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Use of Ground-Water Ages to Assess Recharge

Congress: 2008

Keyword(s): recharge, aquifer sustainability, ground-water ages
AbstractWell constrained water budgets are needed to accurately assess ground-water availability in aquifers throughout the world. Recharge is perhaps the most difficult water-budget component to quantify because of its spatial and temporal variability and because it is difficult to measure directly. There are several reasons why an analysis of ground-water age data could improve our overall understanding of recharge and thereby improve ground -water availability studies. Ground-water ages define timescales for recharge processes that could be used as an indicator of aquifer sustainability. Ground-water age distributions may be considered to be direct measures of recharge, whereas many other techniques for estimating recharge are not. Recharge estimates based on ground- water ages could help constrain other methods. Finally, characterization of climatic conditions associated with age distributions may provide a baseline for understanding how recharge might change in response to future climate- change scenarios. These applications of ground-water age data to understand recharge are demonstrated using existing data from a diverse group of aquifers in Africa, Asia, Australia, Europe, North America, and South America. The synthesis effort focuses on two groups of ground-water age data––modern atmospheric tracers such as CFC, SF6, and 3H/3He (roughly representing the past 50 years) and 14C in dissolved inorganic carbon (roughly representing 1,000 to 30,000 years before present). The analysis is supplemented with modern climatic data and data for proxies of paleoclimate such as δ18O and noble-gas recharge temperatures to characterize recharge and climate for a range of hydrologic and climatic conditions. As an example of the analysis, an unconfined sand aquifer in the humid North American mid-continent had an estimated recharge rate of 150 mm/yr on the basis of CFC-12 apparent ages and more than 90 percent of the water in the aquifer was recharged in the last 50 years. Mean annual air temperature (MAT) for the period of recharge was 5.7°C and mean annual precipitation (MAP) was 740 mm. In constrast, an unconfined sand aquifer in semi-arid western North America had an estimated recharge rate of 15 mm/yr on the basis of 14C apparent ages and more than 90 percent of the water in the aquifer was recharged more than 1,000 years ago. MAT for the record of recharge ranged from about 5.0°C in the Late Pleistocene to a modern mean of 8.4°C and modern MAP is 510 mm. In general, results from this study indicate that a systematic analysis of ground-water age data from important aquifers throughout the world could provide a framework for assessing ground-water availability now and in the future in response to various climate-change scenarios.
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