Water Well Journal
Issue link: http://read.dmtmag.com/i/740475
I have often been told water well drilling is more of an art then a science. And for the most part, I think that is true, but sometimes we need to incorporate a little bit more science into our art. Having spent a good amount of time around drilling opera- tions and groundwater, I have seen some pretty strange things happen. I can recall a site in central Texas where we drilled almost the entire well without seeing any returns, but once the well was installed it had a tremendous vacuum at the surface. On another project, one of my drill crews drilled into a karst tower. The boring actually blew air until it was plugged. I had the experience of witnessing a deep water well in the moun- tains of southwest Texas blow air continuously, long after it was completed. It still blows like there is an air compressor connected to it. Some of these occurrences are difficult to explain while others are not too hard. For example, I had the misfortune of coming across a sole-source family water well improperly completed by a disreputable contractor where the well was too close to the onsite septic system, recirculating effluent water back into the house. That was an example of a human or contractor-related issue, but what about those situations when we're trying our best and run into something out of the ordinary? As ground- water professionals, it is our responsibility to recognize sub- surface geohazards and understand how those features can affect our product—and ultimately our customers. Geohazard What is a geologic hazard? A geologic hazard is a geologic condition or phenomenon that presents a risk or is a potential danger to life and property, either naturally occurring or man-made. (Bates and Jackson 1984) For the purpose of this article we'll discuss faults, frac- tures, and joints as geologic hazards and how they relate to groundwater and water wells. According to R. Allan Freeze and John Cherry, Ph.D., in their venerable book Groundwater: Faults are a structural feature that may be present on rock slopes, and hydrologically, they can play many roles. Faults that have developed thick zones of sheared and broken rock with little fault gouge may be highly permeable, while those that possess a thin (but continuous) layer of gouge may form almost impermeable barriers. (Freeze and Cherry 1979) A fracture occurs when a formation or rock due to stress exceeds its elastic limit and cracks. A joint is a fracture or parting of a rock without displacement. A series of parallel joints form a joint set and joint sets that intersect are called a joint system. (Bates and Jackson 1984) The ability of a fault, fracture, or joint to move water is proportionate to the feature's aperture—in other words, to how wide the opening and to how much fill material is in the opening. Some geohazards can have openings that are wide and clear, allowing significant amounts of surface water and groundwater to flow through like pipeline conduits, while oth- ers are sealed tight with a fault gouge or debris and act like a flow barrier. Additionally, some may be shallow features only crossing a few feet of formation while others may cross several hun- dreds of feet of formation. Often these features may not materialize and are difficult to recognize in alluvial settings. In hard rock settings, these fea- tures can be pervasive and cover many square miles and affect multiple aquifer systems. Locating Geologic Hazards Prior to drilling a new well, it is often a valuable and re- sponsible first step to view and study aerial images of the pro- posed well location. Many unfortunate problems arising later can be recognized beforehand from a simple overview of an aerial image of the site. Though not all potential geohazards can be recognized from an aerial image, many can. Lineaments or linear features are sometimes tell-tale signs of subsurface anomalies. Typically, vegetation or stream channels running in an unusually straight line or areas where streams suddenly disappear are potential structural features that may be associated with a subsurface geohazard. Also, nat- ural drainage patterns like a stream channel with an abrupt FIELD NOTES continues on page 28 A small fault in a road cut associated with larger regional trend in west Texas. All photos by Raymond L. Straub Jr., PG. WWJ November 2016 27 Twitter @WaterWellJournl