Micro zoning of terrain included in sanitary protection zones in karstic conditions
Ante PAVIČIĆ1, Josip TERZIĆ1 & Renato BULJAN1
1Croatian Geological Survey, Sachsova 2, HR 10.000 Zagreb
Keywords: karst hydrogeology, water protection, micro zoning
Introduction
Hydrogeological researches which are performed to determine sanitary protection zones are not adequate to cover every part of the terrain comprised in those zones. Zones determined by minimal investigation works may be taken as a basis, and for the terrain inside the zones there are protection measures and limitations of land use set by Croatian legislation Directives (Narodne novine 55/02; Vlahović, 2003). For certain uses of land inside sanitary protection zones, the directives leave a possibility called "micro zoning". This is needed to carry out hydrogeological researches at an exact location, in a "micro zone". Based on the results, an evaluation of the sensitivity on the micro location can be given, and of its suitability for use in the way applied for. New protection measures and limitations may be prescribed for that exact location. It is well known that protection of karst terrains differs from one case study to another (Bahun, 1989). Dinaric karst in Croatia (Fig. 1) is characterized by very deep and irregular karstification, mainly predisposed by tectonics, compression, reverse faults and overthrusting structures. The structures are additionally neotectonically disturbed because of the change of stress direction from approximately NE-SW to N-S. Otherwise the Dinaric karst is a well-known “locus typicus” and many internationally accepted terms in karst hydrogeology originate from the Dinaric karst area. Nevertheless, Dinaric karst differs from region to region and it can be distinguished into many types. For example, the karst on the Adriatic islands differs from the karst in the Ravni Kotari area near Zadar, and is completely different from the karst in the Lika region. That difference is both morphological and hydrogeological. The protection model for each kind of aquifer needs to be adjusted to the natural conditions.
Croatian legislation on protection zones in karst
According to current Directives for the determination of sanitary protection zones (Narodne novine 55/02), in Croatia there is a whole set of regulations and directives for karst groundwater protection. There are four protection zones (table 1) and they are recognized based on two criteria: the groundwater velocity and the time needed for water to reach the extraction site from the infiltration point. The first zone ("the zone of strict protection regime") is the water extraction site itself, and it must be fenced so as not to allow anyone who is not connected with the water supply to enter. The second zone is called "the zone of strict restriction" and is connected with main drainage and preferential flow lines. "The zone of restriction and control" is the third one and it covers the area surrounding preferential flow zones, as well as highly karstified and permeable rock mass connected with these zones. The fourth zone, "the zone of limited protection" spreads to peripheral parts of the catchment and to terrain formed by low permeable rock mass. It is a question where to place a part of a catchment from where the groundwater flow takes more than 50 days (Table 1) before it reaches the extraction site, because it is also laid down that the whole catchment should be within the fourth protection zone. In that case there is no need to refer to these 50 days. Basically, this model of karst groundwater protection is found in the 1990s (Fig. 1; Fritz & Ramljak, 1992; Biondić et al., 1999; Biondić, 2002).
Table 1. Sanitary protection zones (Narodne novine 55/02) with brief hydrogeological description.
Sanitary protection zone / Hydrogeological description / CriteriaTime for groundwater flow (day) / Apparent groundwater velocity (cm/s)
I / Fence around the extraction site. / - / -
II / The closest part of catchment, preferential flow zones and ponors; highly karstified zones. / < 1 / > 3
III / Terrain close to preferential flow zones; permeable rocks connected with main drainage directions etc. / 1 – 10 / 1 – 3
IV / Peripheral zones in catchment, rocks of lower permeability etc. / 10 – 50 / < 1
Figure 1. Simplified model of sanitary protection in karst which is implemented in present Croatian legislation (Fritz & Ramljak, 1992). 1-groundwater divide; 2-boundary of a sanitary protection zone; 3-sanitary protection zone; 4-ponor (swallow hole); 5-preferential flow line (zone); 6-extraction site.
In a few practical examples it is shown how micro zoning can significantly improve land use in some karst areas comprised inside basic sanitary protection zones. Basic sanitary protection zones would block any kind of habitation in very large karstic catchment areas, so the micro zoning procedure is necessary to permit sustainable development. On one side there is the need for water protection, and on the other the fact that people already live and work in those areas. A Directive of Croatian legislation (Narodne Novine 55/02, article 27), lays down textually: "For any possible land use not permitted in articles 22, 24 and 26 of these Directives, a study must be performed with detailed water explorations applicable to the micro zone. Based on the results, determination shall be made whether that terrain is fit for the claimed purpose, and of any protection measures to be prescribed."
Discussion based on a few case studies
The application of micro zoning research with suggested methods and results is described (Pavičić, 1998, 1999; Pavičić & Ivičić, 2003; Pavičić & Terzić, 2006) in a few examples from the Croatian karst (Fig. 1). Based on experience and achieved results, the following methods could be recommended: detailed hydrogeological mapping, tracer tests, structural geological analyses, geophysical researches, exploratory drilling and hydrochemical investigations. Groundwater tracer tests could be emphasized as the most important method. The apparent groundwater velocities are highly important data in the protection of karst aquifers, because sanitary protection zones are set up in accordance with them. If a tracer test carried out in micro zoning investigations shows lower groundwater velocities than expected for a given zone of the sanitary protection, then limitations in land use could be decreased or even abolished. By detailed research works it is also possible to divide the investigated terrain into zones of different sensitivity, and permit construction or any other kind of land use only in some parts. Tracer tests and detailed hydrogeological mapping could be pointed out as the two main methods in such investigations, and all other methods are simply auxiliary, depending on the task and location specificity. For example, if privileged karst conduit flows are recognized in some researched terrain, then structural geology and photo-geological interpretation could represent a very important tool. If within the boundaries of the investigated terrain there are no karst features suitable for tracer tests (such as ponors, open discontinuities, caves or pits), then borehole drilling should be performed, usually after geophysical researches. Sometimes hydrochemical analyses are essential. Other than those, there are many methods that could be used to improve the designer's conclusions, such as water balance calculations, studies of the climatology and agronomy, geochemistry…
In the Bokanjac area near Zadar there was micro zoning (Pavičić & Terzić, 2006) which consisted of detailed hydrogeological mapping, electrical tomography and seismic refraction profiles, exploratory drilling and tracer test. By mapping it was found that there are no natural karst features suitable for the tracer test. The tracer test was needed because the groundwater velocity is essential in hydrogeological opinion and decision-making for the purpose of land use in such a sensitive karst area. After geophysical explorations, the borehole was drilled, and the tracer test resulted in the relegation of the sanitary protection zone restrictions for the future business district from zone 2 to 3. In the 3rd zone it is practically impossible to have any kind of activity, so the opinion was given that this part of the catchment can be used for the claimed purpose, but certain restrictions must be followed, such as controlled sewage, purification and evacuation out of the catchment. The drilled borehole was an artificial intrusion into the aquifer, and therefore it cannot be considered as a ponor or any kind of natural karst phenomenon. That raises the question: is this method suitable for such terrain? The tracer was poured directly into the aquifer. In natural conditions that cannot happen, because it would take some time for the tracer (or pollutant) to reach the water table. Some purification would happen as well. That is connected with the question of effective infiltration in such terrain. Since there are no real ponors and pits, the infiltration is not discrete. On the other hand it cannot be called diffuse because there is almost no Quaternary cover. This infiltration, between discrete and diffuse, can be called scattered.
When sanitary protection zones are established, there is usually not enough money for the spatial covering of the whole catchment by hydrogeological research. That is the main reason for micro zoning projects. The other important issue is the time, or hydrological moment, because two tracer tests with the same tracer cannot be made in the same hydrological year. Since groundwater velocities for sanitary protection zones must be provided during high groundwater levels at the hydrological maximum, it is sometimes necessary to wait for months, or maybe even more, for suitable conditions.
Conclusion
There are several different types of karst in Croatia, and the procedure for establishing sanitary protection zones needs to be adjusted accordingly. For example, there is no need for tracer tests in catchments where no ponors, ponor zones or swallow holes are present. Tracing from an artificial object cannot be treated just as if it were a natural ponor. A borehole can significantly change the type and amount of effective infiltration, as well as the purification conditions in some part of terrain. In any case, the Dinaric karst is an open system with high infiltration and low purification, so the tracer test data should remain one of the most important criteria. It only has to be taken into account whether the tracing has been done in natural or artificial conditions. It is very important to point out that many "old" tracer tests cannot be taken as facts. In the past a great deal of tracing was done with very low-sensitivity equipment, or even without instruments, simply by observing the occurrence of dye. Most of these tracings need to be repeated.
Current legislation can be taken as a framework, but it needs to be adjusted for the better "covering" of different types of karst. Initial research for establishing basic sanitary protection zones needs to be simplified, but it must be followed with the program of researches that has to be done afterwards. Since there is a lack of funds for initial sanitary protection investigations, together with a need for sustainable land use and micro zoning, the research program should be adjusted:
- Sanitary protection zones must be established as soon as possible for every extraction site. That means even with less money for initial research;
- The elaboration of sanitary protection must comprise "The plan for obligatory micro zoning", which has to solve the main open questions in the catchment, and must be given with a time plan. When sanitary zones are officially acquired, the financial funds for these researches must be secured;
- Other possible micro zoning should be done simultaneously, if possible, or after planned (and proposed) micro zoning researches;
- When all of the proposed micro zoning researches are finished, corrected sanitary protection zones need to be established, even if they are significantly different from the initial ones.
References
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