How
does water move through the ground and interact with sediments and rock? Will
an aquifer recharge
slowly or quickly after water is withdrawn, and where will new groundwater come
from? These questions are central for communities that need adequate drinking
water, farmers tending crops and livestock, and engineers working to keep water
supplies free of contaminants. For example, the 1986 trial recounted in the
book and movie A Civil Action focused on town drinking wells in Woburn,
Massachusetts, that were polluted with industrial chemicals suspected of
causing cancer among residents. Plaintiffs asserted—and an investigation by the
Environmental Protection Agency ultimately confirmed—that chemicals dumped by
several local businesses had flowed through groundwater to the
underlying
aquifer and contaminated the wells.
- porosity: the proportion of total volume that
is occupied by voids, like the spaces within a pile of marbles. Porosity is not a direct
function of the size of soil grains—the porosity of a pile of basketballs is
the same as a pile of marbles. Porosity tends to be larger in well sorted sediments
where the grain sizes are uniform, and smaller in mixed soils where smaller grains
fill the voids between larger grains. Soils are less porous at deeper levels
because the weight of overlying soil packs grains closer together.
-
permeability: how readily the medium transmits water, based on the size and shape of its pore spaces and how interconnected itspores are.Materials with high porosity and high permeability, such as sand, gravel, sandstone, fractured rock, and basalt, produce good aquifers. Low-permeable rocks and sediments that impede groundwater flow include granite, shale, and clay.
Groundwater recharge enters aquifers in areas at higher elevations (typically hill slopes) than discharge areas (typically in the bottom of valleys), so the overall movement of groundwater is downhill. However, within an aquifer, water often flows upward toward a discharge area. To understand and map the complex patterns of groundwater flow, hydrogeologists use a quantity called the hydraulic head. The hydraulic head at a particular location within an aquifer is the sum of the elevation of that point and the height of the column of water that would fill a well open only at that point. Thus, the hydraulic head at a point is simply the elevation of water that rises up in a well open to the aquifer at that point.
this figure show you hydraulic headThe height of water within the well is not the same as the distance to the water table. If the aquifer is under pressure, or artesian, this height may be much greater than the distance to the water table. Thus the hydraulic head is the combination of two potentials: mechanical potential due to elevation, like a ball at the top of a ramp, and pressure potential, like air compressed in a balloon. Because these are usually the only two significant potentials driving groundwater flow, groundwater will flow from high to low hydraulic head.
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