US Geological Survey, 345 Middlefield Rd, MS 434, Menlo Park, CA 94025 USA P. McMAHON and K. DENNEHY
US Geological Survey, Denver Federal Center, MS415, Lakewood, CO 80225 USA
The Great Plains of the United States is a major agricultural region which produces corn and grains. The High Plains aquifer underlies a major portion of the Great Plains and extends from the panhandle of Texas through Nebraska. Since the mid-1950s, irrigation has been utilized in many parts of this region, with water being drawn from the Ogallala Formation.
Irrigation has resulted in a draw-down of the water table over most of the system and has introducted agricultural contaminants into the groundwaters, most likely by movement through the unsaturated zone. In the late 1990’s, the US Geological Survey NAQWA program began a study of the High Plains aquifer to determine recharge rates and potential contamination problems throughout the system. As part of this program, unsaturated zone studies were carried out at several sites. Included were physical measurements, tritium, stable isotopes of water, and various contaminants in soil waters as well as the concentrations of chlorofluorcarbons in vadose zone gasses. This paper discusses results obtained for unsaturated zone studies at three sites in the central section of the High Plains aquifer in Southwestern Kansas.
Two of the sites are agricultural fields (CAL-121 and CAL-122) that grow row crops and have been irrigated with groundwater pumped from the aquifer system since the mid-1950s.
Initially, irrigation was carried out by flooding of furrows in the field but the methodology was changed in 1990 to a sprinkler system which is still in use today. Information about crop rotation, chemical usage and the quantity of water used for irrigation is available for both fields. A third location (CNG), about 60 miles southwest of the irrigated sites, was choosen as a control site. This location is a grassland and it has never been irrigated or used for agriculture. Groundwater levels have declined at all sites, although the decline has been greater at the two irrigated sites with depth to water ranging between 45-50 meters at the time of the study. Soil cores were collected using an ODEX drilling method at all sites and sealed samples were returned to the laboratories for extraction and measurement of various parameters. Wells were also installed at all sites to sample the local groundwater.
Results from groundwater samples collected at the three sites indicate that recharge to the water table has occurred since the advent of irrigation at the two irrigated locations. Tritium concentrations indicate the presence of bomb-produced tritium in groundwater at both irrigated locations whereas the tritium concentration at in groundwater at CNG was below the detection limit (0.3 Tritium Unit). There were also elevated concentrations of nitrate (up to 24 mg/L as N) and atrazine (0.9 micrograms/L) in the groundwater under the irrigated sites.
Concentrations at the control site were 1.2 mg/L and >0.01 micrograms/L respectively for nitrate and atrazine. As there is no possibility of direct recharge for the groundwater, both the
Tritium analyses on water extracted from core material indicate that water moves through the unsaturated zone at the two irrigated sites much more rapidly than at the control site. Tritium concentrations at the control site dropped from about 7-8 TU to below the detection limit at a depth of about 6 meters. No measureable tritium was found below this depth. Stable isotope results also changed very little below the surface layer at the control site, with 18O averaging about –8.5 o/oo throughout the profile. At the irrigated sites, measureable tritium concentrations were found at various depths throughout the core, indicating that post-bomb water was moving through the unsaturated zone. At both irrigated sites, tritium concentrations varied from below the detection limit to over 4 TU at various depths in the profile, indicating that water was not moving down in a simple piston-like fashion. Water may be moving to areas deep within the core via processes such as macropore flow, or by flow around regions where small soil lenses exist where flow is inhibited. Atrazine was occasionally detected in core material at the irrigated sites, usually close to layers where tritium was also present. Calculations using tritium indicate that recharge rates are much greater in the irrigated areas than at the control site. Estimated recharge rates are 54 mm/yr and 39 mm/yr at CAL-121 and CAL-122 respectively, compared to a recharge rate of < 5.4 mm/yr determined at CNG.
IAEA-CN-104/170 ORIGIN AND RESIDENCE TIMES OF THE GROUNDWATER IN THE MULTILAYERED AQUIFER OF TADLA (MOROCCO)
N. ZINE, A. ZEROUALI, M. TALEB
Direction de la Recherche et de Planification de l’eau, Direction Générale de l’Hydraulique (DGH), Rabat, Maroc
M. QURTOBI, M. IBN MAJAH, H.MARAH, N. SAFSAF
Centre National de l’Energie des Sciences et des Techniques Nucléaires (CNESTEN), Rabat, Maroc
C.B. GAYE, P. AGGARWAL
Agence International de l’Energie Atomique (AIEA), Vienne, Autriche M. KRIMISSA, J.L. MICHELOT
Laboratoire de l’Hydrologie et de Géochimie Isotopique (LHGI), Université de Paris Sud, Orsay, France
The conservation and the good management of the groundwater resources necessitate the identification of the recharge areas and the determination of the mean residence times of the groundwater in the aquifers. These parameters are very difficult to determine by the classic hydrogeological methods, particularly in the multi-layered systems like the Tadla Plain in the central Morocco (Fig.1), which comprises four aquifers to depths of about 600 meters.
A number of hydrogeological investigations including some limited hydrochemical and isotopic studies (BRGM-DGH, 1993; Bouchaou et al., 1995; Bouchaou and Michelot, 1997) have been carried out in the basin over the years. A model of groundwater flow and transport in the aquifers has been developed and is being revised to a smaller grid scale. The model was developed on the basis of the following conditions: (i) recharge occurs in the north and northeast at the aquifer outcrops; (ii) there is no recharge or discharge in the southern part of the Plain and its boundary with the Atlas mountains; (iii) there are hydraulic interactions between the different aquifers; and (iv) all the aquifers discharge at the surface springs in the western part of the Plain.
The objective of the isotope investigations was to provide a better understanding and quantifiable estimates for the hydraulic conditions in order to provide a better simulation of the groundwater dynamics and to optimise resource management for the Tadla Plain. More than 100 groundwater samples were collected from the unconfined and confined aquifers of the basin and analysed for their chemical and isotopic composition.
The multidimensional statistical study of chemical and isotopic parameters using the Main Component Analysis (MCA) indicates the existence of two big groups of groundwater corresponding to the unconfined and the confined zones of the different aquifers.
Distinct patterns of isotope composition are found in shallow unconfined and deep confined groundwaters (Fig. 2). Waters in the different unconfined aquifers have relatively enriched
14 values while those in the confined ones are depleted in stable isotopes (oxygen and hydrogen isotope ratios ranging from –6.9 to –5.9 per mil and – 43.4 and –33.1 per mil, respectively) and their tritium and carbon contents are lower (<1 TU). In addition, waters from the springs in the west present very different stable isotopic signatures (oxygen and hydrogen isotope ratios ranging from –7.48 to –7.25 per mil and – 48.8 and –47.3 per mil, respectively) and high carbon contents.
The isotope results clearly indicate that the main sources of recharge for the different unconfined aquifers of the Tadla Plain are located in the northern outcrops. In the Turonian aquifer, a portion of the recharge may represent an Atlasic percolation from outcrops in the east and southwest. The tritium content in the Turonian aquifer is consistent with recharge from local rain and flood waters while for the Senonian and Eocene aquifers, the low tritium values indicate relatively large travel times in the unsaturated zone. There are also indications of intercommunications between the different aquifers “per descendum” in their unconfined parts and “per ascensum” in their confined parts. In the central part of the Turonian the presence of old waters implies that this zone and does probably not receive any modern water from the Atlas. The very distinct isotopic signatures of the springs (with 18O and 2H signatures of high altitude, probable Atlasic recharge type) invalidate the assumption that these are the natural outlets (discharge zones) of the different aquifers of the Tadla Plain.
These isotope results were used to refine the calibration of the existing model of groundwater flow and transport and it is anticipated that the full integration of the results of this work will enable the revision of the model and contribute to the development of a groundwater resource management model for the Tadla Plain.
REFERENCES:
[1] BOUCHAOU, L.; CHAUVE, P.; MANIA, J.; MUDRY, J. et MICHELOT, J.L: 1995- Apports des traçages artificiel et naturel à la connaissance des circulations et des zones de recharge en région semi-aride: cas de l'Atlas de Béni-Mellal (Maroc).
[2] BOUCHAOU, L.: 1995- Fonctionnement des aquifères atlasique et leur relation avec les aquifères de la plaine : cas de l'Atlas de Béni-Mellal et de la plaine de Tadla (Maroc). Thèse Etat, Univ. Cadi Ayyad, Marrakech.
[3] BOUCHAOU, L. et MICHELOT, J.L.: 1997- Contribution des isotopes à l'étude de la recharge des aquifères de la région de Béni-Mellal (Tadla, Maroc). IAHS Publ. 244, 37- 44.
[4] BOUCHAOU, L., MICHELOT, J.L., CHAUVE, P., MANIA, J. et MUDRY, J.: 1995- Apport des isotopes stables à l'étude des modalités d'alimentation des aquifères du Tadla (Maroc) sous climat semi-aride. C.R. Acad.Sci. Paris, série Iia, 320, 95-101.
[5] BRGM - DGH: 1993- Etude du système aquifère multicouche de Tadla.
[6] HSISSOU, Y.: 1991- L'aquifère des calcaires du Turonien (bassin de Tadla, Maroc).
Alimentations locales et lointaines à partir de l'Atlas. Thèse Univ. Besançon.
Fig. 1 – Situation map and sampled wells
-50 -45 -40 -35 -30 -25 -20
-8,0 -7,0 -6,0 -5,0 -4,0 Plio-Quaternaire de
Béni Moussa Plio-Quaternaire de Béni Amir Eocène libre Eocène captif Sénonien Libre Sénonien captif T uronien libre T uronien captif Sources mixtes DMM Linéaire (DMM)
δ18O (‰ vs V-SMOW)
2 H (‰ vs V-SMOW)
Droite Météorique Mondiale
Groupe des domaines libres Groupe des domaines captifs
Groupe des sources
Fig.1: Relationship between deuterium contents and oxygen-18 contents
IAEA-CN-104/180 APPLICATION OF ISOTOPE TRACERS IN CONTINENTAL SCALE HYDROLOGICAL MODELING
B.M. FEKETEa, P. AGGARWALb, J. J. GIBSONc and C.J. VÖRÖSMARTYa,d
aWater Systems Analysis Group, University of New Hamp, Durham, NH, USA
bIsotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria
cWater and Climate Impact Research Center (WCIRC), University of Victoria, Canada
dEarth Sciences Department, University of New Hampshire, NH, USA
Tracing isotopes in hydrological systems is becoming an important tool for hydrologist to study hydrological processes. Stable isotopes such as 2H and 18O are particularly usefull since these elements are building blocks of the water molecules and behave slightly differently in phase changes and diffusion than regular water molecules. Hydrologists working on small and regional scales have demonstrated the value of stable isotope traces in various application such as distinguishing the source of surface whater (old water from the ground-water pool and new water surface runoff), differentiating evaporation (from open water) from transpiration (from plants), snow and smelt glacier mixing, etc.
Application of isotope tracers at large scale is far behind the regional application mostly due to the lack of isotopic data for large regions. The International Atomic Energy Agency has started a major effort inviting experts and instutions from all over the world to change this situation and promote the collection and distribution of isotopic data about various component of the hydrological cycle. IAEA and WMO (World Meteorological Organization) already established a Global Network for Isotopes in Precipitation (GNIP) and IAEA recently initiated a new effort the Global Network for Isotopes in Rivers (GNIR).
The present paper attempts to utilize these emerging isotopic datasets by incorporating isotope tracing in large scale hydrological simulation. The available precipitation and river isotopic composition data are analysed in simple GIS context to demonstarte the consitency of the isotopic data with other Earth system data such as various climate forcings (air temperature, precipitation, vapor pressure, etc.) land characterisation data (land-use, soil types, river networks, etc.) and river discharge data.. After the initial GIS-based analysis, the isotopic data are tested in a modified version of a well established large scale water balance/water transport modeling system (WBM/WTM). The modified WBM allows the simulation of the fractionation of isotopic composition during the evapotransporation processes, while the WTM is used to simulate the mixing of spatially varying runoff during horizontal water transport and estimate the isotopic composition of the river water in a simulated river network.
The paper demonstrates the use of the isotope tracer enabled WBM/WTM by applying in continental scale study. It highlights the power and the challenges of tracking isotopes in a WBM/WTM context and evaluates the performance of the model. The paper provides recommendation for future efforts to make such simulations more realistic and improve their performance.
IAEA-CN-104/183 ISOTOPIC LINKS BETWEEN THE WATER AND CARBON CYCLES