Comparing model-derived to independently-derived estimates of the source and age of groundwater, Idaho National Laboratory and vicinity, Idaho
Abstract
The U.S. Geological Survey, in cooperation with the U.S. Department of Energy, evaluated a three-dimensional model of groundwater flow in the fractured basalts and interbedded sediments of the eastern Snake River Plain aquifer at and near the Idaho National Laboratory to determine if model-derived estimates of groundwater movement are consistent with: (1) results from previous studies on water chemistry type, (2) the geochemical mixing at an example well, and (3) independently-derived estimates of the average linear velocity. Simulated steady-state flow fields were analyzed using backward particle-tracking simulations that were based on a modified version of the particle tracking program MODPATH. The model adequately simulated the areal shape of the 5 microgram per liter lithium contour interpreted to represent the transition from a water type that is primarily composed of tributary valley underflow and streamflow infiltration recharge to a water type primarily composed of regional aquifer underflow. Inconsistences between model- and independently-derived estimates of water type separation in the southern portion of the model area may indicate model error, where unreasonable estimates of hydraulic conductivity result in an over displacement of tributary valley water by the relatively fast moving regional aquifer water. Sources of water at well NPR-W01 were identified using backward particle tracking, and they were compared to the relative percentages of source water chemistry determined using geochemical mass balance and mixing models. The particle tracking results show a good comparison with chemistry results for surface water sources (10 percent difference); however, regional aquifer sources were significantly overestimated by the model. The water samples used in the chemistry analysis were collected from an isolated depth interval in the upper 10 feet of the aquifer, an area likely absent of deeper regional groundwater; whereas, the simulated particles were released in the upper 200 feet of the aquifer to account for a modeled spatial discretization that is too coarse to precisely represent the dimensions of this sampling zone. Independently-derived estimates of the average linear velocity of the young fraction of groundwater within the upper 100 feet of the aquifer were compared to model-derived estimates of the average particle velocity at 23 well locations. A comparison was only possible where velocity estimates were based on equivalent geographical source areas; 12 of the 23 wells met this criterion. Discrepancies between source areas were attributed to: (1) independently-derived estimates of travel time that were only available for young groundwater, (2) the insensitivity of particle tracking simulations to weak-source cells, and (3) model error. Differences between velocity estimates for equivalent source areas ranged from -9.6 to 36.1 feet per day, indicating good agreement; although, velocity differences of 29.9 and 36.1 feet per day for wells USGS 1 and 100, respectively, may indicate an overestimation of the hydraulic conductivity for hydrogeologic zone 1.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.H33H1421F
- Keywords:
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- 1829 HYDROLOGY / Groundwater hydrology;
- 1847 HYDROLOGY / Modeling