WorldWide Drilling Resource

23 WorldWide Drilling Resource ® JUNE 2013 Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Natural Groundwater Gradient - Use It When You Need It Sometimes we forget groundwater is constantly flowing, both laterally and vertically, some distance every day. The speed or velocity can range greatly from almost 0 in deep zones that do not have an area of discharge, to over 1000 feet per day in cavernous lime- stone formations discharging to open springs. “Average” velocities are more often on the order of 0.1 feet per day up to 1.0, or possibly 10 feet per day. The speed is often faster near pumping wells or discharge areas such as seeps or springs. The direction and speed of ground- water should be taken into consideration in the design of groundwater recovery systems intending to remove contaminat- ed groundwater plume(s). A groundwater recovery system is much more efficient if the pumping well is down-gradient of a migrating plume than trying to reverse the natural flow to an up-gradient or side-gradient direction. For instance, assume groundwater is flowing south at 1.0 feet per day and the capture width of a pumping well is 100 feet wide, perpendicular to the flow direction from the north. Also assuming the aquifer is 75 feet thick and has a porosity of 25%, the volume of water passing naturally through the 100-foot- wide area is equal to: (100 feet x 75 feet x 1 foot per day x 7.48 feet per gal- lon) x 25% (porosity), or 14,025 gallons per day. This is equal to pumping 1.0 gallons per minute, continuously, just to capture the volume flowing naturally through this area. If you need to pump against the gradient, to the north, it would take about 2 gallons per minute just to reverse the gradient at the natural flow rate. The natural flow gradient should be considered for the placement of a ground source heat pump geothermal system using open circulation supply and return injection wells. By taking advantage of the natural groundwater flow direction, the distance between the supply well and the injection well can be much smaller, which will lessen the probability of having the heated water travel to the supply well. Another factor in recycling water used in geothermal systems, is to return the water into different water-bearing stra- ta, which are not hydraulically connected, to prevent the water from flowing back to the supply well. Tom Tom Kwader may be contacted via e-mail at admin@ worldwidedrillingresource.com Kwader trans. cont’d on page 26. Make plans to participate in WWDR ’s annual history celebration! Call for details. 850-547-0102