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Dwell Time of Seepage Water in the Non-Saturated Zone 2003

Introduction

The protection of the groundwater as a natural resource is of the greatest importance, since groundwater forms the basis of the supply of the population with qualitatively good drinking water, and thus represents a key element of basic living conditions.

A primary prerequisite for good groundwater is the protection of the soil and the non-saturated zone, which together constitute the groundwater overburden, from contamination, because the percolation of pollutants from the surface into the soil and further into the groundwater represents the main source of contamination for groundwater resources.

The characterization of the pollution sensitivity of the groundwater to substance penetration from the surface is therefore an important basis for planning, as well as for measures aimed at providing groundwater protection, and also for prognostic statements about potential threats to groundwater deposits, as required, among others, for reporting under the European Water Framework Directive WRRL (EU 2000).

Fundamentally, groundwater everywhere is sensitive to pollutants entering from the surface. Spatial differences in pollution sensitivity arise, however, from the different times which substances introduced from the surface require to arrive at the groundwater table and possibly cause contaminations there. The pollution sensitivity of a groundwater deposit can therefore be described as the probability with which a certain amount of a pollutant may reach the groundwater table in a particular time.

Different approaches and methods exist for definition and assessment of groundwater pollution sensitivity. In addition to the concept "pollution sensitivity of the groundwater," the concept "protection function of the groundwater overburden" is used, where a high protection function of the groundwater overburden means low pollution sensitivity of the groundwater, and vice versa.

It is fundamental to distinguish between intrinsic and specific pollution sensitivity of the groundwater, or protection function of the groundwater overburden, respectively.

The intrinsic ("location-inherent") pollution sensitivity refers exclusively to the natural qualities of the groundwater overburden. It can be seen, with reference to Voigt et al. (2003), as a "worst-case scenario of pollutant entry into the non-saturated zone, in which no interaction of the pollutant with the media in the non-saturated zone take place". Intrinsic pollution sensitivity can be estimated on the basis of percolation water speed, or the dwell time of the percolation water in the groundwater overburden.

The specific pollution sensitivity refers to the entry of specific substances or pollutants at the surface. The specific pollution sensitivity thus always refers to a certain use, or a potential or real pollutant entry caused by such use, into the groundwater. Not only the geological-hydrological conditions are an important factor here, but primarily the specific behavior of the substances or pollutants or the substance or pollutant groups and their potentially or actually released quantities. At the same intrinsic pollution sensitivity, at the same location, the specific pollution sensitivity definitely can be very different, depending on the pollutant involved.

Factors which determine pollution sensitivity include:

  • The thickness of the groundwater overburden, i.e. the depth at which the groundwater is located under the surface; the geological lithological and the geochemical qualities of the soils and rocks present, with their components which determine the water conductivity, the water-retention capacity (by cohesive binding to soil components), and the adsorption capacity for certain substances.
  • The quantity of percolation water which penetrates the groundwater overburden, and the amount of the percolation water which ultimately flows to the groundwater (new groundwater formation), since substances are shifted through the percolation water into the groundwater in the non-saturated zone. The percolation water quantity and the amount of new groundwater formation are dependent on the climatic quanta precipitation and evaporation as well as on the geological structure of the non-saturated zone, the surface morphology, land use and surface sealing.
  • The structure of the aquiferous rock below the groundwater surface (aquifers and aquitards), their dimension and thickness, their geological-lithological and geochemical (and in connection with that, their physical) qualities, such as usable porosity and water conductivity as well as, finally, the groundwater slope and groundwater flow direction. These parameters determine how water, and also dissolved substances which have entered the groundwater, can disseminate underground.
  • In the case of the specific pollution sensitivity, also the specific chemical/ physical qualities of pollutants, such as their state of aggregation (solid, liquid, gaseous), their solubility in water, their degradability by chemical or bio-chemical processes, and their ecological/toxicological qualities.

While the general connections between climatic, pedological, geological and hydrogeological conditions and the groundwater pollution sensitivity seems quite simple, the quantification of the processes which specifically determine groundwater pollution sensitivity is quite complicated, and as a rule requires a very comprehensive database and often very complex methods.

As a reference point for the pollution sensitivity of the groundwater, the dwell time of percolation water has been derived from the characteristic quanta for the thickness and qualities of the groundwater overburden as well as the new groundwater formation rate, as a measure for the protection function of the groundwater overburden, and has been represented in the present map.