Groundwater Connection with Springs

Eileen Poeter; Ying Fan; John Cherry; Warren Wood; Douglas Mackay

Groundwater Connection with Springs

Springs are fed by discharging groundwater. They occur where the water table intersects the ground surface. They may form at a depression in the ground surface along a slope that intersects the water table as in Figure 21a, or they may form where a geologic unit of lower permeability perches (traps) water above the groundwater table as illustrated in Figure 21b.

Figure showing the occurrence of a spring where the water table intersects the ground surface — **Figure 21 –** Springs occur where the water table intersects the ground surface: a) at a depression in the surface topography; or, b) where a geologic formation perches groundwater and outcrops at the surface (Poeter et al., 2020, gw-project.org).

Springs have been an important part of human history, and today springs provide drinking water for more than a hundred million people and to businesses that bottle water. Huge springs supplied water to the aqueducts of early Rome and still supply Rome’s water today. Oases (desert springs) were the source of water along much of the Silk Road trading route between China and Europe. The North Silk Road crossed the Badain Jaran Desert in China (the 4th largest desert of Earth) where, even today, over 100 groundwater‑fed lakes are springs nestled within the largest sand dunes in the world (Figure 22 and Figure 23). These lakes maintain the vital oases and ecology in the desert. Similarly, long‑distance groundwater convergence toward, and discharge as springs in, the Great Rift Valleys of eastern Africa has been hypothesized as the life support system for societies of early human ancestors despite centuries‑long droughts.

Map and Landsat image showing where groundwater springs form lakes in dunes and desert — **Figure 22 –** Groundwater springs form lakes among the earth’s tallest sand dunes in the Badain Jaran Desert of China. These lakes range from fresh to extremely saline, and maintain the vital oases and ecology in the desert. a) Location of the Badain Jaran Desert and the lake area (Jiao, 2015); (b) Landsat Thematic Mapper image of the area including most of the lakes in the desert on May 24, 2003, with 10m groundwater contours estimated from ICESat (Ice, Cloud, and land Elevation satellite) data (Jiao, 2015).

Photograph of one of the lakes within the sand dunes of the Badain Jaran Desert of China — **Figure 23 –** A photo of one of the lakes within the sand dunes of the Badain Jaran Desert of China (Jiao, 2017).

The discharge of some springs is much warmer than other surface waters in the same locale. In these thermal springs, groundwater flows to a depth where it is heated either by molten subsurface rock associated with volcanic activity or by deep rocks that are warmed by heat conducted from the cooling core of the Earth to its surface. Density of the water decreases as it is warmed, so the water rises, and when it reaches the surface it manifests as a hot spring. Hot springs are generally located near geologically recent igneous activity as indicated by their frequent occurrence in the tectonically active western portions of North America as shown in Figure 24.

Map showing locations of thermal springs in North America — **Figure 24 –** Thermal springs generally occur near geologically recent igneous activity as indicated by this Google Earth map of thermal springs in North America (map from USNOAA, 2019; data from Berry et al., 1980).

Occasionally groundwater is heated at depth and has a direct conduit to the surface via a zone of fissures or a fault, forming a geyser that periodically ejects a large volume of water. Water cooled by ejection flows back into the reservoir where it is again heated becoming less dense and more pressurized by heated dissolved gasses. At some point the weight of the overlying column of water is insufficient to hold back the water and it erupts. These features (Figure 25) are not as common as thermal springs.

Photograph of Strokkur geyser erupting in Iceland — **Figure 25** – The famous Strokkur geyser erupting in Iceland (Tille, 1996. “Eruption of Strokkur close by” by Andreas Tille is licensed under CC BY-SA 3.0).

License