Wat. Res. Vol. 31, No. 4, pp. 767-776, 1997 © 1997Elsevier ScienceLtd Pergamon All rightsreserved.Printedin Great Britain PII: S0043-1354(96)00365-X 0043-1354/97 $17.00 + 0.00 GEOCHEMICAL STUDY OF SURFACE WATERS IN MOUNTAIN GRANITIC AREA. THE ISKAR UPPER WATERSHED: MASSIF OF RILA, BULGARIA NATHALIE GASSAMM* and SOPHIE VIOLETTE2t ~Laboratoire de G~ochimie des Eaux, Universit~ Denis Diderot & IPG, 2 place Jussieu, F-75251 Paris Cedex 05, France and 2Laboratoire de (~eologie Appliqure, Universit~ Pierre et Marie Curie, 4 place Jussieu, F-75252 Paris Cedex 05, France (Received January 1996; accepted in revised form November 1996) Abstract--The aim of this study is to determine the characteristics of the Iskar upper watershed surface waters as a lirst step of a seasonal evolution study. The geochemical and petrochemical characteristics of the Rila massif are well-suited to study the relative significance of different weathering mechanisms occurring alc,ng the year in a mountain aquatic system. In such an area the atmospheric input contribution to the surface water quality could be assessed. A first sampling survey in summer-time allowed us to get some data on major and minor elements in the soluble phase. The results show that the Iskar upper watershed is well-suited because of no mixing with long residence time groundwaters and of the low population density which results in insignificant anthropic input. © 1997 Elsevier Science Ltd. All rights reserved Key words--mountain surface water system, rivers, lakes, granitic area, major and minor elements IENTRODUCTION systems are highly sensitive to any change in atmospheric input. Anthropogenic impact on natural environments and The Rila massif is a middle mountain south of the especially on aquatic ecosystems is currently a topic Balkani chain, whose highest point, the Musala peak, of increasing concern in Eastern Europe. Aquatic reaches 2925 m (Fig. 1). The area is rich in rivers and ecosystems, which supply water to big cities are lakes, supplying water to Sofia city by the Iskar river beginning to be studied in detail. Most of them are To preserve water quality, this zone has been partly issued from surface waters draining alpine and prohibited for 50 yr. These protected areas are the subalpine zones. For a geochemical and a biological Beli Iskar valley, the Levi Iskar valley, and the upper point of view, l:hese systems are particularly valleys of the Rilska reka and of the Blagoevgrad interesting because of their seasonal contrast. In Bistrizza, the last two ones being affluents of the winter, water circulations are lower because of frost Struma (Fig. 2a). These four valleys and the Musala and precipitation mainly in the form of snow. In peak have been on national park planning since 1993. spring-time, warmer temperatures involve important This massif consists of several independent water runoff. In this season, weathering is mainly hydrological systems which have similar water-rock physical. During summer, both water circulations interaction characteristics (homogeneous petrochem- and exchanges between the different interfaces are istry and fracturation, water-rock surface ratios in maximal (water-atmosphere, dissolved species-par- the same range) but with different atmospheric inputs ticles, water-soil, and also living-nonliving). Chemi- as well in composition as in volume. In the past, cai weathering is preponderant until the first part of mining, lumbering, and hydraulic works have autumn. In crystalline areas, these systems are modified the natural ecosystem. Nowadays, the characterised by thin soils on steep gradient and human activity is insignificant. The aim of this study low-reactive bedrock. Studies of weathering in alpine is to establish that this new ecosystem is homo- and subalpine environments (Driscoll et al., 1989; geneous, that is to say no punctual input can be Mast et al., 1990; Peters and Leavesley, 1995; detected, and that no mixing with long residence time Reynolds and Johnson, 1972)have shown that these groundwaters occurs. In that case the Rila massif hydric systems can be considered as a natural reactor *Present address: Laboratoire de Grologie des Syst~mes allowing to study the seasonal variation of the water Srdimentaires, Universit6 Francois Rabelais, Facult6 des Sciences et Techniques, Parc de Grandmont, F-37200 chemistry as a function of weathering mechanisms Tours, France [mrasl]e-mail: gassama@univ-tours.fr] (physical or chemical), atmospheric inputs, and t[e-maik violette@biogeodisjussieu.fr] biological activity. 767