By Peter Chawaga, Associate Editor, Water Online
Arsenic carries with it one of the most notorious reputations among water source contaminants. As a naturally-occurring element near deep reservoirs of groundwater, arsenic was found to have affected over 137 million people in more than 70 countries as of 2007, and it is known to cause cancer and a host of other serious illnesses.
With this prevalence at top of mind, researchers from Utah State University’s College of Engineering set out to help solve a problem that strikes particularly close to home.
“In the United States, concentrations in excess of the drinking water maximum contaminant level (MCL) in public wells are distributed across the country, but three-quarters of these wells are in the western United States,” said Babur Mirza, a member of the research team. “In basin-fill aquifers in California, Nevada, New Mexico, Arizona, and Utah, 10 percent of domestic wells tested had arsenic in excess of the MCL. The Cache Valley Basin is located 128 kilometers northeast of Salt Lake City… A survey of domestic wells in this basin showed that 27 of the 161 wells tested had arsenic in excess of the MCL.”
To help combat arsenic in water supplies, the researchers wanted to attack it at its source. They saw an opportunity to help by providing a better system for identifying the bacteria that transform arsenate into arsenite, the toxic substance that infiltrates groundwater and leads to widespread contamination.
“Inorganic arsenite is readily soluble in water and, because the ion it forms in water is negatively charged, it does not bind well to soil particles,” said Mirza. “It has a much higher acute toxicity than arsenate.”
The researchers developed a strand of DNA known as a primer. The primer identifies arsenate respiratory reductase (arrA), the gene that turns arsenate into the deadlier arsenite.
“These primers are designed specifically to target the arsenate reductase gene,” Mirza said.
“If a microorganism does not have this gene, none of its DNA will be amplified in the polymerase chain reaction where the primers are employed.”
The researchers designed the primer based on DNA nucleotide sequences from the National Institute of Health’s GenBank database, an annotated collection of all publicly-available DNA sequences. The team extracted DNA from several soil and groundwater samples, using the primer to amplify the gene and then sequencing the amplified DNA to make sure it was identical to what it had gotten from GenBank.
The primer has potential as a powerful tool for better understanding what causes arsenic contamination and therefore, how to stop it.
“When arsenic concentrations above safe levels are discovered in drinking water wells or other drinking water sources, a major question that water resource managers will want to answer is whether the excess arsenic comes from human activities in the watershed or if it is from natural sources,” Mirza said. “Studying the arsenate-reducing microorganism in source water environments and its relative abundance … can help to identify possible factors that enhance or potentially inhibit microbial arsenic reduction and solubilization.”
With a better understanding of where bacteria with the arrA gene are and, therefore, where arsenate is being transformed into arsenite, communities can make watershed efforts to reduce them.
“These primers … can be used to understand the influence of different environmental and/or land management practices, such as agricultural practices and seasonal variation in the groundwater level, on these microorganisms,” said Mirza. “Understanding these processes under natural conditions could inform watershed managers and help them take action to minimize arsenic contamination of groundwater.”
Though it is a small tool measuring an even smaller contributor, the research promises to take a fundamental stance against a massive problem.
Image credit: "Orange travertine at the Crystal Geyser, Utah" Alan Vernon © 2005 used under an Attribution 2.0 Generic license: https://creativecommons.org/licenses/by-nc-nd/2.0/