[en] The stress imposed on the available water resources due to man's impact (exploitation, release of pollutants and agricultural practices) has resulted in depletion of the available reserves as well as deterioration of water quality in many parts of the world. Over wide areas, abstractions are exceeding current natural recharge and it is apparent from scientific studies that these water resources are being mined, especially in arid and semi-arid areas. Sustainable development and management of those water resources needs long term monitoring records to understand the changes and dynamic responses due to the exploitation. These proceedings provide a synthesis of a series of hydrochemical, isotope and geohydrological data sets which will be used for quantitative assessment of the long term dynamic response of the groundwater system. The results show that both stable and radioactive isotopes are excellent tools for characterizing and understanding aquifer systems that are undergoing long term exploitation. Specific outcomes include establishment of methodologies for monitoring and predicting changes in water quality and quantity that will lead to improved water resources management. This publication is a summary of the results achieved during the coordinated research project (CRP) and the various studies performed by the participating institutions are presented as individual presentations. The overall achievements are presented as an executive summary, and the detailed findings are presented in each contribution. These results were presented in the final coordination meeting held in Vienna, 12-16 May 2003. The results obtained from this CRP will be used to improve the predictions of future behaviour of groundwater resources in response to exploitation. The scientific component of this CRP will be a valuable source of information for isotope hydrologists involved in isotope field applications and a useful guide for groundwater managers involved in groundwater resources programmes

[en] It is common to consider that spatial scatter of isotopic composition within fossil aquifers is more significant than variations in time. These observations must be treated with some reserve, since the time of observations is often too short for detecting meaningful changes taking into account the slow rate of water movement in these aquifers. In the other hand, it is also noticeable that intensive uses of groundwater can bring about important changes in aquifer characteristics (flow rate, flow line, ...). In this paper, based on five (5) sampling campaign for chemical and isotopic studies of the 'Complexe terminal' aquifer in the Djerid basin and its borders, we intend to quantify content variation rates for ²H and ¹⁸O according to the exploitation of the aquifer since 1972, date of the first sampling campaign. (author)

[en] This study demonstrates the use of isotope tracers as a tool for monitoring groundwater resources, their depletion and recovery due to a phase of over-exploitation. It is based on spatial hydro-geological data of a Quaternary basin, a three dimensional numerical groundwater flow model and detailed hydraulic, hydrochemical and isotope records over a period of more than 30 years at selected major production wells. Hydrochemical data and tritium values have been used to comprehend the complex flow system throughout this period of exploitation. Tritium values in groundwater samples are influenced by the temporal tritium input function in precipitation, radioactive decay or residence time as well as by the mixing portion of various groundwater components due to exploitation. The mixing portions of groundwater components were dramatically changed by groundwater abstraction, especially during over-exploitation in the late 1960s and early 1970s. The long term tritium record (1969-2001) demonstrates this change for the central part of the aquifer system where main discharge took and takes place. Various snapshots of calculated mean residence times prove increased flow dynamics in the system during the period of over-exploitation and the recovery of the system due to a more sustainable groundwater management. Groundwater hydro-chemistry shows admixtures of NaCl-rich groundwater, that is derived from a waste disposal site. The significant increase in NaCl-concentrations during the 1970s can be attributed to the strongly reduced dilution processes with fresh groundwater from the already over-exploited system. Due to the reduction of groundwater withdrawal, freshwater dilution has improved and the groundwater dynamics have been reduced. The pollutant plume has affected all wells along the flow path into the central cone of depression and is still a problem to groundwater quality. The combination of various investigation methods applied here resulted in a profound understanding of the flow and transport conditions within the studied groundwater reservoir under transient flow conditions over a time scale of more than three decades. The experience of a shallow, fast responding aquifer system, is an encouragement for groundwater managers and experts dealing with (strongly) exploited aquifer systems to use isotope tracer techniques for monitoring purposes and to find adjustment of exploitation to sustainability. (author)

[en] This report demonstrates the possibility of the application of environmental-isotope measurements to construct 'early-warning system (EWS)'. EWS should help to prevent changes of water quality in deep groundwater bodies subject to long term exploitation. A systematic numerical study of flow and transport patterns was performed in system having a depth of 400 m and length of 15 km, and stands for a 'regional aquifer' with stable water divides and constant recharge of 150 mm/y. Results of modelling were shown in more detail for one representative class of multi-layered aquifer. The broader representation underlying the class of models for a shallow aquifer highly exposed to (diffuse-source) contaminations (mainly from agriculture) separated from a deep, 'protected' aquifer by a poorly-conductive layer, with hydraulic conductivity contrasts in the range (from top to bottom) 100:1:10 or 1000:1:10 and with layer height ratios 1:2:5. The anisotropy factor k_z/k_x=4 was assumed. The discharge boundary was approximated by a perfect hydraulic contact over the whole depth underneath the effluent. Generally, it was first more important to obtain qualitative picture of the system's behaviour during, and following, long-lasting water abstractions from the deep layer. In most cases the aquifer was exploited at constant rate for three decades then allow it to relax, and simulate its recovery for further three decades. The abstraction volumes of 10%, 20%, 30% of total recharge were considered. The comparison of turnover balances for various abstraction ratios has shown that, with increasing pumping rate, ever more water is drawn from the upper (contaminated) to the lowest (allegedly protected) layer. Disrupted turnover balances in the presence of pumping were not without effect upon the downward migration of contaminants from the shallow aquifer and it is of particular relevance in the case of persistent components of diffuse-source agrochemicals. It was shown how such a component would, over decades, migrate towards greater depths, and how its migration is accelerated by pumping water out of the deep layer. The computer simulations have shown that ³⁹Ar concentrations at depths of 150-300 m are the adequate indicators for an early warning system. Simultaneously it is shown that tritium measurements can be used for the EWS purposes when the monitoring net is installed in the depths of 100-150 m. (author)

[en] The Southern Negros Geothermal Production Field was put into commercial operation in 1983 initially producing 112.5 MWe and was expanded to the present capacity of 192.5 MWe in 1995. From the start of production until 1988, separated brine is injected to wells located less than 1 km from the production wells. Hydrological connection through permeable structures allowed for the resulted to the immediate return of injected brine to the production wells resulting to thermal declines and consequent reduction in production output. The use of geochemistry and isotopes were invaluable in understanding long term reservoir changes and were used as basis in establishing appropriate strategies. Shift in injection farther from the production wells, controlled injection and prioritization of high enthalpy production wells proved effective to sustain the production capacity of the geothermal field. On the shallow groundwater system of Dumaguete City, the long-term effect of production is not as pronounced compared to the geothermal system. Looking at the interaction of the two systems through the geothermal outflow, mineralized fluids from the natural outflow in Palinpinon extend farther to the east as manifested in the chemistry of wells 55, 49, 53 and 54. Some of the mineralized fluids cross the contact of the volcanic terrane and the Quaternary alluvium, mixes with the groundwater and flows on the subsurface towards the northeast. The mixing of mineralized geothermal fluids in the MDWD wells 55, 49, 53 and 54, possibly through Palinpinon Fault, is a natural occurrence extending from the geothermal outflow in Palinpinon. Further northeast the fluid transport is already governed by convection and dispersion. Long term changes in the shallow groundwater reservoir are controlled only by seasonal variations. (author)

[en] Environmental isotopes (²H, ¹⁸O, ³H and ¹⁴C), hydrochemistry and numerical modelling techniques were used to understand the hydrodynamics, recharge conditions and change in flow pattern due to long term exploitation in the arid regions of northern Barmer and Jalore basins of western Rajasthan, India. It is reported that in these areas groundwater levels are declining at a rate of ∼1m/a because of large-scale abstraction for municipal supply as well as irrigational requirements. In the Barmer basin, the effect of long term exploitation on the deep freshwater lens was studied. Analyses of isotope and hydrochemical data reveal that long term exploitation of deep groundwater has resulted in the influx of more and more brackish and isotopically enriched water having ¹⁴C content :∼27 pMC, which are probably recharged during an arid phase in the past. Different groundwater recharge episodes could also be delineated from the study. Interaction of an ephemeral river with an unconfined alluvial aquifer in the Jalore area was investigated using environmental isotopes and numerical modelling. A three-dimensional transient groundwater flow model was used to interpret the measured isotope and chemical data collected during 1983 and 2000. Analysis indicate that (a) the shallow aquifer receives recharge through river channels during episodic floods caused by intense rain events (b) increased flood events in the recent years have resulted in lowering the brackishness in some places (c) lowered piezometic levels in the deep aquifers induce leakage of relatively enriched younger water from the intermediate zone. The study also helped to identify the affected areas and delineated potential zones for the safe groundwater abstraction. (author)

[en] Fifty-four water samples were collected from sites throughout the Roswell groundwater basin in southeastern New Mexico that were previously sampled in the 1970's. These data sets were used to evaluate the transience of δ¹⁸O values in the basin in hopes of simulating transient conditions with a numerical model. No long term transient trend in the data was established. A compartmental (mixing cell) model was developed and calibrated with the spatial distribution of δ¹⁸O. The model was run as a steady-state simulation and produced estimates of groundwater flow paths, volumetric flow rates, recharge rates and residence times. The results estimate the total annual recharge to the carbonate aquifer as 284 million cubic meters (MCM), slightly lower than previous estimates. Furthermore, the model predicts that 114 MCM of the annual recharge to the carbonate aquifer is provided by underflow from deeper units whose source waters originate in the Sacramento Mountains. Mean groundwater ages in the basin ranged from about 230 to 920 years and were strongly influenced by the fraction of recharge derived from the underflow component. (author)

[en] Alice Springs is located in the arid zone of central Australia. Approximately 10,000 ML of groundwater are taken from the Roe Creek Borefield (∼900 litres/person/day) in the stratified Amadeus Basin. Previous workers used radioisotopes values sampled from groundwater bores to determine recharge rates in the Amadeus Basin. They found evidence of episodic palaeorecharge associated with flooding along the drainage system with two main pulses of recharge at 1400 and 5500 years B.P. The current project resampled the same bores used in previous studies, in order to determine the effect of 25 years of heavy extraction. We found that the primary source of recharge to the Alice Springs area is recent point recharge from the intermittent stream system during rainfall events that exceed 150 mm/month. There is also significant throughflow of groundwater from the regional Amadeus Basin, recharged by rainfall along the margins of the basin. The large depth to water table acts to average out the different pulses of water so that recharge is essentially occurring constantly. The age given by the radioisotopes (∼10-12,000 years) is related to travel time through the unsaturated zone rather than a single flood event. The large drawdown cone is because the groundwater pumping rate exceeds borefield recharge by over an order of magnitude. However, because there are no other major users down gradient of the town borefield, no identified environmental users, and no reasonable alternative source of water in the region the current pumping regime will continue. Implementation of conservation measures as well as spreading the pumping load to the proposed Rocky Hill Borefield would lessen the necessity of continually deepening the Roe Creek Borefield. (author)

[en] Groundwater-surface water interaction, groundwater flow dynamics, and temporal evolution of contaminant concentrations in groundwater and surface water were investigated in a sandy aquifer at Odense, Denmark. Contaminant trends were investigated by evaluating four 30-year long ³H time series from precipitation, groundwater and surface water, and supplemental ⁸⁵Kr, ³H/³He, CFC-12 and SF₆ dating results. The tracer and dating results were used for calibration of a transient 3-D flow model that includes groundwater-surface water interaction. The calibrated model was applied to analyse groundwater-surface water interaction and the general response of the flow system to long-term aquifer exploitation. The applications include simulation of travel times to wells, changes in flow paths and groundwater velocities, water balance, and summer and winter runoff. The results were compared to the general hydrochemical evolution of groundwater and surface water with special emphasis on the nutrients nitrate and phosphate. Results show that significant seasonal variations occur for the content of these nutrients in Odense stream. They furthermore demonstrate that the response of the stream to changes in the isotopic composition and hence the hydrochemistry of precipitation and infiltrating water is fast especially in the winter due to efficient drainage systems. During the winter the observed nitrate concentrations in the stream are 2-3 times higher than during the summer. For phosphate the picture is reversed with summer concentrations being about two times higher than winter concentrations. This illustrates the relative importance of shallow aquifer (drainage systems) and waste water discharge to the stream, respectively. Geochemical modelling demonstrate that the observed sulphate and CaCO₃ increase in groundwater, at an abstraction site abandoned due to this increase, probably is a combined effect of redox and acidification processes in the unsaturated zone induced by lowering of the water table, and nitrate oxidation of pyrite in the saturated zone. (author)

[en] Azraq basin supplies the capital Amman with high quality drinking water, but due to over-pumping from the Amman Water and Sewage Authority well-field (AWSA), tapping the upper unconfined Basalt aquifer; The water level dropped dramatically and signs of salinization and depletion are starting to occur. This study includes hydrological, hydrogeological, hydrochemical and isotope hydrology investigation for the upper unconfined aquifer from the early 80's until present to come up with the causes of groundwater salinization of AWSA wells. The possible salinization scenarios of the upper aquifer could be attributed to: (a) Threat of reverse flow from Sabkha to AWSA well field resulting from low hydraulic gradient of the aquifer water table and increased draw down in the AWSA well field, (b) The probable upward leak of middle aquifer brackish water to the upper aquifer fresh water and the presence of faults that might be conductive, (c) The dissolution processes between water and rock matrix due to lowering the dynamic water levels during pumping which reached the mineralized formations underlying the Basalt. The results indicate that the first scenario is not yet occurring as the hydrodynamic system shows that groundwater flow is still in the direction of Azraq depression. There is no evidence that upward leak from the brackish middle aquifer is taking place; The salinization trend in transient conditions was accompanied by consistent increase in nitrate levels in some wells and high ¹⁴C contents in others while they are both not detected in the middle aquifer. The third scenario is the most likely to occur; The salinization trend of some AWSA wells represented by the gradual consistent increase of EC, Na, Cl, SO₄ and Mg associated with rather constant stable isotopic contents indicate that these constituents originate from the main salts existing in the geochemistry of the lower rock matrix. (author)