Ground-water capture-zone delineation: method comparison in synthetic case studies and a field example on Fort Wainwright, Alaska

Ground-water capture-zone delineation: method comparison in synthetic case studies and a field example on Fort Wainwright, Alaska

Ahern, Julie Anne

Date: 2005-05

Type: Thesisms

Table of Contents: Introduction -- Capture zone delineation background -- Selected capture-zone delineation methods -- Synthetic case studies -- Field case background -- Description of study area -- Geologic framework -- Hydrogeology -- Previous capture-zone delineations on Fort Wainwright -- Field case approach -- Assumptions -- General features -- System parameters -- Model verification -- Model limitations -- Fort Wainwright delineation -- Conclusions and recommendations -- Literature cited.

Abstract:

Ground-water capture zone delineation is an integral part of the recent Source Water Assessments performed nation-wide. Delineations are used to identify where protection from contamination is critical. The objectives of our study were to compare commonly-used methods by quantifying the differences in capture-zone areas and evaluating whether the differences increase with system complexity. We delineated capture zones of hypothetical case studies. We began with a very simple hydrogeologic system and gradually added complexity. Four methods were applied to each case: Calculated fixed-radius (CFR), two analytical solutions (WHPA and UFE-Thiem), and a numerical model (MODFLOW). Area comparisons revealed that, in comparison to the numerical model, CFR consistently overestimated, WHP A underestimated, and UFE- Thiem was variable and the most similar. We then compared the methods in a field case on Fort Wainwright, Alaska. As our numerical method, we used a sub-regional ground-water flow model. Area comparisons were similar to case-study results. Surface-water features were the most influential complexity in the field case. We concluded that each method is only as accurate as its assumptions. Any added margin of error must be appropriate for both the aquifer complexities and required assumptions specific to a given ground-water system.