JAMNIK

Posters Session I

B. JAMNIK

JP Vodovod-Kanalizacija, Ljubljana, Slovenia J. URBANC

Geological Survey of Slovenia, Ljubljana, Slovenia

For almost 15 % of Slovenia's inhabitants living in Ljubljana, the capital of Slovenia, two groundwater sources of drinking water are of great importance. An abundance of groundwater is hidden inside the sandy and gravely Sava river aquifer underneath the urban city area, called Ljubljansko polje, which is one of the largest underground reservoirs of drinking water in Slovenia. Ljubljansko polje is a tectonic basin by its origin and is, together with the second important groundwater resource - Ljubljansko Barje - a part of the Ljubljana basin.

Ljubljansko Barje is highly complicated from the hydrogeological point of view - the variety of unconfined and unconfined aquifers stretching along the city suburbs in the South give us just a misty figure of the processes taking place in the sandy layers, in places covered by impermeable clayey layers and surrounded by karst mountains [1].

The Ljubljansko polje aquifer is one of the most investigated Slovenian areas, because its groundwater has been used for public drinking water supply since 1890. Together with the increasing number of Ljubljana's inhabitants and consequently rising withdrawal quantities, groundwater quality began to show unacceptable deviations from the quality standards. The question of acceptable exploitation quantities that would not cause further decrease in groundwater quality was opened. The aim of isotope investigations was to determine the origin of the abstracted groundwater in more detail. The results served as helpful tools in determining priority tasks in planning future water exploitation and protection.

Isotope investigations had not been applied in groundwater researches of Ljubljansko polje until recently. As an additional tool for understanding the groundwater recharge and flow of Ljubljansko polje groundwater, oxygen isotope composition was being determined during the period from autumn 1997 to autumn 1999. On the basis of results of previous hydrogeological investigations it was concluded that only two important sources of the Ljubljansko polje groundwater exist, local precipitation and the Sava river water. The two sources showed a noticeable difference in oxygen composition, which served as the basis of groundwater origin determination. The pumping wells included into the public water supply system were used as sampling points. Isotope investigations showed that the share of the river water and of local precipitation strongly depend on the sampling point location, namely on its distance from the Sava river recharging area. As a consequence, the values of physical and chemical parameters show annual changes according to the variations in the portion of the two sources. The sampling points with a high proportion of the recharging river water show better groundwater quality. In other words, human impact in the urban city area is the main reason for deterioration of groundwater partly flowing below the urbanised area of the Ljubljana City.

Encouraging results of isotope investigations of the Ljubljansko polje led to the decision to start investigations on the second part of the Ljubljana basin – Ljubljansko barje. The

area could be protected from the anthropogenic influences more easily than the water resource in the close vicinity of the city.

The main aim of the research, based on stable isotope (oxygen, devterium) techniques, was to confirm the existence of different aquifers determined by former geological investigations and hydrogeological observations and to determine the differences in their recharge dynamics.

The investigations started immediately after the conclusion of the Ljubljansko polje research in the autumn of 1999 and took place for two years. The results of isotope composition and chemical parameters, observed simultaneously, contribute to the understanding of groundwater origin, mean altitude of the recharge area, aquifer recharge dynamics and the relation between surface waters and groundwater [2].

Two recent investigations, based on the determination of the stable isotope composition, give important results and form the basis for the decision of the future protection and exploitation of the Ljubljansko polje and Ljubljansko barje aquifers.

REFERENCES:

[1] MENCEJ, Z., Prodni zasipi pod sedimenti Ljubljanskega barja, Geologija 1988, 31, 517-553, Ljubljana.

[2] URBANC, J., Baseflow retention time estimation on the basis of water isotope composition. Proc. Isotope techniques in water resources development and management, 1999, IAEA CSP-2/C, Wienna.

IAEA-CN-104/P-25 RAPID METHOD FOR MEAN RESIDENCE TIME DETERMINATION

N. MILJEVIC, D. GOLOBOCANIN, V. SIPKA

Vinca Institute of Nuclear Sciences, POB 522, 11001 Belgrade, Serbia and Montenegro D. JANKOVIC

Republic Hydrometeorological Service, Kneza Viseslava 66, 11030 Belgrade, Serbia and Montenegro

Environmental isotopes (oxygen-18 and deuterium) are ideal independent tracer to obtain the dynamics of water transport in catchment area because they are constituents of water molecules. The relation between input and output tracer concentrations could be formulated via the convolution integral [1]:

∞

∫

−

=

0

) ( ) ( )

(t c t τ g τ dτ

c_{out in} (1)

where g(t) represents the weighting function of the mathematical flow model (age distribution function), cin(t) and cout(t) are the input and output tracer concentrations, respectively, and t refers to time. We prefer exponential flow model, which is mathematically equivalent to a well-mixed reservoir. In this case g(t) is one parameter function

T

e t

t T

g 1 ^/

)

( = ⁻ (2)

where T stands for the mean residence time (MRT). Any periodical function could be transform in sum of sinusoidal function using the Fourier transform method [2]. The equation (1) is well presented by the first (dominant) harmonics of input and output functions. The average seasonal tracer concentration could be approximated with input (i=in) or output (i=out) sine functions:

12 ) sin(2 )

( _{0i i i}

i

C t C t

C _{= −} π ₊φ

(3)

where Ci(t) represents model function and C0i, Ci, and φi are the mean of δ¹⁸O, amplitude of variation, and phase angle, respectively. This first harmonics has a period of one year. Solving equations (1) and (2) under the condition (3), T can be expressed in months as:

A T = 6 1−A²

π ⁽⁴⁾

where A is the amplitude attenuation, A=Cout/Cin.. The amplitudes of Cin and Cout are proportional to standard deviations of tracer concentrations [3]. Based on Tukey’s work [4], we calculated values for A as the ratio of standard deviations for the input (precipitation),

in out

c

A c

σ

=σ (5)

This relation passed check on simulated sinusoidal curves.

The monthly precipitation and river water samples were collected at location Meteorological Station of Belgrade (Zeleno Brdo, 44^o47’N, 20^o32’E, altitude 243.2 m asl) and Vinca (1145 km from the confluence with the Black Sea), respectively during 1992, 1997, 1998 and analyzed on oxygen-18.

The Danube river is characterized near Belgrade (after mouth the Sava river at Ritopek, 1116.2 km) by a hydrological regime with two maximum flow rates in April-May (6100 m³/s) and November-December (6200 m³/s) and a minimum in August-September (3200 m³/s) for the observed period. The seasonal trend of δ¹⁸O in precipitation fluctuated between –12 ‰ in winter (December-January) and –4‰ in summer (June-July) and it was less pronounced for Danube water from –8‰ (March-April) to -10‰ (August-September).

The δ¹⁸O values in precipitation have been weighted with respect to monthly amount of precipitation in mathematical techniques employed in this work. Obtained values for MRT are between 10 and 12 months for the Danube near Belgrade that is in a good agreement with findings from comparison of δ¹⁸O trend curves for precipitation and Danube water at Vienna (MRT=12 months) [5]. Proposed rapid method can be applied as input for more complex iterative methods for estimation of MRT.

REFERENCES

[1] STICHLER, W., MALOSZEWSKI, P., MOSER, H., Modeling of River Water Infiltration using Oxygen-18 Data, J Hydrology 83 (1986) 355-365.

[2] FISHER, R.A., Tests of Significance in Harmonic Analysis Contribution to Mathematical Statistics, J. Wiley and Sons, New York, 1950.

[3] HAMMING, R.W., Digital Filters, Prentice-Hall, Inc. Englewood Cliffs, N. J. 1977.

[4] TUKEY, J.W, An Introduction to the Measurement of Spectra, Probability and Statistics, Aimquist and Widsell, 1959.

[5] RANK, D., ADLER, A., ARAGUAS A. L., FROEHLICH K., ROZANSKI, K., STICHLER, W., Hydrological Parameters and Climatic Signals Derived from Long Term Tritium and Stable Isotope Time Series of the River Danube, Isotope Techniques in the Study of Environmental Changes (Proc. Int. Symp. Vienna, 1997), IAEA, Vienna (1998) 191-205.

IAEA-CN-104/P-32 NEW CONTRIBUTIONS TO THE ISOTOPIC CHARACTERIZATION OF THE COASTAL AQUIFERS AND THEIR RELATIONS WITH THE SALINE INTRUSION IN THE COSTA DE HERMOSILLO, SONORA, MEXICO

M., RANGEL-MEDINA; R. MONREAL-S.; J. CASTILLO-G.; M., MORALES-M.; H., VALENZUELA

Dpt. de Geologia, Universidad de Sonora, Blvd. Luis Encina y Av Rosales, Centro, Hermosillo, Sonora, Mexico, C.P. 83000

The area of study is located in the coast of the Gulf of California, at the Northwestern of Mexico. It is an Quaternary alluvial plain of continental origin. A new hydrogeologic model is described for the area, consisting of a single unconfined aquifer with an average thickness of 200 m, as well as some located semiconfined zones. We found no evidence of a deep aquifer suggested by previous authors, but there is a deep trap old water. Beneath the unconfined aquifer there exist marine sediments containing Miocene fossils. These marine sediments are more than 500 m thick that fill and cover a number of tectonic basins and horst system (Basin and Range) of variable size and depth. The regional basement is underlying the Miocene sediments, it consist of granites and volcanic rocks. Oxygen-18/deuterium and tritium data support this hidrogeologic model and the origin of the fresh groundwater stored in the aquifer.

There is no evidence of modern water infiltration, even by agriculture return flow, because of the very high real evapotranspiration (>1220 mm/year) and the depth of the groundwater table (135 m). Enough evidence is present in this paper to affirm that the sea water is intruding into the upper part of this aquifer up to 32 km inland. The excesive pumping since 58 years ago depleated the potentiometric surface forming a cone with the actual deepest point in 65 meters under the sea level. The saline intrusion has created a interface zone of 5 to 15 km width and 80 km length of brackish water. As a consequence of the tectonic setting, there is also a fringe zone, which is protecting small areas of the aquifer from the sea water intrusion near the coast, in this place the impervious rock is present, and helps to configure the geometry of the interface zone. Radiocarbon data suggest an average age of 3000 years for the fresh water and the existence of a paleowater stored in both the marine sediments and the volcanic rocks with an age of 30,000 years.

IAEA-CN-104/P-35 COMPARISON OF ENVIRONMENTAL ISOTOPES FOR TRACING GROUNDWATER-SURFACE WATER INTERACTIONS IN A SAND-BED STREAM J. L. PRITCHARD, A. L. HERCZEG, S. LAMONTAGNE

CSIRO Land & Water, Private Bag No.2, Glen Osmond SA 5064 Australia

Groundwater discharge to streams is important for delivering essential solutes to maintain ecosystem health and flow throughout dry seasons. However, managing the groundwater components of stream flow is difficult because several sources of water can contribute, including delayed drainage from bank storage and regional groundwater. In this study we assessed the potential for a variety of environmental tracers to discriminate between different sources of water to stream flow.

Chloride (Cl^-), stable isotopes of water (¹⁸O/¹⁶O, ²H/¹H), radon (²²²Rn) and strontium isotopes (⁸⁷Sr & ⁸⁶Sr) were selected to investigate groundwater - surface water exchange, in particular groundwater discharges to stream flow in a sand-bed stream in SE Australia. These different environmental tracers each provide complementary information on such processes. Since chloride concentrations in groundwaters are generally much higher than atmospheric inputs to stream systems, elevated chloride concentrations in stream water can indicate points of groundwater contribution to stream flow. However, such conclusions are often ambiguous because evaporative processes also cause chloride ions to become concentrated in surface water systems. Because evaporated waters have predictable ratios of ‘heavy’ to ‘light’ water molecules, the isotopic composition of δ¹⁸O and δ²H, can be used to distinguish whether elevated chloride concentrations in stream water are associated with evaporative or groundwater discharge conditions.

It is also beneficial to use additional tracers that specifically target a single process or water pathway, for example ²²²Rn and ⁸⁷Sr/⁸⁶Sr. The presence of elevated ²²²Rn in stream water can only be produced by discharge of groundwater. Because ²²²Rn has a short half-life (3.8 days) and rapidly outgasses to the atmosphere, it does not persist in stream systems for long.

Therefore ²²²Rn does not become concentrated in stream water as it flows downstream, allowing for more precisely locating groundwater discharge to stream flow. Since chloride, water isotopes and radon signatures may all be rapidly altered after reaching the surface (i.e.

via evapo-concentration or decay) they are not always useful for distinguishing between multiple groundwater reservoirs discharging to stream flow. On the other hand, groundwaters develop distinctive ⁸⁷Sr/⁸⁶Sr ratios depending on their mineralogical characteristics and these ratios are not altered by evaporative processes, and persist on time scales commensurate with stream - groundwater interaction.

A case study comparing Cl^-, δ¹⁸O, δ²H, ²²²Rn and ⁸⁷Sr/⁸⁶Sr to investigate the spatial and temporal variability of groundwater inputs to stream flow was conducted in the Wollombi Brook Catchment (SE Australia). The objectives were to characterise the three potential sources of water to stream flow (surface water, groundwater from the near-stream sandy alluvial aquifer system, and groundwater from the regional sandstone aquifer system) and estimate their relative contributions to stream discharge at flood recession and baseflow.

Surface water was sampled at various locations along the Wollombi Brook and from its tributaries during flood recession (Mar-01) and under baseflow conditions (Oct-01). Alluvial groundwater was sampled from a piezometer network and regional groundwater from deeper

bores in the lower to mid-catchment biannually over two years to characterise these potential sources of water to stream flow.

Each of the environmental tracers had distinctive signatures for at least one of the water reservoirs (Fig.1) and their combined assessment facilitated the delineation of water sources to stream flow in different parts of the catchment during flood recession and under baseflow conditions. Chloride identified specific reaches of the catchment that were either subjected to evaporation or received regional groundwater contributions to stream flow. The water isotopes verified which of these reaches were dominated by evaporation versus groundwater contributions. They also revealed that the predominant sources of water to stream flow during flood recession were either rainfall and storm runoff or regional groundwater, and that during baseflow the predominant source of water to stream flow was alluvial groundwater. Radon showed that there was a greater proportion of groundwater contributing to stream flow in the upper part of the catchment than the lower catchment during both flood recession and baseflow. Strontium isotopes showed that regional groundwater contributed less than 10% to stream flow in all parts of the catchment under baseflow conditions.

FIG. 1. The 10^th, 25^th, 75^th and 90^th percentiles represent the variation throughout the two-year sampling period (2000-01) of chloride (Cl^-), deuterium (δ²H), strontium isotopes (⁸⁷Sr/⁸⁶Sr) and radon

(²²²Rn)measured in surface water (SW), alluvial groundwater (AGW) and regional groundwater (RGW), sampled across the Wollombi Catchment.

IAEA-CN-104/P-40 GEOCHEMICAL EVOLUTION AND TIME SCALE OF SEAWATER INTRUSION INTO THE COASTAL AQUIFER OF ISRAEL

O. SIVAN^a,b, Y. YECHIELI^b, B. HERUT^c and B. LAZAR^a

aInstitute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel

bGeological Survey of Israel, Jerusalem 95501, Israel

cIsrael Oceanographic and Limnological Research, National Institute of Oceanography, Haifa 31080, Israel

The dynamics of seawater intrusion into a coast is commonly estimated by a rise in salinity and/or by theoretical hydrological models. Estimations using radioactive isotopes are reported only in several works. Here we present an attempt to quantify the geochemical processes and the time scale of seawater intrusion into a coastal aquifer from the changes in the major ions composition of the waters and the natural distribution of the cosmogenic isotopes ¹⁴C and ³H.

Saline and brackish groundwaters were sampled from observation and pumping wells in the Israeli coastal aquifer. In addition, detailed profiles across the fresh-saline groundwater interface (resolution of 10 cm) were provided using a Multi Layer Sampler (MLS) that was installed three times in this zone. All groundwater samples were analyzed for their chemical composition, stable carbon and oxygen isotopes, ¹⁴CDIC (¹⁴C in the dissolved inorganic carbon) and tritium activity. The coastal rock was analyzed for its chemical contents and stable and radioactive carbon isotope composition of the carbonate and of the organic matter.

The chemical and the stable isotope data revealed three distinct water types (end members) that are placed in different zones in the coastal aquifer: 1. Slightly modified Mediterranean seawater with its salinity (SWS); 2. Slightly diluted (with up to 20 % fresh groundwater) saline groundwater (SDS); and 3. Fresh groundwater (FGW).

The SWS waters show in most cases excess in total alkalinity and DIC and depletion in ¹³C and ¹⁴C with respect to normal seawater, indicating that anaerobic oxidation of organic matter is the first diagenetic reaction that affect seawater during its penetration into the bottom sediments. Later on, the SWS waters dilutes, gain Ca²⁺ and Sr²⁺ and deplete in K⁺ suggesting that mixing with fresh water and cation exchange are the main diagenetic processes that transforms SWS into SDS. At the fresh-saline water interface, SDS waters show conservative mixing with FGW in most cases.

Inspection of chemical data from coastal aquifer around the world indicates that these main paths of seawater during its evolution into saline groundwater, intensive ion exchange and slight dilution with fresh groundwater, are globally important phenomenon.

Most saline waters contain substantial amounts of ³H suggesting that penetration of Mediterranean Sea water and its travel inland to a distance of at 50-100 meter onshore occurred 15-30 years ago. Therefore, the relatively low ¹⁴CDIC activities measured in the saline groundwaters resulted mainly from diagenetic processes and not from simple radioactive decay.

IAEA-CN-104/P-48 STABLE (O, δ, AND C) AND RADIOGENIC (TRITIUM AND ¹⁴C) ISOTOPES STUDIES OF SHALLOW AND CO2-RICH GROUNDWATER FROM THE FOREZ GRABEN (EASTERN MASSIF CENTRAL, FRANCE).

F. GAL, C. RENAC and M. LACROIX

Universite Jean Monnet, laboratoire Transferts Lithospheriques, 23 rue P. Michelon, 42023 Saint Etienne cedex 2, France

The Forez basin, which is located in the eastern part of the Hercynian Massif Central in France, is a non-symmetric Tertiary graben filled with 600 to 700 meters clastic sediments. It is surrounded by Palaeozoic granites ranging from 350 to 320 Ma. Volcanic activity in the basin during Miocene times was related to peri-Alpine tectonic activity.

This tertiary basin is known for its rare thermal and more numerous bicarbonate-rich springs.

For more than two years, groundwater (springs) and rainfall waters have been collected for measurements of temperature, pH, conductivity, alkalinity, dissolved element concentration and stable isotope ratios. A subset of samples were analyzed for tritium content and dated using the ¹⁴C method.

Weighted mean δ¹⁸O values of rain range from -3.5‰ in August (Taverage = 19°C) to –10‰ in September (12°C) to November (6.5°C). Hydrogen isotope analyses of the same samples (in process) should allow us to determine a Local Meteoric Water Line.

Perennial springs located on arenite – basement unconformities (600-1200m a.s.l.) range in δ¹⁸O values from –9 to –11‰ with significant seasonal differences and show exceptionally sudden changes in δ¹⁸O possibly related to local climatic events. These waters will be analyzed as well as rain for tritium (in process). There is a progressive depletion of δ¹⁸O value with altitude of 0.15‰ to 0.25‰ per 100m. Carbon isotope ratios suggest contamination of the recharge waters (δ¹³C = –19‰ to –24‰) by organic matter, whose ¹³C signature is dominated by C3 plants in regions of temperate climate (δ¹³Caverage = -27‰).

Rivers near some of the mineral springs show similar C isotope composition (δ¹³C = -21‰ to –24‰). The CO2-rich springs and the thermal springs have oxygen isotope ratios comparable with those of the shallower springs. Their δ¹³C values range from –2‰ to –6‰ except for one thermal spring (T ≈ 30°C, ¹⁴C age = 2450 ± 150 BP) that has a δ¹³C value of -12‰ (Fig. 1).

Their relatively constant δ¹³C and δ¹⁸O ratios are clearly not affected by any surficial contamination. The C isotope ratio is probably influenced by degassing and hence linked to peri-Alpine tectonic activity in the basin. Stable isotopes ratios will help to constrain both the compositionnal domain of the water and the proportion of modern recharge of bicarbonate- rich ground waters.

Results for each spring have been interpretated using meteorological, topographic and chemical criteria using PhREEQC (http://wwwbrr.cr.usgs.gov/projects/GWCcoupled/phreeqc /index.html) and FLOWPC (Maloszewsk P. and Zuber A.) software. Data and their modelling