Groundwater is a reservoir of water found beneath the Earth’s surface, filling the tiny spaces and cracks within soil and rock. It originates from rain and melted snow that seeps downward, pulled by gravity. Groundwater moves slowly through a network of interconnected pores, not in vast underground rivers. Its movement is governed by the physical properties of the earth materials it passes through and the forces acting upon it.
Where Groundwater Resides
The water resides in the saturated zone, which is the region below the surface where all the voids in the soil and rock are completely filled with water. The upper boundary of this saturated zone is called the water table, a fluctuating level that often mirrors the shape of the land above it. Above the water table is the unsaturated zone, where pore spaces contain both air and water, and water is generally held in place by soil tension.
The ability of the material to hold water is defined by its porosity, which is the total volume of empty space within a rock or sediment. However, simply having space is not enough; the spaces must be connected for water to move through. This connectedness is measured by permeability, which describes how easily water can pass through the material.
A geologic layer that is both saturated and has sufficient porosity and permeability to yield a usable amount of water is known as an aquifer. Materials like unconsolidated sand and gravel typically have high permeability and make excellent aquifers. Conversely, dense clay or unfractured rock may have high porosity but very low permeability, restricting flow and acting as a barrier to the movement of water.
The Forces That Drive Flow
Groundwater movement is driven by potential energy, a combination of two primary forces: gravity and pressure. Unlike surface water, subterranean flow is more complex because the water is not in contact with the atmosphere. Water deep underground can be under considerable pressure from the weight of the overlying water and rock, adding a significant pressure component to its potential energy.
Scientists combine the energy from elevation (gravity) and the energy from pressure into a single measurement called hydraulic head. This is often measured in a well as the height to which water rises above a reference point. Groundwater always moves from a location with a higher hydraulic head to a location with a lower hydraulic head.
This difference in head over a distance is known as the hydraulic gradient, which acts as the slope that dictates the direction of flow. While this is conceptually similar to a ball rolling downhill, the pressure component means that water can sometimes move upward against gravity if the pressure below is high enough. The water will always follow the path of steepest energy descent, moving perpendicular to lines of equal hydraulic head.
Determining Speed and Direction
The rate and direction of flow result from the interaction between the driving forces and the material’s resistance to flow. The fundamental principle used to quantify this movement is Darcy’s Law, which describes the relationship between the force (hydraulic gradient) and the medium’s ability to transmit water (hydraulic conductivity). Hydraulic conductivity is a measure of the material’s permeability, indicating how readily it allows water to pass.
Groundwater typically moves much slower than water on the surface, often measured in meters per year or even per day, rather than per second. For instance, water in a fine-grained sandstone aquifer might only move a few meters in an entire year, illustrating its characteristic slowness. In contrast, water in highly permeable gravels or fractured rock can sometimes move several meters per day.
The direction of flow does not always follow a straight line because the underlying geology causes the flow paths to curve. Variations in rock type, sediment layers, and natural fractures create preferred pathways for the water. Groundwater moves along these complex paths until it reaches a discharge area, such as a river, spring, lake, or the ocean. This destination is always an area where the hydraulic head is low, completing the cycle of its subsurface journey.