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. 9 Few attempts have been made to clear the role of faults in eonveying waters from great depths upwards in plains filled with thick sediment layers [6], A few examples from the exploration of the Great Hungárián Plains will illustrate the role played by the joint utilization of hydrogeology and hydro-chemistry in exploring the deep structure and fault lines even in large hydrogeological regions. P. Treitz explained the origin of alkaline soils in the Great Hungárián Plains—already at the beginningof the century— by the concentration of waters rising from greater depths at fault lines and the intersection thereof [30]. The interrelation of geothermic and ehemical anomalies to fault lines was proved by Sümeghy already a few decades ago [26]. His theory deduced from few data, were verified later by several investigators. The relationship of gaseous wells with deep faults was alsó observed a long time ago [25]. The fact that, that especially the geographical distribution of the chloride content and its representation on maps is highly indicative of structural lines and is helpful in fixing their position more precisely [22, 23], was soon recognized. A detailed investigation of the geothermal anomaly in the central part of the Great Hungárián Plains proved this area to be limited to a narrow beit [ 1], In the same zone anomalies were observed alsó in chloride, totál dissolved salts and specific yield, and it is here that the gaseous wells occur. The gaseous, hot salt water ascending along the open fault from the Pliocene formation, raises the temperature and salt content of the (artesian) water rising is one zone from the coarse grained sand layer between 120 — 250 m in the Pleistocene formation. At the same time — as compared with the surroundings — the yield of somé wells is alsó appreciably higher [6, 7]. Somé time later, detailed geopliysical explorations [1] substantiated the strong positive thermal anomaly near the surface, caused by the rising water. The fractured zone was subsequently penetrated with a deep borehole. in the 2000 m deep borehole, the thermal anomaly was found to be highly positive down to the fractured zone, then— compared to the régiónál average—it assumed a negative character. Further borings for exploring thermal water and geological structure, together with deep seizmic measurements showed along the line of geothermal, chemical and flow anomaly, a fractured fault of several liundred metres slip to exist by which alsó the position of the Tisza river is defined at that particular seetion. By a purely hydrochemical evaluation of the analytical data of karstic thermal waters in Budapest, it became possible to determine that these thermal waters — of four distinct characters — were in connection with four fault lines. The thermal waters rising from great depth retain their character in spite of mixing [27]. Investigations concerning the distribution of gaseous wells, the specific yield, the surface hydrographic pattern and the position of hydrochemical anomalies indicated all these to be located at similar points, especially in belts. This agreement may be regarded as evidence for the fact that according to investigations covering several thousands of square kilometres, even in depression areas filled up with thick and loose sediment layers, the effect of the deep structure is dominant and perceptible on the surface (river pattern, chemical coinposition of groundwater) and in its vicinity [7, 23, 24], The close relation of water quality and tectonics is conspicuous in the Great Hungárián Plains. Results abroad [11, 12, 15] justified the Hungárián method of research. Many explorations of great practical importance could be undertaken if a suitable amount of expedient chemical analyses would be available, as demonstrated by a major exploration and its great economic profit [11]. c) The relation of groundwater seepage and hydrochemistry The velocity of seepage can be determined directly, if the area of recharge, the direction of seepage and the age of water are known. Determination of age became a highly important line of research, though as yet, experts are mainly occupied with its technological aspects. Indirect methods are alsó of help when estimát ing the velocity of seepage. Thus, the seepage velocity can be calculated under favourable circumstances from seasonal changes in the composition of subsurface water. The unavoidable investigation of groundwater pollution, may yield suitable data concerning the seasonal variations in seepage direction and velocity. The geochemical testing of subsurface water is closely related to the problem of determining the velocity of seepage. From the rate at which different minerals and rocks are dissolved information can be obtained on seepage velocity. Owing to the fact that indirect methods are less popular, more laborious and require an extended research program, they are used but rarely, in spite of pro mising results. d) Determination of the water volume with the help of hydrochemistry The fluctuation in the chemical composition of river water may be applied to estimate the amount of recharge to groundwater. Favourable results may be expected in sedimentarv catchment areas with many readily dissolved minerals, or where an area with unusually hard groundwater can be outlined distinctly. 7. Hydrogeological analysis of hydrochemistry data The maximum possible information is difficult to obtain even if reliable data are available. In simpler cases, tabular, graphic or statistical methods may be sufficient.