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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


Underground water

Underground water is formed by the infiltration of precipitation into deeper strata within the bedrock (Figure 1.13). The depth of infiltration, as well as the origin of the aeration and saturation zones, is an effect of the geological substratum; specifically, the arrangement of the strata and their physical characteristics. These are divided into three groups according to their hydrogeological properties: of permeability between 1-0.01D; e.g., sands, grits and gravels.

  • aquifers - having an index
  • aquicludes - having index of permeability between 0.01-0.0001D; e.g., argillaceous sands and arenaceous clays,
  • aquifuges - having an index of permeability of less than 0.0001D; e.g., basalt.

Water penetrates into the pores and crevices of the rocks, filling capillaries between granules, clefts, and even karstic caverns. In this way, precipitation may infiltrate to aquifuge or confining strata (direct alimentation). In aquifers, or those areas wherein the ground water is stored, layers of porosities can be distinguished, with those layers being most porous storing the greatest volumes of ground water. These layers tend to be most sensitive to changes in physicochemical conditions (e.g., air temperature).

Indirect alimentation is related to the presence of hydrological windows that result from sedimentation or erosion processes. The amount of underground water resources and their sensitivity to anthropogenic stress are dependent upon the type of source water. For example:

  • Groundwaters with a free water table occur at different depths, but most commonly near the surface. They are seriously endangered by human activities such as overexploitation, contamination, and heating. Contaminants generally spread very slowly due to the laminar movement of water in the capillaries. They are easily renewable and usually regenerate after rainfalls and spring snow melt.
  • Confined ground water is less susceptible to anthropopression, because they are isolated from the surface within aquifuge rocks. This group is classified as artesian and subartesian waters. They are also renewable, but the process is slow, and they are often over-exploited.
  • Karstic aquifers are extremely susceptible to degradation because pollutants (even suspended solids) can easily and quickly spread over large distances due to the turbulent flow of water within crevices. The rates of water contamination are influenced by humidity and the water balance in the aeration zone, as well as the pressure distribution and water balance in the saturation zone. It also is modified by permeability and the processes of dispersion and dilution occurring in each stratum, and also by contact with surface waters.

A urban catchment overland flow
B rural catchment infiltration and saturation excess overland flow
C Forested catchment Saturation areas, overland flow and subsurface storm flow
D Forested catchment Saturation, impervuous areas overland flow, subsurface flow

Fig. 1.13. Examples of different types of underground waters in areas with differing geological structure

The hierarchy of factors causing vulnerability of ground water resources is as follows: surface morphology, the distribution of strata, petrographic and lithological characteristics of sediments, hydrodynamic conditions, infiltration quantity and quality, and biological conditions in zone of penetration and infiltration. However, the dynamics of underground waters depend mostly on the hydrological regime of the overlying surface waters, which may drain to (at high water levels) or be supplied from (at low water levels) them by means of infiltration processes (Figure 1.14).

Surface water

Surface waters form only 0.014% of the total water resources. However, they are extremely important for wildlife and for the development of human civilisation. There are three types of surface waters: rivers, lakes and marshes. Both surface waters and underground outflows are parts of the same dynamic system. This should be taken into consideration when new artificial reservoirs are planned and constructed, especially along the middle and lower water course. It is also worth emphasising that water resources are renewable, but, when the rate of exploitation and degradation is high, they may become scarce. Therefore, the basis for sustainable use of freshwater resources should be the preservation of the hydrological balance between water uptake and use, and its supply and retention.

Rivers
  are collectors of run-off and underground waters;
  play crucial role in hydrological cycle, because they supply seas and oceans with 45 thousand km3 of water;
  water exchange between river and other ecosystems is very intense - for most rivers it takes only 2 weeks (Kaleśnik, 1975);
  thay drains underground waters, so water table of underground waters is always related to water level of the river.
It is especially true for underground waters in alluvial sediments (basic material covering river valley), where due to shifts between its draining and supplying function river to great extend influences quality of alluvial underground water.
Lakes
  accumulate about 0,26% of the Earth freshwater resources.
Marshes
  are systems saturated with water, which constitute up to 80% of ground volume, because they are result of restricted out-flow of both surface and groundwater, and precipitation much higher than evaporation
  it means that they store about 11,5 mln km3 of water.
both lakes and marshes
  are responsible for water retention in catchment and like rivers they influence quality and quantity of underground water through infiltration and drainage.

 

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