An overwhelming majority of Earth’s water is saltwater, and most of it is stored in oceans. Oceans store over 96% of the Earth’s water supply, so they are a natural jumping off point for thinking about the cycle.1United State Geological Survey, 2022, The Water Cycle, https://www.usgs.gov/special-topics/water-science-school/science/water-cycle (accessed December 20, 2022). Energy radiating from the sun warms the oceans, and some of the stored water evaporates as water vapor.
Water in the atmosphere condenses into clouds in colder temperatures and at higher altitudes, and global wind patterns move these clouds over different parts of the ocean and over landmasses. Water falls from clouds as precipitation either as liquid water (rain) or as snow, ice, or hail.
Water that does not accumulate in snow caps and glaciers flows toward the ocean as surface run off, which takes the form of rivers and streams. Some is stored on land as lakes and reservoirs. However, the ground soaks up most of the water that falls over land through a process called infiltration.
Water that soaks deep into the ground can enter underground sources of drinking water (USDW) also known as aquifers. Water from aquifers returns to the surface through wells and springs, and some groundwater seeps back into surface water bodies or the ocean. When water reaches the ocean, one can think of the cycle as restarting.
One of our subject matter experts will summarize the continuous nature of the global water cycle in the video below.
Transcript
The Water Cycle – David Yoxtheimer – Penn State
The global water cycle is a cycle without beginning or end. Water may be stored at different points, but essentially this finite resource’s cycle is continuous. A good place to start looking at the water cycle is where most water is contained, the oceans. Solar heating evaporates the ocean water, which then condenses in the atmosphere to form clouds. Clouds store, transport and ultimately release water as precipitation, which falls back to the earth as rain, snow, hail or other forms.
When precipitation falls onto the ground, some of it flows over land as surface run-off and collects in streams, rivers, lakes and reservoirs where gravity ultimately draws it back toward the ocean. In high altitudes or latitudes, precipitation may be frozen and stored in snow caps or in glaciers. A portion of the water falling onto land infiltrates through the soil and into the sub-surface. Along the way, plant roots may absorb and release some of it to the atmosphere through evapotranspiration.
The water not taken up by plants flows downward until it reaches aquifers, which allow the water to gradually flow back toward the surface through springs, stream-based flow or wells. A portion of surface waters evaporate and return to the atmosphere as water vapor or flows all the way to the ocean and evaporates to begin the water cycle again.
Humans play an important role in the cycle by withdrawing surface or ground water for a variety of uses such as irrigation or drinking water, but they ultimately return it back to the environment. For example, water used at home and circulated throughout appliances or even the human body returns to the environment through drains to sewers. After sewage is treated, it is returned to surface water as effluent.
The water used for hydraulic fracturing for oil and gas development comes from the water cycle, but the source and amounts used differ by region. For example, operators in the Marcellus Shale rely primarily on surface water while operators in the Barnett Shale rely mostly on groundwater. Because of differences in the rock properties and well lengths, a median water use for a well in the Marcellus Shale is 4.25 million gallons of water while a typical well in the Barnett Shale uses about 3.8 million gallons.
During fracturing operations, water is mixed with additives, pressurized and then injected into the target formation. So how does this water make its way back into the water cycle? Flowback returns to the surface in a matter of days and consists mostly of the fluids initially used to fracture the formation. Produced fluids are brines found naturally in formations, which flow to the surface along with hydrocarbons throughout production.
Operators store produced fluids and flowback in tanks or pits on-site. A portion of the water in pits evaporates, leaving behind dissolved solids and other chemicals. In some cases, water is treated and returned to surface water. It is common for produced water and flowback to be transported for injection and permanent disposal. This has caused some controversy due to concerns about the potential for groundwater contamination or induced seismicity.
For disposal, fluids are pressurized and injected into formations far below groundwater sources. This water, with elevated levels of metals, salts, organics and radionuclides, is not intended to migrate into or interact with other sources of ground or surface water ever again.
Given the large volumes of water used in hydraulic fracturing operations, this has caused concern especially in areas experiencing drought. Beyond the produced fluids injected into disposal wells, water used in oil and gas operations returns to the cycle to continue its journey, replacing other water withdrawn for additional oil and gas operations.
Images: “it is raining on the pier” by lafoto/Shutterstock.com