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The Ganges Kobadak irrigation project is a significant and substantial initiative that is currently experiencing an irrigation insufficiency due to a deficit in water supply from the Ganges River. As a result, the contribution of groundwater to irrigation has increased from 41% in 1982/83 to 93% in 2015/2016 with a current irrigation intensity of 52.95%. However, currently, the utilization of groundwater is even at risk as a result of pollution threats. Some concerns have been raised regarding the possible contamination of the groundwater due to the utilization of chemical fertilizers and pesticides in the tobacco business, alongside the pollution originating from the Gorai River, which is influenced by the activities of the Kushtia Sugarmill. It is essential to assess groundwater recharge sources, such as whether percolation of direct rainfall mixed with agricultural pollutants and/or contaminated surface water seepage contributes to groundwater quality degradation while replenishing the aquifer. In this regard, this thesis investigates commonly practiced geochemical analysis as well as stable isotope technique, as a cost-effective recharge source identification method. Results from lithology confirm unique geological characteristics of Kushtia which include a thick (0-15 m) top clay layer of Ganges alluvium depositions and a single holocene aquifer of 15-175 m deep stretching up to 300 m in some region. Such thick clay layer increases the likelihood of regional groundwater flow rather than local flow. The Gibbs diagram provides further evidence of this possibility, as it reveals significant rock-water interaction with little to no local rainfall dominance within the aquifer. However, according to the Stiff diagram, only deep (76-140 m) tubewell exhibits regional or intermediate flow, whereas shallow (3-75 m) tubewell shows to have an influence of local flow effect. As a result, δ(18O/16O) and δ(2H/1H) ratios of stable isotope analyses show that the main source of groundwater recharge at shallow depths is evaporated surface water, either in the form of rainfall or surface-stored water, with major rivers playing a minimal role in direct recharge at deep water wells. As a result, Kustia Sugarmill’s pollutants coming through Gorai river are not traceable at deep water wells. However, surface or groundwater irrigation activities may be linked to the dissolution of agricultural fertilizers polluting shallow wells. Because, statistical analysis, such as principal component analysis (PCA) of shallow tubewells exhibit high levels of sulphate (SO42+) and chloride (Cl-) and low levels of Nitrate (NO3-). Since tobacco production extensively contaminates surface water and has the potential to contaminate shallow aquifers with highly hazardous pesticides and fertilizers, integrated pest management should be implemented. As the magnitude of the pollutant mixing remains unknown in this thesis work, the groundwater age estimates in a future study can be discussed through tritium (3H) and carbon-14 (14C) isotopes analyses to determine the pattern and spatial extent of groundwater flow in Kushtia. |
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