Groundwater Movement and Recharge

Groundwater is water that occurs below the surface of the ground, where all the pore spaces in the subsurface material are completely filled with water.  Groundwater moves relatively slowly (ranging from several feet per day down to feet per year) and moves downward from higher to lower water table elevations. The age of groundwater (the length of time it spends in the subsurface) ranges from decades to millenia.  Ultimately, it discharges to streams, lakes, wetlands, and the ocean.  This natural discharge is called base flow and is important to many ecosystems.

Recharge is water that infiltrates the lands’ surface and percolates downward to the underlying groundwater system.  It is the source of all groundwater.  Recharge rates are highest in areas with high rainfall, high permeability soils, minimal slope, and sparse vegetation.  The following figure shows the relationship between rainfall and recharge in two typical geologic deposits.

The upper surface of most groundwater systems is the water table.  The elevation of the water table can be measured or predicted at any geographic point through monitoring wells.  Water depths in wells are measured relative to a surveyed measuring point at the top of the well casing (the pipe that transmits groundwater to the land’s surface).  These elevations are then used to produce a map that shows the slope of the groundwater table.  The water table map is a series of contours of equal elevation that resemble land topographic contours.  Groundwater flows perpendicular to the water table contours and moves from areas of higher water table elevation to areas of lower elevation.

The rate of groundwater flow varies tremendously, from a few feet per year in low permeability materials, such as silt or clay, to a few feet per day in high permeability materials, such as sand and gravel.  Faster flow rates are also possible in limestone and in fractured rock.

Calculating Flow Velocities

Flow velocities can be estimated using a simple mathematical equation based upon Darcy’s Law, which governs the movement of groundwater, as follows:

V = Ki/n

Where V = the average velocity of groundwater flow (can vary from a few feet per year in low permeability materials, to a few feet per day in high permeability materials).

K = the permeability of the aquifer (commonly 100 to 500 feet/day in sand and gravel aquifers). 

i = water table slope or hydraulic gradient (measured from water table map; commonly 0.2 to 0.3 percent slope).

N = porosity of the aquifer (the percent of void space filled with groundwater; 0.25 to 0.35 percent for sand and gravel).

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