WorldWide Drilling Resource

Environmental Monitoring by Thomas Kwader, Ph.D., P.G. How deep is deep enough? In the early 1900s, wells were commonly dug, literally by hand, and lined with various materials to keep the sidewalls from caving in. Some of these wells are reported to have been hundreds of feet deep in the U.S., China, and other places. How deep is deep enough to ensure the water is clean and bacteria free? Generally, wells less than 50 foot deep have limited filtering ca- pacity from bacteria present at land surface. Sands and clays generally provide a good fil- tering medium to reduce turbidity and color of the water. As water percolates from land surface, the conditions are aerobic (oxygen rich) above the water table and become in- creasingly anaerobic or oxygen poor below the water table. This change in temperature and oxygen availability is very effective in killing bacteria and viruses. Septic tanks rely on these principles to effectively purify wastewater by rapidly changing oxygen content, filtering, and actually cultivating its own special type of bacteria (denitrifying bacteria) which thrive on removing nitrates from domestic wastewater. This brings up another point regarding depth and location of a new well; the well should not be located immediately below (downgradient of) a source of groundwater contamination - septic tanks, landfills, petroleum sources, fertilizers, etc. Depth of the well is another important factor. The casing (steel, PVC, etc.) should extend most of the bottom of the well, beyond the depth, to the water level. If the casing is too short, the well could provide a conduit for surface water to travel directly to the bottom where the pump will move the untreated surface water back to land surface. Another geologic factor to consider is hard bedrock - slates, granite, limestones, dolomites, etc. - which have a large amount of fracture type porosity generally have high water flow rates through the fractures and are not good at filtering sediments, or bacteria. If a deeper granular aquifer is available, its use as a water-bearing zone should be consid- ered . Tom Tom Kwader may be contacted via e-mail to michele@worldwidedrillingresource.com 37 WorldWide Drilling Resource ® OCTOBER 2015