Climate Change, Mankind and Groundwater

The article below consists of extracts from the USGS, U.S. Forest Service, the National Environmental Health Association YouTube presentation by Dr. Jason R. Barrett, Mr. Joel Pigg, and Dr. Sam Sherchan and the article cited below:

Benz, S.A., Irvine, D.J., Rau, G.C. et al. Global groundwater warming due to climate change. Nat. Geosci. 17, 545–551 (2024). https://doi.org/10.1038/s41561-024-01453-x

 

Groundwater is ubiquitous and represents the largest distributed store of fresh water on earth playing a central role in sustaining ecosystems and enabling human communities and life. The importance of groundwater for global water and food security will probably increase as the climate warms and as more variability in precipitation (more intense droughts and more intense floods), more variability in soil moisture and surface water increase the need for and value of groundwater.

Over the past 50 years, humans have extracted the Earth’s groundwater at an ever increasing rate, largely to provide food and water for the growing global population and the support the economic development needed for all those people. Extracting groundwater beyond what is replenished will slowly over time use up the aquifer and that is a threat we have been seeing for decades. The Gravity Recovery and Climate Experiment (GRACE) satellites and the associated datasets and hydrological models have focused most of their work on resource quantity and the threat to water supply of over using groundwater beyond its recharge rate.  

Population growth and development has increased the threat to groundwater. This growth has Increased the need for water while reducing the open forested areas and natural grasslands that allow for infiltration of precipitation into the ground.  Reducing the replenishing (recharge) of the aquifer while increasing the demand for water is unsustainable combination as documented by the GRACE data trends. Groundwater is used for water supply and serves to support steam flow between rain storms. As groundwater levels fall, perennial steams that feed the rivers become intermittent and then ephemeral. The groundwater becomes disconnected from the surface water network. Groundwater comes from rainwater and snow melt percolating into the ground. 

The most obvious concern is depletion of groundwater as it becomes an increasingly important water source. As precipitation becomes less reliable due to climate change, surface water bodies can drop too low to provide needed water, causing people to turn to groundwater sources. Over-pumping and depletion of groundwater is already a significant problem in many places across the Eastern Region of the United States. Over-pumping will increase as climate change makes traditional sources of water less reliable, but there are other impacts.

Impervious surfaces prevent groundwater from soaking into the ground. Areas with a large amount of impervious surfaces (such as asphalt, concrete, buildings, etc.) not only are susceptible to flooding but are also susceptible to  higher ambient air temperatures because the man made roads, parking lots, concrete surfaces and buildings absorb and trap heat more heat than natural environments. Impervious surfaces where water runs off into local streams instead of slowly soaking into the ground act as direct routes for rainfall to make its way into streams at higher velocities, but also at poorer quality.

Rain that falls on a parking lot that has been baking in the sun all day during summer gets super heated and then runs off into streams. This heated water can be a shock to the aquatic life in the stream and can harm the water quality of the stream. Along with the heat, runoff from parking lots can contain pollutants, such as leaked motor oil, hydrocarbons from exhaust, leftover fertilizer, and normal trash. 

Temperature of water matters because warmer stream water can affect the aquatic life in the stream. Warm water holds less dissolved oxygen than cool water, and may not contain enough dissolved oxygen for the survival of different species of aquatic life. Some compounds are also more toxic to aquatic life at higher temperatures. Increasing global temperatures caused by a changing climate or the heat island effect of the expansion of the urban built environment are impacting our streams.

While work has accelerated to document the using up of our groundwater resources, there has been little work so far into groundwater quality, including temperature. This is now becoming a more important area of research. Groundwater serves as a cooling system and heat sink for the earth and delivers moderate temperature water to streams. As the impact of mankind and a changing climate increase, groundwater’s ability to cool and feed streams is reduced. Climate change is already having impacts on groundwater resources by changing the location, frequency, and intensity of rain storms. Stormwater flowing directly to streams is warmer. In manmade stormwater ponds evaporation increases contaminant concentration of the water that moves into the water table which is also warmer.  

Other research has shown that this could lead to warmer surface water bodies. Many streams depend on groundwater to maintain their cool to cold-water conditions, which are required by many organisms. The U.S. Geological Survey (USGS) has been measuring how much water is flowing in rivers, determining the water levels of groundwater, and collecting water samples to describe the quality of those waters for over a century. Over this time period, they have taken millions of measurements of , groundwater levels, surface water flow and temperature. 

The research has found that warmer precipitation and recharge will eventually reach streams and rivers as warmer baseflow over the next decades and groundwater levels continue to fall. While a permanent water table decrease of one to three feet may not mean anything for a water supply well, it can have severe consequences for surface water bodies and ecosystems dependent on that shallow water table. In the Potomac Aquifer and the Culpeper Basin the water table has fallen by far more than that.

from EPA 2021