PRINCIPLE OF GROUNDWATER PROTECTION IN KARST

Dr. Petar Milanovic, Yugoslavia

The criteria for determining the zones of sanitary protection in karst are essentially different from criteria for nonkarstic terrain. The important difference between the karst aquifer and aquifers in intergranular or fissured rocks is nature of infiltration, type of flow and residence time. Because of fast underground circulation through the karst conduits the residence time in karst aquifer is considerably shorter, i.e. contact of the contaminant with rock matrix is limited. The groundwater velocity in nonkarst terrains is expressed in cm/day, however in karst aquifer velocity ranges in cm/s. In fully saturated aquifer the groundwater flow needs less than one month for 100 km. Extremely fast flows need only 16 h for 20 km distance. The lack of soil deposits at surface and concentrated infiltration through large ponors (swallow holes) enhance rapid transport of contaminant into the aquifer. Consequently, distribution of pollutant is very fast and consequences drastic. Self-purification capacity of karst aquifer is very weak. There is not enough time for degradation of organic materials and elimination of bacteria.

Four zones of sanitary protection are proposed: I. Protection area of a spring or intake structure; II. Zone requiring very severe protection and restriction; III. Protection area; and IV. External protection area. The main criteria for zoning are presented in the article.

As a consequence of hydrogeological anisotropy of karst the boundaries of sanitary protection zones have extremely irregular shape, especially zones II and III which include sinking zones, dolines, faults, deepest sections of base of karstification, zones of water table minimum and other zones containing concentrated underground flows.

By construction of dams, reservoirs, concrete blankets over ponors, plugging karst channels, or by driving tunnels, the previously defined (categorized) protection zone can be the subject of correction. Generally, those structures ameliorate natural characteristics, i.e. some parts of zone II can be recategorized into zone III, or zone III into zone IV.

The case study of sanitary protection zoning of important springs along the 100 km of Dubrovnik littoral is presented in the article. From 72 springs 33 is tapped including 7 for municipal water supply. The catchment area of those springs incorporate the part of eastern Herzegovina, including Popovo polje. Popovo polje is one of the karst areas with most complicated hydrogeology in the world with more than 500 ponors and estavellas, number of intermittent springs and many bifurcation zones. In the main channel zones the water table fluctuates 200 m. Depending on stage of aquifer saturation velocity of groundwater vary from 2 to 6 cm/s.