Quang M. Nguyen
July 7, 2020
Villagers in rural Cambodia use groundwater with arsenic concentrations above allowable levels. [Source: Scott Fendorf]
Arsenic in groundwater is considered as a thread to human health, affecting approximately 137 million people in 70 countries . In the Mekong delta – including the floodplains in Cambodia and Vietnam – arsenic is found in the alluvia deposited by sediments carried downstream from the Himalayas in form of iron oxides. From there, arsenic is released into groundwater through microbial reduction and chemical dissolution . Arsenic with elevated concentrations is detected in the Holocene Alluvium and Pleistocene Alluvium in the deltas across South and Southeast Asia in India , Nepal, West Bengal, Bangladesh, Cambodia and Vietnam (in the Red delta in the North and the Mekong delta in the South (MD/V)) . In the MD/V, besides the shallow Holocene and Pleistocene aquifers, arsenic with high concentrations is also found in the deep Pliocene and Miocene aquifers.
The presence of arsenic in the Holocene and Pleistocene aquifers has long been understood, but its presence in the Pliocene and Miocene aquifers in the MD/V remained a “mystery” until the 2000s with new findings from scientists. This article analyzes these findings in an attempt to understand the source and transport of arsenic in the Pliocene and Miocene aquifers in the MD/V.
Geology and hydrogeology of the MDV
The presence of arsenic in groundwater in the deep aquifers in the MD/V depends on the geology and hydrogeology of the region. Groundwater cannot flow through the impermeable clay layers, but it can flow easily in the permeable sand layers or aquifers under a difference in hydraulic heads or pressure.
Figure 1: The Mekong delta in Vietnam 
The MD/V is created by layers of ancient sediments at depth and those of more recent at or near the ground surface. These layers of sediments comprise of layers of sand (permeable) and clay (impermeable). A cross section of the MD/V from the Cambodia-Vietnam border to the ocean, as shown in Figure 2, shows that the Pleistocene and Pliocene aquifers have increasing thickness toward the ocean. The Pleistocene aquifer, at a depth ranging from 20 m to 90 m below ground surface (bgs) at the border, reaches a depth ranging from 100 m to 200 m bgs at the ocean. Similarly, the Pliocene aquifer, at a depth ranging from 130 m to 180 m bgs at the border, reaches a depth ranging from 240 m to 400 m at the ocean. Based on this inclination, the Pleistocene and Pliocene aquifers appear to reach the ground surface in Cambodia and commingle with the Holocene aquifer without interbedded clay layers.
Figure 2: Geology of the MD/V at Section AA’ (Figure 1) along the Mekong river 
Since the deep aquifers in the MD/V are connected with the shallow aquifers in Cambodia, they have very high pressure. Water levels in wells drilled into these deep aquifers rise very close to the ground surface, ranging from 1 m to 3 m bgs, as shown in Figure 3. These water levels are consistent with those in the shallow aquifers in Cambodia at elevation about 2 m, as shown in Figure 4. As a result, groundwater in these shallow aquifers migrates into the deep aquifers in the MD/V toward the ocean.
Figure 3: Groundwater levels in the MD/V 
Figure 4: Groundwater levels in the Mekong delta in Cambodia (MD/C) 
Scientists had long confirmed that arsenic, in the deltas across South and Southeast Asia, originates from arsenic-laden rocks in the Himalayas carried downstream by the rivers in the region. However, the presence of arsenic in the deep aquifers remains a mystery.
In order to unveil this mystery, Dr. Scott Fendorf, a professor of environmental Earth system science and a senior fellow at Stanford University's Woods Institute for the Environment, and two colleagues, Chris Francis, an assistant professor of geological and environmental sciences, and Karen Seto, now at Yale University, launched a field study in Asia in 2004. They began their investigation in the Brahmaputra River delta in Bangladesh, but the groundwater flow there is highly influenced by irrigation wells; therefore, they moved their work to the MD/C, which was chemically, biologically and geologically similar to Bangladesh but mostly undeveloped, as shown in Figure 5.
Figure 5: Stanford University’s research team in Cambodia
led by Dr. Scott Fendorf (white shirt) [Sources: Stanford University]
In 2009, they concluded that arsenic in sediments transported down by the Mekong river from the Himalayas was released within the first 2 to 3 feet (less than 1 m) bgs and entered the water. They estimated that it would take at least 100 years to migrate down into the aquifers below. They also showed that the 100-year migration of arsenic into the deep aquifers was a natural process that had been occurring for thousands of years. 
Although the Fendorf findings are consistent with current understanding of the source of arsenic in the Mekong delta, they contradict with the principles of hydrogeology when stating that arsenic in water migrates down into deep aquifers across the clay layers. This cannot happen because the deep aquifers are confined with very high pressures.
In 2013, in her “Dissertation Submitted to the Department of Environmental Earth System Science and the Committee on Graduate Studies of Stanford University in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy” , Dr. Laura Erban used a three-dimensional model to prove that “when low-arsenic, deep aquifers were over-pumped during recent decades, clay compaction began, leading to water containing arsenic and possibly other, arsenic-mobilizing solutes being squeezed out of dead-flow storage to adjacent aquifers, a process taking a decade or more.” This hypothesis was also presented in the Proceedings of the National of Academy of Sciences of the United States of America (PNAS) in the same year .
Figure 6: Erban hypothesis 
Figure 7: Pollution of arsenic in the MD/V 
According to available data, groundwater contaminated with high concentration of arsenic in the MD/V is common along the Mekong river between the border and the city of Can Tho. As shown in Figure 7, wells with arsenic concentrations above 100 µg/l are not located within areas of large land subsidence rate, as shown in Figure 8. Therefore, the Erban hypothesis does not seem scientifically justified.
Figure 8: Simulated land subsidence in the MD/V 
Another shortcoming of the Erban hypothesis relates to the amount and concentration of “arsenic-mobilizing solutes of dead-flow storage” in the clay layers that were squeezed out into the deep aquifer.
According to Dr. Erban, “The porewater concentrations of arsenic and arsenic-mobilizing solutes in deep confining clays of the Mekong Delta are not explicitly known, but available evidence suggests they may be high in arsenic-prone regions. Support for high solute concentrations in deep clays derives from a) dissolved arsenic concentrations in shallow clays of the Delta, deposited under similar paleoclimatic conditions, b) dissolved solute concentrations in confining clays in other regions of similar and older age, and c) consideration of the timescales for loss of arsenic from deep clays in the Delta context.”  However, arsenic concentrations in the shallow clays in the MD/V do not exceed 1,000 µg/l while the arsenic concentrations of the deep aquifers may reach 1,500 µg/l. The “other regions of similar and older age” are located outside a delta in North and Central America.
Since the effective porosity of the clay layers is very small, these clay layers cannot contain enough water with high arsenic concentrations to contaminate the adjacent aquifers, if it is squeezed out of the clay.
A reasonable explanation
Based on the geological structure and hydrogeological conditions, as mentioned above, together with the arsenic concentrations in the MD/C, as shown in Figure 9 - with arsenic concentrations in some wells may reach 6,000 µg/l  - it can be concluded that arsenic in the deep Pliocene and Miocene aquifers in the MD/V originates from the shallow Holocene aquifer in the MD/C. From there, arsenic migrates down into the Pliocene and Miocene aquifers and then toward the MD/V.
Figure 9: Arsenic in the Mekong delta in Cambodia 
If the deep Pliocene and Miocene aquifers have an average hydraulic conductivity or permeability) K = 3.15x10-4 m/sec (9.934 km/yr) , an effective porosity Θ = 0.17  and an average hydraulic head along the Mekong river from Phnom Penh, Cambodia to Can Tho, Vietnam i = 2.1x10-3 (a difference in elevations of 400 m in a distance of 190 km measured on Google Map); the average velocity of groundwater in the deep Pliocene and Miocene aquifers is approximately 123 m/yr (v = Ki/Θ). As a result, arsenic would take about 2,000 years to reach groundwater in the deep aquifers underneath Can Tho.
Summary and conclusion
Arsenic-laden rocks from the Himalayas were carried downstream by sediments in the Mekong river and deposited in the delta alluvia. Arsenic with elevated concentrations is found in the Holocene and Pleistocene aquifers in the deltas across South and Southeast Asia. In part of the Mekong delta in Vietnam, arsenic is also found in the deep Pliocene and Miocene aquifers and remains a “mystery” until the 2000s.
To unveil this mystery, the scientists from Stanford University conducted a field study in Cambodia in 2004. They concluded that arsenic in the sediments from the Himalayas was released to the water within the first 2 to 3 feet below ground surface and took at least 100 years to migrate down into the aquifers below. This finding contradicts with the principles of hydrogeology because water cannot migrate across the clay layers into the deep aquifers under confined conditions with very high pressures.
The presence of arsenic in the deep Pliocene and Miocene aquifers was also thought to be squeezed out of the interbedded clay layers, due to land subsidence resulting from decades of groundwater extraction. However, this hypothesis is not supported by scientific evidence.
Based on the geological structure, hydrogeological conditions, and arsenic concentrations in the Mekong delta, a reasonable explanation for the presence of arsenic in the deep Pliocene and Miocene aquifers is that it originates from the shallow Holocene aquifer in Cambodia, and migrates down then toward the deep Pliocene and Miocene aquifers in Vietnam. It may take approximately 2,000 years to reach the city of Can Tho.
About the author
Quang M. Nguyen was a professional engineer of the States of Florida and California. He worked for the National Water Resources Commission in Saigon, Vietnam; the Broward County’s Water Resources Management Division in Florida; and the Stetson Engineers Inc. in Los Angeles County, California, specializing in water resources and groundwater contamination. He retired in 2016.
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