Effect of Groundwater Flow on Forming Arsenic Contaminated Groundwater in Sonargaon, 1
Bangladesh 2
3
Shinji Nakayaa, Haruyasu Natsumea, Harue Masudab, Muneki Mitamurab, Dipak Kumar Biswasc,
4
Ashraf A. Seddiqued
5 6
a
Department of Civil Engineering, Shinshu University, Nagano, Japan
7 b
Department of Geosciences, Osaka City University, Osaka, Japan
8 c
Asian Arsenic Network, Jessore, Bangladesh
9 d
Department of Petroleum and Mining Engineering, Jessore Science and Technology University, Bangladesh
10 11
Abstract 12
Three-dimensional groundwater flow in Sonargaon, Bangladesh is numerically simulated in order
13
to evaluate the flow paths of As-contaminated drinking groundwater in the Holocene aquifer of the
14
Ganges-Blamaptra-Meghna delta plain over a recent thirty-year period. The model indicates that
15
vertical infiltration of surface groundwater into the shallow Holocene aquifer occurs frequently in
16
the Ganges-Blamaptra-Meghna delta plain. It predicts that the water recharged from ground surface
17
moves approximately 10 m to 20 m vertically downward beneath the flood plain, with a gradually
18
increasing horizontal flow, toward the underlying Pleistocene middle mud layer (aquitard). The
19
model also predicts that groundwaters containing highest As concentrations (> 700 μg/L) are
20
formed on the vertical groundwater flow paths where surface water recharges the Holocene aquifer
21
and not on the horizontal flow paths. Combining with the groundwater chemistry, reducing
22
groundwater condition is not essential for the As-contaminated groundwater of the studied area in
23
the Ganges delta plain.
24 *Manuscript
25
Introduction 26
Large-scale natural arsenic groundwater contamination has been a serious problem in numerous
27
areas of the world, especially in Asian countries. In many cases it has a major impact on potable
28
water. Such contamination is typically associated with elevated pH in arid or semi-arid areas, or
29
strongly reducing conditions in geologically young sedimentary basins (e.g., Smedley and
30
Kinniburgh, 2002). In the most polluted area, approximately 35 million people in Bangladesh and 6
31
million in West Bengal, India are exposed to high levels of As (> 50 μg/L) in drinking water
32
(Geological Survey and Department of Public Health Engineering, 2001; Smedley and Kinniburgh,
33
2002; Nath et al., 2008). Elevated levels of groundwater As (> 50 μg/L) in this basin occur on a
34
large scale in strongly reducing alluvium-based aquifers at near-neutral pH, and in flat areas with
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sluggish shallow groundwater flow. The mechanism and triggers of As dissolution from minerals to
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groundwater has been addressed by extensive research, and the microbial reduction and dissolution
37
hypothesis, in which As enriched Fe-oxyhydroxides/oxides are decomposed and release the As into
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the aquifer under reducing groundwater conditions, is widely accepted (e.g., Nickson et al., 2000;
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McArthur et al., 2001; Bhattacharya et al., 2002; Smedley and Kinniburgh, 2002; van Geen et al.,
40
2003, 2004; Horneman et al., 2004; Zheng et al., 2005). In more recent studies, chemical
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weathering of detrital basic minerals such as biotite in the aquifer, due to the enforced infiltration of
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surface water, has been suggested as an essential mechanism for releasing As into the aquifer (e.g.,
Seddique et al., 2008; Itai et al., 2008).
44
In previous studies, it has been well documented that As-contaminated and As-free/less
45
contaminated areas are distributed as patches in close proximity to each other (e.g., Smedley and
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Kinniburgh,2002; van Geen et al., 2003; Ravencraft et al., 2002; Mitamura et al., 2008; Itai et al.,
47
2008). Numerous researchers previously considered that the highly contaminated As groundwater
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appeared in stagnant regions of the aquifer and that dissolved As was flushed out of deep aquifers
49
but not from the shallow aquifers, due to low groundwater mobility. Harvey et al. (2002; 2003),
50
however, documented that the circulation of local groundwater occurred on a scale of a few hundred
51
meters in diameter, and that such small-scale circulation activated in this thirty-year period
52
promoted As release into the groundwater. These results are considered valuable because it
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revealed that the groundwater flow system was an essential factor causing As groundwater
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contamination in the Ganges delta plain (e.g., Geological Survey and Department of Public Health
55
Engineering, 2001; Harvey et al., 2006; Klump et al., 2006; Mukherjee et al., 2007; Michael and
56
Voss, 2009; Neumann et al., 2010; Mukherjee et al., 2011). However, few studies focusing on the
57
relationship between the distribution of varying As contamination levels and groundwater systems
58
have subsequently been published.
59
By conducting a series of studies in Sonargaon, Bangladesh, we realized that the vertical
60
infiltration of surface water into the shallow aquifer promoted As contamination. In those studies,
61
Mitamura et al. (2008) reported that wells installed into fine sediments were highly contaminated by
As (highly As-contaminated wells were occasionally installed into fine micaceous sediments), and
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that the geological structure of the aquifers was an important factor controlling the formation of
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As-contaminated groundwater in Bangladesh. Seddique et al. (2008) reported that detrital biotite
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was a primary source of As and that chemical weathering of this mineral was an essential
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mechanism affecting the chemical composition of groundwater, including the As concentration
67
thereof (Itai et al., 2008). Heterogeneous distribution of major chemical components and the
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isotopic ratios of oxygen and hydrogen in the groundwaters indicated vertical infiltration of surface
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groundwater into the shallow sediments and short groundwater recharge paths (Itai et al., 2008). In
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this study, three-dimensional transient groundwater flow is simulated with realistic assumptions of
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hydraulic constants and boundary conditions of the geological structure, focusing on the
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relationship between the flow paths and the concentrations of As in contaminated drinking
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groundwater over a recent thirty-year period in the shallow groundwater system of the Holocene
74 aquifer. 75 76 Method 77
Transient three-dimensional groundwater flow was simulated using a numerical model to
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document the flow paths and the residence time of As-contaminated groundwater in the Holocene
79
aquifer of the study area over thirty years. The model was constructed based on a data set including
80
topography; geological structure; hydraulic constants; tube well information (numbers, locations
and screen depths of wells drilled less than 40 m deep that are used mainly for drinking water); the
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population living at each levee where there are settlements; transient water head records for three
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observation wells installed at three different depths at the same site; temporal groundwater level
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records at eight hand-auger drilling sites; irrigation; precipitation; evapotranspiration; and assumed
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boundary conditions of aquifers surveyed in 2003 through 2006. The ground surface of all study
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area can be recharge area and the river zone can be recharge or discharge area depending on
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hydraulic gradient between river water head and groundwater head. Of the data used in this study,
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the temporal groundwater level changes and geochemical characteristics such as major and minor
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compositions, H, O, N and S isotopes, tritium units, etc., were already published in the previous
90
reports (Mitamura et al., 2008; Itai et al., 2008).
91
92
Site description and hydrogeological setting 93
The study area is 3.0 km × 3.0 km in size, located 20 km east of Dhaka, in Sonargaon,
94
Bangladesh. The area is composed of terraces in the west and a flood plain, including natural levees
95
along the old Brahmaputra River, which is an abandoned channel of the present Meghna River.
96
Except for the natural levees, the flood plain is inundated during the rainy season. During the dry
97
season, the floodplain dries and is used for cultivation, and the channel becomes narrower and
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consists of a series of disconnected ponds (Fig. 1). The flat surface of the ground, which only
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varies 4 m to 5 m in altitude, and the meandering river channel suggest a very low hydraulic
gradient and/or sluggish groundwater flow beneath the flood plain. Drinking water is drawn from
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groundwater through tube wells installed into shallow and deep aquifers in the populated western
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terrace and levees. Aquifers less than 90 m deep in the study area consist of upper and lower sand
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formations separated by a mud layer; the upper Holocene sand formation is commonly 25 m to 35
104
m thick and overlies the Upper Pleistocene mud formation, which unconformably overlies the lower
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Plio-Pleistocene sand formation (Mitamura et al., 2008) (Fig. 1). The upper sand formation thus
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comprises the shallow unconfined aquifer, and the lower formation is the deep confined aquifer. The
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upper sand formation hosts thin intercalated clay to silt layers in the upper section, and medium
108
sand in the lower section. The upper sand formation can, therefore, be divided into the uppermost
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sand and the upper sand layers at approximately 0 m altitude. The mud formation, which functions
110
as an aquitard, also includes intercalated lenses of very fine sand to silt. The lower sand formation is
111
exposed as terraces in the western part of the study area. The upper sand formation thus abuts the
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lower sand formation with an inferred fault along the boundary between the terraces and the alluvial
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flood plain. The mud and lower sand layers are partly eroded to form valleys in this area.
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115
Model description 116
Three-dimensional groundwater flow in unsaturated-saturated porous media is modeled by the
117
governing equation as follows:
118
t S C q K K x K K x ij i s 3 r r( ) ( ) ( ) ; i, j = 1, 2, 3 (1) 119where is hydraulic head, xi is a spatial coordinate, t is time, Kij is the saturated hydraulic
120
conductivity, is the volumetric water content (= nSr), n is porosity, Sr is the saturation index 121
(ranging from 0 to 1), Kr(θ) is the unsaturated-saturated hydraulic conductivity ratio (also ranging 122
from 0 to 1), C() is the specific water content (=d/d), Ss is the specific storage, and is a 123
function with a value of 1 when Sr = 1 and 0 when Sr < 1. 124
Equation (1) is solved for unknown under appropriate boundary conditions. In the present study,
125
because unsaturated-saturated seepage flow including rain, evaporation and irrigation recharge in
126
unconfined aquifer is analyzed, the unsaturated-saturated three-dimensional seepage flow analysis
127
code AC-UNSAF3D (developed by Okayama University, Japan in 1980s and opened as a standard
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code since 2000 in Japan: http://gw.civil.okayama-u.ac.jp/gel_home/download/index.html) was
129
applied with a three- dimensional finite element method (3D-FEM) scheme (Segol, 1977; Frind and
130
Verge, 1978; Huyakorn et al., 1986; Nishigaki et al., 1992) to document the three-dimensional
131
groundwater flow in this study area using a one month time step. This code is specified for
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advective flow only.
133
134
Model ofstratigraphic structure and hydraulic properties
135
In the present FEM model, a region 3500 m in the x direction, 3000 m in the y direction and
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114 m in the vertical (z) direction is divided into 106567 rectangular voxels of 100 m × 100 m ×Δz
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[m] (Table 1). Because a rectangular grid system with small Δz/Δx and Δz/Δy ranging from
5x10-3 to 2x10-2 was adopted, FEM was used instead of finite difference method from the point of
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analytical precision. The length in the x direction (3500 m) was extended 500 m eastward from the
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study area to avoid boundary effects. Elements above the ground surface are excluded from the
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model. To construct the model, the hydrogeological structure of the area is divided into three layers;
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the upper sand formation, of which the uppermost S1 and S2 layers were defined from sediment
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particle size, the lower sand formation S3, and the mud formation M, which functions as an aquitard
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separating the upper and lower sand formations. The coordinates (x, y, z) of the boundary between
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the aquifers and aquitard are defined based on lithostratigraphic data (Mitamura et al., 2008) of
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sediment columns at 12 drilling sites by linear interpolation using the Surfer code (Golden Software,
147
Inc.) considering well depth of the 230 tube wells. For evaluating the volume of well water
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withdrawn, well depths are assumed to be the same as the screen depths of the 230 tube wells we
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studied. Figure 2 shows the contour map of the ground surface from SRTM 90 m digital elevation
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data originally produced by NASA and the estimated geological structures. Depressions or buried
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valleys, which partly eroded the mud formation, appeared as two WNW–ESE furrows and an
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intermittent NNE-SSW furrow beneath the flood plain. One of the furrows along the WNW–ESE
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direction, which includes drill site DRK-D1 (Figs. 1 and 2(e)) lacks the middle mud formation, and
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the upper and lower sand formations connect directly.
155
Hydraulic properties determined by in situ pumping tests (Mitamura et al., 2008) were assigned
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to each element of the model according to the boundary surfaces between S1, S2, S3, and M (Table
2). The hydraulic constants of aquitard M, the specific storage Ss, and the nonlinear hydraulic 158
properties of suction potential (– relationship) and hydraulic conductivity (Kr– relationship) for 159
each sedimentary formation are defined without hysteresis in the wetting and drying curves in the
160
unsaturated zone of the aquifers (Bear, 1972; Marsily, 1986; Huyakorn et al., 1986; Nishigaki et al.,
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1992). If an element of the model is divided by the layer boundary surfaces, weighted average
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values of the hydraulic constants are assigned to the element according to the volume occupied by
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each aquifer or aquitard.
164
165
Simulation conditions 166
Under the hydrogeological setting described above, groundwater flow in the FEM model was
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primarily determined for a thirty-year period simulating the conditions associated with temporal
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changes in the level of the water table; precipitation, evapotranspiration, and groundwater pumping
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rates for daily use and irrigation. The rate of groundwater pumping for daily use was estimated
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based on data gathered in 2005 on the number of families, population, number of tube wells, and
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well screen depth in each village (Table 3). The average monthly precipitation during 2005 in
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Bangladesh (Fig. 3) was assigned to the nodal points on the ground surface. The floodwater table on
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the flood plain along the old Brahmaputra River was assigned to nodal points on the ground surface,
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set to vary with temporal changes of the groundwater potential observed in 2005 in the shallow
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aquifer (upper sand formation) in wells DRK-W2(-18 m to -27 m) and DRK-W3 (-6 m to -12 m)
(Fig. 4). The monthly evapotranspiration (Fig. 5) was assigned to nodal points on the ground
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surface estimated from the average monthly temperature between 1957 and 1987 at the Rangamati
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meteorological observatory (22.63°N, 92.20°E, 63 m above sea level (ASL)), Bangladesh, using the
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Thornthwaite equation (Thornthwaite, 1948). Fresh water withdrawal for domestic use in
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Bangladesh has been estimated to be 13 L/(personday) in 1987 and 49 L/(personday) in 2000
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(The World‟s Water; http://www.worldwater.org/data.html). 100 L/day/person was assumed as a
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maximum value for recent withdrawals from the tube wells based on these figures. The Bangladesh
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Rehabilitation Assistance Committee Office in Narayanganj, Bangladesh estimated well water
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usage of 4 L/(personday) for this area (pers comm.). Thus, a constant pumping rate of either
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4 L/(personday) or 100 L/(personday) was assigned to nodal points at the well screen positions at
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5 m vertical intervals. The volume withdrawn from each well was calculated from the pumping rate,
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local population, and numbers of tube wells, with flow percentages attributed to each well in each
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village by depth (Table 3). Withdrawal of groundwater from the deep aquifer and recharge via its
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distribution as much as the withdrawal onto the ground surface for irrigation during the dry season
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were also taken into account in the numerical simulation using data for the 2003-2004 irrigation
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season (January to April) in Munshiganj, Bangladesh (Harvey et al., 2006). The withdrawal from
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the deep aquifer for seasonal irrigation (January to April) as much as the α percent of the
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withdrawal (Qw) given by Harvey et al. (2006) (Fig. 3) and the seasonal recharge promoted by 194
irrigation as much as the α percent of the Qw were assigned to the nodal points at z = -70 m ASL 195
beneath the flood plain and at the ground surface, excluding levees, respectively, for the period from
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January to April. In the present study, α was determined to be 75 % in the calibration by trial and
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error as described in the chapter „Result‟. The volume of irrigation water taken from the deep
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aquifer is much greater than that withdrawn for drinking water in this area; the water recharged due
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to irrigation during the dry season is a maximum of 45 mm/month (The World‟s Water;
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http://www.worldwater.org/data.html). The boundary conditions of inflow rate and potential head at
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the ground surface nodes for the thirty modeled years were, therefore, controlled by monthly
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changes in precipitation, evapotranspiration, irrigation groundwater recharge and the level of the
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water table. As described before, horizontal groundwater flow direction will be macroscopically
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from north to south and horizontal hydraulic gradient will be very low due to the flat surface and the
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meandering river channel. Considering the temporally large changes of groundwater head at wells
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and river water head in delta plain, the synchronically systematic potential head difference between
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upstream and downstream must cause constant, horizontal groundwater flow. However, it is
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difficult to obtain the hydrological data on the macroscopic, horizontal groundwater flow from
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upstream to downstream using the only data within this study area. To consider the groundwater
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flow from upstream to downstream, constant inflow and outflow rates were estimated to be the flow
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rates equivalent to a hydraulic gradient of 1 m/10,000 m. The rates were estimated from
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synchronically systematic potential head differences between 3.6 m and 7.7 m in this study area and
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1.5 m and 5.7 m in Munshiganj (Harvey et al., 2006), and were assigned to the saturated nodal
points on the north and the south vertical boundary faces, respectively. The transient
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three-dimensional groundwater flow analysis was carried out by a month as a time step.
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217
Results 218
First, calibration or sensitivity analyses were carried out by checking the concordance between
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calculated and observed results shown in Figures 6 and 7 to indicate the applicability of the
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simulation model. Figures 6(a) and 6(b) show the numerically simulated groundwater head for the
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1st to 15th years (a) and 16th to 30th years (b) in the 30-year simulation of wells DRK-W1,
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DRK-W2, and DRK-W3 for groundwater usages of 4 L/(personday) and 100 L/(personday).
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Figures 6(c) and 6(d) show the variation of simulated groundwater head in the 30th year for the 4
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L/(personday) and 100 L/(personday) groundwater usage levels, respectively, which are the same
225
as the simulations of the 2nd to 29th years of the thirty-year simulation (Figs. 6 (a) and 6(b)) at
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wells DRK-W1, DRK-W2, and DRK-W3 (Fig. 1). In Fig. 3, the irrigation rate is reduced to 75
227
percent of the initial value given by Harvey et al. (2006). The groundwater head variations obtained
228
are consistent with observed values to within 0.4 m. When omitting pumping for irrigation, the
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computed groundwater heads in both the shallow (S1 and S2) and deep (S3) aquifers give the same
230
temporal variation, and the difference between the observed and calculated groundwater heads is 1
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m to 1.5 m in the late dry to early rainy seasons from January to June. It is clear that the uptake of
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groundwater for irrigation causes the groundwater head in the deep aquifer to be lower than in the
shallow aquifers in the dry season. The volume of groundwater withdrawn from tube wells for daily
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use (4 L/(personday) or 100 L/(personday) does not affect the groundwater head throughout the
235
year, suggesting that pumping from the tube wells has a negligible effect on the groundwater flow
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regimen in the study area. The simulated annual variation pattern of the groundwater head did not
237
change for 29 years, from the second year of the simulation, suggesting a recurrent hydrological
238
cycle in the area. Stability of the simulation results under the present assumptions and boundary
239
conditions are also confirmed from the periodicity for those 29 years. Figure 7 shows a map of
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unconfined groundwater head computed for October, early in the dry season, with the observed
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levels at eight hand-auger drilling sites in early October, 2006. Despite uncertainties in the
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hydrologic data, the spatial variation of simulated groundwater heads are concordant with the
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observed levels, indicating that the model used here appropriately simulates the groundwater flow
244
regimen for this study area.
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Figure 8 shows simulated monthly horizontal groundwater flow velocities at Z = -10 m from
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January to December. The horizontal groundwater flow differs markedly between the dry and rainy
247
seasons. In the dry season, from January to May, the dominant groundwater flow in the shallow
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aquifer (S2) is around and toward a WNW–ESE furrow in the southeastern part of the study area,
249
where the middle mud formation is thin or non-existent (Fig. 2(e)). The groundwater flows from the
250
shallow aquifer into the deep aquifer by contact between the aquifer sediments due to the lack of the
251
middle mud formation (Figs. 8(a) to (e) and 2(e)), indicating that irrigation pumping affects the
groundwater flow in the dry season. Between July and September in the rainy season, horizontal
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flow is limited beneath the flood plain, and its dominant pattern appears to be radial flow from the
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levees to surrounding areas (Figs. 8(g), (h) and (i)). Thus the low horizontal hydraulic gradient of
255
the region increases the extent of stagnant conditions in the aquifer during the period of annual
256
flooding. The radial groundwater flow, typically in actively recharge zones, is produced by the
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positive groundwater potential of the large rainy season precipitation to raise the water table of the
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shallow aquifer. The village of Harihardi (Fig. 1) is such an active recharge zone. From October to
259
November, early in the dry season, the stagnant zones become small, and the velocity of the radial
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flow increases (Figs. 8(j) and (k)). In June and December, the groundwater flow changes between
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the dry and rainy season patterns (Figs. 8(f) and (l)). The simulation also shows slight groundwater
262
flows from the terrace into the deep aquifer (S3) beneath the flood plain in both dry and rainy
263
seasons.
264
To evaluate the groundwater residence, the groundwater flow path toward the tube well was
265
traced by forward and backward particle tracking of the particle source placed at an arbitrary point,
266
assuming advection of hypothetical, non-reactive particle (e.g., Mukherjee et al., 2011) using
267
spatiotemporal distribution of the flow velocities computed by the three-dimensional groundwater
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flow analysis. Figures 9(a) to 9(f) show the modeled groundwater flow paths toward the location of
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tube wells Nos. 1 through 6 at Z= 5, 0, -5, -10, -15, -20, -25, -30 m. Water recharged from the
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ground surface infiltrates between 5 m and 20 m vertically downward, with its horizontal flow
component increasing with depth, toward the drilled hole. The groundwater flow path toward the
272
well becomes longer and the horizontal component becomes larger with increasing depth. Figure
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9(g) shows the relationship between z and residence time (model age) (TR) calculated for the wells 274
Nos. 1 through 6. In this study, the residence time (model age) means the time required for particle
275
to reach the target point from a recharge point. Although the simulation is limited to a thirty year
276
run, the obtained model age can be longer than thirty years because the model age can be calculated
277
from the sum of TR on arbitrary segments along the groundwater flow path. The model age of 278
groundwater in the Holocene aquifer increases with depth and is proportional to depth down to
279
approximately -20 m. The correlation between depth, as it increases to about 20 m below the ground
280
surface, and the model age of the groundwater is concordant with the correlation between the depth
281
and residence time of groundwater estimated from 3H/3He isotopes in the Holocene aquifers of the
282
Munshiganji District (Klump et al., 2006) and in Araihazar Upazila (Stute et al., 2007).
283
Figures 10 and 11 demonstrate the simulated groundwater flow paths toward the midpoint of the
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5-meter long screen, along with As concentrations for the 126 surveyed tube wells installed in the
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shallow (Holocene) aquifer (above about -20 m ASL). The present model predicts that vertical
286
infiltration of surface water into the shallow aquifer sediments occurs often; the water recharged
287
from the ground surface (about 4 m to10 m ASL) of the flood plain moves approximately 5 m to 20
288
m downward and the groundwater flow gradually changes toward the horizontal direction as it
289
approaches the underlying middle mud layer (aquitard M). The drinking water pumped up is
derived from the groundwater flowing downward from the ground surface toward the tube well. As
291
noted above, recharge and subsequent radial flow appear in and around levees. A stagnant zone
292
appears in the rainy to early dry season, July to October (figs. 7-11), while recharge is promoted in
293
the stagnant zone in the dry season. The model also predicts that the shallow groundwater in the
294
Holocene aquifer will not recharge in the terrace (figs. 10). The areas with the highest As
295
concentrations (700 μg/L < As < 1200 μg/L) and high As concentrations (500 μg/L < As < 700
296
μg/L) correspond to the zones where recharge occurs in the rainy to early dry seasons (Figs. 7, 8
297
and 10). Thus, local groundwater flow and the As concentration in the flow path must be important
298
for the mechanism that produces As-contaminated groundwater. Figure 12 shows the spatial
299
distribution of As and the concentration of dissolved oxygen (DO) , which was measured after
300
attainment of the stabilities of DO, ORP, EC and pH with enough pumping, of 45 of the total 51
301
points measured from the end of September to early November in 2009 in eastern levees where an
302
active recharge zone appears. DO is detected in 20 of the 51 well samples. The most active recharge
303
zone (Figs. 8, 10 and 11) in this study area appears in the village of Harihardi village (Fig. 1), where
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DO levels of 1.3 mg/L to 3.3 mg/L were detected in 18 of the 28 wells. Two well samples
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containing fairly amounts of DO (1.73 and 1.76 mg/L) relative to DO (0 mg/L) in reducing
306
condition were among the seven that had As > 500 μg/L.
307
Figure 13 shows computational results presenting the relationship between the average length of
308
the groundwater flow path (Lp) and the average residence time (model age) (TR) from the ground 309
surface to tube wells, with the As concentration of each well‟s water, excluding three wells that had
310
residence times >30 years (920 m, 38.5 years, 351 As μg/L), (367 m, 43 yeas, 146 As μg/L) and
311
(532 m, 31 years, 75 As μg/L). The estimated Lp ranges from 38 m to 920 m, and TR ranges from 4 312
years to 43 years. The TR obtained here is concordant with the estimation that the Holocene aquifer 313
(Mitamura et al., 2008) hosts groundwater recharged after 1953 based on 3H measurements of this
314
study area (Itai et al., 2008). Groundwaters in the As hotspots (> 700 μg/L) are characterized by
315
short flow paths and residence times; the hotspots with the highest As concentrations (> 700 μg/L)
316
have flow paths shorter than 220 m and residence times (model ages) <10 years.
317
318
Discussion 319
Here, the As contamination process is discussed in relation to simulated flow paths for a recent
320
thirty-year period in the Holocene aquifer. As noted above, shallow groundwater in the Holocene
321
aquifer has a downward flow path from the ground surface toward each tube well. The sources of
322
that groundwater are rivers, local rainwater, and a small contribution of deeper groundwater
323
distributed on the cultivation field for the irrigation. As groundwater hotspots in this area are closely
324
related to the groundwater flow path after the recharge: our simulation shows that the among the
325
wells we studied As hotspots appear in wells having relatively short Lp and TR as shown in Fig. 13. 326
Figure 14 shows two conceptual models, A (CM-A), in which the As is released mainly during
327
vertical groundwater infiltration, and B (CM-B) in which the As is released over the entire flow
path where As is concentrated in the aquifer sediments. Through the groundwater flow, advection,
329
dispersion and reactive transport of As often occur. In case CM-A (Fig. 14(a)), the As released in the
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vertical flow path would be diluted by the ambient horizontally flowing groundwater. Even if the
331
groundwater passes through As-rich sediment, little As is released. Therefore, As hot spots appear
332
close to the recharge zones in places where the groundwater has short flow paths and residence
333
times. This model can explain the relationship between the TR and the Lp of the groundwaters in the 334
Holocene aquifer shown in Fig. 13. The linear correlation between groundwater age determined
335
using 3H/3He, at depths <20 m and the dissolved As concentration was observed in Araihazar
336
Upazila (Stute et al., 2007). These authors also found that the age of groundwaters containing the
337
highest As concentrations > 1100 µg/L (hotspots) are considerably younger than the others, of
338
which ages are plotted on the linear regression line. Thus, the considerable As release in Araihazar
339
Upazila would occur in the vertical flow path through As-rich sediments, which are distributed as
340
patches, similar to our model CM-A. In case CM-B (Fig. 14(b)), As is not only released in the
341
vertical flow path, but also in the following horizontal path. According to this model, high As
342
concentrations in the groundwater appear to be independent of the length of the flow path and
343
residence time. Therefore, case CM-B cannot explain the As hotspots of this study area. We
344
conclude that the release of As from the Holocene aquifer in our study area is likely to occur
345
primarily in vertically infiltrating water. Arsenic contaminated groundwater in Asian countries is
346
typically found under two different groundwater conditions (e.g., Smedley and Kinningurgh, 2002);
i.e., reducing conditions in humid climates, such as Bangladesh, Vietnam, etc., and oxic conditions
348
in arid to semi-arid climates, such as Pakistan and Mongolia. The formation mechanism in reducing
349
groundwater is believed to be microbial reduction and decomposition of Fe-oxyhydroxides,
350
releasing As, which is adsorbed onto the Fe-oxyhydroxides (e.g., Nickson et al., 2000; Horneman et
351
al., 2004; van Geen et al., 2004; Zheng et al., 2004; Mukherjee et al., 2008). The As release in the
352
oxic groundwater is explained by desorption of As from Fe-oxyhydroxides without decomposition
353
(e.g., Amini et al., 2008). According to these hypotheses, the As in the groundwater of the oxic
354
recharge zone in the study area is likely released via desorption from Fe-oxyhydroxides without
355
decomposition. Some researchers propose silicates as sources of the As; e.g., biotite has been
356
considered to be a primary source (Seddique et al., 2008), although chlorite was demonstrated to
357
enrich As in Masuda et al. (2010). Such basic minerals are easily decomposed via chemical
358
weathering, which is promoted by water flow irrespective of the redox condition, although an oxic
359
condition is preferable to an anoxic condition. In our study area, redox condition of the Holocene
360
groundwaters allows to precipitate FeOOH (goethite) and Fe(OH)3 (ferrihydrite) (Itai et al., 2008). 361
They also documented that the chemical weathering of detrital minerals such as plagioclase and
362
basic minerals are important factors to form chemical composition of the groundwaters. If chemical
363
weathering of As-bearing mineral(s) is the case, higher As concentrations can be expected in the
364
rather oxic recharge zone where the groundwater flow rate is higher than that in the stagnant anoxic
365
zone. This study cannot specify the As contamination mechanism at present, but it is clear that a
reducing groundwater condition is not essential to form As contaminated groundwater of the study
367
area, and presumably even for the other As contaminated groundwater in the Ganges delta plain.
368
369
Acknowledgements 370
The authors would like to thank anonymous reviewers, Associate Editor and Dr Laurent Charlet
371
for constructive reviews and helpful comments. We would like to thank Dr Kazi Matin Ahmed for
372
assistance and support on the field survey. We would also like to thank Ataur Rahman (BRAC) for
373
additional information of well and population for each village in the study area. This research was
374
partly supported by a Grant-in-aid for Science Research, number 15403017, from the Japanese
375
Ministry of Education, Science, Sports and Culture.
376
377
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481
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483
Figure captions 485
Figure 1. (a) Sampling wells and drill sites in study area. (b) Sediment columns at twelve drilling
486
sites. (c) Inferred geologic cross section (E-W). (d) Inferred geologic cross section (N-S).
487
Sedimentary layers are divided into upper and lower sand formations, and successive middle mud
488
formation separating between the upper and the lower sand formations. This figure is modified from
489
Mitamura et al. (2008).
490
491
Figure 2. Contour maps of the ground surface from SRTM 90m digital elevation data originally
492
produced by NASA (a), base of uppermost sand formation (aquifer 1) (b), base of upper sand
493
formation (aquifer 2) (c), base of middle mud formation (aquitard) (d) and thickness of the middle
494
mud formation (e) estimated from lithostratigraphic data (Figure 1) by linear interpolation.
495
496
Figure 3. Average monthly precipitation during 2005 and irrigation in Bangladesh. The averaged
497
precipitation was inferred from the data cited on the web site of Bangladesh Meteorological
498
Department (http://www.bangladeshonline.com/bmd/). The irrigation was obtained from data for
499
the 2003-2004 irrigation season (January to April) in Munshiganj, Bangladesh (Harvey et al., 2006).
500
501
Figure 4. Temporal change of the groundwater potential observed at well DRK-W1 in deep
502
aquifer (lower sand formation), and at wells DRK-W2 and W3 in shallow aquifer (upper sand
formation) in 2005. The screens of observation wells at Darikandi site(Figure 1) were installed in
504
45 to 54 m depth (W1), 18 to 27 m depth (W2), and 6 to 12 m depth (W3).
505
506
Figure 5. Monthly evapo-transpiration estimated from the average monthly temperature between
507
1957 and 1987 at the Rangamati meteorological observatory (22.63°N, 92.20°E, 63 m above sea
508
level (ASL)), Bangladesh, using the Thornthwaite equation (Thornthwaite, 1948).
509
510
Figure 6. Numerically simulated groundwater head for the 1st to 15th year (a) and 16th to 30th
511
years (b) in 30-years simulation of wells DRK-W1, W2, and W3 for groundwater usage of both 4
512
and 100 L/(personday). (c) and (d) show the variation of simulated groundwater head in the 30th
513
year for the 4 L/(personday) and 100 L/(personday) groundwater usage levels, respectively, with
514
observed values at wells DRK-W1, DRK-W2, and DRK-W3 (Fig. 1). H-Lower and H-upper
515
indicate the observed groundwater heads in lower and upper sand formation, respectively, in 2005.
516
Hcal indicates the simulated groundwater heads.
517
518
Figure 7. Contour map of unconfined groundwater head computed for October, early in the dry
519
season, with the observed levels at eight hand-auger drilling sites in early October, 2006.
520
521
Figure 8. Horizontal groundwater flow velocities simulated at Z=-10m in January (a), February (b),
March (c), April (d), May (e), June (f), July (g), August (h), September (i), October (j), November
523
(k) and December (l).
524
525
Figures 9. Modeled groundwater flow paths toward the location of tube wells Nos. 1 through 6 at
526
Z= 5, 0, -5, -10, -15, -20, -25, -30 m i. e. tube well of Nos. 1 (a), 2 (b), 3 (c), 4(d), 5 (e) and 6 (f). (g)
527
Relationship between z and residence time (model age) calculated for the wells Nos. 1 through 6.
528
529
Figure 10. Simulated groundwater flow paths toward the midpoint of the 5-meter long screen, along
530
with As concentrations for the 126 surveyed tube wells installed in the shallow (Holocene) aquifer
531
(above about -20m ASL). (a) As <300 mg/L, (b) As >300 mg/L.
532
533
Figure 11. Vertical profile of simulated groundwater flow paths toward the midpoint of the 5-meter
534
long screen, along with As concentrations for the 126 surveyed tube wells installed in the shallow
535
(Holocene) aquifer (above about -20m ASL). (a) x-z plane, (b) y-z plane.
536
537
Figure 12. Spatial distribution of As and the concentration of dissolved oxygen (DO) of 45 of the
538
total 51 points measured from the end of September to early November in 2009 in eastern levees
539
where a recharge zone appears.
540
Figure 13. Relationship between the average length of the groundwater flow path (Lp) and the
542
average residence time (model age) (TR) from the ground surface to tube wells, with the As 543
concentration of each well‟s water, excluding three wells that had residence times >30 years (920 m,
544
38.5 years, 351 As μg/L), (367 m, 43 yeas, 146 As μg/L) and (532 m, 31 years, 75 As μg/L). V
545
indicates velocity by Lp/TR. 546
547
Figure 14. (a) Conceptual model A (CM-A) and (b) conceptual model B (CM-B) for As release in
548
the groundwater flow system obtained in this study.
549
550
Tables 552
553
Table 1. Finite Element Size Using in 3D-FEM Analysis.
554 555 Depth (m) Δx (m) Δy (m) Δz (m) 556 557 14 – 0 100 100 0.5 558 0 – -70 100 100 1 559 -70 – -100 100 100 2 560 561 562 563
Table 2. Hydraulic Properties of Aquifers and Aquitard Using in 3D-FEM Analysis.
564
565
Soil layer K (m/sec) Ss (1/m) ne 566 567 Uppermost sand 7.4x10-5 10-7 0.3 568 Upper sand 2.6x10-4 10-7 0.3 569 Lower sand 1.6x10-4 10-7 0.3 570 Middle mud 1.0x10-8 10-4 0.6 571
Unsaturated property for sand:
572 Se = (Sr - Srr) / (1 - Srr), φ < 0 573 Se = 1, φ ≥ 0 574 Kr = Se 575 +Se =[ (1+(aφ)b ](1-b)/b, ++a=0.015(cm-1), ++b=5, Srr = 0 576
Where φ ; Pressure head (m), Sr ; Water saturation, 577
Srr ; Residual water saturation and ne ; effective porosity. 578 579 +: van Genuchten (1980), 580 ++: Sakellariou-Makrantonaki et al. (1987) 581 582 583 584 585 586 587 588
589
Table 3. Survey Data for Each Village of the Study Area in 2005.
590
Village name Family Population Number of Persentage of tube well (%) / depth
591 Tube well - 15.24m 15.24 - 30.48 - 45.72 - 60.96 - 91.44m – 592 30.48m 45.72m 60.96m 91.44m 593 594 Muchar Char (MCC) 303 2086 293 0 82 15 0 1 2 595 Harihardi (HHD) 74 379 73 0 85 9 6 0 0 596 Darikandi (DKD) 69 364 40 0 88 0 13 0 0 597 Dolardi (DLD) 169 1062 109 0 54 41 4 0 0 598 Kumarchar (KMC) 113 580 66 0 61 31 6 3 0 599 Gankulkandi (GLK) 65 317 38 0 88 13 0 0 0 600 Mamurdi (MMD) 116 631 68 0 97 0 0 3 0 601 Gulnagar (GLG) 102 536 74 0 43 43 0 0 0 602 Atbari (ABA) 38 226 19 0 100 0 0 0 0 603 Bara Kaitargaon (BKB) 193 1216 95 0 73 0 12 15 0 604 Ledamdi (LDD) 45 251 26 0 88 0 13 0 0 605 Temdi (TMD) 70 346 41 0 86 0 0 14 0 606 607
GLG-D1DLD-D 1 KMC-D1MCC-D1MCC-D2MCC-D3MCC-D4DRK-W1DRK-D 1 HHD-D 1 HHD-D 2 MMD-D1 GLG-D1 DLD-D 1 MCC-D4MCC-D3 HHD-D 1 MMD-D1 0 500 m 0 500 m W E S N MCC-D1 MCC-D2 MCC-D3 DRK-D 1 DRK-W1 0 -20 -40 -60 0 -20 -40 -60 Altitude (m) Altitude (m) 0 10 20 30 40 50 60 70 80 Depth (m) clay - silt silt - v.f. sand v.f. - f. sand f. sand f. - m. sand m. - c. sand c. sand Sampling well : Depth
Lower sand formation
Upper sand formation
Middle mud formation (a) (b) (c) (d) DRK-D1 Dhaka Study area Brahmaputra R. Ganges R. 88oE 90oE 92oE 26oN 24oN 22oN India India Bay of Bengal Old Brahmaputra R. Meghna R.
Fig. 1, Nakaya et al.
0 1000 2000 3000 0 1000 2000 3000 Y (m ) X (m) 0 1000 2000 3000 0 1000 2000 3000 Y (m ) 0 1000 2000 3000 0 1000 2000 3000 Y (m ) 0 1000 2000 3000 0 1000 2000 3000 X (m) 3.82 15 (m) 0 15 (m) -44 15 (m) -54 15 (m) Thickness 1 27 (m)
(a)
(b)
(c)
(d)
(e)
Fig. 2, Nakaya et al.
0
100
200
300
400
500
600
1
2
3
4
5
6
7
8
9
10
11
12
P
re
ci
p
it
a
ti
o
n
,
Ir
ri
g
a
ti
o
n
(m
m
/m
o
n
th
)
Month
Precipitation
Irrigation
Figure 3.Average monthly precipitation during 2005 and irrigation
in Bangladesh. The averaged precipitation was inferred from the
data cited on the web site of Bangladesh Meteorological
Department (http://www.bangladeshonline.com/bmd/). The
irrigation was obtained from data for the 2003-2004 irrigation
Fig. 3, Nakaya et al.
Jan.
Mar
.
May
.
Jul.
Sep.
Nov
.
Feb.
Apr
.
Jun.
Aug.
Oct.
Dec.
Time
0
1
2
3
4
5
Observed (2005)
W1 (45-54 m)
W2 (18-27 m)
W3 (6-12 m)
G
ro
u
n
d
w
a
te
r
ta
b
le
(
G
L
-m
)
Figure 4. Temporal change of the groundwater potential observed
at well DRK-W1 in deep aquifer (lower sand formation), and at
wells DRK-W2 and W3 in shallow aquifer (upper sand formation)
in 2005. The screens of observation wells at Darikandi site (Figure
Fig. 4, Nakaya et al.
0.2
0.15
0.1
0.05
0
Jan.
Mar
.
May
.
Jul.
Sep.
Nov
.
Feb.
Apr
.
Jun.
Aug.
Oct.
Dec.
Evapo-transpiration (m/mon.)
Time
Figure 5. Monthly evapo-transpiration estimated from the
average monthly temperature between 1957 and 1987 at the
Rangamati meteorological observatory (22.63
o
N, 92.20
o
E, 63 m
above sea level (ASL)), Bangladesh, using the Thornthwaite
equation (Thornthwaite, 1948).
Fig. 5, Nakaya et al.
3 4 5 6 7 8 0 24 48 72 96 120 144 168 G ro u n d w a te r h ea d ( m ) Time (month) 3 4 5 6 7 8 180 204 228 252 276 300 324 348 G ro u n d w a te r h ea d ( m ) Time (month) 3 4 5 6 7 8 349 351 353 355 357 359 Z = 0m, -15m, -42m Irrigation + 4L/day/person H-Lower H-upper H H H G ro u n d w at er h ea d ( m ) Time (month) 3 4 5 6 7 8 349 351 353 355 357 359 Z = 0m, -15m, -42m Irrigation + 100L/day/person H-Lower H-upper H H H G ro u n d w at er h ea d ( m ) Time (month)
(a)
(b)
(c)
cal :W2 cal :W2 cal :W1 cal :W1 cal :W3 cal :W3(d)
Figure 6. Numerically simulated groundwater head for the 1st to 15th year (a) and 16th to 30th years
(b) in 30-years simulation of wells DRK-W1, W2, and W3 for groundwater usage of both 4 and 100
L/(person*day). (c) and (d) show the variation of simulated groundwater head in the 30th year for the 4
L/(person*day) and 100 L/(person*day) groundwater usage levels, respectively, with observed values at
Hcal :W1 Hcal :W2 & W3
Fig. 6, Nakaya et al.
Figure 6Groundwater table (m ASL)
6 7 8
23.7
23.71
23.72
90.58
90.59
90.6
90.61
2006 Oct.
N
(
o)
E (
o)
0 1000 2000 3000
3000
2000
1000
0
X (m)
Y (
m)
Observed
Figure 7. Contour map of unconfined groundwater head computed
7.0 m
7.0 m
Fig. 7, Nakaya et al.
90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Sep. - 100L/day/person Vmax=1.30m/mon., Z=-10m Y ( m ) X (m) 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2 -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Jan. - 100L/day/person Vmax=3.00m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Feb. - 100L/day/person Vmax=4.41m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Mar. - 100L/day/person Vmax=4.46m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Apr. - 100L/day/person Vmax=2.85m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in May. - 100L/day/person Vmax=1.57m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Jun. - 100L/day/person Vmax=1.94m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Jul. - 100L/day/person Vmax=1.30m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Aug. - 100L/day/person Vmax=1.36m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Oct. - 100L/day/person Vmax=2.19m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Nov. - 100L/day/person Vmax=2.04m/mon., Z=-10m Y (m ) X (m) -500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500 Groundwater flow in Dec. - 100L/day/person Vmax=2.20m/mon., Z=-10m Y (m ) X (m) (a) (a) (j) (b) (c) (i) (f) (k) (h) (g) (e) (d) (l)
Fig. 8, Nakaya et al. Figure 8
-500 0 500 1000 1500 2000 2500 3000 3500 -500 0 500 1000 1500 2000 2500 3000 3500
Groundwater flow in Feb. (After 30 y.) Vmax=4.42m/mon., Z=-10m Y ( m ) X (m) 90.59 90.6 90.61 2 3 .7 2 3 .7 1 2 3 .7 2
1
2
3
4
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6
-30 -20 -10 0 100 5 10 15 20 25 30 1 2 3 4 5 6 Z ( m )Model Age (year) (g) -40 -30 -20 -10 0 10 20 150 200 250 300 350 400 450 500 No.1 (2050, 450, z) Z (m ) Y (m) A B C D E F G
Model age (year) A : 2.5 B : 7.5 C : 12.4 D : 18.5 E : 28.0 F : 45.5 G : 49.6 2.5 years End point -40 -30 -20 -10 0 10 20 2300 2400 2500 2600 2700 2800 2900 3000 No.2 (2350, 850, z) Z (m ) X (m) AB C D E F G
2.5 years Model age (year) A : 3.4, E : 63.5 B : 9.4, F : 100.4 C : 14.5, G : > 116 D : 23.3 -40 -30 -20 -10 0 10 20 2500 2600 2700 2800 2900 3000 No.3 (2650, 1250, z) Z (m ) X (m)
Model age (year) A : 5.5, F : B : 16.5, G : C : 21.5 D : 39.5 E : 58.9 A B C D E F G 2.5 years -40 -30 -20 -10 0 10 20 1960 1980 2000 2020 2040 2060 No.4 (2050, 1450, z) Z (m ) X (m) A B C D E
Model age (year) A : 3.5, F : B : 11.5, G : C : 19.4 D : 27.6 E : 45.6 F G -40 -30 -20 -10 0 10 20 580 600 620 640 660 680 700 No.6 (600, 600, z) Z (m ) X (m) A B C D E F G
Model age (year) A : 2.5 B : 7.5 C : 14.4 D : 22.5 E : 32.5 F : G : -(a) (b) (c) (f) (e) (d) -40 -30 -20 -10 0 10 20 1000 1100 1200 1300 1400 1500 No.5 (1800, 1000, z) Z (m ) Y (m) A B C D E F G
Model age (year) A : 7.5, F : 62.6 B : 16.5, G : C : 21.5 D : 30.5 E : 44.5
Fig. 9, Nakaya et al. Figure 9
0 500 1000 1500 2000 2500 3000 y ( m ) -30 -20 -10 0 10 0 500 1000 1500 2000 2500 3000 z ( m ) x (m) 0 500 1000 1500 2000 2500 3000 -30 -20 -10 0 10 z (m) -30 -20 -10 0 10 0 500 1000 1500 2000 2500 3000 z (m ) x (m) 0 500 1000 1500 2000 2500 3000 y ( m ) 0 500 1000 1500 2000 2500 3000 -30 -20 -10 0 10 z (m) -30 -20 -10 0 10 1800 1900 2000 2100 2200 2300 z ( m ) x (m) 300 500 700 900 1100 1300 1500 -30 -20 -10 0 10 z ( m ) y (m) As (µg/L) 700-1200 500-700 300-500 100-300 0-100 As (µg/L) 700-1200 500-700 300-500 (b) (a) (a) (b)
Figure 10. Simulated groundwater flow paths toward the midpoint of the 5-meter long screen, along with As concentrations for the 126 surveyed tube wells installed in the shallow (Holocene) aquifer (above about -20m ASL). (a) As <300 µg/L, (b) As >300 µg/L.
Figure 11. Vertical profile of simulated groundwater flow Fig. 11, Nakaya et al.
Fig. 10, Nakaya et al. Figure 10 and 11
500 m
Harihardi
Mamuddi
Darikandi
Temdi
Ledamdi
Fig. 12, Nakaya et al.
