Unit 8: Water Resources // Section 4: Groundwater Hydrology: How Water Flows
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 Will an aquifer recharge slowly or quickly after water is withdrawn, and where will new groundwater come from?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 (footnote 3).
The pore structure of soils, sediment, and rock is a central influence on groundwater movement. Hydrologists quantify this influence primarily in terms of:
- 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 its pores 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 (Fig. 5). 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.
Figure 5. Groundwater flow under the Housatonic River, Pittsfield, Massachusetts
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Source: © United States Environmental Protection Agency.
The 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.
This theory works in the same way that electrical potential (voltage) drives electrical flow and thermal potential (temperature) drives heat conduction. Like these other fluxes, groundwater flux between two points is simply proportional to the difference in potential, hydraulic head, and also to the permeability of the medium through which flow is taking place. These proportionalities are expressed in the fundamental equation for flow through porous media, known as Darcy's Law.
The gradient in hydraulic potential may drive groundwater flow downward, upward, or horizontally. Hydrogeologists collect water levels measured in wells to map hydraulic potential in aquifers. These maps can then be combined with permeability maps to determine the pattern in which groundwater flows throughout the aquifer.
Depending on local rainfall, land use, and geology, streams may be fed by either groundwater discharge or surface runoff and direct rainfall, or by some combination of surface and groundwater. Perennial streams and rivers are primarily supplied by groundwater, referred to as baseflow. During dry periods they are completely supplied by groundwater; during storms there is direct runoff and groundwater discharge also increases. The hydrograph in Figure 6 shows flow patterns in a stream before, during, and after a storm with relative contributions from groundwater (baseflow) and surface water (quickflow, also referred to as storm flow).
Figure 6. Components of a typical flood hydrograph
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