HR: 0830h
AN: H21E-0892    [PDF]
TI: Ground-Water Capture Zone Delineation of Hypothetical Systems: Methodology Comparison and Real-World Applications
AU: * Ahern, J A
EM: ftjaa@uaf.edu
AF: University of Alaska Fairbanks Water and Environmental Research Center, PO Box 755860, Fairbanks, AK 99775 United States
AU: Lilly, M R
EM: mlilly@gwscientific.com
AF: GW Scientific, PO Box 81538, Fairbanks, AK 99708 United States
AU: Hinzman, L D
EM: ffldh@uaf.edu
AF: University of Alaska Fairbanks Water and Environmental Research Center, PO Box 755860, Fairbanks, AK 99775 United States
AB: A capture zone is the aquifer volume through which ground-water flows to a pumping well over a given time of travel. Determining a well's capture zone aids in water-supply management by creating an awareness of the water source. This helps ensure sustainable pumping operations and outlines areas where protection from contamination is critical. We are delineating the capture zones of hypothetical conceptual models that resemble the Fairbanks, Alaska floodplain both in aquifer parameters and boundary conditions. We begin with a very simple hydrogeologic system and gradually add complexity such as heterogeneity, anisotropy, multiple wells, and zones of permafrost. Commonly-used delineation methods are applied to each case. These include calculated fixed-radius, analytical and numerical models. The calculated fixed-radius method uses a mathematical equation with several simplifying assumptions. Analytical techniques employ a series of equations that likewise assume simple conditions, although to a lesser degree than the fixed-radius method. Our chosen numerical model is MODFLOW-2000, which offers a particle-tracking package (MODPATH) for delineating recharge areas. The delineations are overlayed for each conceptual model in order to compare the capture zones produced by the different methods. Contrasts between capture zones increase with the complexity of the hydrogeology. Simpler methods are restricted by their underlying assumptions. When methods can no longer account for complexities in the conceptual model, the resulting delineations remain similar to those of simpler models. Meanwhile, the zones generated by more sophisticated methods are able to change with changes to the conceptual model. Hence, the simpler methods now lack accuracy and credibility. We have found that these simpler techniques tend to overestimate the capture zone. Water-supply managers must consider such inaccuracies when evaluating the costs of each method. In addition to comparing delineation methods, we are using the series of hypothetical MODFLOW cases to aid in modeling a site within the Fairbanks floodplain. We learn how the site-specific complexities may affect the capture zone by examining the effect of each modification to the hypothetical models. The area is located on Fort Wainwright, AK and consists of multiple water-supply wells. They are screened through a formation known as the Chena Alluvium, which is composed of braided-stream deposits. The wells supply water to a power plant for cooling and to a water treatment plant for drinking water use. The power plant discharges heated water into a cooling pond while the water treatment plant releases back-wash water into a small discharge pond. The Chena River is also nearby, approximately 0.4 km from the wells. Data available from the pumping-well facilities, observation wells, and geologic logs are used to determine input parameters for the model.
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2003 Fall Meeting