1. BACKGROUND AND OBJECTIVES
HYDROGEOLOGIC ENVIRONMENT OF HARD ROCKS - GENERAL REMARKS
The IAH Commission on Hardrock Hydrogeology (HyRoC) was established in 1994 with the aim to stimulate international co-operation and facilitate exchange of information between hydrogeologists and other specialists concerning groundwater issues of hard rocks - fractured crystalline (igneous and metamorphic) and consolidated sedimentary rocks.

In terms of hydrogeology, hard rocks represent a heterogeneous and anisotropic, generally very complex environment with irregular distribution of pathways of groundwater flow, typically consisting of three vertical zones, upper weathered, middle fractured and lower massive. Many times a question how to define and limit “hard rocks“ has been discussed. In fact many carbonate and some of volcanic rocks, as well as indurated deposits within hydrogeologic basins characterise similar or the same hydrogeologic properties as “hard rocks”. Hydrogeologic limits between all these rocks are not exactly definable and applied approaches and obtained experience in one rock type might well be transferred to another.

There are two main reasons for international co-operation within the IAH Commission on Hardrock Hydrogeology:

a) Hard rocks are found in many regions of the world. They occur mainly in large areas – shields, massifs, and in cores of major mountain ranges. In addition, these mostly old rocks form the basement of younger sedimentary rocks that are often concentrated into large basins. Thus, hard rocks represent at depths a continuous environment enabling extended, deep regional or even global groundwater flow.
b) In spite of enormous extension of hard / fractured rocks, no adequate attention had been paid to their groundwater issues except for some arid and semi-arid regions, where, under typical non-availability of surface water bodies, groundwater has traditionally represented the only source for water supply. During the last decades, however, increasing interest in hardrock hydrogeology or more generally in hydrogeology of fractured rocks has been noted. Important present-day groundwater issues substantiate the practical importance of this specific hydrogeologic environment, even in temperate climatic zones. Some of these most important issues, that might be also considered as principal objectives of the IAH Commission activities, are listed as follows:
Natural groundwater resources (=natural recharge) in general represent the upper limit for groundwater abstraction and sustainable water management. Natural groundwater resources reflect natural conditions of different climatic zones: arid and semiarid, humid tropical, temperate and arctic regions with permafrost. In contrast to previous opinion, natural groundwater resources in many hardrock areas, mainly those originating in mountains as source areas, are often important. They can cover recharge for groundwater abstraction and maintain flow in water courses in piedmont zones during dry periods.
As to groundwater abstraction, adequately sited water wells or other water intake systems in hardrock areas typically can cover requirements of water supply for small communities, plants or farms and for domestic water consumption. In some areas, groundwater abstraction possibilities are high enough to supply even small towns. It is a matter of economic consideration as need of investment and maintanence expenses whether water supply should be provided from local scattered or concentrated groundwater or surface water sources brought from remote places. Siting of water wells in open fracture systems or thick permeable overburden and depth-related changes in rock permeability, enabling to assess economically adequate depth of water wells, are some of important present-day hydrogeologic issues.
Knowledge of horizontal and vertical zonality of natural groundwater quality is indispensable for any groundwater use: various chemical constituents (incl. minority and trace elements) and physical properties (radioactivity) of groundwater might be detrimental for human health and decisive for specific use (e.g. irrigation, industrial uses) of groundwater.
Many-sided problems are connected with groundwater pollution and environmental protection. Impacts of industrialisation and urbanisation, superficial landfills and deep hazardous waste repositories, of fertilisers and pesticides used in agriculture on soil, rock and water must be monitored and reduced in hardrock environment, too. Character of groundwater flow and contaminant transport is decisive for siting any waste repository.
Clear hydrogeologic understanding and quantitative hydrogeologic assessments are required during various types of geotechnical and engineering-geological activities as construction of tunnels, underground cavities, etc., and in mining where groundwater occurrence is usually considered a restrictive agent. On the other hand, hydrogeologic data obtained at these activities represent an important and useful feedback to results provided by other hydrogeologic methodologies and techniques.
Special attention is paid to depth-related changes in hardrock permeability, possible occurrence and properties of geologic barriers especially in connection with testing and construction of deep waste repositories of radioactive and toxic materials that are often sited in deep zones of crystalline rocks.
Results of recently drilled deep boreholes in crystalline rocks re-opened the issue of deep-seated groundwater flow and brine (connate water) occurrences and discussion on mineral and thermal water origin that are often connected with hard-crystalline rocks. Newly encountered mineral waters might be used for balneological applications, industrial salt or rare elements production and thermal waters as a source of geothermal energy.

All this has been a challenge for hydrogeologists in the last years. There is an urgent need to investigate and define intricate hardrock environment in terms of its permeability distribution, groundwater flow and quality. Better understanding of causes of spatial and time distribution of respective hydrogeologic phenomena and factors will afford a reasonable basis for recent practical implications.

Increasing amount of available data and versatile information concerning groundwater in hard rocks stimulates efforts to regionalise and generalise results from different hydrogeologic environments. Data on hardrock transmissivity and permeability, groundwater resources and quality, obtained by different methodological approaches, offer excellent possibilities of correlative hydrogeologic studies. Thus new important results and conclusions can be drawn in order to understand better hydrogeologic properties of rocks both in local and regional scales. Knowledge of hierarchy of inhomogeneity elements and of a scale effect influencing spatial distribution of hydraulic properties enables simplify adequatly real conditions when defining conceptual and numerical models of groundwater flow and solute transport.

To develope its activities the IAH Commission on Hardrock Hydrogeology apply mainly following principles:

  • Maintaining contacts among interested specialists all over the world.
  • In areas where “hardrock hydrogeologists” are willing to co-operate more closely, as e.g. on joint projects, regional working groups are established.
  • Organisation of short meetings or workshops in different countries.
  • Co-operation within the framework of joint projects.
  • Publications of results by means of Workshop or Conference Proceedings, papers submitted to hydrogeologic journals or monographs.