Covering an area approximately 180 miles long and up to 40 miles wide in Central Texas, the Edwards Aquifer “is one of the most prolific karst aquifers in the world,” according to the Edwards Aquifer Authority (EAA). But water levels across the aquifer have been declining since 2019 due to “a lack of rainfall across the region” totaling about 71.3 inches—“a deficit equivalent to more than 2 years of rain.” The EAA warns that “[s]ignificant regional rainfall events” will be required “to recover from low water levels” because “‘normal’ rainfall will only help to keep conditions steady.”
The EAA was created in 1993 “to manage, enhance and protect the Edwards Aquifer system.” Toward that end, the EAA “took ownership of 151 acres … on the Recharge Zone of the Edwards Aquifer in northeast Bexar County” in 2019, according to EAA’s website. This “natural observatory” is known as the Field Research Park, or FRP, where Our Water Matters recently met with Mark Hamilton and Thomas Marsalia on an extremely rainy day in April. Hamilton is a geoscientist and executive director of Aquifer Management Services, while Marsalia has a background in environmental management and serves as associate director of Aquifer Sustainability at the FRP.
Marsalia explains that the FRP is experimenting with “well known” land management techniques, including brush weirs, log mats, and one-rock dams, in an effort to “put some data behind a lot of those practices.” In the case of one-rock dams, says Marsalia, “You want every rock making contact with the soil … to hold that soil in place to keep water from taking it away … [T]he whole purpose is not just to have rocks there, but it’s to recruit vegetation that’s going to hold the soil in place better than the rocks can do.” According to Marsalia, “The key to [these] practices is simply to slow, spread, and sink surface water runoff while keeping soil and sediments on the land surface. This allows for improved infiltration and water holding capacity in the landscape.”
Another major effort at the FRP seeks to advance the EAA’s understanding of aquifer recharge. “The quantification of groundwater recharge is not an easy task [because] the aquifer is very large and offers a number of challenges for data collection,” says Hamilton. “In layperson’s terms, we simply try to measure the water balance (using stream flows at the top and bottom of the recharge zone and rainfall amounts) and then make qualified assumptions about how much of the loss in stream flow across the recharge zone is recharge.” Hamilton explains that these “stream-based” volumes are known as “discreet” recharge, which is measured by the United States Geological Survey under a contractual agreement with the EAA. Quantifying groundwater recharge also involves “overland or diffuse recharge,” according to Hamilton, in which “factors such as evapotranspiration … surface runoff, and rainfall all need to be accounted for.”
FRP scientists are also investigating how water is interacting in the soil, according to Marsalia. In a YouTube video for the “Go Gardening” series, Marsalia describes the FRP’s continuous soil monitoring plots that comprise “berm and swale” systems equipped with three stations (uphill, downhill, and within the swale or depression) to measure soil water potential, electrical conductivity and soil temperature down to a depth of 30 centimeters. “What we are trying to find out is where is that water going and what is it doing after it infiltrates,” he explains, because the movement of water within the soil is critical to understanding groundwater recharge. Hamilton states in the video that one technique for measuring how much water collects in the soil involves “nuclear magnetic resonance.” While this technique is most commonly used in medical applications to “see things going on inside your body … by aligning the polar fields” with the molecules in your organs and blood vessels, explains Hamilton, “you can also align a water molecule using nuclear magnetic resonance. And when you do that, you can actually measure water content in the subsurface.” The idea is to continuously monitor the movement of water in the soil to understand what’s really going on, according to Hamilton. The “water that infiltrates here, how deep does it go? How much does it spread out? How long does it stay in the ground? … Does it go in the aquifer? … Does it get hung up somewhere in the subsurface?” The movement of water in the subsurface is a “really interesting concept to try to understand,” says Hamilton, “but … it’s really difficult to figure out.”
Perhaps the greatest challenge to the research has been the drought, asserts Hamilton. “It just hasn’t really rained” since the FRP opened in 2019. But on the day that Our Water Matters visited, “that streak came to an end … as near-record rainfall fell across parts of northern Bexar County,” according to the Austin American-Statesmen. Some areas of the Edwards Aquifer recharge zone received up to 6 inches of rain that week, a robust pulse of precipitation that will hopefully keep Hamilton, Marsalia, and the rest of the FRP team very busy in the months ahead.
Visit edwardsaquifer.org to stay abreast of the EAA’s research.