 6.4 190 150 51 5 96 8.11 2.5 1.3 18.4 7.8 160 6.7 160 210 22 6 80 8.22 2.9 1.2 19.2 8.0 140 7.0 140 190 22 7 101 7.73 1.9 1.1 42.1 24.3 160 7.6 230 240 35 8 101 8.18 1.9 1.2 27.3 14.3 190 6.7 260 180 26 9 75 8.03 2.4 1.0 28.1 16.3 140 7.1 200 180 19 10 180 8.18 3.6 1.3 15.6 9.0 110 7.2 98 180 45 11 120 8.48 3.8 1.2 14.8 8.7 110 8.1 82 230 37 12 150 8.28 6.1 2.1 14.0 6.3 300 9.9 370 220 39 13 100 7.83 1.8 0.5 39.3 22.4 68 7.2 10 400 1 14 60 7.65 1.4 0.3 66.6 35.2 230 8.8 390 300 45 15 191 7.64 1.0 0.6 35.3 17.5 65 9.4 33 310 4 a Numbering of sampling points as shown in Figure 2.
310 310 8,0 7,0 6,0 5,0 4,0 3,0 2,0 1,0 0,0 0 100 200 300 ph 8,6 8,4 8,2 8,0 7,8 7,6 7,4 7,2 7,0 Depth of the well (m) 140 2,5 Ca 2+ (mg/l) 120 100 80 60 40 20 B (mg/l) 2,0 1,5 1,0 0,5 0 0,0 Figure 3. The depths of the wells, ph values, Ca 2+, and B concentrations plotted against the F content in groundwater. Filled circles indicate results of the current study; empty circles show previous analyses in the Geological Survey of Estonia. 10,11 Groundwater in the aquifer system studied here is mainly the Ca-Mg-HCO 3 - type; when the Ca 2+ content is high, only low amounts of F are mobilized. The highest F concentrations prevail in wells that have a low Ca 2+ content (r = 0.73) (Figure 3) and a high Na/Ca ratio. Generally, the ph of groundwater and the contents of Na + and Cl increase with depth as the groundwater changes toward the Na-Cl-HCO 3 type. 9 Accordingly, geochemically favourable conditions for high dissolved F in water prevail in deeper portions of such aquifer systems. This is in accordance with analytical results, which show that the highest F concentrations are detected in wells having a depth of 150 200 m (Figure 3). The lowest F values are in shallow wells (depth < 30 m), which open the uppermost fractured part of the aquifer system. Thus, high fluoride concentrations are associated with deep groundwaters that have long residence times in the host rock. Fluoride concentrations are relatively independent of the other water-soluble components. However, F exhibits a good relationship (r = 0.70) with the B content (Figure 3), which points to a similar origin (marine, volcanic) of those elements. The probable source of F in groundwater is the dissolution of carbonate rock matrix, 14,15 secondary (vein) minerals (fluorite), and 2 30 cm thick F-rich K- bentonite beds occurring in Ordovician and Silurian carbonate rocks. 16 Since the Silurian-Ordovician aquifer system is the main source of drinking water in western Estonia, the location and the depth of new water supply wells
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