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Geoelectric Model, Groundwater Potentiality of Salinity Affected Lower Atrai Floodplain Aquifer,

NW Bangladesh: An Approach for Irrigation Management

Chowdhury Sarwar Jahan Dept.Geology and Mining University of Rajshahi Rajshahi, Bangladesh [email protected]

Quamrul Hasan Mazumder Dept. Geology and Mining University of Rajshahi Rajshahi, Bangladesh [email protected]

Abstract-: In the salinity affected Lower Atrai Floodplain aquifer in the NW Bangladesh. Two-fold aquifer system, inter- layered by silt, clay and silty-clay aquitard and aquiclude is classified as: upper aquifer - spatially affected by salinity of varying degrees; and lower aquifer - generally characterized by high salinity. Here the aquifer with resistivity values greater than 69 Ω m is safe for irrigation use. Geochemically, groundwater is hard and saline to fresh water type. Salinity increases with depth, but spatially towards the southern part. Groundwater quality is a product of water-rock interaction, direct mixing and marine spraying or fall-out of airborne marine salts. In general partially or fully salinity affected upper and lower aquifers in the area by except its eastern part is not suitable for STWs and DTWs irrigation. As groundwater demand for irrigation is increasing, the saline water has progressively invaded relatively fresher parts of the aquifer by up-conning. So, special salinity control management approaches can be implemented through engineering techniques such as groundwater abstraction optimization, and as well as scientific and behavioral approaches.

IndexTerms—Geoelectric modeling, Hydrochemistry, Salinity, Lower Atrai Floodplain, Bangladesh.

INTRODUCTION

Bangladesh is an agro-based country, but with increasing population and irrigation demand to meet the country’s food security needs, valuable groundwater resource has been even over-exploited. To improve the quality of life by achieving self-sufficiency in food, the Barind Integrated Area Development Project (BIADP) under the Barind Multipurpose Development Authority (BMDA) was launched more than three decades back in barren agro-based landscape, popularly known Barind area in NW Bangladesh (covering an area of 7500 km² with a population of 5.4 million). Before the implementation of the BIADP, it was mono-cropped land where Aman paddy was the only crop growing in rainfed condition. With the introduction of groundwater based irrigation in the BIADP, double and triple cropping practices have replaced the age-old agricultural practices resulting in increased cropping intensity and production of food grain. As

in the dry period surface water is very scarce, groundwater has been the source of irrigation. Barind area has scope for groundwater abstraction by constructing 8728 DTWs (0.06 m³/sec capacity [1]. At present groundwater is lifted through more than 15000 DTWs tapping.

Saline groundwater was encountered during 1970s in a total area of 61 Km² in Kansopara and Kashob Union of Manda Upazilla (sub-district) and Hashaigari Union of Naogaon Upazilla the NW Bangladesh (Fig.1). Here boro rice is mainly cultivated (80%) in pre-monsoon dry season with wheat (2.07%) and others.

Fig. 1: Study area with locations of shallow tube wells (STWs), deep tube wells (DTWs) and vertical electrical sounding (VES) stations Here large numbers of DTWs were installed without any prior investigation, and in turn saline groundwater badly affects crop yield. A systematic scientific study is therefore essential to find out a suitable way for the inhabitants from the loop of salinisation which affect the socio-economic condition of the area. So the aim of the present study is to identify subsurface layers of hydrogeochemical significance from geophysical study, to demarcate fresh and saline groundwater zone; investigate the present status of salinity; and to find out management approaches for irrigation.

International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015 05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh

www.ru.ac.bd/icmeie2015/proceedings/

ISBN 978-984-33-8940-4

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PHYSIOGRAPHY,GEOLOGYANDHYDROGEOLOGY Geomorphologically the study area lies in the younger and outer part of the Tista Alluvial Fan comprising present day Tista river and its abandoned channels like Atrai [2,3,4,5].

More than 90% rainfalls occur in monsoon (June to September) when major part of the area become inundated by annual flooding depth of 7.6 cm to 1.8 m.

The aquifer condition by gamma ray geophysical well log data in and around the study area to establish hydrostratigraphy [6] and reveals that the area is covered by clay-silt impervious aquitard of Recent age and characterized by multiple layered (two-four) aquifer system of Plio-Pleistocene age at different depth levels, inter-layered by clay and silty clay aquitard and aquiclude. The area has good groundwater development potentiality for irrigation and water supply based on transmissivity value of aquifer (2400 m2/day) [7]. Except in years of low rainfall, groundwater in the area is mostly being well recharged from normal rainfall infiltration [8, 9].

METHODOLOGY

A combination of geophysical and hydrochemical techniques were used to study the salinity affected Lower Atrai Floodplain aquifer. Seven VES surveys were carried out by DC resistivity meter of model SSR-MP-AT-S and the analysis was conducted using software IGIS 2.0. Since the VES positions were spatially and randomly distributed over the area, gridding method like Kriging was adopted to get accurate results, close approximation of input data and output contours and 3-D views. The coefficient of anisotropy () calculated for each VES station is used to project the hydrogeological condition of the area. For hydrochemical study, water samples were collected during irrigation season (i.e., February-March) from 17 DTWs and 23 HTWs, where DTWs under consideration located around the VES stations were selected for the purpose.

HTWs have been installed down to a depth of 30-50 m and DTWs 50-80 m. Average values of field measured parameters of different groundwater samples were calculated for respective VES station in order to have analytical convenience.

Locations of VES and groundwater samples are shown in Fig. 1. Chemical analysis of groundwater samples (both HTWs and DTWs) were carried out in the Analytical Research Division of the Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka and the Central Science Laboratory, University of Rajshahi.

RESULTSANDDISCUSSIONS ANISOTROPY AND GEOELECTRIC SECTION

The co-efficient of anisotropy () as calculated for each VES station ranges from 1.0 to 1.05. Anisotropic contour map (Fig. 2) shows southern part has greater  values that gradually increase towards north. Again contour spaces increase from north to south indicating abrupt lithological variation in the northern part with gradual change of lithologies in the south. In the northern part, aquifer lithology is dominated by coarse to medium sands that gradually changes from medium sands to fine sands towards south. The

multi-layered aquifer is interlayered by clay, silt and silty-clay aquitard and aquiclude. So the aquifer in the northern part has higher groundwater potentiality and hence hydrogeologically important compared to the southern part. Lithological panel diagram constructed with the help of interpreted VES data (Fig. 3) reveals that of the upper and lower aquifers, and inter- layered formations are not continuous.

Fig. 2:Anisotropy () contour map of the study area (Contour Interval: 0.02) HYDROCHEMICAL CHARACTER

Spatial variation of EC values help to divide the area into zones [10] as: Zone-I: up to 250 s/cm; Zone-II: 250-750

s/cm; Zone-III: 750-2250 s/cm; and Zone-IV above 2250

s/cm (Fig. 4). To study the relationship between groundwater salinity and corresponding interpreted aquifer resistivity, EC values of groundwater are plotted as function of layer resistivity (Ωm) from VES data (Fig. 5 & 6). Aquifer resistivity values higher than 69 Ωm (at EC 1033 S/cm) (Fig. 7) treated as fresh water zone while that of lower value brackish water zone. Correlated EC value (1033 µS/cm) indicates that groundwater is of high salinity class [10].

TDS of well waters in the southern part show higher (1000- 2000) values than those of north (<1000). Generally groundwater has low alkalihazard and medium to very high salinity hazard. The spatial distribution of salinity in groundwater (Fig. 7) shows increasing values towards south.

The correlation coefficients (r) between Na⁺ and Cl^-, Na⁺ and salinity, and Cl^- and salinity of groundwater corroborate salinity increase with depth.

In groundwater are Ca²⁺ and HCO₃^- are the dominant cation and anion respectively and contains significantly higher concentration of Na, where the order of relative abundance of the molar concentrations are generally Ca²⁺>Na⁺>Mg²⁺>K⁺ and HCO₃^->Cl^->NO₃^->SO₄^2-. Hydrochemical properties of groundwater are dominated by alkaline earth and weak acids [11, 12] originated from silicate weathering or ion exchange process [13, 14], paleo-sea water or connate water [15] and may be depicted from rock-water interaction involving the dissolution of carbonate and silicate weathering. Genetically groundwater is of ‘Normal Chloride’, ‘Normal Sulfate’

‘Normal Carbonate’ to ‘Super Carbonate’ and is of very hard class [16] that requires softening. Dominance of Na⁺ and Cl^- in groundwater is indicative of the significant effect of sea water via direct mixing, marine spraying, and fall-out of airborne marine salts. Relatively higher concentrations of Cl^-, Na⁺ and International Conference on Materials, Electronics & Information Engineering, ICMEIE-2015

05-06 June, 2015, Faculty of Engineering, University of Rajshahi, Bangladesh www.ru.ac.bd/icmeie2015/proceedings/

ISBN 978-984-33-8940-4

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Mg²⁺ might have resulted from the marine spraying from outside the Holocene shoreline passing near to the area. TDS vs. inter-ionic molar ratios (TDS vs. Ca/Na and TDS vs.

HCO₃/Cl) of groundwater reveals that Ca/Na ratios are controlled by minor influence of marine spraying or factors other than sea water mixing.

Fig. 3: Panel diagram showing subsurface lithological correlation based on vertical electrical sounding (VES) data

Fig. 4: Zonation based on groundwater electrical conductance (EC, µS/cm) GEOELECTRIC MODEL AND GROUNDWATER POTENTIALITY The geo-electric properties of different aquifers are as:

Upper Aquifer: Resistivity values range from 31 to 77 Ωm (Fig. 8a). Iso-resistivity contour map of the upper aquifer reveals that the area is affected by salinity with varying degrees and only marginally fit for irrigation. The thickness of this aquifer ranges from 7 m in the eastern part to < 32 m at the north and northwest.

Lower Aquifer: Resistivity values in range of 31-86 Ωm from the eastern to the western parts of the area (Fig. 8b). Iso- resistivity contour map of the lower aquifer reveals that in generally by the lower aquifer is characterized by high salinity having little scope for groundwater development for irrigation purposes, whereas the thickness ranges from 8 m at the south to 24 m in the eastern part of the area.

The groundwater potentiality of the area based on geoelectric resistivity survey and hydrochemical character reveals that except eastern part where lower aquifer is fresh and suitable for irrigation, the upper and lower aquifers are partially or fully affected by salinity. So the area is not suitable for STWs and DTWs irrigation in its eastern part.

MANAGINGSALINEGROUNDWATER

Actions and measures for saline groundwater management can be broadly grouped into categories like engineering techniques and scientific and behavioral approaches.

Engineering Techniques: In the area the number of DTWs and STWs are increasing with increasing demand for irrigation water (Fig. 9). In the western and northeastern parts of the area the numbers of STW and DTW have increased over the period of 2005-2010. In the southern part the number of DTW remained the same throughout the period due to high salinity of groundwater, while the number of STW has increased. As a result, groundwater withdrawal rate has increased and saline water invaded relatively fresher parts of the aquifer by upconning. So, management approach should be aimed at keeping the groundwater withdrawal at an optimum level, especially in the pre-monsoon period to prevent further increase of the salinity hazard.

Fig. 5: Correlation between aquifer and groundwater resistivity

Fig. 6: Correlation between aquifer resistivity and groundwater electrical conductance (EC, µS/cm)

Fig. 7: Spatial distribution of salinity in groundwater

Fig. 8: Iso-resistivity contour maps of a) upper and b) lower aquifers

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A management approach for fighting against salinity especially in areas of up-conning is the artificial recharge of aquifers. During the monsoon period (July-November), 90% of the northeastern part remains inundated by flood water. So the beel (swamp) areas in the north-eastern part can be used in the post-monsoon and winter seasons by constructing surface water recharge basins using Water Spreading Method of Artificial Recharge [17], that will increase infiltration of fresh water into the aquifer, and will help to recover the aquifers already salinized rendering them unsuitable for irrigation development.

Fig. 9:Number of shallow tube wells (STWs) and deep tube wells (DTWs) operating for groundwater abstraction

Scientific and Behavioral Approaches: Especially in the southern part of the area where Boro rice highly dependant on groundwater irrigation is cultivated (60-80% of total crop production) in winter and pre-monsoon period. Minimum groundwater irrigation is needed for wheat production (1-10%

of total land) that consumes 75% less groundwater than Boro rice cultivation. Moreover, it is not justified to cultivate Boro rice (e.g., 1 kg rice with more than 2000 litre groundwater) in such area suffering from saline water upconning. This measure must be undertaken immediately as a demand management approach for groundwater conservation.

In the study area, groundwater irrigation is needed for Boro cultivation which must be avoided especially during the pre- monsoon summer period (March to May). So the crop diversification program should be undertaken to change the pattern of intensive crop cultivation, especially from traditional paddy cultivation in summer (pre-monsoon) to crops like wheat. It involves cash inflow to the out of cash crop farmers;

proper marketing of agro-products and shifting to surplus generating farming for their survival and growth.

CONCLUSIONS

1. The area has two-fold aquifer system and inter-layered by silt, clay and silty-clay aquitard and aquiclude. The aquifer resistivity greater than 69 Ωm is safe used for irrigation use.

2. Genetically groundwater is of normal chloride, normal sulfate and normal carbonate to super carbonate, very hard and slightly saline to fresh water type. Groundwater quality is a contribution of water-rock interaction, anthropogenic pollution as also, effects of sea water mixing and marine spraying.

3. Groundwater salinity increases depth wise and spatially increases towards the south. Upper and lower aquifers in that area are partially or fully affected by salinity. So the area is not generally suitable for STWs and DTWs irrigation in the eastern part.

4. Due to increased groundwater withdrawal, the saline water has progressively invaded in relatively fresher parts of the aquifer by upconning and thereby increasing groundwater salinity. So the measures for irrigation management, namely, groundwater withdrawal in optimum level especially during summer; construction of surface water recharge basins; crop diversification program etc. should be practiced to prevent salinity hazard.

REFERENCES

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