FLUORIDE LEVELS IN THE SILURIAN-ORDOVICIAN AQUIFER SYSTEM OF WESTERN ESTONIA

Transcription

1 307 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 ( 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 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 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, Tartu, Estonia. enn.karro@ut.ee

2 308 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: ,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 ,11 are marked. Waters in eastern Estonia are characterised by low F, whereas high F ( 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).

3 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 a Numbering of sampling points as shown in Figure 2.

4 ,0 7,0 6,0 5,0 4,0 3,0 2,0 1,0 0, 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) 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 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

5 311 must be selected very carefully to avoid need for later construction of expensive raw water treatment facilities and increase in the price of water. The wells that are currently operating should be carefully mapped and monitored. If needed, water treatment should be provided or the wells closed. ACKNOWLEDGEMENT This study was financially supported by Grant No from the Estonian Science Foundation. REFERENCES 1 Burgstahler AW, Limeback H. Retreat of the fluoride-fluoridation paradigm. Fluoride 2004;37(4): Lahermo P, Sandström H, Malisa E. The occurrence and geochemistry of fluorides in natural waters in Finland and East Africa with reference to their geomedical implications. J Geochem Explor 1991;41: Grimaldo M, Borja-Aburto VH, Ramirez AL, Ponce M, Rosas M, Diaz-Barriga F. Endemic fluorosis in San Luis Potosi, Mexico. Identification of risk factors associated with human exposure to fluoride. Environ Res 1995;68: Moturi WK, Tole MP, Davies TC. The contribution of drinking water towards dental fluorosis: a case study of Njoro division, Nakuru district, Kenya. Environ Geochem Hlth 2002;24: Alarcón-Herrera MT, Martín-Domínguez IR, Trejo-Vázquez R, Rodriguez-Dozal S. Well water fluoride, dental fluorosis, and bone fractures in the Guadiana Valley of Mexico. Fluoride 2001;34(2): Saava A. Health hazards due to drinking water. Proc Latvian Acad Sci 1998;52: Ministry of Social Affairs (EST). Joogivee kvaliteedi-ja kontrollinõuded ning analüüsimeetodid [The quality and monitoring requirements for drinking water and methods of analysis]. Tallinn; Perens R, Vallner L. Water-bearing formation. In: Raukas A, Teedumäe A, editors. Geology and mineral resources of Estonia. Tallinn: Estonian Academy Publishers; p Perens R, Savva V, Lelgus M, Parm T. The hydrogeochemical atlas of Estonia (CD version). Tallinn: Geological Survey of Estonia; Savitskaja L, Viigand A, Jaštšuk S. Siluri-Ordoviitsiumi veekompleksi põhjavee mikrokomponentide ja radionukliidide uurimistöö [Investigation of minor components and isotopic composition of Silurian-Ordovician aquifer system]. Tallinn: Geological Survey of Estonia; Report No.: Savitskaja L, Viigand A, Jaštšuk S. Siluri-Ordoviitsiumi veekompleksi põhjavee mikrokomponentide ja radionukliidide uurimistöö [Investigation of minor components and isotopic composition of Silurian-Ordovician aquifer system]. Tallinn: Geological Survey of Estonia; Report No.: Handa BK. Geochemistry and genesis of fluoride-containing ground waters in India. Ground Water 1975;13: Saxena VK, Ahmed S. Dissolution of fluoride in groundwater: a water-rock interaction study. Environ Geol 2001;40: Carpenter R. Factors controlling the marine geochemistry of fluorine. Geochim Cosmochim Acta 1969;33: Vingisaar P, Gulova H, Kiipli T, Taalmann V. Distribution of microcompounds in the Estonian Ordovician and Silurian carbonate rocks. Proc Estonian Acad Sci Geol 1981;30: Huff WD, Bergström SM, Kolata DR, Sun H. The Lower Silurian Osmundsberg K-bentonite. Part II: Mineralogy, geochemistry, chemostratigraphy and tectonomagmatic significance. Geol Mag 1998;135: Copyright 2005 International Society for Fluoride Research. Editorial Office: 727 Brighton Road, Ocean View, Dunedin 9051, New Zealand.