Groundwater is the vast majority of the planet’s liquid fresh water, existing beneath the surface and filling the spaces within soil, sand, and rock layers. Though out of sight, this subsurface water is an indispensable resource that sustains ecosystems, agriculture, and public water supplies globally. Understanding how groundwater is stored and moves, along with the legal framework governing its use, particularly the concept of overlying rights, is necessary for its long-term management.
Where Water Is Stored Below Ground
An aquifer is a geologic formation capable of storing and transmitting usable quantities of groundwater. Aquifers are comprised of materials like gravel, sand, or fractured rock. The ability of a material to hold water is quantified by its porosity, which is the total volume of open space within the rock or sediment.
The ease with which water can flow through an aquifer material is called permeability. High permeability requires the pores to be interconnected and large enough for water to move freely, such as in coarse-grained sand or highly fractured rock. Aquifers are broadly classified into two types based on their geological setting. An unconfined aquifer has its upper boundary as the water table and is in direct contact with the surface through the overlying soil.
A confined aquifer is trapped between two layers of impermeable material, such as clay or shale, which are known as confining beds or aquicludes. Water in a confined aquifer is under pressure from the weight of the overlying formations, which can cause water in a well to rise above the aquifer’s upper surface, sometimes even flowing freely at the ground level. These confining layers offer a degree of protection from surface contamination, but they also mean that the water is replenished much more slowly than in unconfined systems.
How Groundwater Moves and Replenishes
The water table marks the upper surface of the zone of saturation, where all pores and cracks are completely filled with water. Above this boundary lies the unsaturated zone, where pore spaces contain both air and water. The configuration of the water table often mirrors a subdued version of the land surface topography, being higher under hills and lower near valleys and streams.
Replenishment is called recharge, occurring when precipitation or surface water infiltrates the ground and moves downward through the unsaturated zone to the water table. This natural recharge can happen broadly across a large area, known as diffuse recharge, or it can be focused in specific locations like streambeds, sinkholes, or wetlands. The rate of groundwater movement is very slow, often measured in feet per day, meaning water can take years, decades, or even centuries to flow through some aquifers.
Groundwater eventually leaves the aquifer system through a process known as discharge. Discharge zones are typically areas where the water table intersects the land surface, such as at natural springs, seeps in wetlands, or by contributing base flow to rivers and streams. Water can also be discharged to the atmosphere when plant roots draw it from the upper part of the saturated zone in a process called evapotranspiration.
The Concept of Overlying Water Rights
Overlying water rights are fundamentally tied to the ownership of the land situated directly above a groundwater basin or aquifer. A landowner possessing this right has a priority claim to extract water from the aquifer for reasonable and beneficial use directly on that overlying property. This use typically includes domestic consumption, irrigation for crops, and livestock watering on the parcel itself.
These rights are considered “correlative,” meaning that all overlying landowners must share the available water supply during times of shortage, proportional to their reasonable needs. Overlying rights are considered senior to other types of groundwater claims, granting them the highest priority for use. They are not lost through non-use, as they are viewed as an inherent part of the land ownership.
In contrast, an appropriative groundwater right is acquired when water is pumped from the basin for use on non-overlying land or for commercial purposes, such as selling it to a municipality. Appropriative rights operate on the principle of “first in time, first in right,” where the oldest user has a higher priority than newer users. However, in a situation where the groundwater basin’s supply is insufficient, the overlying rights holders must have their reasonable needs met before appropriators can take any water, demonstrating the overlying right’s superior legal standing.
Safeguarding Groundwater Resources
Groundwater resources face two primary threats: depletion and contamination. Depletion occurs when the rate of water extraction, primarily through pumping, exceeds the natural rate of recharge, causing water tables to decline. This over-pumping can lead to reduced well yields, increased pumping costs, and land subsidence, a geological phenomenon where the ground compacts and sinks.
Contamination results from pollutants on the surface migrating downward into the aquifer. Common sources include industrial waste, leaking septic systems, and agricultural run-off containing pesticides and fertilizers. Once an aquifer is contaminated, the slow movement of groundwater means remediation can be extremely costly, time-consuming, or technically infeasible.
Protecting this resource requires implementing proactive management strategies. One effective method is Managed Aquifer Recharge (MAR), which involves diverting excess surface water, such as storm runoff or treated wastewater, and intentionally allowing it to percolate into the ground to artificially replenish the aquifer. This action, combined with effective monitoring programs for water levels and quality, helps balance supply and demand, ensuring the resource remains available for future generations.