Natural sources of arsenic - Workplan
The information on the natural occurrence of arsenic and other, accompanying harmful elements in bedrock, Quaternary overburden and in groundwater existing in research institutes, municipalities and other parties will be gathered into a common database. Supplementary sampling will focus on areas where the level of knowledge is vague. Additionally, detailed studies related to the arsenic phases (solid and dissolved) will be carried out in selected key areas to provide adequate background information on the long-term behaviour of arsenic in the environment.These studies also include the sampling of agricultural soil and crops (potato, wheat and timothy grass) from selected farms located in areas of elevated arsenic levels. Some tests for forest berries and mushrooms will be done to monitor their possible intake of arsenic. The data will be used for the risk assessment and the compilation of GIS-based risk area maps, which will be made available for the authorities and public.
The natural arsenic in the bedrock of the Pirkanmaa region occurs predominantly in arsenopyrite, which is a mineral composed of iron, arsenic and sulphur. From the primary minerals arsenic may dissolve into groundwater and arsenic may be transported along fractured zones to the surroundings. A well drilled in such a system may suffer from very high arsenic concentrations.
The Ministry of Social Affairs and Health has announced a 10 µg/L safety limit for the arsenic in Finnish drinking water following the internationally agreed standard. The corresponding safety limit for arsenic in soil is 50 mg/kg. Also the arsenic in soil is derived from the bedrock. The upper part of the bedrock and the overlying overburden became a subject for a rigorous wearing and reworking during the latest glaciation, which retrieved from the Finnish territory about 10 000 years ago.
Summary of Results
Natural arsenic in the area is derived from the arsenic bearing minerals, which are locally enriched in the metamorphosed, crystalline bedrock. Due to the action of geological and geochemical processes, arsenic has transferred to groundwater and soils. The glaciogenic events were particularly important in dispersing arsenic into the surrounding areas. The study area divides into three units based on geological grounds. In the northern half of the area granitic bedrock dominates and the arsenic concentrations in all geologic media were at the average level encountered in the country. The arsenic problem is clearly focused in the southern part of the Tampere Region (also known as Pirkanmaa), where metamorphosed volcanic rocks are common constituents of the bedrock (Backman et al. 2006).
Arsenic concentrations in shallow groundwater and surface waters are generally low, below 1 µg/l. Hence, arsenic is not an issue for the public water supply, which are based on these shallow water reservoirs. The major concern is focused on drilled wells, which are used by private households and other small units. Altogether, 1237 arsenic analyses from drilled wells were available. In 22.5 % of the wells the limit value, 10 µg/l, was exceeded. All these arsenic wells are located in the southern part of the study area. Most of the samples that had arsenic speciation analysis were arsenate (As5+) dominated.
Elevated arsenic concentrations in soils are related to till, which is the main soil type in the region. The regional arsenic anomaly extending from the Tampere Region towards south was already recognized in the nationwide geochemical mapping of till. The median value for arsenic in the study area is double compared to the rest of the country (5.3 mg/kg vs. 2.6 mg/kg). There are areas where the arsenic concentrations exceeded the limit value for contaminated soil (50 mg/kg for residential areas and 100 mg/kg for industrial areas). The highest encountered concentration was 9 280 mg/kg. Arsenic concentrations tend to increase downwards in the soil profile and the highest concentrations are generally in the basal part of the sequence. This observation has important implications for the handling of arsenic-bearing till. Arsenic concentrations in other soil types are generally low, although slightly higher than elsewhere in the country.
Locally high arsenic concentrations in bedrock groundwater may pose a risk for public health in the southern part of the Tampere region. In shallow groundwater and surface water the arsenic concentrations were low. In some cases the high arsenic content in bedrock and soil may give rise to environmental problems and demand careful consideration in land-use planning. RAMAS project produced a series of geochemical maps presenting the arsenic distribution in various geological media. In addition, an integrated geochemical risk area map was compiled, where the observed arsenic concentrations relative to the guideline values for drinking water (10µg/l), soil (50 mg/kg) or bedrock (50 mg/kg) were applied to evaluate the source of the risk.
The contents of arsenic and other elements in arable and forest soils and crops were investigated in selected farms. The 13 farms studied were located in areas where the arsenic concentrations in till were known to be elevated. The aims were to compare arsenic concentrations between the arable and forest soils, between soil layers, between crop species and between the high- and low-arsenic areas. Wheat grains (Triticum aestivum L.), potato tubers (Solanum tuberosum L.) and timothy grass (Phleum pratense L.) were selected crop species because they are important in the human food chain (Mäkelä-Kurtto et al. 2006).
Arsenic contents in arable soils ranged from 2.90 to 6.80 mg/kg dry matter (dm) in the plough layer and from 2.84 to 4.82 mg/kg dm in the subsoil. These values are at the national level despite of the elevated arsenic concentrations in the surroundings. Only about 1% of total arsenic was in a soluble form in the soil plough layer. Arsenic content in corresponding forest soils were somewhat higher, but distinctly lower than in till. This is due to the differences in source and transport distance of the geogenic material forming these soil types. The source for clays and other fine-grained soils, typically cultivated in this region, is further away in low-arsenic bedrock areas, while tills represent the local, arsenic-rich bedrock. A major source of arsenic in the arable and forestland seemed to be of geogenic origin. Obviously, the surface layers have received minor amount of additional arsenic from anthropogenic sources, like atmospheric deposition and fertilizer preparations.
Contents of arsenic in the crops were at a low level. Arsenic contents increased in the following order: wheat grains (0.005 mg/kg dm), potato tubers (0.011 mg/kg dm) and timothy grass (0.014 mg/kg dm), on the average. Peeled potatoes contained less arsenic than unpeeled ones. Soil-to-plant uptake factors of arsenic were also low 0.001 for wheat grains and potato tubers and 0.004 for timothy grass, on average. Arsenic had one of the lowest soil-to-plant uptake factors among the elements studied. Limited data on forest berries and mushrooms collected by the project did not evidence any arsenic uptake either.
References:
Backman, B., Luoma, S., Ruskeeniemi, T., Karttunen, V., Talikka, M. & Kaija, J. 2006. Natural Occurrence of Arsenic in the Pirkanmaa region of Finland. Geological Survey of Finland, Miscellaneous Publications, 88 p.
Backman, B., Eklund, M., Luoma, S., Pullinen, A., Karttunen, V., 2007b. Natural and anthropogenic arsenic contents in the Pirkanmaa region. Arsenic contents in different soil horizons, in tailing sand and dust in water at quarries, at CCA wood preservative plants, and at landfills and in natural berries, mushrooms and birch sap. Geological Survey of Finland, Miscellaneous Publications, 33 p (in Finnish).
Mäkelä-Kurtto, R., Eurola, M., Justén, A., Backman, B., Luoma, S., Karttunen, V. & Ruskeeniemi, T. 2006. Arsenic and other elements in agro-ecosystems in Finland and particularly in the Pirkanmaa region. Geological Survey of Finland, Miscellaneous Publications, 116 p.
