WorldWide Drilling Resource

Environmental Monitoring by Thomas Kwader, Ph.D., PG Factors Affecting Groundwater Recharge Rates and Water Levels Have you ever wondered after a heavy rain how much the water level will rise in a well? To accurately estimate the water level rise, a number of factors must be known or estimated as follows. We will first make a few qualifying assumptions: A) The well is relatively shallow and the sediments are relatively permeable from land surface to the water table. B) The land surface soils are relatively flat, and let’s assume about 20% of rainfall runs off to nearby creeks or streams. C) There are no other large pumping wells nearby (>0.25 mile). Let’s also assume a 10-inch rain over a one-week period for ease of calculation. What portion of this 10-inch rain will reach the water table? 1) Of the 10 inches hitting the ground, 2 inches will run off as surface water. 2) 1 inch will saturate the pore spaces between land surface and the 25-foot water table. 3) 1 inch will be absorbed in the root zone by the plants and trees in the area. 4) ½-inch will be evaporated by the sun heating the soil after the rain until the next “rain event”. This would leave 5½ inches to percolate downward to the 25-foot water table. So how much would the water level rise in the aquifer - 5½ inches? No. Assuming an effective porosity of 25%, every 1 inch of percolating water would cause the aquifer to rise 4 inches in the well. [5½ inches x 4 would equate to a rise of 22½ inches] There are other factors to consider relating to the water level in the well. Water in most aquifers is “draining” constantly, even during rainfall events. A typical lateral drain rate could be ½-inch to more than 2 inches per day, depending upon permeability, temperature, or slope of the water table. Note the loss is near 0 inches per day when the water level is “flat” (drought conditions). Factors favoring water level rise include: streams providing recharge through their bed bottoms, ponds and lakes with leaky bottoms, large-scale irrigation (from deeper aquifers), etc. Also, storm events occurring in cold periods (low evaporation) which last a long time (days) recharge groundwater much more than summertime “flash floods” which tend to provide more surface water runoff. Tom Tom Kwader may be contacted via e-mail to michele@worldwidedrillingresource.com VirginiaWaterWell Association 2018 Winter Conference Double Tree by Hilton - Koger Center Midlothian/Richmond, Virginia Office: (804) 387-8395 Info@VAWaterWellAssociation.org www.VAWaterWellAssociation.org WorldWide Drilling Resource ® 7 JANUARY 2018