Quang M. Nguyen
July 7, 2020
Villagers
in rural Cambodia use groundwater with arsenic concentrations above allowable
levels. [Source: Scott Fendorf]
Introduction
Arsenic in groundwater is considered as a thread to human health,
affecting approximately 137 million people in 70 countries [1]. 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 [2]. Arsenic with elevated concentrations is
detected in the Holocene Alluvium and Pleistocene Alluvium in the deltas across
South and Southeast Asia in India [3], Nepal, West Bengal, Bangladesh, Cambodia
and Vietnam (in the Red delta in the North and the Mekong delta in the South
(MD/V)) [4]. 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
[5]
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 [5]
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 [4]
Figure 4: Groundwater levels in the
Mekong delta in Cambodia (MD/C) [6]
Fendorf Investigation
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. [7]
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.
Erban Hypothesis
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” [8], 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 [4].
Figure 6: Erban hypothesis [4]
Figure 7: Pollution of
arsenic in the MD/V [4]
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 [5]
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.” [8] 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 [9] - 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 [6]
If the deep Pliocene and Miocene aquifers have an average hydraulic
conductivity or permeability) K = 3.15x10-4 m/sec (9.934 km/yr) [10],
an effective porosity Θ = 0.17 [8] 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.
References
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