Hidrológiai Közlöny 1971 (51. évfolyam)
1. szám - Dr. Erdélyi Mihály: Hidrogeológiai tényezők hatása a felszínalatti vizek minőségére
8 Hidrológiai Közlöny 1971. 1. sz. Special Conference Number, Szebellédy, L. the direction and rate of flow. This is especiallv of importance when utilizing the groundwater in coarse-grained, alluvial deposits. The paper of Gy. Gondos indicates the decisive enrichment which may be caused by the dissolving of "heavy minerals" in groundwater with a verv low iron- and manganese content, if the topographical (depressed areas), geological (lack of impermeable cover) and biological factors (deeaying plants) enable a major recharge from the surface and the development of a reductive environment. It should be pointed out here, that the disturbing effect of water with a chemical composition that cannot be explained by geological conditions, may be eliminated in a number of cases. In this way the area of infiltration and the direction and rate of groundwater flow can be determined by comparing data of water analysis and geochemical rock-analysis. With the help of this method alsó the factor causing the change of "hvdrochemistry" can be detected. It is tacitly assumed that at the start of the hydrological cycle the infiltrating precipitation is practicallv pure. The composition of rain water is well-known from a number of analyses, but that of the water getting into a saturated zone is much less known. It is thus quite impossible to follow the route of subsurface water, the changes in its quality, if the initial member of the process (the water entering the aquifer) is not known well enough. Ample data are available on the enrichment of rainwater by sodium chloride of sea-water particles transported by the wind. At the same time, very üttle is known about the sodium carried by wind in arid and semi-arid areas, from dry beds of shallow lakes, marshes into rainwater and further to the permeable formations (sand dunes) and thence into the groundwater. The same, though minor effect may be caused by pollutants from irrigated areas when the irrigated soil is dry (after harvesting and ploughing or when leaving fallow). Similarly, Httle is known of the changes in groundwater quality caused by the dust mixing with air in denselv populated, industrial areas from the rapidly drying blast-furnace slag piles, dumping places, refuse heaps and backfills. Few data are available on the effect of polluted air on groundwater — through precipitation. In recent years, however, growing attention is paid to the effect on the quality of surface- and groundwaters- and thus alsó on water- supplv- of various backfills and slags. Let us emphasize here again that pollution control can be made more effective by geological methods by comparing the geochemical analysis of the pollutant with water analysis. b) Tracing the route of subsurface waters Groundwater flow, may be traced in liighly different scales, ranging from a distance of a few metres (civil engineering), to several hundred kilometres (régiónál hydrological investigation of large hydrogeological units). Regardless of scale, chemistry is always a decisive factor of research. The chemistry of groundwater and the mineral composition of aquiferous rock may be related in different ways. This relation is a very simple one when, from the point of infiltration to the spring (or artificial withdrawal) the water seeps through a single, geochemically well defined waterbearing formation, witliout communicating with any other waterbearing layer or surface water. The situation may become much more complicated, however, if waters of different composition from one or several waterbearing rocks mix in an unknown ratio. The resulting chemical processes, as for instance, base exchange, absorption of dissolved ions and other factors render the investigation of water motion more difficult. The situation is even more involved when waterbearing layers of different rocks communicate not only by normál stratification but alsó tectonically or only tectonically. In such instances the sudden change in pressure and temperature along the faultline, the mixing of waters of often widely different character complicate the situation even more. In case of complex hydrogeological conditions, especiallv when the aim is to explore large hydrogeological regions, or to collect hydrological data from their parts, the evaluation of hydrogeological, chemical and geohvdrological data is indispensable, together with a complex approach of these specialized branches of science. Cheboterev [4] and alsó others, have demonstrated that the composition of subsurface waters becomes more and more similar to that of sea water, as they penetrate deeper into the earth's crust or as the they get nearer to the sea. The change in the predominant ions—towards sea water—is as follows: HC<V - HCO 3-+Cl" - ci-+hco 3- - CI- + S0 4" or S0 4— + Cl" — Cl" . This complete sequence is valid in generál, however, in practice it occurs but exceptionally. Very often, due to somé easily soluble rock, and depending on geological circumstances, the series changes somewhat, vertically, under the effect of the dominant ion: HC0 3--HC0 3--(-S0 4- and HC(V + Cl~ - Cl~+ + HC0 3- Cl-[13]. Once the generál series of Cheboterev or those differring therefrom owing to local geological conditions are known, and the complex approach is adopted, a few analyses may suffice to obtain an overall picture about the hydrogeological conditions of great areas, especiallv about the limits of recharge areas and directions of flow [2, 6, 11, 13, 19, 22, 24, 27, 32], Frequent local anomalies in the quality of water can be caused by ascendant water only. This theory is as yet rejected by many experts. However, more and more data indicate, all over the world, that even in the great plains, ascendant water— often from great depth—is a major factor in water quality [6, 7, 19, 22, 23], According to many data, alkaline soils indicate not only in the Great Hungárián Plains [19, 20] but alsó in other parts an area where upward flow is predominant [3, 13].
