Understanding Groundwater

Read below to learn about the science behind groundwater and the basics of groundwater management. Click the figures to view larger images. View the groundwater primer to learn more.

The Sustainable Groundwater Management Act defines the following six indicators of groundwater sustainability. Learn more about the Sustainable Management Criteria here. 

Hydrologic Cycle

Figure 1: Schematic of the Hydrologic Cycle, showing how water moves from the oceans by evaporation, falls as precipitation back to the land and moves through streams, into lakes and into the subsurface as groundwater.

All water on the surface of the earth and underground are part of the hydrologic cycle (Figure 1), driven by natural processes that constantly transform water from liquid to solid or vapor and back to liquid while moving it from place to place.

Water evaporates from ponds, lakes, oceans, reservoirs, and soils. Plants take water from the ground and emit water vapor into the air, a process called evapotranspiration. Water vapor forms clouds that eventually condense and return to the earth’s surface as precipitation such as fog, rain, sleet, or snow.

During storms, water runs off the surface into streams or water bodies or seeps into the ground. Water that sinks into soils or surface rock becomes groundwater, “recharging” groundwater reservoirs or aquifers.

Groundwater can discharge at seeps or springs, or into rivers, streams, lakes and oceans, or wells. In arid areas and during the summer, precipitation may first infiltrate into the ground, but much of it quickly returns to the atmosphere through evapotranspiration.

What is Groundwater?

Water infiltrates and collects underground, and is held either in an upper unsaturated zone or a lower saturated (“groundwater”) zone (Figure 2).

The unsaturated zone is located immediately beneath the ground and above the water table. Here, the pore spaces contain both air and water. Since an unsaturated zone contains both water and air, a well drilled into the unsaturated zone will not produce water.

In the saturated zone, all the open spaces are filled with groundwater. The top of the saturated zone is called the water table. The saturated zone supplies water to wells. Like surface water, groundwater flows from higher to lower levels, under the influence of gravity (Figure 3).

While aquifers may be thought of as underground reservoirs, storing water underground is more complex than in a surface reservoir. An aquifer is something like a sponge, but made of various rock materials with variously-sized open spaces. The elevation and movement of water within an aquifer is not visible from the surface, and groundwater reservoirs take much longer to fill and extract water from than a surface water reservoir.

Aquifers can be either unconfined or confined (under pressure). Unconfined aquifers are connected to the atmosphere through porous rock and are bounded at the top by the water table, which forms the transition between the unsaturated and saturated zones. Confined aquifers, in contrast, are separated from the atmosphere by low permeability silt or clay confining layers (or aquitards), and are typically found beneath unconfined aquifers.

Figure 4: (A) Gaining streams receive water from the groundwater system. (B) Losing streams lose or add water to the groundwater system. (C) Disconnected streams are separated from the groundwater system by an unsaturated zone.

Thus, in some areas (gaining streams) groundwater aquifers can help support streamflow during dry weather conditions and in other areas (losing streams) surface water bodies are important sources of recharge to groundwater aquifers. Streams can also shift between gaining and losing flow along their courses and over time when the hydrology, underlying geology, local climate, or streamflow conditions change.

Groundwater Recharge

Figure 5: An infiltration pond in Sonoma County, used to filter surface water into the ground

Figure 6: Water leaves the surface of the earth through evapotranspiration – the combination of evaporation from surfaces and transpiration from plants.

Natural recharge generally occurs in permeable soils, sediments and fractured bedrock during periods when precipitation exceeds evapotranspiration or when streams are flowing. When rain falls faster than it can soak into the ground, runoff occurs and limits the amount of water that is recharged. Other factors that can limit recharge include the presence of hard surfaces, such as roads and driveways, the presence of shallow clay layers, and areas with a shallow water table. In many areas of California, stream channels typically provide a significant source of recharge to groundwater basins. The identification and protection of groundwater recharge areas is an important step in sustaining groundwater basins.

Seepage from canals, surface reservoirs, septic and leaking water supply systems, urban watering and storm runoff also contribute to recharge. Recharge can also result from irrigating crops. When applying more irrigation water than a crop can absorb or evapotranspirate (convert to water vapor), the excess irrigation water sinks through the root zone and gradually recharges groundwater. Since recharge rates vary widely depending on soil conditions, climate and crop, it is difficult to estimate the value of irrigation to aquifer health.

Recharge can be enhanced by placing water in spreading basins to percolate into the ground, modifying stream channels and surface topography to slow the rate of runoff and increase infiltration of stormwater, and inserting clean drinking water into confined aquifers directly through recharge wells.

Groundwater Discharge

The speed and manner of groundwater movement from recharge areas to discharge areas where it exits the aquifer (Figure 6), depend on the permeability of the various materials (sediments, soils, rocks) that the water must flow through, the hydraulic gradient of the water moving underground (slope), and the cross-sectional area of the aquifer (if the aquifer is a hallway, a door frame in the hallway is the cross-sectional area).

Most natural discharge occurs at springs, and through seepage into stream channels, wetlands and seasonal lakes in desert areas. In some basins, significant amounts of groundwater flows into another basin or discharges into the ocean. Humans can also significantly contribute to discharge by pumping groundwater from water wells.

Wetlands or lakes can form when the water table lies close to or at the ground surface. The wetland may disappear if groundwater levels are lowered. Water evaporates from damp soil at the edges of the wetland and is evapotranspirated by wetland plants.

A well is a hole drilled below the ground surface into the saturated zone (Figure 8). Wells typically have screens at intervals along their length to allow groundwater to flow into the well from the most productive aquifer(s). All water wells are required to prevent foreign substances from entering the well and the aquifer. This is called surface completion and consists of installing casing and sanitary seals. The casing also keeps the material of the aquifer from falling into the well and provides space for a pump and a filter pack. Well casing typically consists of steel pipe, but can also consist of plastic pipe (e.g., PVC) for some applications. The most common pumps used in wells today are line-shaft turbines and submersible turbine pumps. A well also could be an “open hole” completion below the sanitary seal and casing, as in many bedrock wells. A well that is built correctly and is properly developed will not impede the flow of groundwater from the aquifer into the well or allow sediment from the aquifer to enter the well.

Until the well pump is turned on, the groundwater level in the well will be that of the local water table for an unconfined aquifer or the potentiometric surface in a confined aquifer. When the pump is turned on, the groundwater inside the casing is pumped to the surface, and the water level within the casing is lowered. The difference in pressure between the water level in the casing and the water level in the aquifer creates a hydraulic gradient between the aquifer and the casing. As a result, groundwater flows from the aquifer into the casing to replace the water that is being removed.

The rate at which groundwater flows from the geologic formation into the well is determined by the permeability and storage capacity of the aquifer material and efficiency of the well during pumping (affected by well design and construction).

To learn more about sustainable management, read about Petaluma Valley’s completed activities, or Groundwater Sustainability Plan.