Physics Institute, University of Bern, Switzerland
For groundwater management the question of SUSTAINABILITY is getting more and more important. This question contains two components: can the amount of extracted water be kept constant? and; will the concentration of possible contaminants remain below official limits?
The understanding of water budgets and more over how they change in response to human activities is a key issue in this context.
Although forecasts of future evolutions that are based on calibrated numerical flow models are imprecise, they nevertheless may represent the best available decision making information available. The predictions are expected to be surveyed by appropriate monitoring programs.
In the following the question of sustainability will be discussed separately for three age ranges of the extracted groundwater:
a) for water younger than about 50 years
b) for waters in the age range between about 50 years and several thousand years and c) for waters older than several thousand years.
Each time range will be illustrated in the talk by corresponding case studies.
Young groundwaters (age less than about 50 years)
The dominant drinking water resources in Europe are shallow and relatively recent groundwaters. These may be water from river infiltration, water from recent precipitation and infiltration with a considerable recent interaction between unsaturated and saturated zones.
Quality and quantity of the extracted young water may show relatively fast temporal variations. The water composition is usually far away from the natural (baseline) conditions and man made contaminants may already be present in the water. Because of the short timescales involved, effects of overexploitation may even be reversed and a system may recover from contamination within a few years. Therefore the questions of water balance and of sustainability are of minor importance for water authorities. However, predictions on water quality and quantity are still needed; predicting models should include reactions of the flow system, if the extraction rate is changed.
Tools to answer these questions are on one side direct measurements of effective or possible contaminants. On the other side chemical, physical and isotopic parameters are useful for model calibration and for the detection of trends. For instance: nitrates, salt contents, pressure heads and man made isotopes are indicators of a change of the flow system. He-3/H-3, SF6, Kr-85 and CFCs can be used for the detection of changes in the age structure, in the mixing components and in the origin and flow path of the water. Time series of these parameters and their gradients help to quantify on-going changes of the flow system and may induce restriction of groundwater extraction.
Water with residence times between about 50 years and several thousand years
Sustainability for use of waters in this age range is the main criterion for water management:
Man made tracers are absent and the risk of their appearance should be low enough to guarantee high water quality also in the future. Extraction and infiltration rate should be in a steady state. Groundwater managers should be aware of the high quality of these water reservoirs; information and education are important aims.
Calibrated flow- and geochemical models will allow to predict future water quality and to estimate water balances. Time information for the flow models are obtained from Ar-39, C-14 and He-4, whereas the presence of man made tracers like nitrates or Freons would indicate a shift of residence time distribution to the youger side. Monitoring programs and measured time series are useful for early warning: a change of the age structure and/or of the admixed components are clear signals to re-evaluate the model predictions. A difficulty in using chemical tracers are their large natural variations in time and space; the talk emphasises the distinction between real trends and natural variations. In an ideal situations natural “Baseline”
concentrations of chemical compounds remain constant.
Water older than several thousand years
Sustainability is not a good criterion for exploitation of so old waters. Water authorities should be aware that such a reservoir of water with excellent qualities is limited and that the exploitation of these groundwater is very often a “mining of a resource”. Tools for the identification of such old waters should be based on a dating and a climatic information: for dating long lived isotopes like He-4, C-14, Cl-36 and Kr-81 can be used, stable isotopes and noble gas recharge temperatures are main climate indicators. Changes of these tracers are strong signals for water authorities for overexploitation.
IAEA-CN-104/123 ISOTOPE CHARACTERIZATION OF MAJOR RIVERS OF INDUS BASIN, PAKISTAN
M. AHMAD, M.A. TASNEEM, J. A. TARIQ and M.I. SAJJAD
Radiation and Isotope Application Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan
Pakistan lies between latitudes 24° and 37° North and longitudes 61° to 76° East. It possesses quite complicated and attractive physiographical features. There are very often a series of mountain ranges possessing deep broad valleys in-between. It includes the famous valley of the Indus that has been the cradle of ancient civilization like those of the delta area of Nile and the valley of the Tigris Euphrates. Indus River is one of the longest rivers in the World. It has five major tributaries viz. Bias, Satlej, Ravi, Chenab and Jhelum joining from eastern side, while a number of small rivers join the Indus on the right side. All these main rivers are perennial. They originate from the mountains. Physiography and climate of the catchments of these rivers vary widely. Going from the catchment of the River Satlej to the catchment of Indus River, altitude increases and temperature decreases. In Northern Areas, mountains are covered with glaciers and some of the peaks are higher than 8000m, which get snowfall even in summer season. The basic sources of these rivers are snowmelt, rainfall and under certain conditions seepage from the formations. For certain rivers the source of snow is seasonal which falls in winter and melts in summer. From the middle of March to the breaking of monsoon, in mid July, river water is drawn from the melting of snow. During monsoon, rainfall run-off is added to the rivers over and above that from melting of snow so their discharge increases manifold. During 1980-84, samples were collected on monthly basis from the river Satlej at Sulimanki, the river Ravi at Baloki (upstream including Qadirabad-Baloki Link Canal originating from the river Chenab) and Sidnai including two link canals originating from Trimu Headworks just after the confluence of the rivers Chenab and Jhelum, the rivers Chenab at Marala, mixed water of the rivers Chenab and Jhelum, the river Indus at Taunsa and Panjnad (after joining the other tributaries). The samples were analyzed for 18O,
2H and 3H isotopes. This paper presents the isotopic data of and different correlations. Ranges, mean values and δ18O-δ2H are tabulated below.
δ18O (‰) δ2H (‰) Tritium (TU) d-excess (‰) Station
Min Ma Avg Min Ma Avg Min Ma Avg Min Ma Avg
δ18O-δ2H
Suleman -11.3 -7.4 -9.1 -73 -43 -57 25.3 58.0 41.9 12.1 20.9 16.2 δ2H = 7.4 δ18O+10.4 Baloki -11.7 -6.9 -8.8 -71 -36 -53 24. 8 68.0 41.4 10.8 22.4 17.0 δ2H = 6.8 δ18O+ 6.3 Sidnai -11.1 -6.3 -8.6 -54 -74 -37 25.0 78.0 45..3 10.3 19.7 14.8 δ2H = 7.6 δ18O+ 11.4 Marala -12.8 -6.4 -9.7 -79 -35 -59 23.0 88.0 50.1 9.0 20.7 15.1 δ2H = 6.2 δ18O+1.2 Trimu -13.0 -6.8 -9.6 -80 -38 -59 23.9 70.0 44.9 13.2 24.1 18.4 δ2H = 7.1 δ18O+ 9.1 Taunsa -13.5 -8.6 -11.8 -88 -55 -74 36.8 75.3 60.0 13.4 25.6 19.3 δ2H = 6.1 δ18O - 2.7 Panjnad -10.8 -8.6 -9.8 -68 -52 -61 43 64 52 15.8 21.7 17.6 δ2H = 7.0 δ18O+ 7.5 All the rivers have vide ranges of stable isotopes and tritium. The river Indus at Taunsa has relatively the most depleted values of δ18O and δ2H because of major contribution of snowmelt coming from glaciated peaks in Northern Areas. Tritium is also higher due to some contribution of snow fallen during high tritium period in 1960s. Isotopic data of pure snowmelt collected during 1992-94 show that δ18O (-15.9 to -12.2‰) and δ2H (-115 to -82‰) are even more depleted along with still high tritium ranging from 25 to 65TU, which supports
the above finding. Isotopic signatures of the river Indus at Panjnad get enriched due to contribution of other tributaries, which have enriched isotopic values. The rivers Sutlej and Ravi have he most enriched values of δ18O and δ2H because their catchments have relatively low altitude and contribution of snowmelt is also less. River Chenab at Marala has the widest ranges of δ18O and δ2H because of mixing of snowmelt originating from higher altitudes and rainfall of piedmont areas. Data of Trimu, which show the combined effect of the rivers Chenab and Jhelum is almost similar to that of Marala. The δ18O and δ2H monitored at both of these stations i.e. Marala and Trimu during 1990-93 have average values of -10‰ & -61‰
and -9.4 & 59‰ respectively, which are slightly different than the previous record. It also observed that temporal variations of both the δ18O and δ2H in rivers are cyclic especially in the rivers Indus, Jhelum and Chenab (Fig.1 and Fig.2) depending on the contributions of snowmelt and rains i.e. enriched during monsoon. The δ18O and δ2H data also give information about source of moisture. The winter runoff and snowmelt have relatively depleted isotopic signatures and higher d-excess indicating the source of moisture from the West (Mediterranean Sea) while the d-excess in monsoon is relatively less along with enriched isotopic values, which is also confirmed by the meteorological information. The correlations between δ18O and δ2H for all the give the slope lees than 8, which is mainly due to high d-excess in winter precipitation or snowmelt and low d-excess of monsoonal/summer rains. The lowest slopes for Taunsa at Indus and Marala at Chenab (Fig.3) are attributed to high variations in discharge contributions of resulting from both the sources of moisture through precipitations.
-15
-14
-13
-12
-11
-10
-9
Time (Months)
18 O (‰)
1200 1250 1300 1350 1400 1450 1500 1550 1600
Lake Level
Fig.1. Temporal variation of δ18O of River Indus (outflow from Tarbela Dam Reservoir) and lake level (Sep. 1987 to May 1997)
-13 -12 -11 -10 -9 -8 -7 -6 -5
Oct-83 Feb-84 Jun-84 Oct-84 Feb-85 Jun-85 Oct-85 Feb-86 Jun-86 Oct-86 Feb-87 Jun-87 Oct-87 Feb-88 Jun-88 Oct-88
Time
18 O (‰)
River Chenab River Jhelum
Fig. 2. Temporal variation of δ18O of Rivers Chenab and Jhelum
-90 -80 -70 -60 -50 -40 -30
-12 -11 -10 -9 -8 -7 -6
δ18O (‰) δ2 H (‰)
River Chenab at Marala Average
GMWL
Fig. 3. δ18O and δ2H of the river Chenab at Marala Barrage
IAEA-CN-104/132 ASSESSING AQUIFER CONTAMINATION VULNERABILITY USING TRITIUM- HELIUM AGES IN PUBLIC DRINKING WATER WELLS IN CALIFORNIA, USA J.E. MORAN, G.B. HUDSON, R. LEIF, and G.F. EATON
Lawrence Livermore National Laboratoy, Livermore, CA, USA
We have sampled over 800 public drinking water wells as part of a study to assess relative contamination susceptibility of the major groundwater basins in California. This project is sponsored by the California State Water Resources Control Board, and is carried out in collaboration with the U.S. Geological Survey. The parameters used to rank wells according to vulnerability are groundwater age dates (using the tritium-3helium method), stable isotopes of the water molecule (for water source determination), and occurrence of low level Volatile Organic Compounds (VOCs). The project, carried out in collaboration with the US Geological Survey, uses these observational data in a probabilistic approach to assess the vulnerability of public water supply wells to contamination by anthropogenic compounds.
Sources of contamination to groundwater occur near the earth’s surface, and have been present mostly since World War II. Therefore, wells that receive water that has recharged in the recent past (young groundwater ages) are more likely to intercept contaminants transported by advection.
Because of the large number of samples collected, the major basins used for drinking water supply can be compared and contrasted in terms of relative vulnerability. A large volume of both imported and locally captured water is artificially recharged in these urbanized, intensively managed basins. Imported recharge water from higher elevations is identified by lighter δ18O signatures, while the downgradient flow pattern of recent recharge is outlined by increasing groundwater age. The presence of a continuous confining unit can be the key feature for protecting deep aquifers in areas with ubiquitous surface contamination. For example, an effective confining unit in the Santa Clara Valley basin (Silicon Valley) prevents widespread vertical transport of contaminants down to drinking water wells. In the Los Angeles County and Orange County basins of southern California, groundwater age and the frequency of occurrence of low-level VOCs are spatially correlated, with more recently recharged water likely to have VOC detections. In particular, the gasoline additive methyl tertiary butyl ether (MTBE), which has had widespread use only in the last decade or so, occurs only in the youngest groundwater. ‘Pre-modern’ water is nearly always free of VOCs, except when a suspected ‘short circuit’, (e.g., loss of integrity in well casing) allows near surface contamination to reach ‘old’ water.
Long-screened production wells used for public drinking water supply clean, high quality samples, and sample the resource that is being exploited. However, the groundwater age distribution from production wells may be quite broad, and comparisons to the predicted initial tritium value for the measured mean age, along with analysis of radiogenic 4Helium are used to de-convolute the mixed age. The tritium that was present at the time of recharge is well defined from measurements of tritium in precipitation. A groundwater sample for which the measured age gives a decay-corrected tritium value that falls on or near the curve, does not have a significant component of water that dilutes the tritium measured. Samples that fall below the ‘initial tritium’ curve contain a fraction of water that recharged before 1955 (‘pre-
The analysis of the age distribution of groundwater at production wells allows estimation of the dilution factor for a contaminant introduced in the young water fraction, and furthermore provides a predictive capability for the future trend to be expected for the contaminant. Water resource managers can use these vulnerability assessments to focus monitoring efforts, site new wells, plan land use, and evaluate remediation activities.
0 100 200 300 400 500
1960 1970 1980 1990 2000
Recharge Year
3 H at recharge (pCi/L) GW samples0% pre 1955
50% pre 1955 75% pre 1955 90% pre 1955
From Talbert Gap
"W ater Factory 21"
FIG. 1. The zero percent pre-modern curve shown represents tritium measured in precipitation over the last several decades. Curves for mixtures containing equal fractions pre-modern water are shown for reference. Well water samples from public supply wells in southern California, USA, are plotted according to the mean tritium-helium groundwater age measured, and the corresponding expected initial tritium value at the time of recharge. Many wells draw a large fraction of pre-modern groundwater. Two samples above the curves indicate the presence of a local industrial or medical tritium contribution.
This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W- 7405-ENG-48.
IAEA-CN-104/135 HYDROLOGICAL PROCESS STUDIES USING COSMIC RAY PRODUCED RADIONUCLIDES
D. LAL
Scripps Institution of Oceanography, Geosciences Research Division, 0244, La Jolla, CA 92093, USA
The field of hydrology is presently assuming new dimensions in response to dramatic advances made in our understanding of the evolution of past climates. It is now apparent that even changes in the solar flux alone do not regulate climates. To understand mechanisms of climate regulation, it is essential to fully understand the role played by the water and carbon cycles, and therefore also, the characteristics of the principal reservoirs where water and carbon are stored; the atmosphere, biosphere and the hydrosphere. How water is transported through these reservoirs and what factors regulate the transport of water, are the central questions which must be answered to understand the global hydrological cycle, which are a prerequisite to understand climatic changes.
The global hydrological cycle is the vastly complex dynamic of transfers of all phases of
‘water’ molecules, gas, liquid and solid, through the atmosphere, land and the oceans. Its complexity ultimately owes itself to the properties of the phase diagram of water in part, and in part to the appreciable latent heat involved in the change from one phase to the other.
Transport of water is therefore closely linked with the transport of energy. The operation of the hydrological cycle is therefore closely related to the climate; climate controls the hydrological cycle, and as the cycle evolves with climate, it produces strong feedbacks which regulate the climate in an important manner. An example of extreme coupling between the hydrological cycle and climate is the concept of ‘Snowball Earth’, which could have happened in climates different from what we are used to thinking about!
Tracers provide a convenient way of obtaining space-time integrals of motion of ‘water’
molecules over both large and small range of space and time. It is now well recognized that a large number of cosmic ray produced isotopes are used for dating and tracing components of geological/geophysical/geochemical systems. Some of the tracers are well suited to study the time scales involved in the transport/mixing of water molecules since they follow the motion of water molecules. Others, which do not follow the motion of the water molecules (as a result of their removal from the fluid by biogeochemical processes), serve as useful tracers to characterize properties of the reservoir.
Both natural and artificial, stable and radioactive nuclides are employed as tracers. Their usefulness in hydrology has been demonstrated since the emergence of the field of isotope geochemistry in the fifties. The earliest applications of isotopes as tracers was the identification of a clear-cut isotopic relationships between 18O and 2H in worldwide fresh surface waters, arising due to isotopic fractionations occurring in the operation of the hydrologic cycles (Craig, 1961). The relationship, called the “ global meteoric water line” is only global in application; it is the average of several regional meteoric water lines, and emphasizes the central point that meteoric waters evolve in a predictable fashion, and that therefore one can model the water trajectories. Subsequently both naturally and artificially
Earth, and (ii) radiogenic and nucleogenic processes associated with the 238,235U and 232Th.
Artificially produced radionuclides are released from testing of nuclear weapons and operation of nuclear reactors.
Each class of isotopic tracer has its own niche, providing detailed information on specific processes, and time scales. For obtaining information about the hydrological cycle in its operation through different scales of space and time, it becomes necessary to simultaneously study two or more tracers. In fact any model(s) used to treat a tracer data has to be consistent with the information based on all the tracers. Thus whilst it may not be apparent in some cases, multiple tracers are indeed often being deployed to study hydrological cycles.
In this presentation, we will consider the applications of the cosmic ray produced (cosmogenic) isotopes in hydrology, with a brief discussion of the history of their applications, current applications, and finally the promise of cosmogenic isotopes in hydrology.
Cosmic rays produce nine radionuclides of half-lives ranging between 10 years and 1.5 my, and five, between 2 weeks and 1 year (Lal and Peters, 1967). Useful radionuclides are also produced directly in the oceans by cosmic ray interactions Lal et al. (1988). In Table 1, we present a broad overview of the applications which the cosmogenic nuclides have found to date. With increase in sensitivity of measuring small amounts of isotopes, the field of cosmogenic tracers in hydrology is expanding, finding new applications in hydrology, including studies of ground water infiltration rates and soil dynamics (Lal, 2001). It should be realized that the cosmic ray source function is a very weak one! The global average flux of cosmic ray nuclei at the top of the atmosphere is ~ 1/cm2.sec. The incident energy flux is ~ 5 x 10-3 cal/cm2/yr, which is ~ 4 orders of magnitude smaller than global heat flow, and more than 8 orders of magnitude smaller than the solar flux. However, the mean energy per cosmic ray particle is much larger, and sufficient to induce nuclear reactions in matter, causing significant changes in the composition of matter, and thereby injecting tracers in the geospheres. And it is this reason that even the weak cosmic ray source is able to make a significant contribution to earth sciences (cf. Lal, 1991).
Finally, in closing it must be pointed out that tracers, stable or radioactive are not a
“panacea’’; their proper utilization requires a very good understanding of the characteristics of the system under study. This information has to be obtained through studies of conventional hydrological methods and that based on distribution of stable isotopes. Tracer radionuclides modeled appropriately provide information on time scales and rate constants.
Naturally produced radionuclides have an important attribute in that their source functions are well known. However note that in some cases, very useful information can be obtained by pulsed injections of tracers, as in the case of radionuclides injected in the atmosphere by nuclear weapons’ testing. Overall, since one employs tracers using “black box” models, representing the mixing and exchange properties of atmosphere and the hydrosphere, it is necessary to employ multiple tracer types to validate the results of tracer models (cf. Phillips 1994). And it is indeed gratifying to see that generally we have available a great variety of tracers available in hydrological studies.