Igor Mucha, Dalibor Rodák, Zoltán Hlavatý and Ľubomír Banský
Ground Water Consulting Ltd., Kolískova 1, 84101 Bratislava, Slovak Republic
In: C. Ray (ed.), 2002 Riverbank Filtration: Understanding Contaminant Biogeochemistry and Pathogen Removal, 177-219. Kluwer Academic Publishers, Printed in the Netherlands.
There exist a number of practical reasons for studying groundwater recharge via riverbed, so called riverbed infiltration. The most important are quantitative and qualitative reasons:
The groundwater recharge (or river water infiltration) is increasing the yield of wells.
Intrusion of river water can result in degradation or improvement of aquifer groundwater quality, mainly when groundwater extraction is large enough and long lasting.
Pollution of river water and intensive use of groundwater for drinking purposes requires special attention to substances, which may contaminate groundwater. Except this, natural decrease or increase in groundwater quality, directed by redox processes, is common, when using riverbank wells and aquifer recharge with river water. Our health is today not so much endangered by micro-organisms as by chemical substances, which may remain in the water due to insufficient filtration via river bed and a part of an aquifer, and due to subsequent insufficient water processing in waterworks.
In general, there exist two hydrogeological approaches for use of groundwater, recharged by the river water:
The first, the riverbank filtration wells, situated close to the river. They are pumping the river water filtered by the riverbed sediments and a small fraction of an aquifer. This water is usually not suited for direct supply without treatment.
The second, the aquifer wells, situated in such distance from the river that the so called aquifer “self purification processes” have sufficient time and space to change the river water into groundwater with satisfied quality, usually suitable for direct water supply.
The first approach, wells situated near the riverbank, is for example, used in Hungary at the Danube.
The second approach, wells situated in some larger distance from river, we are using generally in Slovakia (see for example Figure 1). Depending on local hydrogeological conditions, various measures were applied to ensure convenient groundwater quality. Waterworks well field at Rusovce, near Čunovo, is protected by filling polder in front of it by gravel. This well field is situated close to the Danube. The content of manganese is reduced using in situ treatment (input of oxygen rich water into aquifer - Vyredox). The well field of Kalinkovo originally situated in some large distance from the Danube is at present close to reservoir, therefore sealing bottom with loam was used to ensure the infiltration from the original river bed. For minimizing of riverbed clogging in front of system of wells in front of Šamorín waterworks, the linear hydraulic guiding structure was constructed. Some other measures and situation of experimental hydrogeochemical profile of piezometric monitoring wells are shown in the Figure 1.
Municipal water supply of Bratislava uses for more than 100 years alluvial aquifer water recharged by the Danube. Bugél , when giving reason for groundwater supply in Bratislava, describes groundwater pumped from wells situated near the Danube as only naturally filtered river water. Rosenau , for example, is writing that the water, obtained from wells situated close to river, filters through the sand slowly and is thus purified. Examples of typical water analyses from production-wells recharged by the Danube from Šamorín and Rusovce waterworks well fields are included in Table 1. Also included are values of Slovak standards for drinking water. The symbol < means under the detectable limit.
Except classical approach of hydrogeological survey, special tests and studies have been carried out in the framework of the Gabčíkovo – Nagymaros hydropower project. The goal of these studies was to protect, and on the right site of the Danube to improve, the groundwater quality and quantity.
The Gabčíkovo part of the Gabčíkovo Nagymaros hydropower project (Figure 2), in addition to the flood control, ensures an improvement of the navigation parameters in the Danube, and produces electric energy. The project influences groundwater level, groundwater flow and quality in surrounding area. The project has used knowledge and errors from construction of hydropower plants on the river Danube and Rhine, and this above all, in the context of the impact of these constructions on the natural environment, including groundwater resources. The situation is on the Danube unique, because the bypass canal (navigation and derivation canal) is lying outside of the inundation area, to save its valuable ecosystem. River branches of the inundation are supplied with water on the both Danube sides. An independent commission of experts of the European Union writes in its report of the 23 November 1992 in Budapest: “In the past, the measures taken for the navigation constrained the possibilities for the development of the Danube and the floodplain area. Assuming the navigation will no longer use the main river over a length of 40 km a unique situation has arisen. Supported by technical measures the river and the floodplain can develop more naturally” (Figure 2).
On the particular research basis, since the year 1953, and based on the research and monitoring of natural environment during the preparation of the hydropower system Gabčíkovo - Nagymaros, many various measures were proposed and realized. Some measures were realized directly in the reservoir and in the river branch system, which were not touched through the construction works on the both sides of the Danube. The goal was to improve the quality of the surface and groundwaters, to increase groundwater levels, and to minimize possible undesirable impacts. Some suggestions served the improvement of the ecological conditions, and others were realized in the whole area with the goal to improve the hygienic circumstances and life conditions of the population. Furthermore, all constructions were realized so that the surface and groundwater flow and levels, are adjustable to the sufficient extent.
The Gabčíkovo project protects not only the Danube floodplain ecotopes, but comprises also a wide scale of facilities for the management of the surfaces and groundwater regime. Furthermore, it includes facilities for the simulating of floods in the inundation area, influencing of erosion and sedimentation processes in the reservoir and river arms, the groundwater quality, and other factors of the natural environment.
2. General approach
The geological environment of groundwater resources is characterized by complex interplay of physical, chemical, geological, biological, climatic and hydrological forces, landscape-ecological processes, including surface and river morphological characteristics. There are many variables of land and groundwater use, contaminant sources, and natural and manmade impacts on groundwater quality and quantity.
There exists no single “standard” approach for assessing, protecting and management of groundwater quality. Aquifer survey, monitoring, modeling of groundwater quality, groundwater flow, and environmental processes are the most complex approaches to optimal groundwater resources development and sustainable groundwater management, including management of the groundwater quality processes. Reduction and oxidation (redox) processes are often the key mechanisms controlling groundwater quality. In the same time the groundwater modeling methods are used for environmentally sustainable long-term groundwater development and mitigation of the previous damages, already caused by the urban development. Modeling methods and related groundwater survey and monitoring are giving the opportunity to study the possibilities of the groundwater quality protection, and potential possibility of improvement of present already influenced natural conditions.
Sustainable development can continue to develop only through improvement in knowledge, organization, technical efficiency and wisdom. Groundwater and environmental monitoring hand in hand with the modeling methods are the future tools of environmentally sustainable long-term groundwater management and environmental protection. Preservation of good groundwater quality and sustainable quantity has high environmental and social priority. There is high responsibility from both the scientific and the non-scientific points of view.
3. PRESENT STATE OF KNOWLEDGE OF GROUNDWATER QUALITY PROCESSES
Water of natural rivers and mountainous lakes has generally oxidizing conditions. Groundwater, according to hydrogeological conditions, can have oxidizing or reducing conditions. In oxidizing conditions the most of anaerobic microbiological metabolism is suppressed by the presence of oxygen. Distribution of oxygen, organic and inorganic components in groundwater, and impact from the surface via groundwater recharge, and mainly groundwater recharge from the river water infiltration into an alluvial aquifer, inducted by groundwater extraction, are the key factors responsible for groundwater quality. These factors are also main tools for groundwater quality protection and management. Reduction and oxidation (redox) reactions are often the key mechanisms controlling groundwater quality, including migration of toxic organic and inorganic components. The redox processes require the catalytic effect of enzymes produced by microorganisms in order to proceed at significant reaction rates. The most important organisms involved in this bio-catalysis are in infiltration process bacteria in aquifer zone and bacteria, algae, fungi, yeasts, and protozoa on riverbed zone. Redox processes deliver bacteria the energy required for their metabolism. Although bacteria play an important role, the investigation of microbial processes in groundwater zone is still in the beginning stages of development. One of the first models of transport and chemical reactions including activity of microorganism is described by Kinzelbach and Schäfer  and Schäfer , which were used for development of our vertical 2D model, used also at Kalinkovo .
The sequence of important redox processes is described in various textbooks, for example, in the book Geochemistry, groundwater and pollution by Appelo and Postma  . This book presents the state of theoretical knowledge in groundwater chemistry.
Sequences of reaction along the flow path starting at the Danube from highly oxygenated water that enters an aquifer are as follows:
Aerobic respiration – oxidation of organic carbon dissolved in water (DOC), oxidation of solid organic matter in the infiltration zone (river bed sediments and zone of clogging), and aquifer organic matter, in aerobic conditions. Oxygen saturated groundwater may react with any reduced substance in the aquifer sediment, such as organic matter (plants and wooden rests, peat, lignite and coal fragments), Fe(II)-bearing minerals (like pyrite, siderite, etc.).
Oxidation of ammonium ions and nitrites to nitrates, precipitation of manganese in form of oxides, in aerobic conditions.
Denitrification, reduction of nitrates by organic carbon in water and in aquifer, in anaerobic conditions, accompanied by production of nitrites and gaseous nitrogen.
Dissolution of manganese minerals, which are present in the sediments – reduction of oxides or hydroxides of manganese by the organic carbon dissolved in groundwater or by re-oxidation of nitrites to nitrates, respective by dissolution of carbonates of manganese.
Precipitation of manganese in the form of minerals (oxidation of manganese by nitrates, accompanied by production of manganese hydroxides and nitrites, respectively precipitation of manganese in the form of carbonates or catching of dissolved forms of manganese by sorption and ion-exchange processes).
If there is still enough organic carbon dissolved in water or present in matrices of sediment, and nitrates are reduced (anoxic conditions), reduction of Fe-oxyhydroxides present in the Danube sediments occurs, and dissolution of Fe(II)-bearing minerals (pyrite, magnetite, siderite, some silicates and clay minerals) take place. In aquifer, having originally oxidation conditions, as the Danube aquifer originally is, considerable amount of iron is present as limonite brownish-yellow coating on the surface of quartz, sandstone and feldspar grains. Limonite - αFeOOH - various amorphous iron hydroxides and minerals as goethite, ferrihydrite occurs widely as the weathering product of all iron-containing minerals. The reductive dissolution of FeOOH plays the crucial role when an electron acceptor (such as dissolved organic matter, H2S, or methane) enters aquifer, for example, by groundwater recharge, river bank infiltration, or aquifer contains organic rich sediment layers or lenses.
The most obvious control on Fe2+ concentration in groundwater is by oxidation to Fe3+ and precipitation in form of limonite. This process occurs in zones where Fe2+ rich groundwater meets with O2 rich groundwater, e.g. from recharge or infiltration from the river. These principles are used by so called “in situ” or “Vyredox” groundwater treatment.
Because of heterogeneous aquifer, all these processes are close related. Course of oxidation-reduction processes is slow. It is function of reaction kinetics and hydraulic mixing of aquifer water. By real groundwater flow, water is not in equilibrium with all dissolved and not dissolved matters, which is for example shown by existence of nitrites and manganese in presence of nitrates.
groundwater, according to local hydro-geological situation, can have oxidizing or reducing conditions. In the past, shallow aquifers in alluvial sediments had mostly oxidizing conditions. These conditions are often present in uncontaminated surface regions and traditionally driven agriculture and forestry, where organic compounds are scarce and where groundwater is replenished with oxygen-rich rain or surface water. In oxidizing conditions, the most of anaerobic microbiological metabolism is suppressed by the presence of oxygen. The state of oxidizing conditions is in fact the pre-requisition of the utilization of groundwater as a direct source for water supply, without treatment.
In regions where soil, and water, which recharges an aquifer, contains too much organic compounds and less or no oxygen, aquifer become anaerobic. The oxidized compound is generally organic matter, which arises sometimes by the man activity. There exist also groundwater where the reducing conditions are of natural origin, for example, in alluvial wetlands, river branches, and often flooded areas. Reducing groundwater contains often Mn, and Fe in soluble form. Such groundwater, if not polluted, is easily to be treated by aeration and sand filtration, and may supply very good drinking water. Reducing groundwater conditions are even good for denitrification, for example in agricultural areas. If the reducing groundwater conditions are reaching deeper stand of reduction, than increases content of SO4, CH4 and NH4, and various other organic components, and water cannot be used for water supply without expensive treatment. State of redox conditions is therefore important by groundwater management and by choosing an optimal situating of production wells. Ensuring of the oxidizing conditions in groundwater, including protection against groundwater pollution, is the main goal of the river groundwater recharge quality management.
Practical applied state of knowledge of groundwater quality modeling and possibility of management are described, for example, in Migration processes in the soil and groundwater zone by Luckner and Schestakow : “Conjunctive use and management of surface and groundwater have proved increasingly necessary. groundwater and surface water must be considered as inseparable components of an interactive system. Any management of water quantity must also include consideration of water quality. It is therefore imperative that groundwater is used only to the extent and in a manner which also guarantees its sufficient protection. Two fundamental goals of subsurface water management must be met. The first fundamental goal is protection against overuse and ultimately against exhaustion. A second fundamental goal should be the protection against degradation and contamination”. This second goal, protection of groundwater quality against degradation, and management of the groundwater quality for optimal water supply, was the main topic of the experimental study, which will be described in following chapters.
Schematization, approximation, and transformation of theoretically known processes are required to formulate a simulation model, which should be compatible with the planed solution procedure. This means in fact simplification of natural conditions, and may therefore cause serious errors in modeling process. The study was aimed to extend such present state of knowledge and eliminate modeling technique errors. Therefore, it is based on careful monitoring of groundwater quality processes and local hydrological and geological conditions.
4. GROUNDWATER QUALITY RESEARCH OBJECTIVES
Suitable redox conditions, which in practice mean some proper ratio and distribution of oxygen, organic, and inorganic components in groundwater, are the basic groundwater quality management objectives (in areas where direct pollution sources, as for example oil pollution, are already eliminated).
Possibilities of management of redox conditions in groundwater are as follows:
Protection of groundwater and aquifer, to avoid organic and inorganic pollution from the surface.
Choose of aquifers with low content of organic matter, avoiding inundation areas, old and buried river branches.
Management of riverbed and river flow velocities to avoid clogging and sedimentation of fine sediments containing organic rests and remnants.
Use of proper agricultural soil techniques, irrigation and drainage, and proper forestry cultivation, with the goal to reduce organic leftovers.
Support of the groundwater level fluctuation in optimal depth.
Optimization of groundwater level depth to avoid soil water logging.
Improvement and management of river water quality.
Encouragement of groundwater recharge with water of suitable quality.
Control of the groundwater flow direction, portion of groundwater recharge from various sources (rain, river infiltration), water extraction quantities, etc.
Improvement of riverbed quality to be suitable for the water seepage into aquifer, increase of permeability, decrease of organic components. This is important also from the microbiological point of view, because the quality of the riverbed sediments (content of clay and mud) is influencing also the existence of pathogenic organisms .
Optimizing relation between surface water infiltration and evapotranspiration.
Artificial input of oxygen and other oxidants and compounds into aquifer.
Optimization of microbiological activities in aquifer, soils, and zone of the aeration.
Optimization of groundwater flow path and proper distribution of extraction wells.
Minimizing or rehabilitation of surface water bodies with low water quality and possibility of recharging aquifers in areas used as a source of groundwater for municipal water supply.
Proposing optimal well discharge in relation to situating of extraction well filters.
The goals of groundwater quality management consist in preparing the management conclusions to ensure oxidizing groundwater conditions for the direct water supply, or if not possible, to prepare suitable reducing conditions as prerequisite of groundwater treatment for water supply.
The groundwater project objectives usually include preparation of suitable monitoring tools, tools for interpreting long term water quality monitoring data, modeling tools for groundwater quality management and for decision making.
Research objectives are usually focused on methods, which should be used, improved or elaborated for groundwater quality monitoring and management:
Groundwater sampling and chemical analyses.
Monitoring of parameters and interpreting redox related processes.
Methods of construction of simple groundwater quality monitoring wells and elaboration of sampling methods related to redox identifying parameters.
Creation of database system related to interpretation of groundwater quality.
Elaboration of methods for estimation of aquifer and dispersion parameters including macro-dispersion, suitable for estimation of mixing zones or redox unstable zones in heterogeneous, layered and thick alluvial aquifers.
Use and improvement of 3D and vertical 2D non-stationary groundwater flow models.
Improvement of existed groundwater quality models, including oxygen reduction, denitrification and reduction of NO3, Mn(IV) and Fe(III).
Elaboration of methods how to influence processes in aquifer by changes in river bed infiltration, groundwater flow and level fluctuation, depth of groundwater level, irrigation, etc. Elaboration of conditions and sensitive analyses for use of such methods.
5. SIGNIFICANCE OF THE ALLUVIAL groundwater QUALITY RESEARCH
Alluvial sediments, alluvial deposits, alluvium, is defined as detrital material which was transported by a river and deposited at points along the flood plain of a river. Alluvium is commonly composed of high permeable sand and gravel with low content of organic carbon. Alluvial aquifers, a water-bearing alluvial strata, are the largest reservoirs of groundwater. This water is easy extractable, and, unlike to other types of aquifer, alluvial aquifer is rechargeable not only from the rain, but also directly from the rivers, under suitable hydraulic conditions. Most of European water supply systems of wells are situated in river alluvial sediments, or other sediments, which are hydraulically related to rivers (for example glacial sediments in northern Europe, or Tertiary sediments, e.g. sandstone, in Czech Republic or Germany).
Relationship and attribute between river and alluvial aquifer is usually as follows:
In general, natural groundwater discharges to the stream are larger than the recharge of aquifer from the stream. This natural water exchange changes according to geological situation and at places where the aquifer is exploited. The portion of groundwater recharge from the river increases with increase of aquifer exploitation. In most cases of waterworks, which are situated in alluvial aquifers, the aquifer recharge from the river is the main source of groundwater exploited.
Groundwater quality in alluvial aquifers is function of water quality recharging an aquifer. This means the rain water seeping via the soil and aeration zone and the river water flowing into an aquifer via the river bed sediments. Soil and riverbed are of a main interest in agricultural and rural development, industrial development, flood protection, navigation improvement, and environmental protection and mainly as a recharge source of high quality groundwater for water supply.
The main attributes of alluvial aquifers are the high and mostly permanent water source of excellent quality, easy exploitation, easy usually natural recharge of water resources, and relatively high vulnerability by man made actions.
6. Danube water, source of groundwater recharge
The upper part of the area downstream from Bratislava represents an important groundwater resources area. The damming of the Danube and the new water level regime of the surface waters (Danube, reservoir, seepage canals) evoked changes in the conditions of the groundwater recharge and of the groundwater flow. For example, they caused a shifting of the groundwater divides, changes in the groundwater recharge areas, changes in the river bed permeability of the Danube and the reservoir, alterations of the flow velocity, and changes in the flow directions of groundwater (Figure 3). These changes subsequently influenced the qualitative groundwater regime. This will be explained on example of changes of groundwater quality at waterworks at Rusovce.
The interpretation of the processes of groundwater recharge and groundwater quality in the river Danube bank zone and aquifer leans on evaluation of the long-term water level regime and the water quality monitoring of the Danube (Figure 4). Beside new data and measurements, accessible historic data were processed, and a purposive database was prepared.
The characteristics of the Danube water quality show an important seasonal trend (Figure 5). Especially, seasonal is the water temperature, the content of solved oxygen, the electric conductivity, the content of nitrates, nitrites, ammonium ions, phosphates, hydrogen-carbonates, sulfates, chloride, sodium, potassium, calcium, magnesium. For example, content of solved oxygen fluctuate, on average, from 8.5 to 12.5 mg/l, and the content of nitrates from 7 to 17 mg/l. The chlorides fluctuate from 14 to 25 mg/l, with the maximum values in the winter months and minimum in summer period. For characteristics, which are decisive for the course of oxidations-reduction processes, their average seasonal fluctuation was expressed in the yearly cycle for the period after filling of reservoir (Figure 5). Danube water with high content of oxygen is continuously infiltrating through riverbed into aquifer and oxygen is therefore an important oxidant in aquifer system. (In this case the Danube water is usually saturated with oxygen; saturated dissolved oxygen content at 25 OC is 8.26 mg/l and at 5 OC 12.8 mg/l.) Pumping water from aquifer wells or riverbank wells increases the import of oxygen rich water into an aquifer, mainly because redox reactions are relatively slow and the real riverbed sediments, containing some increased content of organic matter, are usually thin. Oxygen rich groundwater may react with any reduced substance in the aquifer sediment, such as organic matter, Fe(II) bearing minerals like pyrite and others, and ensure existence of limonite, which is an excellent absorbent for various organic pollutants (they usually need longer time to be oxidized) and for heavy metals.
7. Experimental hydrogeochemical profile of wells
System of experimental multilevel observation piezometers at Kalinkovo (observation wells with short screens) have been constructed for the special study of hydrochemical changes and “self” purification processes in groundwater infiltrating from the Danube into the aquifer (Figure 1) [9, 10]. Wells were constructed mainly for studying of the hydrochemical transport processes in the riverside aquifer of the Danube, changes in oxidation-reduction state of Danube water after seeping into the thick gravel aquifer, processes of oxidation of organic matter in the aquifer, etc. Design principles of monitoring wells and sampling method are in Figure 6.
The aquifer consists of highly permeable poorly graded sandy gravel and irregular lenses of sands. The system of wells consists of 11 multilevel wells (Figures 7 and 8). Individual wells were situated along the forecasted groundwater flow, corresponding to the situation after filling the Čunovo reservoir with water. The distance of individual wells from the Danube and reservoir, and the depth of their filters, is defined according to the geological profile and presupposed course of geochemical processes (Figure 9). The configuration of wells and their filters – horizontal and vertical distribution of filters – enables one to follow the development of the groundwater quality from the Danube downwards through the aquifer. The filter part of the piezometers is 0.5 m long, which enables taking a representative samples even in the case of expressive vertical variability of chemical composition of groundwater. Inside of 50- or 70-m-deep boreholes, 6 or 8 observation levels were built in, respectively. During the drilling works, drilling mud was not used, so as not to influence the future water samples by long-term ion exchange with the clay introduced by the drilling mud .
The definite well casing and screen consists from polyethylene pipes with diameter of 90/110 mm. The inner diameter of the pipes was designed to comply with the method of sampling used, which requires lowering both, the submersible pump and the measurement equipment, to the filter level. The filter and casing, made from polyethylene, corresponds to the trend of geochemical studies. Polyethylene well-casing material with a low content of plasticizers is resistant to electro-chemical corrosion. Casings are also mechanically resistant. In the proposed method of sampling, adsorption and leaching of some microcomponents (e.g. Sn and Sb from thermoplastic stabilizers utilized in thermoplastic pipe production) are negligible even when organic micro-contaminants are studied . Before installation, all filters and casings used for monitoring wells were washed with fresh water.
As a filter pack and seal between filters, the excavated borehole sieved sandy gravel was used (Figure 6). As a rule, this material was placed at the same depth from which it was taken. For the gravel pack, the coarse-grained fraction of the sieved sandy gravel material was used. The seal between individual screen parts was filled with the fine-grained fraction. There are therefore neither negative cation-exchange processes nor increasing pH levels in water samples, which is the case e.g. of a bentonite seal . The fact that the excavated material was used restricted possibility of additional contamination in the process of gravel-pack construction. In addition, it eliminated the input of foreign rock materials into the well surrounding.
Using the suggested method of two pumps (submersible and suction, Figure 6) overcomes the negative influence of stagnant water in a well after a short period of pumping. Pumping by suction pump, localized just below the water level in a well, ensures vertical up-flow of previously stagnant water in the casing and an inflow of fresh groundwater via the well filter into the well. Fresh aquifer water flows towards the sampling submersible pump and measuring electrodes situated near the filter. Sampling is carried out by submersible pump, which does not cause such marked changes in the composition of gases in water as a suction pump does.
This system of observation wells, situated at Čunovo reservoir near Kalinkovo village, is still in observation and can be used for any groundwater quantitative, qualitative, and experimental studies. In addition, long term monitoring on all water supply wells and special observation wells spread over the whole area is carried out. Selected wells of this system are monitored also in the framework of the Joint Slovak-Hungarian monitoring.
The observation objects of the hydrogeochemical profile at Kalinkovo, originally drilled in the framework of PHARE project, were readjusted for recent monitoring purposes in 1996.
Collection of data in the year 1997 was oriented towards groundwater level, groundwater flow and the groundwater quality (chemistry) monitoring. Using these data a comparison with the period of the preceding examination, which was performed in the year 1994 after filling the reservoir, was possible.
In the near riverbank zone the groundwater flow is following the direction from the Danube via PZ-1 to PZ-8 well, until a distance of approximately 1,800 m from the Danube. The infiltration area is lying across the Danube riverbed (Figure 10). Thickness of sandy-gravel alluvial Danube Quaternary aquifer is near the Danube about 40 m and is increasing with the distance to 70 to 80 m at the well PZ-8. At the well PZ-11, it is probably 130 m. Under the quaternary gravel Neogene sand and clay occurs. At the surface, there is 1 to 2 m soil and alluvial fine sand. Wells PZ-1 – PZ-8 are equipped with 6 piezometers.
The influence of the water level fluctuation in the Danube, as well as in the reservoir, is well visible in the wells situated between the Danube and the more or less stabile water level in the seepage canal, following the Danube water level fluctuation in some regular ratio. The water level difference between shallow and deep piezometers reaches, in the area close to the Danube, up to 0.17 m. This confirms the important vertical component of the groundwater flow, infiltrating via the Danube and reservoir riverbed.
The following parameters were measured in situ during the water sampling: pH, electric conductivity, temperature, dissolved oxygen, redox potential, and alkalinity. Water samples were analyzed for Cl, SO4, NO3, NO2, NH4, Fe, Mn, CHSKMn, Ca, Mg, Na, K, Si, PO43-.
Oxidation - reduction processes were identified as responsible for co called “self” purification processes in the water infiltrated from the Danube river. The sequence of the oxidations-reductions processes forms interconnected geochemical zones in the aquifer . An example based on measured values is in Figure 11. A zone close to the Danube river bed reflects a quality of the infiltrating water, which is characterized with high content of dissolved oxygen, with high content of organic matter, and nitrates. Behind this zone along the flow paths a zone with increased content of manganese and nitrite values is lying. At the end, a zone with lowered content of nitrates and organic matters appears (Figure 11).
The seasonal alterations of the composition of the Danube water (Figure 5) and the fluctuation of the water level in the Danube causes that a penetration of oxygen rich water occurs into more deeper and distant parts of the aquifer, which again causes that the high content of manganese and nitrite declines. With other conditions, when the content of oxygen in the Danube water is smaller, the dissolved oxygen is consumed by the oxidation of organic materials before the water reaches the closest well, PZ-1, which is situated about 5 m from the Danube river bank (approximately after 10 m of flow path). Widening of the zone with elevated manganese and nitrite content occurs (Figure 11).
From Figure 11, can be seen, that the system of observation wells with piezometers situated in various depths can be used for complex studying of natural hydrochemical and microbiological processes of riverbank filtration and the processes further in the aquifer.
8. Hydrogeochemical model
For the detailed study the groundwater flow-model and hydrogeochemical model in vertical cross-section were applied . The model is based on the method of final differences. Two-dimensional setup was prepared in vertical cross-section. Transport is simulated using random walk method. For simulation of oxidation – reduction processes a special geochemical model was prepared.
Using data from the monitoring wells the groundwater flow and quality model was calibrated and verified. The model is composed for stationary and non-stationary groundwater flow in vertical cross section. Model is capable, for example, to simulate the influences of the heterogeneity of the aquifer, the influences of the water table storage capacity, the influences of the water level fluctuation in the Danube and in the seepage canal, water pumping from the aquifer, the influences of precipitation, and evaporation. It includes permeability and shape of the riverbed sediments. It is possible to simulate the variable quality of the infiltrating Danube water, water from the seepage canal, mixing of the water under influence of the hydrodynamic dispersion, mutual reactions to the chemicals dissolved in the water and reactions of these chemicals with minerals in the sediments of the aquifer.
The aquifer is discretized in the network of 52 x 405 grid nodes. The model is extended as far as on the right side of the Danube. Among other things, it contains the topography of the Danube riverbed, the influences of the left-side seepage canal, the heterogeneity in accordance with the results of the geological survey, and the measuring results of the piezometric water levels. The model contains a transportation, and a reaction part, that takes into consideration the system of N02, - O2, - N03, - Mn2+ - Corg , where Mn2+ and Corg can appear in the mobile and immobile phase. The model can be improved or changed according to new requirements, new results of monitoring, and other hydrogeochemical processes or principles.
In the framework of the study by Rodák , a series of model solutions were realized, that explain the principles and interconnections between the groundwater recharge (river bank infiltration) and groundwater quality, and the influences of different other factors.
As an example, we quote a modeling solution of the groundwater flow for the stationary average water level in the Danube after filling the Čunovo reservoir. In Figure 10, there is example of flow lines and velocities, piezometric contour lines, groundwater divide.
In Figure 12, there is an example of conservative transport of chlorides for a long-term stationary average water level in the Danube, and the variable content of chlorides in the infiltrating Danube water. The water quality alteration in the Danube that infiltrates into the groundwater is shown in Figure 5. The geochemical model of groundwater quality changes (redox processes) of the Danube water containing 3 mg/l of organic carbon and 12 mg/l of oxygen infiltrating into the aquifer is given in Figure 13, and resulting modeling solution in vertical cross-section is given in Figure 14. These figures, to some degree, correspond to monitored results and geochemical zones shown in Figure 11.
9. Oxidation-reduction processes
Oxidation-reduction processes in aquifer were identified as the key processes responsible for so called “self purification” processes in infiltrated Danube water. The sequence of the oxidation-reductions processes form zones in the aquifer profile [6, 13]. These zones are in space and time not stable. The reaction system and the situation of the zones are in the condition of a dynamic balance that reacts sensitively also to low changes of the hydrological and hydro-chemical circumstances of the Danube water. On the basis of the interpretation of data from the monitoring it can be implied, that the main processes go alongside the infiltration track in following sequences (described earlier):
Aerobic respiration, oxidation of organic carbon in the water and in the sediment under aerobic conditions,
Oxidation of the nitrite, ammonium ions and the manganese which occurs in the Danube water under aerobic conditions (oxidation of the nitrites to nitrates and precipitation of the manganese in form of oxides under aerobic conditions),
Step-by-step denitrification, reduction of the nitrates by organic carbon in the water and in the sediment, accompanied by production of nitrites and gaseous nitrogen under anaerobic conditions,
Dissolution of minerals of the manganese in the aquifer sediments (reduction of oxides or hydroxides of the manganese in the water through dissolved organic carbon or through re-oxidation of nitrites to nitrates, as well as dissolution of manganese carbonates)
Precipitation of manganese in form of minerals in the aquifer (oxidation of manganese by nitrate), accompanied by production of hydroxide of manganese and nitrite; eventual precipitation of the manganese in form of carbonates, and catching dissolved forms of the manganese by the ion exchangers.
The mentioned processes are close joined together and are in principle function of time and mixing of water, which is again function of hydraulic dispersivity and flow velocity. The course of some oxidations-reduction reactions is relatively slow. Its course hangs from the kinetics of the reactions, and from the groundwater flow velocities, including mixing processes. The water is not in the entire chemical balance with all dissolved and unsolved materials. Taken samples are, in spite of the short well filters, mixture of water from various progresses of processes (for example, the existence of nitrites, or existence of manganese with presence of nitrates). It means that the theoretical computations of equilibrium states are for evaluation of samples not fully applicable. However, the equilibrium model is still an important approximation for the general estimation of the distribution of individual components and for the general evaluation of the actual state of chemical reactions. In addition, it is an important characteristic because the processes are directed towards such equilibrium along the groundwater flow originated from the Danube water riverbed infiltration.
The interpretation of the content of nitrites and manganese appears complicated. From the relatively low decrease of the nitrates alongside of the aquifer profile, it can be deduced that the reactivity of the aquifer sediments is low. In the experimental hydro-geochemical profile, the gradual reduction of the nitrates to nitrites proceeds to the complete denitrification with decrease of the nitrogen components, however, only in the low extent. It becomes implied that the intermediate products of the denitrification become again oxidized through minerals of the manganese, which slow down the process of a complete denitrification, and elevated nitrite concentrations remain maintained in the groundwater. Little by little the nitrites become oxidized or reduced along the profile, and this via chemical and biochemical processes.
In accordance with the previous knowledge, the appearance of the manganese with denitrification processes is interconnected with the sub-oxidizing groundwater state. The source of the manganese in the groundwater is not the Danube water, but the minerals of the manganese in the sediments of the aquifer. Elevated concentrations of the manganese in the groundwater are tied at a restricted zone, similarly like this of the nitrites, which are disappearing approximately at the observation well PZ-7B. If we exclude the possibility that a mixture of different water types occurs in the area between the wells PZ-1 and PZ-8, it is likely that the manganese in form of minerals precipitates gradually from the water and that the ion exchange processes apparently have a more inferior meaning.
An interpretation of the saturation degree of the groundwater against selected minerals of the manganese - Manganit, Rhodochrosit, or Pyrochrosit, Hausmannit and Pyrolusit - showed that the groundwater near the Danube, regarding all minerals of the manganese taken into consideration, is not in the balance . Rhodochrosit and Pyrochrosit have the tendency to dissolve. Regarding the remaining minerals, the water is over-saturated or under-saturated. Approximately after 150 days of the flow of the water in the aquifer a stabilization of the system in the groundwater was determined. The state of approaches the thermodynamic balance regarding the Manganit and Rhodochrosit plays probably an important role by the migration of the manganese in the Danube alluvial aquifer .
The geochemical model solutions (Figure 13) and modeling solution in vertical cross-section (Figure 14) show that 2 to 4 mg of active organic carbon oxidize in the aquifer within 80-100 days after infiltration of water from the Danube through in water dissolved oxygen and/or nitrate. The oxygen dissolved in the Danube water, infiltrating in quantities of 8.4 to 13.6 mg/l, is reduced by oxidation of organic materials as well as other materials like nitrite (manganese), in dependence on the quality of the Danube water, within 40 to 150 days after infiltration.
Nitrate shows changes in concentration according to the presence of oxidants. With high content of dissolved oxygen and low content of organic carbon, nitrate content sinks only little after consumption of the oxygen. The process is than determined by the content of organic matter in the sediments of the aquifer. However, a fast reduction of the nitrates can occur by high content of organic matter in the Danube water, already in the course of 250 days. With average content of organic matter, only a moderate decrease of the nitrates with stabilization of its concentration is noted after oxidation of the organic materials, which is dissolved in groundwater.
In correspondence to the prerequisites that the model assumes, the nitrite concentration increase in the groundwater with reduction of the nitrates, and the electron donator can be the organic carbon or in water dissolved manganese. Nitrites are instable in water and they are subject to further reduction to gaseous nitrogen, when the organic materials are present. In the case of an average and higher quality of the infiltrating Danube water (more oxygen and less organic carbon), the nitrite and manganese concentrations are lowered again after 250 to 300 days after infiltration of the Danube water. In the case of a lower quality of the infiltrating Danube water, after complete consumption of the nitrates, the long-term increase of manganese concentrations can be expected. This is steered by further geochemical processes as ion exchange and geochemical balance processes between water and sediment.
There exist various possibilities to speed up the oxidation process of organic carbon and to speed up the so-called self-cleaning processes. One method is the artificial conveying of oxygen into an aquifer, realized, for example, in the Rusovce waterworks (Figure 1). Another very common method is the aeration of suitable water in waterworks. From the hydrogeological point of view the classical method is to keep the surface water not polluted, with the low content of organic carbon, high content of oxygen, and mainly, to keep the river bottom sediments high permeable without fine sediments and without high content of organic matter. An example can be the waterworks well field at Šamorín (Figure 1).
A directed influencing of the groundwater quality development in an aquifer, and use of this development for water supply purposes, must be supported by interpretation and explanation of facts and opinions obtained by groundwater survey, pumping tests and water quality monitoring. The monitoring offers information about alterations in surface water quality, flow regime of surface water and other factors included into the groundwater quality processes.
The principles and results of the monitoring program in the area were incorporated into the groundwater quality model. The basic management principles for riverbank infiltration and groundwater recharge from river are:
To ensure good surface water quality in the Danube, characterized by low content of organic carbon, high content of oxygen, optimal content of nitrate in relation to previous two.
To leave large enough time intervals between groundwater recharge and exploitation, allowing passing the important geochemical processes. For example, there should be high enough distance of exploitation wells from the river, well defined pumping quantity, depth of well screen, etc..
To ensure good quality of riverbed sediments and riverbanks and to ensure suitable permeability, allowing suitable water infiltration. Use of some hydraulic measures and river training can help to realize such a goals.
To avoid areas of river branch system, wetlands, and flooded areas, including old meanders and buried valleys fulfilled with organic sediments. To avoid areas influenced with stagnant surface water and not proper cultivated agricultural areas or forestry.
The experimental hydrogeochemical profile at Kalinkovo is suitable to use for further, more complex, experimental field studies of geochemical microbiological and other processes in an aquifer, including the detailed study of riverbed and riverbank infiltration, from both quantitative and qualitative point of view. Such a study calls for international cooperation.
11. Examples of groundwater quality changes
11.1. The Rusovce waterworks
The Rusovce waterworks (Figure 1), located on the right side of the Danube between the villages of Rusovce and Čunovo parallel to the Čunovo reservoir, utilizes groundwater recharge from the Čunovo reservoir on the places of the previous Danube riverbed. The system consists of 23 wells located about 120 m from the seepage canal, and 500 to 600 m from the reservoir. The distance between the individual wells is 100 m. The capacity of the whole waterworks wells, after setting the hydroelectric power structure of Gabčíkovo step into operation, equals 2,480 l/s. Typical space groundwater flow from the Danube towards the system of wells is shown in Figure 15.
A lowering of groundwater level occurred on the right side of the Danube, during the long-lasting pre-dam lowering of the riverbed of the Danube. The groundwater flow from the inland agglomeration of Petržalka and Austrian territory supported the transport of contaminants towards the wells of waterworks at Rusovce and other local municipal wells (Figure 3). The rise of the water level in the Čunovo reservoir constituted a radical change in the groundwater level and flow. The groundwater level in the area of water works and also the right side of the Danube River branches rose approximately 2 to 4 m. The rise of groundwater level has had a positive impact on the discharge of the waterworks wells, on the inundation area of Rusovecké Ostrovy (Rusovce Islands) and agriculture. At present, the prevailing direction of groundwater flow is from the Danube towards the water supply well field at Rusovce and further inland (Figure 3). Since the total dissolved solids in the groundwater were originally very high (as much as 1,000 mg/l) in some localities), a decrease in these solids is regarded as the dominant positive change in the area. The content of chlorides and sulphates, and the total dissolved solids, rapidly decrease at wells, which were directly affected by the water from urbanized territory in pre-dam conditions (Figure 16). Therefore, a very positive change with respect to groundwater quality, linked with the new reservoir and increase of the proportion of water infiltrated from the Danube arises. New conditions constitute, from the both, quantitative and qualitative points of view, an unambiguous profit.
11.2. The Šamorín waterworks
The waterworks well system at Šamorín (Figure 1) consists of six wells with filter depths from 45 to 90 m. The waterworks were set in operation in 1975. The amount of exploited groundwater depends on demand. The capacity of the waterworks is at present 900 l/s. Observation well HGS-2, which is in continual operation, was selected as representative for the monitoring of the groundwater quality development. Setting the Gabčíkovo project into operation caused some changes, mainly increase of groundwater level and slight decrease of some component, mainly Cl, SO4, NO3, Ca, and others. There was also an increase of TOC to values about 2 mg/l, slowly decreasing in the two last years. Figure 17 shows how sensitive the groundwater composition and mainly the redox sensitive parameters, are on changes in recharge, groundwater flow and pumping conditions. Despite the fact that all water quality parameters are fulfilling the required drinking water quality standards, it is clear that careful monitoring and decision-making are necessary.
Based on previous knowledge and experience and taking into consideration existence of some technical means and observation wells it is proposed to carry out a more complex study entitled "Sustainable long-term development of groundwater resources in river alluvial conditions".
The objectives and summary of the proposed project are as follows:
Sustainable development of groundwater resources can continue only through improvement in knowledge, organization, technical efficiency and wisdom. groundwater and environmental monitoring hand in hand with the modeling methods are the future tools of environmentally sustainable long-term groundwater management.
Reduction and oxidation processes exert an important control on the groundwater quality under natural conditions. They also play a major role in aquifer pollution problems.
Protection of groundwater quality against degradation and management of groundwater quality for optimal water supply is the main topic of such research project.
Alluvial aquifers are the largest reservoirs of groundwater. This water is easily extractable, and rechargeable not only from the rain, but also from the rivers and artificially. Most of European water supply systems of wells are lying in river alluvial sediments. The main attributes of alluvial aquifer are the high permanent water resources of excellent quality, easy exploitation, easy recharge, but relatively high vulnerability by man made actions.
The need for maintenance and at some places for restoration of groundwater quality in alluvial aquifers, and optimal sustainable aquifer exploitation, asks for very complex decisions. Decision-making should be based on reliable tools for the prediction of groundwater responses to various human interferences. An integrated database, GIS and modeling system, based on experimental field measurements and careful monitoring, is to be developed. Such system should be able to run simulation models of groundwater flow, groundwater transport and groundwater quality processes, including groundwater recharge and discharge.
The proposed research and cooperation is based on experience in various countries, both of theoretical and experimental nature. Project can be divided into following parts:
Inception phase, in which the problems are analyzed, defined and detailed program for following activities is set up.
Experimental fieldwork, using special system of multilevel monitoring wells, under various typical conditions. To define changes in groundwater quality it is required regular sampling and analyzing of groundwater chemical composition.
Development of database suitable for processing and interpretation of monitoring data, modeling works of the final phase appointed to calibration and verification of the groundwater quality processes. In other words, a great portion of the assumed conditions will be experimented and compared with the real situation.
International cooperation in the framework of the project shall be aimed to develop a dialog and exchange of scientific and practical experiences with the goal to improve the knowledge of groundwater quality processes. This is in the same time the only way to ensure sustainable groundwater quality use and management. Alluvial aquifers were chosen, because these are common in all European countries. International co-operation includes in Europe typical geological situations, Germano-tectonic, Alpino-tectonic and glacial geological condition. Benefits from such co-operation will be surely of at least European dimension.
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|Cover page of publication||Table 1. Example of water analyses from the waterworks at Šamorín and Rusovce|
|Figure 1. Example of waterworks at the Čunovo reservoir||Figure 2. Gabčíkovo part of the Gabčíkovo-Nagymaros hydropower project|
|Figure 3. Groundwater level and flow direction before and after creation of the Čunovo reservoir||Figure 4. Some parameters of the Danube water quality at Bratislava, Slovakia|
|Figure 4. - Continued||Figure 5. Occurrence of some parameter values in the Danube during the year at Bratislava, Slovakia|
|Figure 5. - Continued||Figure 6. Groundwater sampling - design of monitoring well, method of sampling and in situ field measurements |
|Figure 7. Hydrogeochemical cross-section of Kalinkovo: Location of monitoring well (PZ-1 to PZ-13)||Figure 8. Hydrogeochemical cross-section of Kalinkovo: Location of individual well screens|
|Figure 9. Hydrogeochemical cross-section at Kalinkovo: Interpretation of conductivity coefficients from the granulometric analyses||Figure 10. Hydrogeochemical cross-section at Kalinkovo: Groundwater flow lines|
|Figure 11. Hydrogeochemical cross-section at Kalinkovo: Example of geochemical zones||Figures 12. Hydrogeochemical cross-section at Kalinkovo: Example of conservative transport of chloride|
|Figure 13. Hydrogeochemical cross-section at Kalinkovo - example of groundwater quality changes of the Danube water during the flow in the aquifer ||Figure 14. Example of modeling solution in vertical cross-section based on data given in Figure 13|
|Figure 15. Groundwater flowing toward exploited wells at Rusovce waterworks (model result)|
|Figure 16. Some water quality parameters in the wells at Rusovce waterworks||Figure 16 - Continued|
|Figure 17. Some water quality parameters in the wells at Šamorín waterworks||Figure 17 - Continued|