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Up From The Ground University of Alabama researchers set out to understand the role of groundwater in Mobile Bay

Initial research by a team of University of Alabama marine scientists hints groundwater contributes contaminants into Mobile Bay, a situation that could worsen as rising sea levels awaken contaminants dormant in the sediment. Scientists are searching for clues from the past to see if this happened before.

Daniel Montiel, a UA doctoral student under Dr. Natasha Dimova in geological sciences, scans Mobile Bay as part of field work in summer 2017. “We explore the shoreline of Mobile Bay for groundwater discharge,” he said.

By Adam Jones

Photos by Zach Riggins

Mobile Bay is the fourth largest estuary in the United States fed by the second largest delta in the country, Mobile–Tensaw River Delta. Hugged by rivers and the Gulf of Mexico, it is difficult to imagine groundwater as a major player in the Bay’s ecology.

But, simply put, it is. Accounting for a trickle of water in the bay compared to water from the Mobile River, groundwater delivers half of the nitrogen derived from ammonium into the bay, according to studies by Dr. Natasha Dimova, assistant professor and a marine and environmental geochemist in UA’s geological sciences department. Nitrogen is critical for the health of the estuary, but too much of it brings problems that can affect water quality and economic activity such as the lively seafood industry.

Top left: Water is pumped into sensors to detect radon, an indicator of groundwater. Top and bottom right: Tazmul Islam, a graduate student in geography, advised by Dr. Sagy Cohen, collects samples of water to determine sediment concentrations in the bay. Bottom left: Montiel had to keep detailed records of where and when data was collected.

Since groundwater takes longer to reach the ocean than a river, groundwater has the time to accumulate much more chemicals, nutrients and heavy metals.

Dimova’s work studying submarine groundwater discharge around the Mobile Bay and Delta point to the possibility of something else boosting groundwater’s potency just before it leaks into the bay – coastal sediments.

“All the data falls in this corner tells us the quality of the groundwater coming out is largely determined by what happens in the coastal sediments just before the discharge."

Dr. Natasha Dimova

A motivation to study submarine groundwater discharge in Mobile Bay is a unique phenomenon called jubilees, moments when huge numbers of marine life such as fish and crabs swarm shallow waters. They can have severe economic impact on the seafood industry and tourism that thrive in and around the bay.

One of the important triggers of jubilees are an increase in algae, or small marine plants, in what are named harmful algal blooms. These blooms occur when an abundance of nutrients that algae feed off release into the bay.

While rivers bring spikes of nutrients, Dimova and team's work clearly shows hot spots of submarine groundwater discharge coincide with frequently observed areas of jubilees in Mobile Bay during the dry season.

Dr. Dimova, in the grey shirt, and Dini Adyasari, a visiting doctoral student from University of Bremen, Germany, check monitoring stations that detect the properties of groundwater just before discharge into the bay, left, and just after entering the bay, right.

When sorting out the influence of groundwater on the health of the bay and influence on harmful algal blooms, Dimova wants to know what is in the sediment. Below the surface, the sediment can tell the history of the bay, shedding light on when there have been harmful algal blooms.

Initial work shows the sediments are storehouses for nutrients in an organic layer that Dimova’s team discovered around Mobile Bay just below the surface, seen when a rod, or core, of sediment is carved out and pulled from the soil. The sediment also holds onto toxic metals such as mercury, which, when consumed by fish, hurts the seafood industry.

“Imagine that sediment that has never been touched by sea water and now that sea water is going up, it’s going to shake everything,” she said. “It’s a ticking bomb to me.”
Top left: Montiel and Dr. Rebecca Totten Minzoni, UA assistant professor geological sciences, use a UA-owned, truck-mounted GeoProbe coring system to drill a sediment core, on the right. Bottom left: Jacob Honeck, an undergraduate student in geological sciences at UA, Dimova and Totten Minzoni collect a core of sediment pulled using the vibracoring technique from the shallow water of Mobile Bay.

Along with the nutrients affecting the bay today and could affect it in the future, the sediment tells an intricate history of sea level and environmental changes in the bay. Sediment cores provide a longer timeline to help understand how and when increased contributions of nutrients occurred.

Not only can this long perspective show information about storm impacts, floods and ecological responses to these events before instrumental records began or humans settled the area, but sediment cores help researchers learn about how the coast responded to past rates of sea level rise.

“With the cores we collect in Mobile Bay, we can step back in time with each layer to tell us about how environments changed, and ultimately produce a long-term frequency of the harmful algal blooms over time."

Dr. Rebecca Totten Minzoni

Answers to questions about the current influence of groundwater on water quality of the bay by natural or man-made sources combined with a historical picture of nutrient fluxes and harmful algal blooms can be used to influence restoration efforts as well as economic development decisions along the bay.

“It’s only through collaboration that we can provide the past and the current state of Mobile Bay and get the full picture,” Dimova said.

“The broad picture is all about maintaining water resources in Alabama."

Dr. Natasha Dimova,

Aspects of this research are funded by the National Science Foundation through the EPSCoR program investing in innovations at the nexus of food, energy and water systems.

Created By
Adam Jones
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Credits:

Zach Riggins, The University of Alabama

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