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Environmental Monitoring by Thomas Kwader, Ph.D., P.G. How “Fast” Does Groundwater Flow? Have you ever wondered how fast groundwater flows through an aquifer? There are many factors that influence the direction and speed in which groundwater flows. There are two main factors that influence the rate of groundwater flow - permeability of the ground sediments and gradient. Permeability is the degree of “connectiveness” of the pores in the formation through which the water flows, or the ease at which the water can move laterally in the aquifer. Gradient is the slope of the water level surface from where the water enters the ground to the discharge area. Groundwater recharge often occurs in a porous sandy area where precipitation can enter the subsurface quickly and travel vertically until it reaches the water table where the water will then tend to travel horizontally and deeper into the aquifer. The greater the gradient (slope of the water) surface, the faster the water will move through the aquifer. Groundwater flow rates can be very slow, as little as inch- es per day, per year, or even slower. The speed of the ground- water is a function of the permeability of the flow zone. Clays generally have a very low permeability due to the very small pore spaces which is also slowed by the “natural cohesion” of water and its ability to cling together, such as forming water droplets on a smooth surface like glass. Also, many geologic materials may have “cementing” properties which plug the spaces between the grain and may even cement the pores “shut”. An example would be dense limestone or cemented sandstones. If the permeability (ability of the formation to transmit water) is very low as with a dense clay, a formation could have a high porosity, but almost zero permeability, thus little movement. The second factor influencing groundwater flow velocity is the slope or gradient of the water surface. If a groundwater table (or piezometric) surface has a gradient of zero or almost flat, there will not be any force available to push or move the water. Generally, groundwater is faster near the recharge areas where rainfall drives the water downward, which in turn hydraulically pushes the water laterally towards a discharge area. Discharge areas can be seepage into a stream, river, or ocean; or discharge can be from wells pumping in an area where the water levels are drawn down compared to surround- ing areas where pumping is low or nonexistent. Groundwater flow rates can range from near zero feet per day to more commonly a few feet or tens of feet per year. Groundwater flow can be very fast in fractured formations such as limestone, dolomite, or shales. Recently, groundwater rates in Florida have been measured using fluorescent dye tracers resulting in the rates of over a mile per day into springs along rivers. Tom Tom Kwader may be contacted via e-mail to michele@worldwidedrillingresource.com 44 MARCH 2019 WorldWide Drilling Resource ® ENV