Isotopic and Hydrochemical Approaches in

Isotopic and Hydrochemical Approaches in Understanding the Potential of Deep Aquifer in the Meghna Basin, Eastern of Bangladesh 1

Nasir Ahmed, 2Alan H. Welch, 3Pradeep K. Aggarwal, 3Bill G. Wallin, 1Ratan K. Majumder, and 1iahadat Hossain 1

Isotope Hydrology Division, Institute of Nuclear Science and Technology, Atomic Energy Research Establishment, Savar, Dhaka, Bangladesh (e-mail: [email protected]). 2Water Resources Division, USGS, Carson City, NV 89706, USA (e-mail: [email protected]). 3Isotope Hydrology Section, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Vienna, Austria (e-mail:[email protected])

Abstract The shallow aquifers (depths permissible limit 50 g/L). A comprehensive isotope geochemical study was conducted in four high-arsenic upazilas - Laksham, Chandpur Sadar, Kachua and Lakshmipur to understand the potential of deep aquifer. Two major hydrochemical problems were identified in the deep aquifer, specifically high salinity and unacceptable concentrations of trace elements like iron and manganese. The range of arsenic concentration in all shallow wells is 244–733 g/L. Except for two deep wells in Kachua, which are at intermediate depths ranging from 128 to 137m, contain high arsenic concentrations (range 115– 200 g/L). These two samples have heavier stable isotope compositions of oxygen-18 (–2.23 & – 3.11 ‰) and deuterium (–16.3 & –17.6 ‰) compared with water samples from similar depths. The compositions also appear to have been affected by evaporation. The isotopic composition of these two wells appear to be affected by downward flow of water due to improper well construction or pump induced recharge from the shallow aquifer. Most of the deep wells (depth range 90–352 m) are found with lower concentration of arsenic having range 1.0–35 g/L. Residence times deduced from 14C measurements showed older water in the deep aquifer. Water samples from two deep wells (depths >300 m) in Kachua show relatively fresh water with low concentration of chloride, iron, manganese and barium. This deeper or third aquifer may be used as a safe alternate source of drinking water.

upazilas – Kachua, Laksham, Chandpur Sadar and Lakshmipur Sadar in the year 2002–2004 under the technical cooperation project (BGD/8/018) sponsored by International Atomic Energy Agency (IAEA). Environmental isotopes including stable isotopes (2H, 18 O and 13C) and radioactive isotopes (3H and 14C) allow investigation of a variety of problems and development of management practices for the groundwater resources in Meghna basin. It aimed to study known groundwater resources in the Meghna basin and explore potential ones in the region. The paper reflects on the application of isotope techniques in understanding groundwater recharge process and residence time.

Introduction The main challenge facing the present and future availability of groundwater in the Meghna basin is “How to identify the source of safe drinking water and protect it from degradation?”. The main source of water in this area is groundwater. The region faces acute crisis of potable water. The young (Holocene) alluvial deposits in the Meghna basin are mostly affected with high arsenic water, resulting in a major public health crisis for millions of people. It is widely believed that the source of arsenic in Bangladesh groundwater is geological in origin, not anthropogenic. As reported by the local people, in many places the groundwater is brackish or saline. Because deeper aquifers in this region generally have low arsenic concentrations they represent a potential source of safe drinking water. An assessment of the potential for development is needed, however.

Site Description The study area consisting of four upazilas lies between 90°35' to 91°15' E longitude and 22°47' to 23°28'N latitude covering an area of about 1420 km2 with population of about 1.35 million. The location map of study area is shown in Fig 1.

A comprehensive isotope geochemical study was conducted on the deep aquifer in four high-arsenic

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Int. Conf. GeoDev Oct 26-31, 2009, Dhaka, Bangladesh

Fig 1. Location map of the study area

The geology of the study area can be divided into three broad categories and these are (i) Chandina Formation, (ii) deltaic deposit and (iii) alluvial deposit. Among them Chandina Formation covers only the northern extreme portion of Laksham upazila and north-eastern margin of Kachua upazila. The south-western edge of Chandina Formation (Tippera surface and equivalent) grades transitionally into the Meghna flood plain (Bakr, 1977). The greater part of Laksham and Kachua upazila is covered by the deltaic deposit. Lower Meghna deposit covers most part of Lakshmipur Sadar upazila, where as Chandpur Sadar upazila is covered by Meghna flood plain deposit. Physiographically the study area lies within Meghna Estuarine Floodplain (Brammer, 1996) under Tippera surface of Bengal basin which is bounded by the Meghna river in the west, Lalmai hills in the east and Old Meghna Estuary at its south. The elevation of north and northeastern part of study area are relatively higher, i.e. Kachua and Laksham are 7.25m and 11.0m above mean sea level (MSL) respectively. Two upazilas Chandpur Sadar and Lakshmipur Sadar in the western which is close to Meghna river are situated at a relatively lower (5.80m and 3.60m MSL respectively) elevation. The average annual rainfall is

2290mm in which about 97% is received between June and September. The temperature during premonsoon period is highest and rise up to 33.5°C in May. Relative humidity is high throughout the year with average 84%.

Materials and Methods A total of 46 groundwater samples were collected from nested observation wells, local hand tubewells, Tara pumps and production wells within the study area in November 2002 and January 2004. The numbers of deep (depth range 90–352m) and shallow (depth range 10–70m) groundwater are 32 and 14 respectively. The sampled shallow hand tubewells were located in vicinity of the surveyed deep wells. Four samples were collected from the major river named Meghna which flows through the western boundary of Chandpur Sadar and Lakshmipur Sadar. Three samples were taken from the ponds located in the vicinity of the shallow hand tubewells in Laksham. Two additional samples were taken from the distributary of Meghna river named Dakatia. During sampling campaign, the Dakatia river was found dead. The water samples were collected for

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both isotopic and chemical analyses. The well purging was performed using the RediFlo2 submersible pump. Physio-chemical properties such as pH, electrical conductivity (EC), dissolved oxygen (DO), temperature (ºC) and total alkalinity of water samples were measured on-site. The samples for hydrochemical analysis were filtered through 0.45 m membrane filters. All samples for cation analysis were acidified to pH≈2 with HNO3 in the field in order to avoid any precipitation of trace elements.

IAEA, Austria and other reference laboratories in Europe.

Hydro-Geological Investigation Bangladesh Water Development Board (BWDB) constructed five nested piezometric wells and one production well at Vill. Srirampur, Karaia union, Kachua upazila for performing the aquifer test; and monitoring and assessing the water quality (Table 1) parameters. The production well (PW) was installed down to depth of 365m. The depth of observation wells are defined below: (a) P-4: 25m deep, completed in the shallow arsenic contaminated aquifer (b) P-5: 180m deep, completed in the first deep fresh to brackish water aquifer (c) P-1: 280m deep, completed in an unusable brackish water zone (d) P-2: 335m deep, completed in the deepest fresh water zone about 47m from the production well (e) P-3: 352m deep completed in the deepest fresh water zone about 80m from the production well.

The hydrochemical analyses were performed in the United States Geological Survey (USGS) laboratory. The major cations (Na+, K+, Ca+2 and Mg+2), Arsenic and trace elements (Fe, Mn, Ba, Pb, Zn, Cu and Al) of water samples were analyzed by Inductive Couple Plasma-Mass spectrophotometer (ICP-MS). The major anion concentrations (Cl-, SO4-2 and NO3-) were measured by Ion Chromatograph, using standard methodologies (APHA, 1998). The stable and radioactive environmental isotope analyses were performed in the Isotope Hydrology laboratory,

Table 1. Physical parameters including hydro-chemical and isotopic results of water samples of nested observation wells installed by BWDB at Vill. Srirampur, Union-Karaia, Upazila-Kachua Sample ID

Depth (m)

pH

EC (µS/cm)

HCO3 (mg/L)

Cl (mg/L)

As (μg/L)

Fe (mg/L)

Mn (μg/L)

Ba (μg/L)

δ18O (‰ VSMOW)

δ2H (‰ VSMOW)

14 C (pMC)

P-4

25

7.06

1667

598

232.5

100

10.60

536

122

-3.15

-19.6

38.7

P-5

180

6.80

1523

326

298.7

1.0

0.98

40.4

45.2

-4.03

-22.5

--

P-1

280

6.51

5090

203

1379.6

1.0

1.83

374

99.3

-3.18

-16.7

39.0

P-2

335

6.64

1347

182

284.0

1.0

3.30

241

95.4

-3.14

-14.6

38.4

P-3

352

7.03

754

256

122.8

5.5

3.35

277

32.0

-3.01

-15.8

38.9

A lithological cross-section (Fig 2) is drawn, based on borelogs of BWDB and Geological Survey of Bangladesh (GSB), to observe the formation thickness as well as aquifer system in the Kachua area. The aquifer system can be divided into three fairly distinct systems. The upper or 1st aquifer system is essentially the shallow aquifer, the middle or 2nd aquifer system has generally been called “deep aquifer” and the deeper system or 3rd aquifer is newly discovered at this site. The other six nested piezometric wells installed by Department of Public Health Engineering (DPHE) in Lakshmipur Sadar were also investigated for hydrogeochemistry and environmental isotope. The lithology of a well including isotope results of six wells is shown in Fig 9. The depth of the wells are 10m (LPW1), 20m (LPW2), 30m (LPW3), 40m (LPW4), 50m (LPW5)and 150m (LPW6).

Remarks

P-3

Upper or 1st Aquifer

Deep or 2nd Aquifer

High Arsenic Zone

High Iron Zone

High Chloride Zone

Deeper or 3rd Aquifer

Suitable for Drinking Purpose

Fig 2. Aquifer System in Srirampur Area, Upazila-Kachua (Source: BWDB)

Results and Discussion Hydrochemistry Proceedings Volume

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with highest value of 733 g/L in Kachua. Most of the deep wells (depth range 90–352 m) in the study areas are found with lower concentration of arsenic having range 1.0–34.7 g/L. Although these deep wells may be acceptable from the standpoint of arsenic, the salinity is not acceptable in some wells. Fig 4 shows that the two deep tubewells in Kachua upazila affected with higher As concentration exceed the Bangladesh standard 50 g/L. Are these deep wells really contaminated with arsenic? It is assumed that the migration of shallow arsenic contaminated water to deeper parts of the aquifers may be due to improper well construction or as a result of groundwater pumping.

The minimum and maximum ranges of physiochemical properties and hydro-chemical analysis results of 46 groundwater samples are given in table 2. Six parameters i.e. electrical conductivity (EC), chloride (Cl), arsenic (As), iron (Fe), manganese (Mn) and barium (Ba) are mainly focused. Hydrochemical data implies two major hydrochemical problems in the deep aquifer, specifically high salinity and unacceptable concentrations of trace elements like iron and manganese. Mostly, the groundwater of the deep aquifer is Na-Cl type and for the shallow aquifer is Ca-Mg-HCO3 type. In most of the shallow hand tubewells (depth less than 70m) concentration of arsenic exceeds the Bangladesh standard (50 g/L). The groundwater is generally in reducing condition with low concentrations of SO42– and NO3– but with high concentrations of Fe and Mn. Most of the deep tubewells are in the depth range 200–250 m.

Limiting value 600 mg/L

400

350

300

EC values for the deep wells range from 494 to 5220 µS/cm with an average value of 1940 µS/cm. The Cl values for the deep wells range from 10 to 1475 mg/L with an average 191 mg/L and most of the samples fall beyond the maximum permissible limit of 600 mg/L. Higher EC and Cl values in the deep wells are usually observed in Kachua and Lakshmipur upazila. One deep sample (150m) in Lakshmipur has exceptionally high EC (~10000 µS/cm) and chloride (7141 mg/L) values. Shallow aquifers with well depth variation 10-50m show the EC range 293 to 7500 μS/c.

Deep wells Shallow wells

Kachua Laksham

Depth (m)

250

Lakshmipur Sadar (LPW6) Depth= 150m EC= 9999 S/cm

Kachua

200 Chandpur Sadar

150

100 Lakshmipur Sadar (LPW4) Depth= 40 m EC= 7050 S/cm

50

0 0

1000

2000

3000

4000

5000

6000

7000

8000

Cl (mg/L)

Fig 3. Chloride (mg/L) ranges in shallow and deep aquifers of four upazilas in Meghna basin Arsenic (g/L)

Table 3 shows the description of samples from the nested observation wells at DPHE compound, Lakshmipur. For the shallow aquifers, the higher EC values exceeding 2000 μS/cm are seen mostly in Lakshmipur. The deep wells of depth 200–250 m have mostly intolerable limits of Cl concentration exceeding the permissible limit. The Cl of shallow aquifers (Fig 3) with well depth variation 10-50m shows the range 5.35 to 1960 mg/L. Figure 3 also reflects that the deep wells having depth band 200250 m are affected with high chloride ions. It is observed that the five deep wells have crossed the salinity level of sea water (i.e. 1000 mg/L) and contain abruptly higher EC values with range 3790– 10000 μS/cm implying the presence of high amount of NaCl. Some residents of Kachua and Lakshmipur find the salinity objectionable and do not use the deep wells for drinking.

0

100

200

300

400

500

600

700

0

50

100

Depth (m)

150

Is Deep Groundwater Contaminated ?

200

300

350

Limiting value 50 g/L

250

Laksham Chandpur Sadar Kachua Lakshmipur Sadar

400

Fig 4. Distribution of As (g/L) in shallow and deep groundwater of Meghna basin

It is observed from Fig 4 that most of the shallow wells having depth 10-50m are contaminated with As

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Table 2. Water quality variation in shallow and deep aquifer in four upazilas of Meghna basin. Well type

Range

Depth (m)

pH

EC (µS/cm)

HCO3 (mg/L)

Cl (mg/L)

As (μg/L)

Fe (mg/L)

Mn (mg/L)

Ba (μg/L)

Upazila: Laksham Min.

90

6.03

556

51

12.19

1.7

3.23

210

19

Max.

251

7.61

3790

261

1121.8

18.2

43.9

2806

125

Avg.

188

6.56

2031

125

542.5

7.3

24.6

1863

80

Min.

24

7.29

293

255

5.35

14.8

0.90

35

4.0

Max.

70

7.70

734

300

43.5

368

4.79

467

15

Avg. 39 Upazila: Chandpur Sadar

7.54

521

283

20.2

209

2.71

224

11

Deep

Shallow

Min.

174

6.29

503

110

64.0

1.1

1.85

73

31

Max.

235

6.84

2440

201

665.1

3.2

13.6

703

328

Avg.

207

6.61

1131

166

247.1

2.1

5.76

351

116

Min.

23

7.06

744

396

40.0

622

5.64

92

90

Max.

26

7.17

1099

498

93.4

632

6.58

712

131

Avg. Upazila: Kachua

25

7.12

922

447

66.7

627

6.11

402

111

Deep

Shallow

Min.

125

5.91

494

133

10.23

1.0

0.24

16

17.2

Max.

352

7.03

5220

336

1474.6

115

16.4

1230

599

Avg.

210

6.55

2159

213

544.9

25

5.39

276

170

Min.

24

7.06

1667

246

232.5

100

0.40

25

9.8

Max.

27

8.50

2800

617

754.5

733

10.6

536

122

Avg. 25 Upazila: Lakshmipur Sadar

7.62

2272

486

468.8

365

4.06

216

49 127

Deep

Shallow

Deep Shallow

--

150

6.66

9999

240

7141.1

34.7

12.1

1470

Min.

10

7.00

1703

490

290.8

150

2.64

221

24

Max.

50

7.48

7060

713

1959.5

711

19.1

1041

543

Avg.

30

7.22

5759

586

1125.1

524

7.87

562

341

Table 3. Description of samples of the nested observation wells at DPHE compound, Lakshmipur Sadar. Sample ID

Depth (m)

pH

EC (µS/cm)

HCO3 (mg/L)

Cl (mg/L)

As (μg/L)

Fe (mg/L)

Mn (μg/L)

Ba (μg/L)

LPW1

10

7.48

1703

518

290.8

150

2.64

456

23.9

LPW2

20

7.15

5480

525

--

432

4.56

1041

338

LPW3

30

7.27

7500

490

--

711

7.82

635

441

LPW4

40

7.00

7050

713

1959.5

646

19.1

221

543

LPW5

50

7.20

7060

685

--

679

5.21

458

357

LPW6

150

6.66

9999

240

7141.1

34.7

12.1

1470

127

The variability of Fe and Mn in the shallow and deep aquifer is shown in Fig 5. High Fe concentrations are typical of many of the shallow and deep groundwaters exceeding the permissible limit 1.0 mg/L. The highest average Fe value is found 24.6 mg/L in the deep wells in Laksham. Like Fe, Mn concentrations are also high in the groundwaters as a result of the strongly reducing conditions. Almost all the deep wells have Mn concentrations higher than Bangladesh Standard 100 g/L. The highest average Mn value is found 1863 g/L in the deep well in Laksham. In most

shallow wells in Lakshmipur, Mn concentrations are found higher than 100 g/L. From table 2, it is observed that almost all the deep wells have Barium concentrations higher than the permissible limit 100 µg/L. The highest Ba value is found 599 µg/L in the deep well in Kachua. Most of the shallow wells in Lakshmipur are found Ba concentrations higher than 100µg/L. The variability of Fe and Mn in the shallow and deep aquifer is shown in Fig 5. High Fe concentrations are

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typical of many of the shallow and deep groundwaters The highest average Fe value is found 24.6 mg/L in the deep wells in Laksham. Like Fe, Mn concentrations are also high in the groundwaters as a result of the strongly reducing conditions. Almost all the deep wells have Mn concentrations higher than Bangladesh Standard 100 g/L. The highest average Mn value is found 1863 g/L in the deep well in Laksham. In most shallow wells in Lakshmipur, Mn concentrations are found higher than 100 g/L. From table 2, it is observed that almost all the deep wells have Barium concentrations higher than the permissible limit 100 µg/L. The highest Ba value is found 599 µg/L in the deep well in Kachua. Most of the shallow wells in Lakshmipur are found Ba concentrations higher than 100 µg/L.

exceeding the permissible limit 1.0 mg/L. high salinity (~10000 µS/cm). The isotopic value of this sample is high (~ –3.70‰ for oxygen) and plot below the meteoric water line, but not towards the sea water. Most of the shallow samples (less than 70 m) have stable oxygen isotope values between –1.71 and –3.63 ‰ with greater evaporation effect indicating recharge from present day rain and flood waters. Shallow groundwater samples from Lakshmipur have different isotopic characteristics. The shallowest sample (10m) has enriched oxygen isotope value of –2.52 ‰ and the rest shallow samples have depleted values of around –7.20 ‰ (Fig 9). The specific conductivity (or salinity) of the shallow samples (depth 20–50 m) from Lakshmipur are quite high (~7000 µS/cm) compared to other shallow samples in this study. The 10m sample is clearly impacted by evaporation before or during infiltration, as indicated by its isotopic values that plot below the meteoric water line, but not towards the sea water. Much lower isotope values of other shallow samples from Lakshmipur (20–50 m depth) plot on the meteoric water line and are very similar to Meghna river water in this region. High salinity in most of the shallow samples in Lakshmipur and Kachua, therefore, are likely due to geochemical reactions with soils during recharge and leaching of salts, rather than due to mixing with seawater (Aggarwal et al., 2000).

The water samples from two deep nested observation wells (depth >300 m) in Kachua show relatively fresh water with low concentration of chloride, iron, manganese and barium. This deeper or 3rd aquifer (Fig 2) may be used as a safe alternate source of drinking water for the rural community in the eastern of Bangladesh.

0 Deep Wells Shallow Wells -10

Pond

(Fresh water in Kachua, >300 m)

Dakatia River Meghna River

(Laksham, 24m)

-20

2H (‰ VSMOW)

(Lakshmipur, 10 m) (Lakshmipur, 150 m)

-30

Meteoric Water Line

-40

(Lakshmipur, 20-50 m)

-50

-60 -9.0

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

18O (‰ VSMOW)

Fig. 5. Iron and manganese concentrations in shallow and deep groundwater of Meghna basin

Fig 6. Stable oxygen and hydrogen isotope composition of shallow & deep groundwater and surface water in Meghna basin

Environmental Isotopes of Water The deep groundwaters sampled in the study area have stable oxygen and hydrogen isotope ratios ranging mostly from –3.0 to –6.0‰ and –14.6 to –35.6‰ respectively (Fig 6). These samples plot on or slightly below the meteoric water line, but do not appear to be affected by sea water. The stable isotopic composition suggests an origin from local rain and rivers, with or without some evaporation before infiltration under different climatic regimes about 2 to 20 thousands years ago. Deep groundwater sample (depth 150m) from Lakshmipur has

The two deep wells in Kachua, which are at intermediate depth 128–137m, are found contaminated with higher arsenic concentrations ranging 115–201 g/L (Fig 4). These two samples have enriched stable oxygen (–2.23 and –3.11‰) and deuterium (–16.3 and –17.6‰) isotope values with greater evaporative loss. It indicates that these two wells might be affected from arsenic by downward flow of water, as resulted from improper well construction or pump induced recharge from the shallow aquifer. The groundwater samples from depth greater than

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300 m plot on the meteoric water line and have high (about –3.10‰) oxygen isotope values. These waters are relatively fresh with Cl of less than 600 mg/L.

the sampled groundwater recharged the aquifers a long time ago. An integrated interpretation of hydrogeologic and isotopic data of nested observation wells at Lakshmipur Sadar has been illustrated in Fig 9. Shallow Lakshmipur samples with low oxygen isotope values (about –7.2‰) and high salinity (Table 3), have relatively lower carbon-14 of about 65 pMC and higher carbon-13 values of about –5.8 per mil. These relatively high (less negative) carbon-13 values indicate that the low carbon-14 content of these samples do not reflect old groundwaters, but present-day recharge that undergoes substantial geochemical reactions with the sediments before reaching the aquifer (Aggarwal

The tritium content of the deep samples varies from 0.01 to 0.52 TU indicating older groundwater having no recent contribution to recharge. Two shallow samples in Laksham has slightly higher tritium values (2.57 and 4.41 TU), but these samples have very high 18O values (–1.71 and –2.74‰), indicating drawdown of water from a nearby pond. One shallow sample in Laksham having depth about 70m has high stable isotope composition (18O= –3.38‰ and 2H= –19.8‰). Although, this shallow sample shows a slightly positive tritium value 0.50 TU, but it has higher carbon-14 value 79.8 pMC. Evidently, the low tritium content of shallow groundwater in Laksham is consistent with recharge from local rain and flood waters. The absence of tritium in many of the shallow samples in Lakshmipur and Kachua indicates relatively large travel times in the unsaturated zone (several years to tens of years), resulting in the loss of tritium by radioactive decay before reaching the aquifer. Higher contents of tritium (2.4–7.4 TU) in the samples indicate the age about 40-50 years, consistent with their origin during the nuclear test era.

100 Deep Wells 90

Shallow Wells Lakshmipur Nest

Carbon-14 Activity (pMC)

80 70 60 Kachua Nest

50 40 30 20

Lakshmipur Nest

10 100 Deep wells 90

0 -30.0

Shallow wells

-25.0

Carbon-14 Activity (pMC)

80

60 50

-5.0

0.0

et al., 2000).

Lakshmipur Nest

70

-20.0 -15.0 -10.0 13  C (per mil. VPDB)

Fig 8. Stable carbon isotope and carbon-14 compositions of deep and shallow groundwaters in Meghna basin

Kachua Nest

Kachua Nest

40

Conclusions

30

The groundwater quality results reveal that upper/1st aquifer zone is contaminated by arsenic. Although the deep/2nd aquifer appears to be free of arsenic, in some areas and at certain depths the deep aquifer contains large quantities of Cl, Fe, Mn and Ba that makes it unsuitable for drinking purposes. Salinity is a more severe problem in Kachua and Lakshmipur upazila, where most deep wells produce slightly saline water. On the otherhand, arsenic, chloride and iron concentrations in the deeper confined/3rd aquifer are found within tolerable limits for drinking purpose. This deeper or third aquifer may be used as a safe alternate source of ground water for the rural community. High arsenic concentrations are present mostly in shallow groundwater (depth less than 70 m). Deep wells with high arsenic concentration likely contain shallow groundwater in the well, most likely due to improper well construction or pump induced recharge from the shallow aquifer and do not necessarily imply the contamination of deep aquifers.

Lakshmipur Nest

20 10 0 0

50

100

150

200 Depth (m)

250

300

350

400

Fig 7. Carbon-14 content vs. depth of shallow and deep groundwaters in Meghna basin

The carbon-14 contents of groundwater in study area range from 13.4 to 95.2 pMC. Samples with no tritium have lower C-14 values of 38.7 to 74.2 pMC. The carbon14 content also shows a good correlation with sampling depth (Fig 7). Samples below 100 m depth generally have a carbon-14 content less than 60 pMC. The carbon-13 values of most of the deep groundwater samples, spanning the range of observed carbon-14 values, vary from –5 to –24‰ (Fig 8) and show no good correlation with carbon-14 content. The carbon-14 contents of deep samples ranging from about 13 to 60 pMC thus indicate groundwater ages of about 5 to 17 thousand years, that is, Proceedings Volume

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Results of isotope techniques have provided adequate information on recharge conditions and age of groundwater in the basin, and open up prospects for further investigations using isotope techniques.

18O (‰ VSMOW)

 2H (‰ VSMOW)

-2.52

-20.7

2.35

91.4

effectively isolated from the shallow groundwater under the present hydrologic conditions. - The isotopic patterns indicate that shallow groundwater is being continually recharged with a residence time of 10s of years while deep groundwater may not be recharging under present conditions. Deep groundwater may be either recharged on a long time scale of 1000s of years (Laksham, Kachua and Lakshmipur) or nor recharged at all with 10000–20000 years old water (Chandpur Sadar and Kachua).

-7.5

-51

0

74.2

Acknowledgements

-7.1

-47

0

64.3

-7.14

-47.1

0

61.8

-7.1

-47

0

61.0

Piezometric Nests at DPHE Compound, Lakshmipur Sadar Depth (m)

0

Lithology

As ( µg/L)

3 H (TU)

14 C (pMC)

Grey Silt Grey Very Fine Sand

 150

10 Grey Silt and Clay

20

Grey Silt



Grey Very Fine Sand

432

Grey Fine Sand

 711

30

40

Grey Silt and Clay



Grey Fine Sand

646

The study was conducted within the frame of technical cooperation project (BGD/8/018) with IAEA. The IAEA is acknowledged for doing the environmental stable and radioactive isotope analysis for the water samples in isotope hydrology laboratory of IAEA and other reference laboratories in Europe. The cooperation of local Department of Public Health Engineering (DPHE) office during field sampling is highly appreciated. We appreciate the assistance rendered by Groundwater Hydrology, Bangladesh Water Development Board (BWDB) in providing us hydrological & hydro-geological data. We are thankful to BAEC for providing us the logistic supports and field assistance in water sample collection.

Grey Silt and Clay Grey Very Fine Sand

50

60

70

Grey Silt and Clay

 679

Grey Fine Sand

Grey Silt Grey Clay Grey Fine Sand

80 Grey Silt

90 Grey Fine Sand

100

References

110

Aggarwal, P. K., Basu, A. R., Kulkarni, K. M., Froehlich, K., Tarafdar, S. A., Ali, M., Ahmed, N., Hussain, A., Rahman, M. and Ahmed, S. R. (2000): A report on isotope hydrology of groundwater in Bangladesh: Implications for characterization and mitigation of Arsenic in groundwater. No. IAEA-TCR0159, International Atomic Energy Agency, Vienna, Austria. APHA (1998): Standard methods for the examination of water and wastewater. (20 ed.) American Public Health Association (APHA), American Water Works Association (AWWA) and Water Environment Federation (WEF), Washington D. C., USA. Bakr, M. A. (1977): Quaternary geomorphic evolution of the Brahmaputra-Noakhali area, Comilla and Noakhali district. Geological Survey of Bangladesh, Records, 1-2. Brammer, H. (1996): The geography of the soils of Bangladesh. University Press Ltd., Dhaka, Bangladesh.

Grey Clay

120 Grey Fine Sand

130 Grey Very Fine Sand

140 Grey Silt and Clay

150

 35

-3.72

-25.6

0.36

32.4

Fig 9. Lithology of a well at Lakshmipur with arsenic and isotopic information of six nested observation wells

Isotopic and chemical analyses of groundwater indicate that: - The stable isotope data of oxygen and hydrogen indicates the origin of groundwater from local rain, rivers and flood waters, with or without some evaporation before infiltration. High salinity in the shallow groundwaters are likely due to geochemical reactions with soils during recharge and leaching of salts rather than due to mixing with seawater. On the otherhand, deep saline groundwaters are connate marine waters which were entrapped during late Pleistocene interglacial stage. - Shallow groundwaters (