Hidrológiai Közlöny 1988 (68. évfolyam)

3. szám - Alföldi László:A felszín alatti vizek mikrobiológiai kérdése

142 HIDROLÓGIAI KÖZLÖNY 1988. 68. ÉVFOLYAM, 3. SZ

ANt Abstract: A review oil groundwater microbiology Alföldi, L. Groundwater microbiology has recently became an independent field of the sciences, utilizing the results of geomicrobiology, hydrocarbon microbiology, soil microbiology, pathology and general microbiology for the analysis of the water quality conditions of subsurface waters, while creating new research subjects as well. Due to steep thermal gradients, the reduction of nutrient availability and to marked physico-chemical changes, microorganisms of the infiltrating water are retained in the unsaturated soil zone. From the point of view of microbiology this zone plays a screening, selecting and transformation role. The density, viscosity, isothermal compressibility and vapour pressure of water change with temperature (Fig. 1) and, depending on the magnitude thereof, change also the living conditions of the bacteria. The temperature in the regions close to the surface displavs daily, seasonal and annual periodicity (Fig. 2), which, depending on the depth to the groundwater table, is reflected also in the temperature of water. The temperature barrier means thus inevitably that the organisms migrating downward from the surface encounter temporarily or permanently temperature conditions adverse to them, which results in many cases, if not in total extinction and the complete interruption of biological functions, but in minimal prolification rotes. The zone referred to bv the hvdrologists as three-phase, or unsaturated is situated between the surface and the groundwater table. Three-phase conditions prevail here over the greater part of the year, but the voids between the particles are filled several times bv infiltrating rainwater ( Fig. 3 ) The chemistry of these waters depends on the rates on finfiltretion and leaching, further on the degree of saturation, the temperature being also modified. The organic content of the percolating water decreases sharply relative to that present in the moisture, or water within the topsoil. The nutrient content, the concentration of dissolved ions in the aquifers, which communicate hvdraullicallv with the deeper formations are definitely reduced, the nutrient environment is much roorer than in the topsoil, so that there are little nutrients available for the heterotrophic bacteria, although a total absence of organic matter occurs under exceptional conditions onlv. The nutrient and electron acceptor quantities, the change in the chemical composition of water alone is a non-negligible factor. In the transition zone not onlv a rapid drop in the concentration of dissolved substances in general, but also a drpstic change in ionic ratios must be anticipated resulting certainly in new living conditions ÍFig. 4 ). In infiltration periods it mav also become saturated temporarilv. gravity mav induce downward seepage, so that the soil bacteria, or any bacterial pollution of the topsoil, such as pathogenic bacteria, may be varried rapidly into the first ponifer. The bacteria, or anv other microbes transported in this wav rapidly into the ppturpted zone pre exposed over a distance of a few metres, within a few davs to profound cbopges in pH, temperature, concentration and redox conditions. The abundance. virtually unlimited supplv of free oxygen in the three-phase layer is replaced gradunllv in the saturated zone by limited oxvgen supplv, a shortage of oxygen, then bv reducing — in other words — anoxic, anaerobic conditions. The changes in Eh and pH will be controlling environmental factors to a number of microbes in the groundwater as well (Fin. 5). The microbiological point of view the transition zone reprepents a definite filter, with selecting, eleminating and probably transforming functions in the path of the micro-organisms migrating, or being transported downwprd from the surface biosphere and the topsoil. Under natural conditions, the vertical distribution of the micro-organisms is bound to reveal some stratification in accordance with the changing environment within the transition zone, the predominance of heterotrophic sneeies being

1 replaced bv autotrophic ones, oxidation changing into reduction over a section, whereafter with increasing depth the special electron acceptors or sr>ppies with selective biological functions assume roles of arrowing importance ( Fig. 6 ). The bpc+erin entering the topsoil irrigated with treated sewage or from sludges disposed on the lends hpi'e been observed to become arrested within the top, about 20 cm thick le ver (Vit

7, 7 ). The nrobebilitv of pathogenic bacterial pollution across the tree-phase trpnsitipn zone is low under normal conditions. Pollution exposures of extended duration pnd high concentrption irwv create conditions under which the natural barriers and ohs+neles the spreedinf of bncteris. pre decomposed, allowing the pollutants to enter the pnnifer. The susceptibility to pollution of pnuifers close to the surface depends to P Iprre extent on the hvdrogeologv of the area. Well drilling methods and the extent of hpeteripl pollution have been positively correlated (Fig. 8 ). The well penetrates below the impervious lover, providing access to the pollutants into the aqifer. During the well drilling operations extended contacts are created with the surface, moreover, the flushing fluid, the drilling' tools and the well construction installed introduce microbes into the nnnifer. giving thus birth to a special ecosvstem in the close surroundings of the well. Microbes uncapable of autonomous movement mav penetrate along the well construction down to the screen or into the aquifer itself (Fig. 9). The impossibility of using operpting wells to retrieve from deeper pnuifers water samples containing exclusively microbes indigenous to the virgin aquifer will also be realized from the foregoing. Tn core sampling anv part, of the sarnie exposed in any wav to the environment pen he sterilised so ns to store the undisturbed water in the interior of the core (Tin. m I. pltbongh the original pressure would even then be extremely difficult to mointpin. Pr+hofenic bpcterín mav find pccess over a wide variety of potential paths to open pnuifers close to the surface. These aquifers respond sensitively to the changes inducer! bv withdrawals, since anv concentrated pumping mav result in sharplv increased flow velocities and in a reversal of the original flow direction. Corrolarv thereto the pollutants, in this particular instance the pathogenic bacteria, will also change the