WorldWide Drilling Resource

Notes from the Groundwater Guy by Thomas E. Ballard, P.G., C.H.G. Southeast Hydrogeology, PLLC Declining Well Production All wells will experience performance declines over their lifespans, some wells more rapidly than oth- ers. For instance, wells in alluvial deposits, while generally more productive than bedrock wells, as a rule tend to be more prone to well performance issues and require more frequent rehabilitation. The major causes of reduced well performance over time are as follows: Filter Pack Plugging: Wells typically have a one-way flow of groundwater into the well which, over time, results in the mi- gration of fine-grained material such as silts and clays into the filter pack, the accumulation of which eventually impedes ground- water flow into the well. Smaller sand particles can often form sand bridges over time for the same reason. These situations result in increasingly inefficient flow of water into the well and even full plugging of sections of the filter pack in some instances, resulting in a decline in well performance. Mineral Incrustations: Depending on the chemistry of the groundwater and well de- sign factors, mineral incrustations can develop on the screen and even in the filter pack. Calcium carbonate, iron, and manganese deposits are the most common types of mineral incrustations seen in wells, but there can be other types, as well. Mineral incrustations can develop as a result of turbulent flow in the filter pack and at the well screen, velocity drops at the well screen, and oversaturation of minerals in the groundwater. There is evidence that lower quality screens may be more subject to the development of mineral incrusta- tions. Biofouling: It is estimated that biofouling is involved in more than 80% of all well plugging cases. The usual suspect in biofouling situations is iron-reducing bacteria, which grows well in the aerobic zone of the well. Iron-reducing bacteria form a protective biofilm which can often shield the bacteria themselves from straight chemical applications, such as chlorine. In the aerobic environment of the well, iron-reducing bacteria can thrive and reproduce to the point they can completely plug well screens. Many times, sulfur-reducing bacteria can grow under the iron-reducing bacteria, and in anaerobic zones of the well. The danger of sulfur-reducing bacteria is less to do with plugging of well screens, rather they produce hydrogen sulfide gas, which is highly corrosive to the well. Indicators of iron-reducing bacteria are increased iron concentrations in the well water over time, and indications of sulfur-reducing bacteria are “rotten egg” odors from the hydrogen sulfide gas. Less common bac- teria-related performance issues can also arise, so it is important to monitor the total bacteria trends in a well. A Note on Open-Hole Bedrock Wells: Open-hole bedrock wells are not immune from developing well performance issues over time. Iron-reducing bacteria can still be an issue, for instance. Mineral incrustations usually take the form of deposits which can reduce the size and productivity of the fractures supplying water to the well. Over time, productive factures can squeeze down due to overlying rock pressure, reducing groundwater flow through those fractures. Due to the volumes of water which can pass through the fracture and solution channels in karst areas, silt and clay accumulations can cause plugging to the point where flow can be reduced. Some or all of these problem situations can occur in the same well, so it is important to use proper diagnostic tools to de- termine the full cause of reduced well performance before deciding on an appro- priate course of action for well rehabil- itation. Tom Tom Ballard may be contacted via e-mail to michele@ worldwidedrillingresource.com We have stainless steel and metal valves, as well as plastic cycle valves! For more info call: (800) 364-2124 or visit us online: www.barrettsupply.com Proudly made in the USA Sales rep territories available. 70 DECEMBER 2017 WorldWide Drilling Resource ® The magazine that you send to me is a great tool for my business. There I found suppliers [to keep] running my business. Arturo Celaya, Guanajuato, Mexico The WWDR Team loves to hear from our subscribers!