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Biogeochemistry

An arsenic surprise: How our buried past will catch up with us

Article: Sea Level Rise Induced Arsenic Release from Historically Contaminated Coastal Soils. Joshua J. LeMonte, Jason W. Stuckey, Joshua Z. Sanchez, Ryan Tappero, Jörg Rinklebe, and Donald L. Sparks Environmental Science & Technology  DOI: 10.1021/acs.est.6b06152

 

Rising seas and sinking lands

Figure 1: Sea level rise indicators on a beach in Western Australia. (Image courtesy of Flickr user go_greener_oz)

Sea level rise is becoming a big concern especially for islands and coastal communities. Global sea level is expected to rise 0.8 to 1 meter by 2100 but this is not evenly distributed everywhere (Figure 1). Some places, such as the Mid-Atlantic coast of the U.S., will experience greater sea level rise due to a combined effect of the rising seas and coastal subsidence (sinking of land). These combined effects, along with an increase in tropical storms, will flood land with sea water.

 

Unexpected consequences of sea level rise

In addition to the more obvious implications of sea level rise, scientists are starting to consider what will happen to the biogeochemical processes of the local soil and water as it is inundated with sea water. Pollutants, including arsenic (As), have leached into the soil over time as a result of industrial activities. These chemicals are now a time capsule of our industrial history. Unfortunately, sea level rise may cause some of these chemicals, formerly trapped in the soil, to be released.

 

Arsenic

Currently, a third of the U.S. EPA’s superfund sites have elevated levels of arsenic. Arsenic is released during the smelting process in order to purify metal ores. Over time at industrial sites, this by-product arsenic increased the concentration in soil well above natural background concentrations. All over the world arsenic poisoning is a concern in the food and groundwater. Rice is one of the largest sources of arsenic poisoning since rice readily absorbs arsenic from the surrounding water and soil. Arsenic poisoning leads to cancer and other adverse health effects, especially in children.

Figure 2: A schematic of how arsenic can get into the water system as a result of sea level rise (LeMonte et al., 2017)

Arsenic can remain bound to the soil for a very long time. As a system becomes anaerobic (low or no oxygen), arsenic changes chemical form and is no longer bound so strongly, allowing it to enter into plants and water (Figure 2). Chemical reactions of iron (Fe) in these anaerobic conditions are thought to be a primary driver releasing arsenic.

 

Study Site

Researchers collected soil samples from an arsenic contaminated site in Wilmington, Delaware. This particular area is expected to see a 1 meter seal level rise by 2100. Like most cities along the Eastern Seaboard, Wilmington has a history of industrial activities which includes ore processing, leather tanning and chemical processing. The soil was collected on the banks of the tidally influenced Christina River. Part of this study was focused on understanding how freshwater and seawater will impact the arsenic release.

 

Experimental Set-up

The soil samples were set up in an automated biogeochemical microcosm reactor. This is basically a mini representation of outdoors conditions where seawater or freshwater can be added to the system and different parameters tested without the additional complications of outdoor measurements. They measured pH, temperature, particle size, metal concentration, total organic carbon, and more.

 

What is Eh?

Along with the above parameters, scientists also measured Eh. Eh measures the reduction potential or the likelihood for a chemical to acquire electrons. This tells scientists if the system is characterized by oxidizing or reducing conditions. An oxidized system has plenty of air (oxygen). Reduced systems usually do not have good air flow (i.e. swamps). Scientists controlled the Eh of the system by adding oxygen to increase the Eh or nitrogen to decrease the Eh.

 

Results

Before the addition of the contaminated soil collected from the site in Wilmington, arsenic in both fresh and sea water was negligible. When the soil was added, the arsenic increased in both water types as the Eh decreased. Figure 3 shows the results of this study for both water systems. Arsenic’s release from soil was found to be positively correlated with Eh as well as alkalinity and dissolved organic carbon .

Alkalinity is essentially the buffering capacity of the system. How much acid can you add to the system without changing the pH? It helps prevent rapid changes in pH which is important for protecting aquatic life.

Increased dissolved organic carbon can be used as an indicator of microbial activity.  As soil microbes eat all the delicious carbon in the system, they make dissolved organic carbon and release arsenic that was bound up in their food.

Figure 3: The relationship between arsenic and Eh, pH, alkalinity, iron (Fe2+), dissolved organic carbon (DOC), and sulfide (S2-) present in river and sea water experiments. (LeMonte et al., 2017)

River vs. Sea

As you can see in Figure 3, river water released almost twice as much arsenic as seawater. The researchers offer a couple of different explanations for this. It may be that seawater stresses out the soil microbial community. When microbes are stressed out they can’t eat as much which keeps more of the arsenic (and other chemicals) in the solid phase and not released to the water.

 

Who cares?

If saltwater stresses out the local microbial community so that they don’t eat as much and don’t release as much arsenic, shouldn’t sea level rise be a good thing?  The more salt water in the system the less likely it is arsenic will be released, right?

Take a moment and think about how sea level rise works. You know it is predicted to be 1 m by 2100 but that change will not be overnight. In fact, the slow change may give the microbial community time to adjust so it isn’t stressed out and we may see arsenic releases more similar to the river water in this study. No matter what, traditionally dry soil areas will release arsenic when they flood in the future. Flooded systems that become reduced (low or no oxygen) will be favorable for the release of arsenic regardless of freshwater or seawater. Scientists are still not clear on the extent of contaminant release with sea level rise, but they know that this is a serious concern, in addition, to the other daunting changes that sea level rise will bring to coastal communities.

Victoria Treadaway
I am a PhD candidate at the Graduate School of Oceanography at the University of Rhode Island. I am an atmospheric chemist studying organic acids in the troposphere to better understand their role in ozone processing. I flew on a Gulfstream V and a C-130 all in the name of science!

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