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Groundwater Quality Assessment of BARIsal City Corporation in Bangladesh

Sukhen Goswami
Department of Geology and Mining, University of Barisal, Barisal 8200

A. K. M. Moshiur Rahman
Department of Geology, University of Dhaka, Dhaka 1000

M. I. M. Fuad
Department of Geology, University of Dhaka, Dhaka 1000

Dhiman Kumer Roy
Department of Geology and Mining, University of Barisal, Barisal 8200

Abdur Rahim
Department of Disaster Resilience and Engineering, Faculty of Disaster Management, Patuakhali Science and Technology University, Dumki, Patuakhali 8602

Abstract/ Executive Summary:

The southern coastal belt of Bangladesh holds importance for its dramatic hydrogeological nature and aquifer systems. An attempt has been carried out to assess and evaluate the groundwater quality at Barisal City Corporation in the southern coastal city of Bangladesh. Samples were collected for the dry and wet periods from the nested wells to assess the groundwater chemistry and vertical variations for different seasons. During both periods, high concentrations of Na+ and Cl- were measured along with high electric conductivity (>7000 µS/cm) which is telltale of brackish water, whereas, at greater depth (>150 m) these parameters played reverse role. Dominant water types in wet season were Na-Cl- HCO3, Na-Cl and Mg-Cl while distinguished type of water was identified in different depths. Copiousness of the major ions has followed this order: Na+ >Mg2+> Ca2+>K+ > Cl- > HC03 2- > SO4 2- > NO3 - >PO4 2- . Vertical distribution of Arsenic (As) shows significant trend in which shallower wells have higher concentrations than the deeper one. Water from shallower depth is proved to be highly and injuriously contaminated by seawater intrusion but in deeper piezometers water is identified as slightly contaminated by seawater ingression. Variations in water quality may be influenced by the presence of thick silty clay layer that prevents injection from upper zone.

Keywords: Nested piezometer, Groundwater quality, Cl- / ratio, Hydrochemistry, Salinity Variation.

Location: The southern coastal region of Bangladesh

Start Date:

End Date:

Program Area: Crop-Soil-Water Management

Commodity: Groundwater, Water quality

Objectives:

To assess and evaluate hydrogeochemistry and water quality for both dry and wet seasons of the Barisal city corporation in Barisal Sadar Upazila.

Methodology:

The study area is situated in the southern coastal region of Bangladesh. It lies in 90.36^o E longitudes and 22.68^o E latitudes and is in the Barisal city corporation of Barisal district. BWDB installed a nest with four piezometers in different depths to assess the vertical and seasonal variations of groundwater chemistry. This area is situated on the bank of the Kirtonkhola River. The geology of this area determines the groundwater potential, vertical and seasonal hydrogeochemical variations. This study area is composed of primarily Quaternary sediments having huge thickness which form the shallowest upper aquifers. The tertiary formation, having good potential of groundwater development, is composed of sandstone argillaceous and pebble bearing grits, clays (variegated) with lignite seams and pebble beds forms the main and deeper aquifers (BGS-DPHE, 2001). At present water-bearing formations deeper than 150-200 m are being exploited on a great quantity in the coastal zone to cater the need of municipal water supply and in the rural areas for drinking purpose (Zahid et al, 2009). Large-scale extraction has not been encouraged in the coastal areas due to possibility of seawater intrusion or leakage from the upper aquifer (Aggarwal et al, 2000; Zahid et al, 2009). Sampling and Analyses: This study is principally based on data which were collected from the study area by BWDB in 2012-2013. A total of eight groundwater samples were collected from nested wells, marked as BSBSPZ_1, BSBSPZ_2, BSBSPZ_3 and BSBSPZ_4 for both wet and dry periods across a river along a line. Nested wells are the piezometric well screened at different depth to evaluate the vertical and seasonal changes in hydrochemistry. Collected samples were then analyzed in the BWDB laboratory to assess major cations like Na⁺, Ca²⁺, K⁺, Mg²⁺ and anions like CO₃ ^2-, HCO₃ ^-, NO₃ ^-, SO₄ ^2-.

Source of Information: Barisal University Journal Part 1, 4(2):339-350 (2017); ISSN 2411-247X

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The groundwater quality of the study area is controlled by various physiochemical parameters and seawater intrusion. Groundwater quality is mainly controlled by mixing of the surface water which is basically saline and ion exchange reactions. The Mg2+ and Ca2+ are derived mainly from dissolution of carbonate precipitates along with ion exchange process in the groundwater. Dominant water type in wet season was Na-Cl-HCO₃^- , Na-Cl and Mg-Cl while distinguished type of water was identified in different depth. Copiousness of the major ions has followed this order: Na⁺ >Mg²⁺> Ca²⁺>K⁺ > Cl^- > HCO₃ ^2- > SO₄ ^2- > NO₃ - >PO₄ ^2- . Vertical distribution of Arsenic (As) shows significant trend in which shallower wells have higher content than the deeper one. Shallower piezometers (up to depth of 106 m and 152 m) encountered more or less finer sediments while medium to coarser particles were found dominant from deeper piezometers (237 m and 268 m deep). A thick layer of silty clay has segregated the upper aquifer from lower aquifer. Chemistry of the brackish water collected from shallower piezometers suggests that it’s vulnerable to neither drink nor irrigation. Meanwhile, extraction from the greater depth would be a good option for daily use as its proven slightly injurious by means of Cl- / (CO₃ ^2- + HCO₃^- ) ratio. Electric conductivity and chemical parameters also indicate the same phenomena for both cases. Thick silty clay layer may have played a tremendous role in making difference in water quality between shallower piezometers and deeper piezometers. It may be preventing sea water to intrude in large amount in lower subsurface.

Knowledge Management: Journal