RD&D-Programme 2004 - SKB

19.2.18 Microbial processes
Conclusions in RD&D 2001 and its review
Microbes affect the groundwater chemistry by speeding up reactions which otherwise proceed
very slowly. Redox reactions are above all affected, but weathering reactions can also be
catalyzed.
Research in the Äspö HRL and elsewhere has shown that different types of microbes live in the
fractures in the crystalline bedrock. Some live on organic carbon compounds that accompany
infiltrated meteoric water from the earth’s surface, while others can live on methane and
hydrogen from the earth’s mantle /19-86/.
Microbes under ground can live without oxygen, in fact some are so sensitive to oxygen that
they die if oxygen is present in their environment. However, many of the underground microbes
are capable of consuming oxygen, as the Rex experiments have shown /19-87/. When oxygen
is not present as an oxidant, different microbes can instead make use of other compounds, such
as the sulphur in sulphate ions (SO 4
2–
) with formation of sulphide (S 2– ), and the iron in various
minerals (Fe 3+ ), which then dissolves in the groundwater (in the form of Fe 2+ ). Manganese
(Mn 4+ ) in brownstone can also be used by microbes, and then dissolves in the groundwater
(Mn 2+ ). Numerous microbes in groundwater break down organic carbon compounds to carbon
dioxide to obtain energy, while others use methane gas as an energy source. Together, the lives
of these microbes in the groundwater have an important influence on the geochemical environment
in the water-filled fractures in the rock /19-88/.
Newfound knowledge since RD&D 2001
Bacteriogenic iron oxides (Bios) are formed by bacteria when anaerobic iron-bearing groundwater
reaches environments with oxygen. Large quantities of iron oxides mixed with microbes
and their excretory/secretory products have proved to be very effective filters for trace metals
/19-89/. The tests have been conducted at the 300-metre level in the Äspö HRL. Retardation
of trace metals in Bios considerably exceeds retardation in inorganic iron oxides. Bios are
formed where groundwater seeps up to the earth’s surface. Any migrating radionuclides will be
effectively captured by Bios, leading to locally increasing concentrations but regionally lower
concentrations compared with a situation without Bios.
Laboratory investigations of capacity to mobilize metals have been carried out on strains of
bacteria isolated from great groundwater depth. The results show that the studied organisms
excrete/secrete complexing agents which can, for example, mobilize uranium out of Ranstad
shale /19-90/. Further investigations have shown that these complexing agents are also able to
mobilize radionuclides from solid phases of silicon and titanium oxide. The tests have been
conducted in an aerobic environment. Investigations are currently being conducted under
anaerobic conditions.
Fungi, mainly yeasts, have been found in groundwater from repository depth in small but
significant quantities /19-91/.
Microbial oxygen consumption in a fracture has been modelled /19-92/. The calculations also
included inorganic processes, for example pyrite oxidation, and evaluated a case where an
infinite quantity of oxygenated water flows through a fracture. The modelling results show that
microbial processes should have a great effect in the short term, but that inorganic reactions take
over in a long-term perspective.
Programme
A laboratory at a depth of 420 metres in the Äspö HRL permits continued work on microbes
and their importance in the deep repository in a project called Microbe. Boreholes have been
instrumented and the chemistry and microbiology of the groundwater have been characterized
264 RD&D-Programme 2004