Groundwater quality processes
after the
bank infiltration from the Danube at Čunovo
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.
1. Introduction
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 [1], when giving reason for groundwater
supply in Bratislava, describes groundwater pumped from wells situated near
the Danube as only naturally filtered river water. Rosenau [2], 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 [3] and Schäfer [4], which were used for
development of our vertical 2D model, used also at Kalinkovo [5].
The sequence
of important redox processes is described in various textbooks, for example, in
the book Geochemistry, groundwater and pollution by Appelo and Postma [6]
. 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 [7]: “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 [8].
-
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 [6].
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 [11]. 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 [12]. 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 [10]. 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 [5]. 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 [5], 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 [5].
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
[5].
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).
10.
Conclusion
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.
12.
Proposal
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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