Scott Fendorf (white shirt) poses with his team above a seasonal wetland that was dug out and flooded to simulate a permanent wetland environment. Credit: Scott Fendorf

Groundwater in South and Southeast Asia commonly contains concentrations of arsenic 20 to 100 times greater than the World Health Organization's recommended limit, resulting in more than 100 million people being poisoned by drinking arsenic-laced water in Bangladesh, Cambodia, India, Myanmar, Vietnam and China.

Stanford scientists have solved an important mystery about where the microbes responsible for releasing dangerous arsenic into groundwater in Southeast Asia get their food. Their findings, published in the journal Nature Geosciences, could guide future land management and future development.

Arsenic is bound to iron oxide compounds in rocks from the Himalayas, and gets washed down the major rivers and deposited in the lowland basins and deltas. Scientists know that in the absence of oxygen, some bacteria living in those deposited sediments can use arsenic and iron oxide particles as an alternative means of respiration. When they do this, however, the microbes separate the arsenic and iron oxides and transfer the toxin into underlying groundwater.

The mystery in this system, though, is an obvious source of energy that the microbes can tap to fuel the separation process.

"The question that really limits our ability to come up with predictive models of groundwater arsenic concentrations is how and why does the food they use vary across the landscape and with sediment depth," said Earth system scientist Scott Fendorf, a professor at Stanford's School of Earth, Energy & Environmental Sciences.

Field experiments

In their study, Fendorf and his team wanted to determine if the arsenic-releasing microbes were feeding on recent deposits of plant material located near the surface, or whether they were tapping into older biomaterial buried deeper in the subsurface. A second question they wanted to answer was, How does arsenic release vary across the landscape in Asia?

To address these questions, Fendorf and his team performed a series of field experiments. They collected sediment cores from two types of environments in the Mekong Delta in Cambodia: seasonal wetlands -- where the soil is saturated by rainwater for only part of the year -- and permanent wetlands, which are continually inundated.

"We focused on wetlands because that is the dominant type of landscape found in Cambodia, Vietnam and other countries affected by arsenic contamination," said Fendorf, who is the Huffington Family Professor in Earth Sciences and also a senior fellow, by courtesy, at the Stanford Woods Institute for the Environment.

Mixing sediments collected from different depths in vials with artificial groundwater revealed that the oxygen-deprived bacteria living in the upper few feet of permanent wetlands were releasing arsenic. However, water mixed with sediments gathered from the same shallow layers of seasonal wetlands was arsenic free.

The Stanford scientists hypothesized that bacteria residing in the shallow layers of seasonal wetlands were eating all of the digestible plant material during dry periods, when sediments are exposed to air and the microbes have access to oxygen. As a result, no food is left for the microbes when the floods returned, rendering them unable to cleave arsenic particles from iron oxides.

This hypothesis was confirmed when the scientists added glucose -- a carbon-rich and easily digestible sugar -- to the seasonal wetland vial and the microbes began releasing arsenic.

"The arsenic-releasing bacteria living in the shallow regions of seasonal wetlands are 'reactive' carbon limited -- that is, they don't release arsenic into the water because there isn't enough carbon available in a form they can use," Fendorf said.

The same experiment repeated with samples taken from about 100 feet underground -- the depth of most drinking wells in Asia -- showed that bacteria living deep beneath permanent and seasonal wetlands are similarly limited and and do not release arsenic into groundwater under normal conditions. The careful sleuthing has identified the bacteria in the permanent wetlands as the primary culprit of arsenic release.

Effects of land development

The work suggests that, under normal conditions, microbes in seasonal wetlands don't pose a significant threat for adding arsenic to groundwater. But what if the conditions changed, the scientists wondered, as could happen when the land is developed for other uses?

To answer this question, the team conducted a second type of experiment, in which they simulated the conversion of a small, remote seasonal wetland into a permanent one by digging out a seasonal wetland plot and keeping it permanently filled with water. As predicted, this resulted in the release of arsenic. (The amount was small and transient, Fendorf said, and people were never threatened by the experiment.)

The findings have large-scale implications for projecting changes in arsenic concentrations with land development in South and Southeast Asia and for the terrestrial carbon cycle.

"If you change the hydrology of a region by building dams or levies that change the course of the water, or if you change agricultural practices and introduce oxygen or nitrate into sediments where they didn't exist before, that will alter the release of arsenic," Fendorf said.

The above post is reprinted from materials provided by Stanford's School of Earth, Energy & Environmental Sciences.

Post a Comment

Sudarsan Sahu said... January 12, 2016 at 11:01 AM

Many many congrats to Dr Scott Fendorf. He has done really a great job. So much pain he has taken to reach the goal through conducting so much in-situ experiments. We people never realize the thing without such field demonstrations.
Earlier, we have drawn more than similar of such conclusions for the middle Ganga Plain (MGP), where arsenic level at places exceeds even 2000 ppb. Being people from Central Ground Water Board, we possess lots of field experience (geomorphology) and have exposure to number of shallow as well as deeper borehole lithologs (sedimentology and stratigraphy). Our understanding was also in the same line as Dr Fendorf. At different times through different publications, we emphasized on the following points:
1. Distribution pattern of groundwater arsenic bears significant correlation with the floodplain geomorphologic elements with characteristic stratigraphic sequence.
2. Elevated concentrations of arsenic are observed close to the organic matter (OC) rich clayey deposits in the abandoned/palaeochannel cut-offs.
3. The sandier areas (point bar ridges, levees) in the plain are low in arsenic.
4. The thickness of the dark grey to black coloured clay/mud cover overlying the shallow aquifer and the availability of OC are the controlling factors for arsenic release in MGP.
5. The spatial variability of groundwater arsenic is closely associated with the variability of OC in argillaceous sedimentary bodies and their release to groundwater.
6. The spread of the reducing environment depends on the volume of organic carbon release, hydraulic conductivity in the aquifer, groundwater flow, and the volume of fresh oxic water recharge.
7. We also hypothesized a source–distance relation between the arsenic concentration and the OC rich clay plugs.
8. Enhanced groundwater withdrawal and increase in frequency of withdrawal, causes high agitations and fluctuations in water table, causing release of more OC to groundwater, consequently increasing the level of arsenic in groundwater.
We do also say the microbes at shallow level consuming OC at the expense of available oxygen, thereby producing a reducing environment. Repeatedly we are talking of the variation in geomorphic environment in the floodplain and underlying lithology to be the cause behind spatial variability of groundwater arsenic. Again, I congrat Dr Scott Fendorf and his team, who have finally helped to erase the mystery of spatial variability of groundwater arsenic. Now, at least, no one sholud ponder over this and think beyond for its remedial measures and providing arsenic free potable water to lakhs of people threatened with arsenic poisoning. Below are given some of our publications with the above said findings.
Saha D, Sahu S, Chandra PC (2011) Delineating arsenic-safe deeper aquifers and their hydraulic parameters in parts of Middle Ganga Plain, Eastern India. Environ Monit Assess. doi 10.1007/s10661-010-1535-z.
Sahu S, Saha D (2013) Geomorphologic, stratigraphic and sedimentologic evidences of tectonic activity in Sone-Ganga alluvial tract in Middle Ganga Plain, India. Jour of Earth System Science. 123, No. 6, pp. 1335–1347.
Sahu S, Saha D (2014) Correlation between stratigraphy, flood plain geomorphology and arsenic distribution in groundwater of in Middle Ganga Plain, Bihar, India. Environmental Earth Science. ISSN 1866-6280. DOI 10.1007/s12665-014-3637-3.
Saha D, Sahu S (2014) A decade of investigations on groundwater arsenic contamination in Middle Ganga Plain, India. Environmental Geochemistry and Health. ISSN 0269-4042. DOI 10.1007/s10653-015-9730-z.
Sahu S, Saha D (2015) Locating safe shallow aquifers in groundwater arsenic contaminated areas of Middle Ganga Plain, India: A geomorphic and stratigraphic approach. Abstract Volume. India Water Week, Pragati Maidan, New Delhi.
Sahu S (2013) Hydrogeological conditions and geogenic pollution in parts of western Bihar. PhD thesis submitted to Department of Geology, Banaras Hindu University, Varanasi. Uttar Pradesh.