Introduction to Basic Ground-Water Flow
By the earthDr!
Examples of Ground-Water Flow: Influent and Effluent Streams
Ground-Water Flow is just water movement through a porous media. Ground water can flow between soil particles; and, through crevices, cracks, joints, and fractures of consolidated rock. Ground-water flow needs to be viewed from both a micro-scale and a macro-scale; and, also from views in between. Arrows are used in the figure below to demonstrate ground water flow routes. Ground water flow may or
may not predominate in the larger soil pores, known as the macropores. Navy blue arrows are used to distinguish ground water flow in the macropores. Individually less flow occurs through the smaller and smallest soil pores, known as the mesopores and the micropores, respectively; however, collectively this may not be so. Obviously more flow of water can occur in a larger soil pore, but if there are few large-sized soil pores relative to the shear number of small-sized soil pores in a given soil, then flow will predominate in the small-sized soil pores. Flow through these smaller soil pores is depicted by the light blue arrows. Ground water cannot flow, along a straight line, from left to the right. Individual water molecules must move around soil particles: up, down, left, and right. These herky-jerky movements are ground-water flow on a micro-scale. These herky-jerky movements are called mechanical mixing. This figure depicts overall ground-water flow from left to right. This is ground-water flow on a macro-scale. Individual water molecules follow a tortuous path in going from left to right. This constant change in direction that individual water molecules must undergo in traversing a distance results in much mixing of the ground water along with any contaminants that may be dissolved in the water. Along with diffusion, mechanical mixing is one of two processes responsible for dispersion of contaminants in the water table.

What is the driving force pushing water between soil particles; and through crevices, cracks, joints, and fractures in rock? The answer to this can be illustrated by the figure below. Water is lost from the overlying stream. This water must flow between individual soil particles, or more precisely, the water
must flow through the soil pores. Just to reiterate, the soil pores are analogous to water pipes. When you want to drain your water pipes in your home, you shut off the water supply to your house, then open the faucet in your upstairs bathroom and the boiler drain in your basement; and then, let gravity take over. Then, the water drains to the basement. Obviously the water that is lost from the overlying stream bed, drains vertically-downward through the soil to recharge the water table below. Typically, water flows from a higher elevation to a lower elevation, because of gravity. If the water cannot freely drain through the soils, mounding of the water table will occur in the soil demonstrating an impedance to flow. Since water cannot typically freely drain or freely flow through the soil, a gradient (analogous to relief) is established at the water table. Water will flow from a higher point on the gradient (the upgradient point) to a lower point on the gradient (the downgradient point) due to gravity.

This last figure on this page, depicts three different ground-water flow regimes. In this figure, shallow ground-water flow from either side is to the stream. Deeper ground-water flow is not captured by the
stream, but continues to flow in a downgradient direction from right to left. Only the shallow portion of the water table is captured by the stream. Deeper ground-water flow paths may be influenced, but not captured by the stream. Remember that this is just an example. In some hydrogeologic settings, even the deeper ground-water-flow paths may be captured. Why does ground-water flow from one point to another? The ultimate driving force is gravity, just as it is your home plumbing pipes. However, this force gets re-defined as the water moves from one elevation to another elevation and it is known as total head: composed of elevation head, pressure head, and if the fluid has sufficient velocity, velocity head. Normally, velocity head in the ground water is not measurable and can be neglected.

Please note that water in your plumbing system can also flow upwards just as can ground water. Water in your plumbing system can flow uphill because of the driving force, called total head, and the pipes to contain this head. The ground water is also contained in pipe-like lines that just happen to be somewhat leaky. These pipe-like lines are variously called soil pores; or cracks, crevices, joints, and fractures of consolidated rock.