Arsenic in bedrock groundwater in the Pirkanmaa region of Finland
|Location||International Geological Congress,oslo 2008|
|Author||Backman, Birgitta۱; Luoma, Samrit۱; Ruskeeniemi, Timo۱; Karttunen, Virpi۲|
|Holding Date||11 October 2008|
This study aims to describe the occurrence of arsenic in bedrock groundwater in Pirkanmaa. This area of 14700 km2 with 450000 inhabitants is located in southern Finland, about 160 km northwest of Helsinki. The work was conducted within an EU Life-Environment project entitled RAMAS, which developed a risk assessment and risk management procedure for arsenic for the Pirkanmaa area.
The crystalline Precambrian bedrock in the area constitutes a considerable and useful groundwater reservoir. Drilled bedrock wells are, however, mainly used for private households or for small water cooperatives. Groundwater sampling in the Pirkanmaa region has been performed many times since 1994. The supplementary sampling by the RAMAS project was carried out in 2005 and 2006. The total number of samples was 1272. Some of the wells were monitored but the data mainly consisted of wells sampled once. Dissolved arsenic was analysed in all samples, but in some cases the total arsenic was also determined. The detection limit for arsenic by ICP-AES/MS was 0.05 µg/l. Arsenic speciation (As5+ and As3+) was determined in 14 well waters, one of which was monitored.
The arsenic content in the bedrock groundwater of the Pirkanmaa region consists of natural, geogenic arsenic. Natural arsenic in the area is derived from arsenic-bearing minerals, which are locally enriched in the bedrock. The Pirkanmaa region can be divided into three geological subdivisions: the Central Finland Granitoid Complex (CFGC) in the north, the Tampere Belt (TB) in the centre, and the Pirkanmaa Belt (PB) in the south. The arsenic problem is clearly focused in central and southern Pirkanmaa, where the arsenic-rich bedrock is composed of mica schist, felsic and mafic metavolcanic rocks and mica gneiss. Based on 603 lithogeochemical samples, the arsenic concentrations varied from 0.1 to 377 mg/kg.
Due to the action of geological and geochemical processes, arsenic has transferred to groundwater. In the dataset of 1272 arsenic analyses from drilled wells, the dissolved arsenic concentration varied from < 0.05 to 2230 µg/l. In 22.5% of the studied wells the health-based limit value of 10 µg/l, recommended by the WHO and the Finnish Ministry of Social Affairs and Health, was exceeded. In the region of the CFGC the median arsenic content of bedrock groundwater was 0.6 µg/l, while respective values for the TB and PB were 5.5 µg/l and 1.6 µg/l. The median total content of arsenic analysed from 46 samples was 47.4 µg/l. In this dataset the median dissolved arsenic concentration was 37 µg/l, and a positive correlation coefficient of 0.98 was recorded between total and dissolved arsenic concentrations. Based to the speciation analyses, groundwater samples were dominated by arsenate As5+ in 9 out of 14 well waters.
If exposure is not limited by appropriate measures, locally high arsenic concentrations in bedrock groundwater may pose a risk to public health and the environment in the southern part of the Pirkanmaa region.