As a World-First, the Geological Survey of Finland Conducts a Nationwide Survey of Black Shale Deposits
Black shale is a rock type that contains sulphur, and it can be found in various parts of Finland. Black shale can have an adverse effect on surface water quality if the rock surface is broken or soil containing black shale is excavated. Because of this, identifying areas where black shale is present is crucial in construction, forest ditch network maintenance and other land use. As a world-first, the Geological Survey of Finland GTK has mapped the occurrence of black shale units countrywide. In addition to the dataset, a guide on how to assess and manage the environmental impacts of black shale has been published.
Sulphur-rich black shales are abundant in the Precambrian of Finland. Particularly large formations – up to hundreds of metres thick – can be found in Eastern and Northern Finland. Black shale deposits constitute an environmental risk if they are exposed to weathering. Weathering causes black shale to release sulphur compounds and potentially toxic elements and compounds. In anaerobic conditions, sulphur takes the form of sulphide and does not cause issues. Acidification takes place when soil is excavated or when the groundwater level drops, bringing sulphur-containing material into contact with oxygen and surface water.
A black shale formation does not always pose an environmental risk.
“When the layer of black shale in the bedrock is over three metres thick, contains more than one per cent sulphur and is located close to the ground surface, such as exposed at the bottom of a ditch or at the edges of a stream, which allows surface water to wash over it, the area can be considered to have an increased environmental risk. It is essential to know both the sulphur content of the black shale formation as well as the environmental state of the receiving water bodies. Carrying out a site-specific survey is advisable if land use is being planned in an area where black shale is present, particularly if the pH value of the surface water is already lower than the average concentration in Finland,” says Senior Researcher Kirsti Loukola-Ruskeeniemi from GTK.
GTK has published datasets on the occurrence of black shale in Finland. The survey was based on the geophysical low-altitude flight data of the GTK, as only some 3% of Finland’s bedrock is exposed. The locations of the black shale deposits were determined by using the electrical and magnetic properties of the bedrock. Geophysical surveys were supplemented with samples drilled from the bedrock. The maps depict the locations of black shale deposits but determining the environmental effects in detail requires on-site research.
Black shale is often located near ore deposits
Black shale is commonly found in the rock assemblages of sulphide mines. For example, Terrafame Oy’s Talvivaara mine in Sotkamo processes black shale ore.
Finland’s black shale deposits were formed in anaerobic conditions as seabed layers approximately two billion years ago. Black shale contains at least 0,5% organic carbon. Anaerobic conditions in seabed depressions allow both sulphur-containing black shale precursors and sulphide ore solutions to be stored in geological layers. Because of this, black shale and sulphide ore deposits are often found near one another.
During the last glaciation, Finland was covered by an ice sheet. Black shale material was transported in the same direction the glacier was moving – usually southeast – either as ground-up and mixed with the glacial till or as rocks or even blocks.
“The direction in which the glacier was moving has turned out to be essential. The largest amounts of black shale in basal till are most likely to be found along the first hundred metres in the direction where the glacier was moving. The most common glacier flow direction in Finland is from the northeast and therefore, areas where environmental risks are greater can be found in the southeast, while on the northeastern side of the black shale unit, the till contains less black shale material, and the risk is smaller. However, the amount of black shale material in glacial soil depends on many factors, such as the flow conditions of the glacier and the size of the black shale formation,” explains Geologist Jaakko Auri from GTK.
Managing the environmental effects of black shale
The presence of sulphur-rich black shale is reflected in the chemical compositions of the soil, surface water and groundwater. In areas where black shale units are present, groundwater can contain more heavy metals than on average, which is why drilling bedrock wells in sulphur-rich black shale is inadvisable. However, the effects always depend on the size and the sulphur content of the black shale. These characteristics should be investigated particularly in the Category 1 groundwater areas and peat production areas. Additionally, other land use such as construction can put a strain on the environment.
“Identifying areas where black shale is present is extremely important in planning forest ditch maintenance operations,” says Senior Scientist Tiina M. Nieminen from the Natural Resources Institute Finland.
In land use, the environmental effects of black shale can be managed in different ways, such as by neutralising extracted material, preventing oxidation or by stabilising soil material. In the case of construction sites, it may also be necessary to assess the corrosion caused by black shale-rich soil, which may affect underground steel and concrete structures.
Different types of black shale can be found in different parts of the world. For example, almost half of China’s land area also contain black shale deposits although some of the units are fairly thin. Wherever black shale is found, taking it into consideration during regional planning both preserves the environment and lowers costs.
Loukola-Ruskeeniemi mentions an example from Canada. “In Eastern Canada, an international airport was built in an area where black shale is present. If the airport had been built just half a kilometre to the west, environmental damage, and additional costs could have been avoided. During spring floods, water that had flushed road cuttings and become acidified was drained into a nearby river, which caused fish deaths. Road cuttings have had to be covered since then, which has of course been costly. The environmental impacts should always be investigated by means of site-specific studies.”
Further information
Kirsi Loukola-Ruskeeniemi
Senior Researcher
Geological Survey of Finland GTK
kirsti.loukola-ruskeeniemi@gtk.fi
tel. +358 29 503 2850
Jaakko Auri
Geologist
Geological Survey of Finland GTK
jaakko.auri@gtk.fi
tel. +358 29 503 5222
Tiina M. Nieminen
Senior Researcher
Natural Resources Institute Finland (Luke)
tiina.m.nieminen@luke.fi
tel. +358 29 532 5457
Country-wide exploration for graphite- and sulphide-rich black shales with airborne geophysics and petrophysical and geochemical studies (open access)
ScienceDirect peer reviewed article on mapping black shale deposits.
Black shale dataset in GTK’s Hakku data service (abstract and metadata in English):
Bedrock black shale data
Black shale datasets in GTK`s map services:
Map service Mineral Deposits and Exploration
Map service Happamat sulfaattimaat
