Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Dr. Erdélyi Mihály: Hidrogeológiai tényezők hatása a felszínalatti vizek minőségére

Special Conference Number, Szebellédy, L. Hidrológiai Közlöny 1971. 1. sz. 7 based on a comprehensive interpretation of many thousands of water analyses, borehole profiles and somé rock investigation. In connection with the waters in individual formations, the territorial and depth rangé of certain water types, their relation with the bordér of the basin and its rocks are discussed. The central part of the Hungárián Basin, the Great Plains, is especially suited for investigating the mixing of fossile and infiltration waters, the processes of dissolving and recharging. Great significance is att ributed to t he movement of artesian water rising from great depth in the basin. This was little studied so far. A number of phenomena for which there seemed to be no explanation up to now can be interpreted by this approach. In the Great Hungárián Piains three water horizons of different character can be distinguished: 1. The top horizon extends down to 30—50 m below the surface. The water here is of two types. The groundwater in sand areas with readv infiltration, as well as that in zones in the vicinity of rivers is of a character resembling precipitation. The groundwater under impermeable surfaces (loess, clay) is high in dissolved salts, especially so in alkaline soil areas. Their water is of an alkaline character, with a totál salts content up to 2000—3000 mg/l. The ascendant water from great depth plays a great role in enrichment, because of the slow seepage in the fine-grained rocks and the high degree of dissolving. 2. Under the subsurface groundwater of variable chemical character a largely uniform layer of fresli water extends to great depth, with a slightly increasing salt concentration downwards in the coarse-grained sediment. 3. Below this more or less fossile waters high in sodium chloride are found [10, 28]. The character of water is investigated by geological formation quoting many references. Á relation is suggested — valid in big areas — between the constituents and granulometrv of rock, and the character of water. The effect of the geographical situation, the distance from the bordér of the Great Hungárián Plains and the geological conditions are analyzed. By this approach a grouping supported by thousands of data is obtained for the waters in the Great Hungárián Plains according to geological formations. The water in the lower-Pannonian subhorizon (except the layers of the borders near the surface) is of sodium chloride tvpe. The artesian water in the upper-Pannonian subhorizon contains sodium hvdrogene carbonate. The artesian water of the upper-Pliocene and the lower Pleistocene formations, if fine grained, and with little and slow recharge from the surface, is alsó of the sodium hydrogen carbonate type. However, mainly in the coarse grained lower Pleistocene formations, with much surface infiltration, the water contains calcium magnesium hydrogen carbonate, with a negligible amount of sodium. The transition between the waters in the upperand lower Pannonian aquifers mav be sharp or gradual, depending on the thickness and area of the impervious layers. Near the eastern bordér of the Great Hungárián Plains, even the lower Pannonian formation contains water with a high amount of HC0

3, indicating a certain modest recharge from the bordér area. (5. General considerations The rapid growth in both qualitative and quantitative demands for water directed increasinír attention to the path of subsurface water in the earth crust. Its chemical character, if assessed well, discloses the "history" of surbsurface water. This explains the rapidlv increasing importance of chemistry in water researcli. a) The origin and supply of subsurface waters considered from tlie point of hydrology and hydrochemistry. When investigating the circuinstances of infiltration and recharging of subsurface water the main problem is concerned with time and location when and where the infiltrating water reaches equilibrium witli the aquifer. Once equilibrium is reached the quality remains more or less constant and the path of water can be traced without difficulty. A sudden change in quahtv indicates a change alsó in the geological surroundings (rock boundarv, aquiferous structural hne, or rock boundary and structural line). In crystalline rocks and "pure" quartz sand equilibrium is reached rather slowly. However, in fine grained rocks it may be verv quick. Of special interest is the researchon fme-grained rocks, which disclosed the relation of minerals, rocks and water in alkaline soils, as a determinant of soil texture[17, 29], The importance of this work may be appreciated best by remembering the difficulties arising in the agriculture of aridand semi-arid regions all over the World, especially in connection with irrigation. Chemistry is useful for investigating even in bigger areas the direction of flow if the waterbearing formation is a carbonate rock, readily soluble in water. The data should, however, be evaluated in the knowledge of geological conditions. A good example for this type of work is the paper by Karácsonyi and Scheuer. When investigating larger hydrogeological units, special attention is necessarv if the waterbearing formation contains small amounts of readily soluble rocks, especially if these rocks are present in small spots difficult to explore geologieallv. In such cases the infiltration area, and the direction of flow are not readily determined, owing to the change in the chemical character of water, caused by the small mass of rocks dissolving easily [13]. The role of small rock masses pertaining mainly to the heavy mineral fraction, which under normál circumstances do not dissolve "irregularly", may be verv important. The fact that under certain hvdrological conditions they dissolve rapidly may cause difficulties in exploring the chemistry of water entering the aquifer, the point of entrance,