Posters Session I
F. HUNEAU
Université Bordeaux-1, CDGA, Talence, France B. BLAVOUX, Y. TRAVI
Université d’Avignon, Laboratoire d’Hydrogéologie, Avignon, France
The European Water Framework Directive makes necessary to identify and define every groundwater system across European member countries. The concept of groundwater body, according to the Water Framework Directive, imposes to estimate a “good status” of groundwater and thus encourages restoring this initial good status in places where strong deterioration is occurring. Isotopic techniques might be useful to define such pre-exploitation conditions and particularly in complex systems were paleowaters preserved in confined aquifers can provide pristine waters.
The Miocene aquifer of Valreas is located in Provence close to the Rhône valley. It consists of multilayered sandstones of approximatly 600 m depth in the central part of the basin. The structure of the aquifer is complexe and the southern part of the system is confined under a marly Pliocene cover of around 100 m depth. Recharge takes place in the northern part of the basin through the outcropping area of the Miocene sediments. The groundwater flow direction in the aquifer is from north-east towards south-west. The paleo-flow conditions in the aquifer were presumably controlled by the presence of the Pliocene cover, causing a rapid flow in the unconfined part of the aquifer compared to the confined part of the aquifer. At the boundary between the two systems, several springs were present. Since the 1950’s, extensive abstraction of groundwater has been carried out from the confined part of the aquifer for irrigation purposes. As a result, the springs at the confined/unconfined boundary are now dry, and groundwater flow takes place at a more uniform velocity of around 2 m/yr in the entire aquifer.
More than 100 boreholes were sampled, and stable isotopes of water molecule (18O, deuterium) as well as 13C concentrations were measured on each sample. In addition, 25 boreholes located along the main flow line were sampled for radiocarbon dating in order to evaluate groundwater residence time and to precise groundwater velocity [3]. Major ions and trace elements concentrations were also investigated with a special interest in arsenic concentrations. These results have been used to set up a conceptual model of the functioning of the aquifer [1]. Some of the most interesting results are the evaluation of groundwater residence time which is clearly over 30 000 years, and the reconstruction of paleotemperatures prevailing during the recharge of waters at the end of the Pleistocene [2].
Owing to the large number of isotopic data available it has been possible to draw maps covering the whole area and showing the distribution of 18O, deuterium and 13C (FIG. 1 &
FIG. 2). These documents are useful and allow locating the recharge area with accuracy and thus potentially sensitive areas. It can also be used for the estimation of groundwater dynamic or to detect the intrusion of young waters under the Pliocene cover. These waters are
can be of major interest. The age of groundwater increases gradually from the recharge area towards the confined aquifer. Along the down gradient direction, the increasing in 13C, related to matrix exchange reactions with carbonates, can be considered as a good qualitative age indicator. Thus a strong enrichment in 13C will indicate the occurrence of old pristine waters in the aquifer. This can be easily realised and at low cost compared to radiocarbon. 18O distribution maps were used to locate long residence time waters thanks to the strongly depleted signature of paleowaters. 18O data were also used to precise the extension of the recharge area by comparison with modern precipitation content.
The chemical data obtained from paleowaters can be considered as the baseline concentrations of the aquifer and they will be used as a goal towards restoration of water quality especially in the unconfined Miocene aquifer. The methodology developed here might be applied in other cases, and would be of help towards achievement of good groundwater status.
FIG. 1 : Distribution of 18O in the aquifer. FIG. 2 : Distribution of 13C in the aquifer.
Valréas Valréas
Boundaries of the Pliocene cover
N Boundaries of the
Pliocene cover
N
0 5km 0 5km
Cretaceous Cretaceous
Tortonian hills Tortonian hills
δ13C‰ of GW δ18O‰ of GW
Borehole Borehole
REFERENCES:
[1] HUNEAU F. (2000): Fonctionnement hydrogéologique et archives paléoclimatiques d’un aquifère profond méditerranéen. Etude chimique et isotopique du bassin miocène de Valréas (S-E de la France). Thèse de doctorat, Université d’Avignon, France, 192 p.
[2] HUNEAU F., BLAVOUX B., AESCHBACH-HERTIG W. & KIPFER R. (2001):
Palaeogroundwaters of the Valréas Miocene aquifer (South-eastern France) as archives of the LGM/Holocene transition in the Western Mediterranean region. Proceedings of the International Conference on the Study of Environmental Changes using Isotope Techniques, IAEA, Vienna, 23-27th April 2001.
[3] HUNEAU F., BLAVOUX B. & BELLION Y. (2001): Differences between hydraulic and radiometric velocities of groundwater in a deep aquifer: example of the Valréas Miocene aquifer (South-eastern France). C. R. Acad. Sci. Paris, Earth and Planetary Sciences, 333, 163-170.
IAEA-CN-104/P-56 ISOTOPE TECHNIQUES APPLICATION IN UNDERSTANDING THE RECHARGE PROCESS OF THE DAVAO CITY AQUIFERS
S. CASTAÑEDA, T. GARCIA, L. FERNANDEZ, L. ASCAÑO, A. RAMOS, L. DEL CASTILLO
Philippine Nuclear Research Institute, Commonwealth Avenue, Diliman, Quezon City, Philippines
H. MASPIÑAS, E. CALONZO, J. DIAZ, R. CABANAG, E. REGALALDO Davao City Water District, Davao City, Philippines
Davao City, one of the Philippines’ major cities, is undergoing extensive urban and eco- tourism development. Generally, groundwater is the most important source of freshwater, supplying 97% of the city’s water requirements. Davao City is generally mountainous, characterized by extensive mountain ranges, uneven distribution of plateaus and some lowlands. The city’s urban and development areas are concentrated along a narrow coastal strip averaging 5 km in width and extending 56 km facing the Davao Gulf. It is in the coastal area where most of the production wells are located and where heavy abstraction is concentrated. Chemical and isotopic characterization of the water sources in Davao City were undertaken to provide an insight into the processes of groundwater occurrence, particularly on the origin and rate of recharge into the groundwater and evaluation of the aquifer’s vulnerability to pollution. Field investigations were conducted from October 1998 to February 2002.
The study area, 42km x 33km, is within the Talomo–Lipadas-Sibulan (TLSS) catchment basin. The groundwater aquifer in the TLSS is composed of reworked and redeposited overlapping flows of Quaternary pyroclastics. It has an upper unconfined aquifer composed of sand, gravel and occasional boulders which is tapped by shallow domestic wells. The deeper aquifer which is being tapped by wells of the Davao City Water District at depths ranging from 46 to about 140 meters below ground level is multi-layered aquifer separated by thin, relatively less permeable layers of clay. Three river systems, Lipadas River to the west, and Talomo and Davao Rivers to the east traverse the study area. These flow through the city and empty to the Davao Gulf, south of the city.
Chemical composition of the groundwater shows that most of the waters in the Talomo–
Lipadas–Sibulan catchment (TLSS), except for Well #1, are classified as Ca+Mg-HCO3
waters. The fluid in Well #1 is a mixture of Ca+Mg-HCO3 and Na+K-Cl waters. The high Cl, about 200 mg/L, is attributed to the presence of connate water in marine sediments at deeper levels. Composition diagrams of the water sources show three groups of water; one, mixed with river end members (Talomo River in the east and Lipadas River in the west);
second, mixed with Well #1; and a third group of unmixed waters, found in the deeper aquifer.
The mean isotopic composition of precipitation in Davao City has been established from data obtained for the period December1999 to January 2002 from four stations located at different elevations in the watershed. δ18O values ranged from –13.51 ‰ to –3.54 ‰ and δ 2H values
surface waters in Davao City showed small variations, clustered along the LMWL.
Groundwater from production wells with depths ranging from 90 m to 152 m, exhibited isotopic compositions ranging from -49.9‰ to 39.90 ‰for δ2H and - 7.64 ‰to - 6.38 ‰ This suggests a uniform groundwater recharge derived mainly from local precipitation.
Differences in recharge altitude and mixing of different water origin could explain the slight variation in isotopic contents.
The isotopic composition of precipitation exhibited an altitude effect. A gradient of -0.23 ‰ /100m was obtained for 18O while a gradient of -1.6 ‰/100 m resulted for 2H. From these altitude effect lines, the elevations of recharge for the groundwater in the study area were estimated.
The profile of ‰ 18O in the groundwater from the Talomo-Lipadas catchment area reveals a distinct trend. The trend goes from less negative values inland to more negative values near the coast. As seen from water chemistry, three groups of water are revealed from the isotopic composition trend of the waters. The inland groundwater has isotopic composition similar to that of low altitude rainfall or river fed from low altitude. This has been estimated to be coming from elevations < 250 m. Groundwater with the most negative values is tapped along a 2-km wide region adjacent to the coast. This coastal water is derived from higher altitudes estimated to be at elevations of not greater than 500m. The coastal waters, on the other hand, exhibit isotopic enrichment approaching the Talomo River and Well 1 located on the eastern end of the study area. Isotopic enrichment and a corresponding increase in chloride concentration in adjacent wells indicate connection with Well#1. Isotopic values gradually decrease towards the coast. Mixing between the shallow low altitude groundwater and the deep high altitude groundwater produces this decreasing trend in isotopic composition.
Tritium values from precipitation ranged from 1 TU to 11 TU while values from groundwater ranged from 0 TU to 6. Groundwater along the coast generally showed negligible or very low TU values. Representative samples from inland wells and from shallow wells had measurable tritium. Further investigation using CFCs showed representative wells from the coast yielded water with no detectable CFC indicating waters older than 50 years. Representative waters from inland had detectable CFCs with indication of mixture of old and recent waters, confirming the trend observed from stable isotopes. Recent recharge ages of 27 (55%), 22 (44%), 17(56%), and less than 10 years (8%) have been determined from the binary mixing model.
IAEA-CN-104/P-60 STUDY OF THE POLLUTION IMPACT FROM THE USAGE OF SEWAGE AND DRAINAGE WATERS ON THE GROUNDWATER OF THE QUATERNARY AQUIFER, WEST CAIRO, EGYPT