DELINEATION OF WELLHEAD PROTECTION AREAS IN FRACTURED ROCK – ADVANCES DURING THE LAST 10 YEARS

Over ten years ago (1991) we helped prepare a USEPA Technical Guidance Document entitled Delineation of Wellhead Protection Areas in Fractured Rocks. This document evaluated approaches for the delineation of wellhead protection areas in unconfined fractured rocks and summarized techniques in use by practitioners at that time. These techniques included vulnerability mapping, flow-system mapping, geochemistry/isotopes, and numerical modeling using porous-media methods. Now, over ten years later, a re-evaluation of these techniques and of advances in the field seems appropriate.

We believe the methods of fractured-rock aquifer evaluation have advanced in five key areas. First, improved borehole geophysical methods, especially methods using flowmeters and their interpretation using dynamic flowmeter logging, allow much improved analyses of hydraulic properties and better correlation of heterogeneities between wells. Second, improved field instrumentation, including new types of water-level, temperature and electrical conductivity sensors and dataloggers, allow us to acquire more accurate and abundant field data at lower cost than ever before. Third, geochemical and isotopic methods for groundwater dating and source area verification continue to evolve. Fourth, we have improved porous-media modeling tools, including both graphical user interfaces (GUIs) for existing models and analytic element techniques for rapid modeling of large areas. The improved GUIs allow better incorporation of thin high-hydraulic conductivity features into models and the more efficient use of stochastic methods for evaluating uncertainty. And fifth, we now have improved fracture analysis tools and fracture network models, and a willingness of more practitioners to employ these techniques.

In spite of these methodological advances, the delineation of capture zones in fractured-rock terrain is often still more art than science. Fracture dynamics within the pumping well are rarely considered. Complete analysis of fracture networks around the well is rarely done, and fully-developed fracture flow models are never constructed. There is still no good way to verify the mapped contributing areas, and the resulting uncertainty is high. Application of these new techniques to capture zone delineation remains a fruitful area for hydrogeologic research.