Abstract. To examine the occurrence and distribution of arsenic in groundwater,
137samples were collected from private wells in Southeastern Hanoi. According to
the concentrations of major cations and anions, groundwater compositions vary from
Ca-HCO3 to Na-HCO3 type. Groundwater has a very large range of chemical
compositions and the spatial variability of arsenic concentrations even between
samples collected within a few km of each other. Arsenic was detected in 103 out of
137 groundwater samples with high spatial variation, ranged from less than 5 to 549
µg/L (MDL = 2 µg/L, average 56.3 µg/L). Total Fe concentration varies over a wide
range, from below quantity limit (0.25 mg/L) up to 49.4 mg/L (average 9.7 mg/L).
Manganese concentration ranged from less than 0.1 to 8.1 mg/L with an average of
0.8 mg/L. Approximately 58%, 44% and 34% of samples exceed the Vietnam
National technical regulation on groundwater quality for Fe, Mn and As, respectively.
Arsenic showed poor correlations with other compositions in groundwater even with
the parameters which are common in reducing conditions and relate to releasing of
arsenic. These finding showed that more deep studies need to be carried out to have
better understanding about arsenic occurrence in groundwater.
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HNUE JOURNAL OF SCIENCE DOI: 10.18173/2354-1059.2017-0053
Chemical and Biological Science 2017, Vol. 62, Issue 10, pp. 44-51
This paper is available online at
CHEMICAL CHARACTERISTICS OF GROUNDWATER
IN SOUTHEASTERN HANOI
Tran Thi Mai
1
, Pham Thi Kim Trang
1
, Vi Thi Mai Lan
1
, Vu Thi Duyen
1
, Dao Viet Nga
1
,
Dao Manh Phu
2
, Bui Van Minh
1
, Pham Hung Viet
1
and Benjamin C. Bostick
3
1
Research Center for Environmental Technology and Sustainable Development (CETASD),
University of Science, Vietnam National University
2
Vietnam Environment Administration, Ministry of Natural Resources and Environment
3
Lamont-Doherty Earth Observatory (LDEO), Columbia University, Palisades, New York 10964, USA
Abstract. To examine the occurrence and distribution of arsenic in groundwater,
137samples were collected from private wells in Southeastern Hanoi. According to
the concentrations of major cations and anions, groundwater compositions vary from
Ca-HCO3 to Na-HCO3 type. Groundwater has a very large range of chemical
compositions and the spatial variability of arsenic concentrations even between
samples collected within a few km of each other. Arsenic was detected in 103 out of
137 groundwater samples with high spatial variation, ranged from less than 5 to 549
µg/L (MDL = 2 µg/L, average 56.3 µg/L). Total Fe concentration varies over a wide
range, from below quantity limit (0.25 mg/L) up to 49.4 mg/L (average 9.7 mg/L).
Manganese concentration ranged from less than 0.1 to 8.1 mg/L with an average of
0.8 mg/L. Approximately 58%, 44% and 34% of samples exceed the Vietnam
National technical regulation on groundwater quality for Fe, Mn and As, respectively.
Arsenic showed poor correlations with other compositions in groundwater even with
the parameters which are common in reducing conditions and relate to releasing of
arsenic. These finding showed that more deep studies need to be carried out to have
better understanding about arsenic occurrence in groundwater.
Keywords: Arsenic, groundwater, chemical characteristic, Southeastern Hanoi.
1. Introduction
The presence of trace elements in groundwater is an important issue because it affects
possible use of water. Particularly, arsenic contamination of groundwater that used for
drinking is major concern to human health due to its neurotoxicity and potential to cause
cardiovascular diseases and different types of cancer, of which skin and bladder cancers
are most common [6]. Natural As in groundwater at concentration above the World
Health Organization Maximum Contaminant Level (WHO, 2004) of 10 µg/L have been
Received November 22, 2017. Revised December 9, 2017. Accepted December 16, 2017.
Contact Pham Hung Viet, e-mail address: phamhungviet@hus.edu.vn
Chemical characteristics of groundwater in Southeastern Hanoi
45
reported in many countries such as Bangladesh, India, Vietnam, China, Argentina, Chile,
Mexico, the US and Spain [1] with at least 150 million people are directly affected by As
contamination all over the world [6].
There are several hypotheses to explain how arsenic released from sediment into the
groundwater. Most research suggests that arsenic initially is present as Arsenate (As V)
within the Fe-oxide crystal lattice and during microbial or/and chemical reductive
dissolution of the Fe-oxide, arsenic are released in arsenite (As III) ion states [3]. An
alternative, but somewhat complementary release mechanism suggested by some
researchers is that arsenic released by competition the absorbed sites on the Fe-oxide with
other ions such as HCO3
–
, PO4
3 –
[8], many of which are created as byproducts of the
reductive dissolution of iron oxides by microorganisms. In reality, both of these
mechanisms could be important but separated in time and space, because groundwater
travels through sediments in aquifers, and thus separates dissolved arsenic, phosphate and
bicarbonate from their sources. The composition and environmental condition of sediment
and groundwater is heterogeneous, and, combination with the mobility of dissolved As,
this leads to the complicated distribution of arsenic in groundwater. Within the Red river
delta in Vietnam the spatial variability of arsenic concentrations is considerable over
distances of a few km, next to the location has low arsenic concentration (less than 5
µg/L) is the location has very high arsenic level (~700 µg/L) [5]. Therefore, it is
necessary to have detail studies in specific areas to access groundwater quality, especially
the arsenic distribution in groundwater, as a basis to evaluate and avoid the impact of
contaminated water on human health.
The main objectives of this study were to characterized the chemistry of groundwater
and investigated the extent of natural As distribution and the correlation between As and
other compositions in groundwater in Southeastern Hanoi, and to use this information to
understand in greater detail what controls the spatial distribution of arsenic, and its
evolution.
2. Content
2.1. Materials and methods
2.1.1. Sample collection
A total of 137 samples private wells were sampled during 2013 - 2014 over an area of
about 200 km
2
in Southeastern Hanoi, included some districts: Dong Da, Thanh Xuan,
Hai Ba Trung, Hoang Mai, Thanh Tri, Thanh Oai, Long Bien, Gia Lam - Hanoi city and
Van Giang district - Hung Yen province. Samples were collected from random private
wells with the density of one sample per about 2 km
2
. The private wells screened at
various depths from 7 to 80 m, but most were screened at 30 – 40 m.
Samples were taken after 5 - 15 minutes pumping, when the dissolved oxygen
concentration (DO) and electrical conductivity (EC) in the water reached stable values.
Water temperature (T), pH and redox potential were also recorded on-site by HACH and
Mettler-Toledo equipment.
T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick
46
Figure 1. The investigated area in Southeastern Hanoi
(The circles show the well locations and As concentration depicted by color and size)
Groundwater was filtered (0.45 µm cellulose acetate filters) on-site in two
polypropylene bottles. One bottle for the analysis of metals, ammonium and phosphate
was acidified with concentrated nitric acid to reach a pH < 2. Dissolved organic carbon
(DOC) samples were filtered with nylon 0.2 µm filters 24 ml glass vials (washed and
baked at 500
o
C in 6 hours before sampling) and acidified with concentrated hydrochloric
acid to pH < 2. All samples were stored in a refrigerator at 4
o
C until laboratory analyses.
2.1.2. Chemical analysis
Metal elements were analyzed by flame atomic absorption spectrophotometer on a
Shimadzu AAS 6800 instrument. Total arsenic was determined on the same instrument
using HVG hydride generator connected to AAS 6800. Determination of total arsenic
requires that all inorganic arsenic compounds be in the As (III) state. Therefore, As (V) in
groundwater was reduced to As (III) with sodium iodide before reaction with sodium
borohydride to produce arsenic hydride AsH3 by HVG. The minimum detection limit for
theses analyses is 2 µg/L for As, and similar values for other elements. Accuracy was
tested by analyzing ICP Multi element standard IV (1.11355 - Merck), ICP Multi element
standard IX (1.09494 - Merck), IC multi element standard I (1.70398 - Merck). Replicate
analyses were carried out to check precision of the results; accuracy and precision were
within the range of ±10% and relative standard deviation (RSD) less than 5%. Major
anions like chloride (Cl
–
), sulfate (SO4
2–
), nitrate (NO3
–
) and other anions were
determined by ion chromatography using a Shimadzu LC20AD/HIC-20ASuper.
Groundwater samples were PTFE 0.2 µm filtered before analyzed by instrument. While
NH4
+
, SiO3
–
and PO4
3-
, were determined by UV-VIs (Shimadzu UV-3101) using
respectively the nitroprusside, the ammonium molybdate and ascorbic acid methods. The
minimum detection limit for theses analyses is 5 - 10 ppb for most anions.
Chemical characteristics of groundwater in Southeastern Hanoi
47
DOC concentration was analyzed on Total organic carbon analyzer (Shimadzu,
TOC-VCSH). The principle of DOC analysis is determined carbon dioxide generated by
680
o
C combustion catalytic oxidation after acidified and sparged to remove inorganic
carbon and purge-able organic carbon. This method required blank subtraction, but
resulted in stable and reproducible DOC concentrations when DOC exceeded ~0.1 mg/L
(as it did for all samples).
2.2. Results and discussion
2.2.1. Chemical characteristic of groundwater
Groundwater in studied area is about neutral; with pH ranging from 6.1 to 7.6.
Arsenic levels were usually higher in groundwater with pH of 7 or greater. Temperature is
around 26
o
C and specific conductivity varies from 147 to 2040 µS/cm (average
569.5 µS/cm). Field measured Eh values ranged from -169.5 to +160.8 mV. Almost 70%
investigated wells have reduced Eh values, indicated reducing conditions that is favorable
for process of release arsenic from sediment into groundwater and 30% studied wells have
oxidizing conditions.
According to the concentrations of major cations and anions, the groundwater
composition varies from Ca-HCO3 to Na-HCO3 type (Fig.2), as a results of alumino-
silicate weathering with carbonic acid, as well as dissolution of carbonate minerals,
followed by Ca and/or Mg ion exchange with Na on clay minerals [8]. The SiO3
-
concentration ranges from 6 to 32.1 mg/L, but most were 25 - 30 mg/L, near the solubility
limit for most SiO2 polymorphs. These concentrations reflect dissolution of silicate
minerals in the aquifers, followed by re-precipitation of secondary phases.
Figure 2. Piper diagram consisting of major ions in groundwater
in Southeastern Hanoi
SO4
2–
, NO3
–
and Cl
–
may trace anthropogenic input to groundwater in agriculture
areas, or be derived from sewage. However, in most of the wells, SO4
2–
, NO3
–
and Cl
–
exhibit relatively low concentrations. 53% and 94% of the samples contained less than 0.2
mg/L of SO4
2–
and NO3
–
, respectively, while Cl
–
concentrations range from 1.4 to 180.4
mg/L (average 36.4 mg/L) with a medium value of 19.9 mg/L. These low levels suggest
T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick
48
that anthropogenic inputs are not significant in the area. Alternatively, the SO4
2–
, NO3
–
could be removed by reduction in reducing waters, but the salty solutions would like
persist, resulting in variable Na and Cl
-
concentrations. Given the occasionally higher Cl
-
levels found in the environment, it is possible that some of these components have entered
groundwater but not been preserved.
Total Fe concentration varies over a wide range, from below quantity limit (0.25
mg/L) up to 49.4 mg/L (average 9.7 mg/L). The generally high concentrations of Fe found
in most wells indicate that most of the waters are reducing. Manganese concentration
ranged from less than 0.1 to 8.1 mg/L with an average of 0.8 mg/L. Almost 58% and 44%
of samples exceed 5 mg/L Fe and 0.5 mg/L Mn, respectively, which are the Vietnam
National technical regulation on groundwater quality (QCVN 09-MT:2015/BTNMT).
These regulations are based on the suitability of water for use rather than health impacts,
but indicate that the water is often sufficiently metallic as to have problems with metallic
taste and will change the color of food such as rice prepared in untreated groundwater.
2.2.2. Arsenic in the groundwater and the correlation between As and other chemical
parameters
The spatial distribution of arsenic in groundwater in Southeastern Hanoi is illustrated
in Fig.1. High arsenic concentration wells are mainly in Thanh Tri and Hoang Mai district
(center of the map in Fig. 1 and Fig. 3a), those areas could be contaminated due to arsenic
mobilization caused by extensive pumping in the water supply plants. Arsenic was
detected in 103 out of 137 samples from study area with concentration ranging from less
than 5 to 549 µg/L (average 56.3 µg/L). Approximately 34% of the examined wells
exhibited arsenic concentration higher than 50 µg/L, which is the Vietnam National
technical regulation on groundwater quality. More than half of wells that contained
measurable As exceeded 10 µg/L (about 43% of all wells), which is WHO guideline.
Several wells have high As concentrations (on the right side of Red river in Fig.3a, b),
showed relatively high values of Fe concentrations. However, in most of the wells showed
As concentrations are not well correlated with the concentration of Fe (r = 0.13, p = 0.129),
this is not the same with what is generally expected in case of As release from reductive
dissolution of Fe oxide in such reducing environments [9, 10], which implies additional
mechanisms affect either Fe levels, or As levels. The wells had lowest Fe concentrations;
also showed the highest pH values and contrast in other wells. These observations may be
relate to an increase of the net Fe precipitate rate, when pH increases to the neutral range,
resulting in lower Fe concentration in solution [8], even when reduction can release As
into solution.
From the redox perspective if conditions are reducing enough to release As, they are
certainly reducing enough to release Mn, therefore when As concentration has high value,
Mn also has high value of concentration. However, the groundwater data in this study
reveals that sites where As concentrations is high (from 100 to 549 µg/L), also have
relatively low Mn concentrations (less than 0.1 to 1.6 mg/L) (Fig. 3a, c). This contrast
can be explained that Mn re-precipitated with dissolved carbonate which is created from
biodegradation organic matter, resulting decrease Mn concentration in groundwater [7].
Indeed, HCO3
-
concentrations showed relatively high values, ranged from 65 to 1310
mg/L (average 327 mg/L).
Chemical characteristics of groundwater in Southeastern Hanoi
49
Figure 3. Spatial distribution of As (a), Fe (b), Mn (c), NH4-N (d) in groundwater
in Southeastern Hanoi (the well depths range from 7 to 60m)
The high NH4-N concentration of some studied wells (Fig. 3d) is explained to be
derived as the product of microbial decomposition of organic matter or the
ammonification of organic nitrogen in peat therefore NH4-N is the factor express reducing
condition and normally has a positive correlation with As concentration in groundwater [4].
However, in this study, the NH4-N concentration was not significantly correlated with As
level (r = 0.142, p = 0.09), which is not similar with some previous researches in
Bangladesh, Nepal and Vietnam [4]. The detail role of NH4-N concentration in increasing
As concentration cannot be clarified from the chemical parameters in this study, and must
be examined further. We tentatively suggest that the ammonium is retained in sediments
through exchange, or that most of the waters have traveled from their sources, resulting in
the redistribution of arsenic and ammonium during transport, and the apparent decoupling
of As levels. For example, at the source, arsenic and ammonium levels are correlated in
solution because of reductive dissolution, but the arsenic is retarded by interacting with
the sediment (retardation factor of 10-15, based on a partition coefficient of 10 - 20, van
Geen et al., 2013), while ammonium is both taken up as a nutrient by plants through
geological window, and travels with a different retardation factor. Thus, at a given
distance away, the two parameters can be either positively or negatively correlated even
though they are mechanistically related.
T. T. Mai, P. T. K. Trang, V. T. M. Lan, V. T. Duyen, D. V. Nga, D. M. Phu, B. V. Minh, P. H. Viet and B. C. Bostick
50
Table 1. Pearson correlation coefficients
AsT FeT Mn
2+
NH4-N HCO3
-
NO2
-
SO4
2-
NO3
-
AsT 1
FeT 0.130 1
Mn
2+
-0.137 -0.207 1
NH4-N 0.142 0.075 -0.176 1
HCO3
-
0.240 0.215 -0.127 0.685 1
NO2
-
-0.103 0.091 -0.023 -0.074 -0.033 1
SO4
2-
-0.208 0.017 0.257 -0.143 0.097 0.198 1
NO3
-
-0.085 -0.135 -0.054 -0.055 0.026 0.438 0.309 1
Pearson correlation coefficients have been calculated to examine possible
relationships among the measured parameters (Table 1). Arsenic exhibited weak
correlation with HCO3
–
(r = 0.24). Although, the relationship between arsenic and
bicarbonate are well known that bicarbonate can play an important role in the
mobilization of arsenic through the competition for adsorption sites [1, 2]. Weak negative
correlations of arsenic with NO3
-
, SO4
2-
and NO2
–
are showed by minus correlation
coefficients. One caveat in examining correlations is that the correlations assume linear
forms, something limiting. In the case of sulfate, this is extreme. Sulfate concentrations
are nearly universally low (a small dynamic range) but variable in high As wells, and
when sulfate is high the As is universally low. Thus, this inverse relationship is obvious,
but it is not well fit by a line, and thus results in a low quality fit with negative correlation
when regressed onto a line.
3. Conclusion
The groundwater composition varies from Ca-HCO3 to Na-HCO3 type. Groundwater
has a very large range of chemical compositions and the spatial variability of arsenic
concentrations is considerable over distances of a few km. Arsenic was ranged from less
than 5 to 549 µg/L (average 56.3 µg/L) with spatial distribution are possible affected by
extensive pumping by water supply plants. Approximately 58%, 44% and 34% of samples
exceed the Vietnam National technical regulation on groundwater quality for Fe, Mn and
As, respectively. Arsenic concentration showed weak correlations with other
compositions in groundwater even with the parameters which are common in reducing
condition, reflecting that secondary processes are occurring that also affect the levels of
As, Fe and other elements, and also suggesting that the arsenic may have traveled from its
source, which weakens or changes correlations spatially during travel. Need to be
examined further on sediment in the studied area to understand these correlations.
Acknowledgements. The authors acknowledge the valuable support through the
PEER-based Research Project Grant 544 funded by US-AID/NSF (2013 - 2015) named
"Evaluating the Sustainability of Ground Water Resources: Academic and Scientific
Gaps".
Chemical characteristics of groundwater in Southeastern Hanoi
51
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