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Investigating the
seepage rate
in the reservoir of
an alpine Austrian river
with high water
quality
CHRISTIAN HASENLEITHNER
Ennskraft, Resthofstraße 2,
A-4400 Steyr
Tel. 07252 81122 253
Albert
Brugger
Universität Wien, Institut
für Zoologie, Abteilung Meeresbiologie
Althanstr. 14, 1090 Wien,
Österreich
Tel. 0222 31336 1351
A8940639
@unet.univie.ac.at
ABSTRACT
International
research has been busy trying to find out large-scale changes of the water
quality in connection with the seepage rate. It was found out that the biggest
change of the water quality happens in the first few meters of the filtration.
The experience of water suppliers confirms this aspect. The changes occurring
in this section and the simultaneous decomposition processes have up to now
only been examined in its basic form, what has lead to the project which is
described here.
Experiences
along the river Enns show that the already given proof concerning the
functioning of a natural filtration has to include the aspect of location and
process of decomposition and the stability of these processes throughout the
year. Otherwise the regularly emerging questions about the safety of the water
supply from filtrates in riparian areas cannot be answered with scientific
proof.
Therefore
a special infrastructure to investigate the upper sediment layer had to be
developed. With the help of monitoring stations at the bank of the river the
quality development is pictured in hydraulic profiles, especially in the most
active upper sediment layer (0 - 1.5m).
Keywords: Seepage rate, bank filtrat, ground water, water quality, water supply, drinking water
Introduction:
The
Ennskraft, an Austrian power supply enterprise, operates 12 hydroelectric power
plants along the rivers Enns and Steyr. From approximately 50 wells near the
river Enns (distance 50 to 100 meters) the enterprise has been supplying its
own plants as well as surrounding communities with drinking water for 50 years.
The discussion about the protection of these wells as well as the exploitation
of new wells has lead to the concept of the project introduced here.
The
Project is carried out in co-operation with Univ.-Prof. Dr. Ingerle (University
Innsbruck, Inst. for technic and environment), Doz. Dr. Herndl (University of
Vienna, Inst. for marine biology), Ass.Prof. Dr. Blaschke (Technical University
of Vienna, Inst. f. Hydraulics), Hofrat Dr. Schöller (Chairman of the austrian
national comitee of international water-supply association), Donaukraft and
Ennskraft.
Present situation and
general conditions
Topography
and geology
With
254 km the river Enns is the longest river which flows through Austria only. It
drains an area of 6,080 km2. The average annual water volume is 6,594 million
m3. Since the opening of the power plants Schönau (1972) there has been an
almost closed chain of ten hydropower stations run by Ennskraft. In the
surveyed section of the river it flows through three geological zones which are
characterised by the respective impervious layer.
·
the Limestone Mountains
from the "Gesäuse" to Ternberg
·
the "Flyschzone" from
Ternberg to Steyr
·
the Foothills of the
Alps north of Steyr
On
both sides of the river Enns the last two glacial periods lead to the formation
of huge gravel terraces. The melted snow and ice transported enormous amounts
of rubble from the Alps, which covered the area.
During
the interglacial periods the ice of the glaciers melted. Consequently the
rubble was transported even further, resulting in the formation of new valleys.
In the following cold periods they were filled up again. The final result of
this repeating process during the various climatic periods is the present
relief of the valley landscape along the river Enns.
The
geological division of the area in question corresponds largely with the
structure of the utilisation of the area. As shown in the chart, the section
Altenmarkt - Steyr is mainly used for forestry and grassland, in the regions
north of Steyr the area is more intensively used for agriculture. Additionally
it is dominated by industrial and urban development.
The
water quality of the river Enns
Over
the years the water quality up to the city of Steyr has remained quite high and
stable. This is also certified in the "Gewässerschutzbericht Oberösterreich
19/1998". The only parameter which in all upper austrian rivers are higher than
recommendations by EU are bacteria. All other parameter of the Enns are i.e.
minor as statet in guidelines for quality standards at surface waters for the use
of drinking water supply (75/440/EWG) and in guidelines for the quality of
water for saving the life of fish (78/659/EWG).
In
the urban area of Steyr the quality deteriorates slightly. Downstream of the
city the quality of the water improves due to the self purification of the
river. The organic burden, as seen from the consumption of potassium
permanganate (KMn04) and BOD5, was rather high up to 1980. From then on it
decreased due to wastewater treatment and the closing of industry. It is now
nearly constant along the river.
The
oxygen concentration of the river is high (>10mg/l) and largely constant.
The chemical parameter such as hardness (about 10°dH), conductivity (about 350
µs/cm) and nitrate (<10mg/l) correspond to the geological circumstances. In
the longitudinal section a minor rise of these concentrations can be observed.
Ground-water
quality
Along
most parts of the river there is a free exchange between ground water and the
surface water of the river. Exceptions are short, artificially sealed dam
sections. When judging the quality of the ground water the two sections
Altenmarkt to Steyr and Steyr to the river mouth of Enns into Danube have to be
taken into consideration separately.
·
Altenmarkt - Steyr:
Along the southern section, the well-wooded catchment area between Ternberg and Altenmarkt the quality of the ground water is generally satisfactory. Downstream, between Ternberg and Steyr, there are negative tendencies due to the agricultural utilisation and settlement.
·
Steyr - river mouth
into Danube
The experiences of Ennskraft with wells near the river are confirmed by the nitrate concentration as an indicator for the quality of the ground water. Near the river Enns, along the section dominated by bank filtrate, the nitrate concentration is below 10mg/l. In the mixing zone between bank filtrate and real ground water the concentration is between 10 and 30mg/l. Off the river the nitrate concentration is higher than 50 mg/l, sometimes even up to 100 mg/l. The situation is similar as far as conductivity and the pesticides Atrazin and DEA are concerned.
RESEARCH PROJECT
Infrastructure
Two adjoining research stations near an existing well (50m off the river) work simultaneously (figure 1):
·
OGÖLAB1: It consists of
six perforated Plexiglas bores to extract sediment specimen. Additionally three
multi-level bores (length: 1.5m) with five openings, each to extract
undisturbed water samples in depth of 0.1, 0.2, 0.4, 0.6, 0.9 and 1.5m, were
rammed. All bores are located in the in the upper section of the bank sediments
and stream bottom.
·
OGÖLAB2: is a
high-grade steel box with windows. The box, which has a flood-safe entry on the
top, was inserted about 1.5m below the average level of the river Enns. Besides
the video recordings of the riverbed clogging, water samples are possible due
to the 15 bores (length: 30cm) which stand out up- and downstream from the box.
Additionally it is possible to measure the water pressure in these bores
constantly. The bores MLD and MLE are situated in a filter layer consisting of
fine sand. The bore MLF is situated in natural substrate.
·
WELL: It is situated
50m off the reservoir of hydropower plant Garsten. The pump has a capacity of
25l/sec. The aquifer thickness is approximately 5m, the absolute thickness of
the rubble layer is 15m.
Figure 1: Infrastructure on the bank of the river
Enns upstream hydro power station Garsten
Aims
of the project
· the documentation of the permeability of the river
banks in connection with the hydrologic condition
· the documentation of bacterial activity in the
longitudinal section between river bank and well
· the development of the water quality in the same
section
· the proof of accumulation or dissolution of compounds
in the bank sediments
· the modelling of microbiological and chemical processes
Samples
of water quality
· data collection system in Enns and well
· water level in 35 monitoring stations (weekly)
· chemical/physical samples of 35 monitoring stations
(monthly)
· bacterial samples of 15 monitoring stations (monthly)
· micobiological samples of 15 monitoring stations
(monthly)
· special chemical samples of 8 monitoring stations
(twice a year)
Hydrology
Already known from early experiments, the water needs approximately two to three days to get from the Enns to the pumping well.
The permeability of the
aquifer is on average 2*10-3m/s. As expected all monitoring stations show gradual
riverbed clogging after a one-year run.
As
an example the bores MLE2 to MLE4 are shown (figure 2), which cover the first
meter of the bank sediments (located in OGÖLAB2).
As
shown in the graph the riverbed clogging results in a reduced water level of
about 30cm. The clogging takes place in the artificial filter layer of fine
sand in the upper 10 to 20cm of the bank sediment. In the layer of natural
substrate clogging takes place 1m below surface.
The
clogging also results in a reduced water level of about 30cm after one year.
Till a depth of 50cm this factor is reached after a relatively short time. In
monitoring stations between 1 and 1.5m riverbed clogging reaches the same
factor but considerably later. At some periods a partial destroying of the
clogged riverbed can be detected. One reason for this could be sediment
transport during floods.
Figure 2: OGÖLAB2, river-bank, differences between
waterlevel of Enns and bores MLE2 to MLE5 (Datei MLWSP01.xls - Dia WSP MLE Diff
Absolut GB)
Figure 3: Hydrograph of oxygen concentration between
Enns, MLD and PLF
(Datei qualo2.xls - Dia O2 Ufer MLD GB)
Chemical/physical
parameter
The
oxygen concentration of the river Enns changes with the seasons.
In
summer and autumn it reaches 10mg/l and in winter 13mg/l. In bores in the upper
sediment layer the oxygen concentration is just slightly below that of the river
in winter. In summer there is a decrease of 3 mg/l in the first 10cm of the
bank sediments at most. A further decrease until bore PLF (18m from the river
bank) is insignificant (figure 3).
At
the bores in the stream bottom the oxygen concentration decreases between 2 and
5 mg/l in the course of a year. Two thirds can be found in the first 10cm, the
rest in the first meter. Up to bore PLF the oxygen concentration increases by 1
mg/l. These results correspond with the bacterial count.
The
nitrate concentration is an indicator for the pollution of the ground water
mainly due to agriculture. The concentration in the river Enns fluctuates
between 3 and 5 mg/l. The same results can be found in a 10m strip on both
sides of the river. 20m off the river (bore PLF) the nitrate concentration
without bank filtrate is at 40 mg/l, 50m off the river (well) it is higher than
45 mg/l.
One
week after putting the well into operation the concentration decreased to 4
mg/l in PLF (20m) and to 20mg/l in the well.
The
river water dissolved organic carbon (DOC) concentration was ≈ 1.5 mg/l
at the beginning of the study period and decreased down to ≈ 0.9 mg/l in
summer (Fig. 5). A pronounced peak in July 1998 (2.2 mg/l) conincided with high
particle load of the river Enns. The DOC declined linearly with depth showing a
concentration of ≈ 0.5 mg/l at the pumping well.
Figure 4: Water sample MLD1 to MLD5 - rate of
reduction of bacteria koli in the first meter of the sediment. (Datei
baktgr01.xls - GB Dia Ekoli48 MLD Abbau 1m)
Fig. 5: Seasonal distribution of porewater dissolved
organic carbon (DOC) within the artificial filter layer (MLD) of the river
bank.
Figure 6: Seasonal distribution of porewater bacteria within the artificial filter layer (MLD) of the river bank. Dotted lines are 3rd order polynomial fits of respective data.
The
bacteriological tests show a speedy reduction of the bacterial count in the
first meter of the sediment. 50m off the river no bacteria relevant for the
supply of drinking water could be detected. As an example the situation of
E.Coli (48h) is portrayed.
in
comparison to the water of the river Enns in the first 10cm (MLD1) there is a
reduction in the bacterial count with factor 3 to 80; in the first meter (MLD5)
the factor is 5 to infinite. In the summertime (where the reduction is
generally higher) a total reduction to zero was detected after 1m (figure 6).
The
reaction was slightly lower. As on the river bank no bacterium coli could be
traced in a distance of 20m to the river Enns.
With
the other bacteria relevant for drinking water the results were the same.
Conclusion
A
main drainage river of a high water quality is the base for the use of not
treated bank filtrate in areas near the river. After a one-year operation
period the existing results show that that the essential reduction of total
bacterial numbers as well as of fecal indicator strains relevant for drinking
water takes place in the upper layer of the river bank. A few days for the
transport of the water from the river through the river bank to the well are
enough to guarantee the quality of the drinking water.
The
location of wells can be co-ordinated bearing the following two factors in
mind:
·
the quality of the
ground water in the hinterland (agriculture)
·
the quality of the main
drainage river
By
intermixing according to these two factors the necessary
chemical/physical/bacterial parameter can be obtained. Additional safety is
achieved by sterilisation (i.e ultra-violet light)
The
riverbed clogging of the Enns has no considerable influence on the water
quantity. Even if a high clogging of the river bank is assumed the sediment
transport during floods led to a partial destroy of the clogging layer on the
river bottom.
The
data existing after one year are analysed momentarily and will result in
modelling the biological and chemical processes. Special reports will be
published.
Ingerle K.: Uferfiltriertes
Grundwasser aus Stauräumen von Flußkraftwerken, Schriftenreihe der Forschung im
Verbund, Band 18, 1993.
VanHusen: Die jungquartäre
Entwicklung der Ennsterrassen zwischen Steyr und Altenmarkt und deren mögliche
Eignung für eine überregionale Grundwassergewinnung, unpublished, 1993.
Ennskraft: Trinkwasser aus
dem Ennstal und Verbund, unpublished, 1995.
Bencala, K.E.: A perspective
on stream-catchment connections. J. N. Am. Benthol. Soc. 12, 1993, p. 44-47.
Hendricks, S.P.: Microbial
ecology of the hyporheic zone: a perspective integrating hydrology and biology,
J. N. Am. Benthol. Soc. 12, 1993, p. 70-78.
Valett, H.M.; Fisher, S.G.;
Grimm, N.B.; Camill, P.: Vertical hydrologic exchange and ecological stability
of a desert stream ecosystem, Ecology 75, 1994, p. 548-56
Vervier, P.; Gibert, J.;
Marmonier, P.; Dole-Olivier, M.J.: A perspective on the permeability of the
surface freshwater-groundwater ecotone. J. N. Am. Benthol. Soc. 11, 1992, p.
93-102