The Bangladesh Arsenic Problem
Arsenic contamination of the groundwater in Bangladesh is a serious problem. In the Ganges Delta, the affected wells are typically more than 20 meters and less than 100 meters deep. Groundwater closer to the surface typically has spent a shorter time in the ground, therefore likely absorbing a lower concentration of arsenic; water deeper than 100 meters is exposed to much older sediments which have already been depleted of arsenic. Human activities are not the primary cause of arsenic found in groundwater in Bangladesh. A team of researchers from Lawrence Livermore National Laboratory, Barnard College, Columbia University, University of Dhaka, Desert Research Institute and University of Tennessee found that the arsenic in groundwater in the region is part of a natural process that predates any recent human activity, such as intensive pumping.
Symptoms of arsenic poisoning begin with headaches, confusion, severe diarrhea, and drowsiness. As the poisoning develops, convulsions and changes in fingernail pigmentation may occur. When the poisoning becomes acute, symptoms may include diarrhea, vomiting, blood in the urine, cramping muscles, hair loss, stomach pain, and more convulsions.
Arsenic is related to heart disease(hypertension related cardiovascular), cancer, stroke, chronic lower respiratory diseases, and diabetes.
Bacterial respiration of organic carbon releases naturally-occurring arsenic from sediment into groundwater, but the source of this organic carbon remains unclear.
The Bangladesh arsenic crisis came to international attention in 1995. The study conducted in Bangladesh involved the analysis of thousands of water samples as well as hair, nail, and urine samples. They found 900 villages with arsenic above the government limit.
Brian Mailloux of Barnard College and his team isolated microbial DNA from several depth intervals in arsenic-contaminated aquifers in Bangladesh and analyzed the DNA's radiocarbon signature, which reflects whether the organic carbon used by the microbes derives primarily from younger, surface-derived sources that are transported by groundwater into the aquifers, or older, sediment-derived sources.
Using "bomb pulse" radiocarbon analysis, Lawrence Livermore scientist Bruce Buchholz dated the DNA of groundwater bacteria. He found that the DNA samples were consistently younger than the sediment, suggesting that the microbes favor using surface-derived carbon.
The surface-derived carbon has flowed into the aquifer over hundreds to thousands of years - a rate that is approximately 100 times slower than groundwater flow. The results suggest that recent human activities, such as intensive groundwater pumping, have not yet significantly affected the release of arsenic into the groundwater at this site.
Above-ground testing of nuclear weapons during the Cold War (1955-1963) caused a surge in global levels of carbon-14 (14C), and remains in all living things. Carbon-14 or radiocarbon is naturally produced by cosmic ray interactions with air and is present at low levels in the atmosphere and food. Although nuclear weapon testing was conducted at only a few locations, excess levels of 14C in the atmosphere rapidly dispersed and equalized around the globe.
According to Buchholz, "The bomb curve forms a chronometer of the past 60 years."
The radiocarbon signature of DNA is a direct measure of the carbon used during microbial respiration and growth. In this study, the team developed a method to filter, extract and purify DNA from groundwater aquifers for radiocarbon analysis to determine the organic carbon pools fueling microbial reduction.
"We were able to separate the recent bomb pulse radiocarbon from the natural carbon signature and found the arsenic levels are now directly tied to a natural process as opposed to being driven by human activities," Buchholz said.
For further information see Bangladesh Arsenic.
Sampling image via Lawrence Livermore.