Superfund Research Program
Arsenic and Manganese Mobility: Land Use, Redox Shifts, and Environmental Sensors
Project Leader: Charles F. Harvey (Massachusetts Institute of Technology)
Grant Number: P42ES016454
Funding Period: 2010-2015
- Project Summary
Studies and Results
Researchers have known for over a decade that organic carbon can drive the biogeochemical reactions that mobilize toxic metals, particularly arsenic and manganese, from soils and aquifers into groundwater supplies. However, the source of this carbon is often very poorly understood. Is it carried in from the surface? If so, what is its origin? Or, is it derived from the sediments themselves? To address these questions the research team recently performed a set of experiments on water they collected from soils and aquifers at their field site in Munshiganj, a region where the groundwater is quite heavily contaminated by arsenic. Charles Harvey and his research team used radiocarbon analysis of the dissolved organic carbon in this water to determine whether the younger or older fraction of the organic carbon was more labile or mobile in the system. The researchers simply radiocarbon-dated the dissolved organic carbon, inoculated it with native microbes, added oxygen, and then radiocarbon-dated it again. Surprisingly, the radiocarbon age decreased, indicating that the older fraction reacted rather than the younger -- in other words, the fraction of carbon that is centuries older is more labile and therefore moving throughout the system. This result is surprising as one might think that if the old organic carbon was labile, it would have been oxidized over the last centuries. To test the generality of these findings the researchers collected water samples from an arsenic contaminated region of Vietnam, and observed the same phenomena. The researchers hypothesize that changes to the soils and aquifers can mobilize old organic carbon that drives reactions that release arsenic from the old sediments. This changes the way they think about the contamination problem -- they now need to determine what causes labile organic carbon from sediments to be released.
The researchers have begun looking forward to some of the work proposed in the renewal. They have collected data comparing rice yield to arsenic concentrations in rice field soils and irrigation water. They simply asked farmers to map the productivity of different areas of their fields in order to develop maps showing where yields are great, good, fair or bad. Then the researchers analyzed for arsenic in soils samples and irrigation water from these areas. The results have been striking -- there is a strong correlation between arsenic level and reduced rice yield. The researchers are now testing for nutrient levels and other potentially confounding factors. So far, it appears that arsenic alone is the most significant factor reducing rice yield.
The research teams' previous work revealed there is little relation of arsenic concentrations to manganese concentrations in the range of 0.2 to 1.0 mg/L, but when manganese exceeds 1.0 mg/L arsenic concentrations are extremely high, above 500 μg/L. This relationship is consistent with the reductive dissolution of arsenic-bearing minerals including those of Mn (III/IV), and suggests that dangerous manganese level will occur in water that also has dangerous arsenic concentrations. Now, in addition, the researchers have a better understanding of the sources of available and labile organic carbon in these systems, which is an important driver of potential exposures. Finally, although the researchers had long hypothesized that rice yield and arsenic concentrations were related, this had not been quantitatively shown. Now they have data to support that hypothesis, and can work with farmers in these regions to develop strategies for mitigating arsenic concentrations, thereby improving rice yields and by extension overall quality of life.