Evaluation of recharge in selected aquifer systems of the United States using tracers of groundwater age (Invited)
Abstract
Well constrained water budgets are needed to assess groundwater availability and manage aquifers sustainably 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. Understanding of recharge could be improved through an analysis of groundwater age data because groundwater age distributions integrate recharge processes at relatively large spatial and temporal scales and can be direct measures of recharge. In this presentation, we synthesize existing datasets of groundwater age into a consistent analysis of the timescales and rates of recharge at a national scale. Timescales of recharge, as defined by 2,746 distributed tritium measurements and 224 radiocarbon measurements from 18 flow path studies, varied widely across the United States and were dependent on factors such as aquifer confinement, climate, geologic features like karst conduits and fractures, and changes in sea level. On average, 78 % of sampled flow path lengths in confined aquifers contained pre-Holocene recharge compared to 23 % in unconfined aquifers. Age distributions in aquifers determined from 650 sulfur hexafluoride, chlorofluorocarbon, and tritium/helium-3 measurements from 25 flow path studies and 27 water-table well networks were used to calculate recharge rates for young (0-50 year) groundwater. Recharge rates of young groundwater ranged from 34 to 1,200 mm/year and were dependent on factors such as climate, geology, land use, and topography. Recharge was inversely related to air temperature for a group of 10 flow path studies receiving similar precipitation from South Carolina to Massachusetts, presumably because of the diminishing effects of evapotranspiration on recharge with decreasing temperature. Recharge was directly related to precipitation for a group of 13 flow path studies having similar air temperatures from Massachusetts to Oregon. Regional characterization of climatic conditions associated with recharge may provide a baseline for understanding how recharge might change in response to future climate scenarios. Comparison of groundwater age-based recharge estimates to a national map of natural recharge based on stream base-flow separation (Wolock, 2003) showed similar overall patterns but substantial local differences, for example in areas of irrigated agriculture where the age-based estimates were larger. The two methods have important differences in what they represent so local comparisons could help resolve distinctions between different definitions of “groundwater”, “recharge”, and “base flow”. Also, groundwater age data provide additional information, for example about residence times. Results from this study show that groundwater age-based recharge estimates can provide useful insights into the spatial and temporal variability in recharge at a national scale and the factors controlling that variability. But gaps exist in the spatial distribution of groundwater age studies at a national scale, particularly with respect to their representation of certain climatic, geologic, and land use conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H14D..06M
- Keywords:
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- 1829 HYDROLOGY / Groundwater hydrology;
- 1876 HYDROLOGY / Water budgets