How is climate change affecting the thermal springs?

Word of the day: Climate Change

1. A significant and long-lasting change in the Earth's climate and weather patterns.

Merriam-Webster. (n.d.). Climate change. In Merriam-Webster.com dictionary. Retrieved August 24, 2021, from https://www.merriam-webster.com/dictionary/climate%20change

Thermal Spring

The footprints of persistent and significant climate change are seen with extreme weather events such as heavy rainfall and wildfires resulting from drought. More subtle impacts are occurring right in Arkansas’s backyard to one of its most cherished entities: the thermal waters in Hot Springs.

Hot Springs National Park Natural Resource Program Manager Nathan Charlton said climate change has a direct effect on the health, quality and quantity of the 4,000 year old thermal water people everywhere are in love with.

“The impacts that we can see are the shallow groundwater interactions with the thermal water during storm events and/or droughts, et cetera,” Charlton said. “We would suspect that if there was a long-term drought, we might see a drop in the flow rate as the hydraulic head may lessen.”

Although the health of the thermal springs has been studied for many years, in 2015 the park began a more concentrated study to monitor the health of its 35 thermal springs.

“We’re monitoring those just to keep our pulse on the health of the water,” Charlton said.

“We do one a week ... via hydraulic technician, and they're looking for, just to see how the water is doing in general. ... We’re looking for temperature-specific conductivity, pH and dissolved oxygen.”

One reason the park has continued this study for the past six years is to see how climate change and development is affecting the springs long-term, specifically in their recharge zone, which is where all of the water comes from and discharges into Hot Springs. Charlton said the recharge zone refers to the geographic boundaries where all the water that falls as precipitation will enter an aquifer. Hot Springs’ recharge zone extends east towards Indian Mountain nearing Jessieville, Arkansas.

“Precipitation falls within the recharge zone and is able to percolate downward into the earth due to the high fracturing of the Ouachita Mountains,” he said. “As the water is allowed to get deeper, it is able to heat up due to the geothermal gradient of the earth. Because the aquifer is confined by impervious layers below, the water travels downward and then westward toward Hot Springs. Once this water reaches a certain geological fault it is allowed to shoot upwards at a much speedier pace towards the surface, retaining much of its heat. The location where this water eventually does reach the surface is on the western slope/bottom of Hot Springs Mountain. This water cycle takes about 4,400 years on average.”

Charlton said what they see and notice more is seasonal variation, but when there is a big rain event, which may be caused by climate change or drought, they see a different flow in their spring measuring, and a different temperature.

One example he gave was in 2011, after a large rain event, they measured six springs to see how it affected their temperatures. They dropped about 6 degrees.

When precipitation increases, temperature decreases and flow increases. Since the more concentrated study on the water’s health began in 2015, Charlton said they have seen temporary drops in temperature and flow during high rain events due to the shallow groundwater mixing.

“We have two different types of water that’s coming into our springs,” Charlton said. “We have the ancient hot water, thermal water that’s 4,000 years old that falls as rain in the recharge zone and eventually works its way through the geological system it's in, the water system, and comes out.

“But it’s not just that water. Up to 30% can be just regular shallow cold ground water that mixes with it right before it emerges. Not quite enough to cool it down a lot, but it cools it down some. So when you have these large rain events and suddenly there’s a ton of water in the shallow ground water, it’s still mixing and so we’re going to see differences in our spring because we have more cold water entering the system faster and a lot more of it.”

If the water, which is usually about 143 degrees, decreases significantly in temperature, the risk of growing different types of harmful bacteria begins to increase.

“Right now it’s so hot that when it comes out of the earth it’s so hot you can drink it,” Charlton said. “It’s free of bacteria, harmful bacteria that people are worried about when they’re drinking water. If it gets too low it wouldn’t have that same property of killing the bacteria that are harmful to humans, so it would become less palatable.”

“Bacteria grow at many different temperature ranges in all environments,” he said. “If the water were to eventually lower in temperature enough than theoretically that would mean some bacteria could thrive and others may diminish. Longer-term monitoring would be needed to speak on the temperatures decreasing over time.”

If more extreme rain events occur in the recharge zone, it will result in more runoff.

“Ideally what you want in a healthy, equal water system is kind of a slow, steady rain so it has time to infiltrate,” he said. “If you have these bigger, violent, harder rain events that climate change is causing then you have more run-off, and if you have too much water falling, it can’t infiltrate. It’s running off of the sides of the hills and leaving the recharge zone.”

Also, climate change causes drought, which brings about an unhealthy forest that is susceptible to pests and will lead to tree death — which is then susceptible to wildfire.

“We’re trying to manage prescribed fire so that we can eliminate big giant wildfire events, which climate change is going to cause with drought,” Charlton said.

“Drought is and will continue to be an issue for Hot Springs,” he added, “but there is no single most-likely projection of climate change. All global climate models indicate the area will become warmer, but they differ in their projections of precipitation. Some models indicate an increase in total annual precipitation, others indicate a reduction. Either way, more extreme events are expected be it drought or increased high-intensity precipitation events.”

Fewer trees provide more run-off, therefore less infiltration, which will reduce the recharge and the flow of the springs.

Large rainfall events can also potentially transport surface-derived pollutants like pesticides and herbicides into the system.

“(If) we have a big event, there’s more likelihood that that water is going to pick up those surface contaminants, pesticides and transport them quicker, with more power, into our system,” he said, adding that no matter how hot the temperature of the water gets, it won’t kill these inorganic chemicals that are harmful to humans.

Drought and heavy rainfall seen can also affect the springs’ temperature and flow.

“Rainfall can cause rapid changes to the temperature of some springs — as much as 24 °F for the most sensitive monitored spring,” Charlton said.

“Long periods without precipitation cou also reduce spring flow until rain returns. Water flow from the hot springs has demonstrated responsiveness to drought, with a 6 to 8-week time lag between the initiation of drought and a reduction in spring discharge.”

Charlton said climate change’s current effects are going to eventually impact the health, quality and quantity of the thermal hot water in Hot Springs, and he feels like this is already evident.