Recent geoscientific information relating to deep crustal studies

Geoscientific information relating to conditions deep in the earth's crust, as a basis to the deposition of radioactive wastes in vertically drilled boreholes to depths of 4-5 km, has been compiled already in several reports. The objective of this present document is to provide an update of geoscientific information that has become available in the open literature since 1998. Emphasis has been put on crystalline rocks to conform to SKB's disposal concepts for radioactive wastes. These conclusions are arranged under various headings of importance with respect to deep hole disposal of radioactive wastes at 4,000-5,000 m depth. Geothermal considerations: The rock thermal conductivity is an important parameter to help dissipate the heat generated by the waste package and depends on rock composition. In this respect, since mafic-rich varieties are more conductive, a granitic host rock may be considered more problematic. Fractures tend to increase the thermal conductivity; however due to the rock stress the frequency of fractures decreases with depth so that a deterioration in thermal conductivity with depth is to be expected. Evaluating the temperature/heat production gradient is an important pre-requisite for disposal, for example the long-term stability of engineered materials. The experience from the German KTB programme has underlined that it may not always be possible to extrapolate near-surface derived predictions of temperature variations to repository depths, especially in a heterogeneous bedrock environment. Hydraulic conductivity: Main problems are the presence of gas phases and groundwater flow; of lesser importance is fluid circulation initiated by the temperature gradient resulting from the temperature increase of the waste package. Rock permeability is expected to be in the order of (1 to 3) x 10-13 m2 over a 1-2 km zone if exogenic fracturing is assumed, in the order of 10-17-10-16 m2 at repository depths, and in the order of 10-20-10-19 m2 at still greater depths. This reduction in permeability is matched by a reduction in porosity. In areas of subdued topography (e.g. typical Baltic Shield terrain), a zone of active downward moving meteoric water exchange exists in the upper 0-2 km; at greater depths the highly saline fluids and brines are extremely ancient and no recent meteoric water input is observed. In areas of more extreme topography the zone of active meteoric recharge may reach 4-5 km before highly saline fluids are encountered. At the KTB site highly saline fluids appear to be present throughout the rock matrix to at least 9 km. Most, however, are present in microfractures which lack connectivity due to the very low permeability of the host rocks and therefore do not participate in any active groundwater circulation. If hydraulic circulation is indicated at favourable fracture zones of higher hydraulic conductivity, the presence of highly saline to brine fluid compositions may be expected to minimise such circulation. Evidence shows that active saline groundwater circulation does exist but is restricted to intermittently occurring hydraulically conductive fracture zones. Nevertheless at some locations (i.e. the KTB site) it was concluded that in a high brine environment relatively rapid solute transport in fracture systems is possible. Hydrogeochemistry: At repository depths long-term hydrochemical stability appears to be assured where hydraulic conditions are favourable (i.e. low topography; weak hydraulic gradients; low permeability). This is further supported by the presence of highly saline fluids to brines and associated gases at great depths which reveal ages of millions of years with no evidence of recent meteoric water exchange. Fracture mineral chemistry and fluid inclusion studies support long-term stability. Highly saline fluids, whilst undesirable from a near-field viewpoint (e.g. corrosion potential), are less conducive to radionuclide mobilisation and transport. Solute transport through the bedrock will be diffusion dominated provided major hydraulically conductive fracture zones are avoided. Microbial activity: The potential for microbial activity appears to be feasible in the Baltic Shield bedrock environment as it is unlikely that the temperature threshold of 115 deg C will be exceeded at repository depths, i.e. the temperature above which life microorganisms cannot be sustained. However, provided major hydraulically conductive fracture zones are avoided (i.e. potential sources of microbes and nutrients), microbial activity may not be an important issue