Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Abos Bruno: A talajvízdúsítás vízminőségi problémái

Special Conference Number, Szebellédy, L. Hidrológiai Közlöny 1971. 1. sz. 29 ment occurs usually in chemical composition. Of course, owing to the mixing of infiltrating surface water and groundwater the effects cannot be separated and the mutual influences are often very difficult to clear. The recharging theories and the anticipated benefits are considered mainly in the case of recharging groundwater of high iron content. Concerning the oxidation of iron compounds the assumption by Schwertmann is introduced, who indicated ferri-oxide (Fe

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3—hematite mineral) as the end product. In Hungárián practice the ochre earths förmed along the gravel pack in aquifers consisted mainly of high-iron, ferrous calcium carbonates. A considerable merit of the paper is that the correct theoretical considerations are illustrated by a practical example, namely the results attained at the Borsod,szirák Waterworks. Careful preliminary studies and correct considerations are claimed to permit considerable reductions in the iron content of water at producing sites coinbined with groundwater recharging. This wouldsimplify the removal of iron, or this stage of treatment could be eliminated altogether. The coinbined method suggested by the author is believed to be potentially applicable at several waterworks both at home and abroad. The conclusions arrived at in the paper by A. Fázold [7] are belived to be incorrect. In the introductory part it is stated that: "The large number of chemical, biological and bacteriological tests performed during the recharging operations at the Borsodszirák Waterworks offered opportunity for deriving conclusions of generál validity for designing pusposes". In subsequent parts the quality of water is classified mainly on the basis of the colicount. Results obtained in a single water producing area are necessarily inadequate for arriving at design criteria of generál validity, since different results have actually been attained at other sites in Hungary, as well as abroad, with well fields supplied from recharging basins (see paper [4] by Widmer). The evaluation of water quality according to the coli-count is known already to be strongly biased. Many types of coli bacteria have been demonstrated to live in the soil and to find access to water, which are not of faecal origin and are thus not indicators of pathogenic bacteria. Moreover, when evaluating the quality of well water, reliance on bacteriological data alone is insufficient. These must be supplemented by chemical and biological tests as well. Detailed and complex research appears to be still necessary in the field of groundwater recharging, before even approximate design criteria can be made available on the distance of wells from the recharging basin under a particular set of site conditions. Ón the other hand, the author is to be congratulated for having raised this problem in his paper, this being among the least cleared ones in groundwater recharging. The paper by Widmer [4] of Switzerland deals with the watersupply of the town Basel, where groundwater recharging is practiced extensively. Plans are deseribed for enlarging an existing waterworks, where water from the Rhine river is pretreated. Two aspects are deemed worthy of being mentioned separately. Over an infiltration area of 23 hectares, of which a part only is operated, the rate of recharging is 120 000 cu. m/day. Individual infiltration areas are operated for about two weeks, with infiltration rates of more than 1 metre/day. The inflow is then shut off, water is permitted to infiltrate and the basin is permitted to stand empty for about 2 weeks. During this period the retained organic substances are minerlized and the filter layer is regenerated. The basin is refilled subsequently and the recharging cycle is started again. Of course, this economical method of operation is only possible where the raw water is adequately pretreated. In this area it is contemplated to increase the rate of recharging by 20 percent. Preliminary studies have indicated, however, that the present quality of withdrawn water cannot be guaranteed, unless the surface water is pre-treated with great care before recharging. Clarification with iron chloride, double rapid filtration, oxidation with ozone, treatment on activated carbon filters and subsequent disinfection with chlorine dioxidé are contemplated for this purpose. The conclusion arrived at from the paper is that groundwater recharging with high efficiency and yielding high-quality well water cannot be attained unless the raw water is pre-treated carefully. The advantages accruing from preliminary studies in the hydraulic laboratory are pointed out in the paper by Mrs. Dávid [8], who performed model tests in order to find the optimum spacing of the wells and their distance from the recharging basin. The significance of the study is increased by the fact that changes in the groundwater level and the influence thereof on the yield of wells was alsó taken into consideration. The results yield hydraulic information on the location and spacing of the wells. It is pointed out — correctly — that problems of a hydraulic character can be solved in the laboratory, but the process in the field may be affected by several dominant factors, the influence of which could be cleared by field experiments, or pilot-plant tests only. In the two papers by Dr. G. Öllős (published in Hidrológiai Közlöny, No 1. 1970. and Vízügyi Közlemények, No. 1. 1970.), a critical review is presented on the earlier theories and practice of slow filtration, with special regard to groundwater recharging. Numerous waterworks are deseribed on the basis of personal experience, where groundwater recharging is practiced and slow filtration with artificial closed basins is applied. The process of slow filtration is analysed from chemical and biological points of view alike and a systematical classification is presented. The complex approach is adopted in these publications. It is emphasised — and we are in complete agreement in this respect — that only carefully pretreated surface water must be used for slow filtration, or else the quality of the water produced may be objectionable and neither the yield of slow filtration during individual filter runs will be satisfactory. Great significance should be attributed to the statement that slow filtration is essentially design-