Physicochemical Evaluation of Groundwater Quality Emphasizing Salinity and its Environmental Impacts in Selected Areas of Satkhira District, Bangladesh

Abstract

The present study was carried out in the south western coastal adjoining area of Satkhira district along the Indian boarder, to assess the groundwater quality measure for drinking and irrigation purposes. However, the salinity intensity, sources, mechanisms have also been identified based on the physicochemical parameters. The impact of salinity was evaluated by the field observation and secondary crops production data in the study. In this context, there are 55 groundwater and 10 surface (River and Pond) water samples were collected from different well locations for pre-monsoon and post-monsoon periods of 2011. The average concentration of the major quality controlling parameter's such as EC (2585.38 µSiem), TDS (1630.43 mg/1), Na+ (406.20 mg/1), er (748.50 mg/1) and Mg (60.25 mg/1) are objectionable level in the most of the groundwater samples for pre­monsoon period exceeds the WHO (2006) and DoE (1997) standards limit and unsuitable for drinking purposes. As the trace element Mn concentration (0.245 mg/1) is also cross the recommended permissible limits in most of the samples, while other parameters in average of pH (7.19), Ca (72.43 mg/1), K (11.14 mg/1), NO3-(0.152 mg/1), so/-(27.224 mg/1) and Fe (0.78 mg/1) are not objectionable in this period. But during the post monsoon period, most of the values decrease except NO3-and K. The Pie diagram result shows that Na+ and er are the dominant ions for both seasons that are individually exceed 50% of the total cations or anions in groundwater of the area Strong correlation was found among the Ca and Mg, er and so/-, and Na+ and er, which indicates that they are originated from the same sources. The Piper plots indicate that Na+ and er are dominant ions that were made the alkaline (Na-type) and Saline nature (Cl-type) of groundwater in the area. The Wilcox diagram, US salinity diagrams and various recommended irrigation water classification result based on certain parameters-EC, %Na and SAR, which shows that about 40% of the groundwater samples in the area are moderately suitable and unusable for irrigation purpose. A significant variation was found among the groundwater and surface water, where salinity, temperature, pH and EC values of surface water are comparatively high than the groundwater, while TDS is high in river water but in pond water its values lower than groundwater. Na+, K+, er, so/-and NO3-concentrations are high in both river and pond water than the groundwater, but Ca, Mg and HCOl- concentrations are comparatively lower than the groundwater. In the salinity study, the field observation result shows that there are 19 groundwater samples in each season (out of 55 samples) are "very light salty or light salty to salty taste", most of which are found in the area of Shyamnagar and Kaligonj Thana, while 36 samples are salt free observed by tongue. Salinity varies in the area from 0.2 to 3.9 ppt during pre-monsoon period, but its intensity slightly declines during post-monsoon period ranges between 0.1 to 3 .1 ppt; EC results indicates that 19 samples are fallen saline categories, 4 samples are brackish and rest of 32 samples are fresh groundwater during pre-monsoon periods, while few variations was found for saline and brackish water during post-monsoon period. Salinity is also evaluated by TDS which result shows 11 samples are moderately saline, 10 samples are slightly saline and rest of 34 samples are fresh water during pre-monsoon period, while few variations was found during post-monsoon period. Comparatively high salinity found within the depth of 27 to 230 feet in coastal adjoining area, but light salty groundwater also found in deep tube well (>600 feet) away from the coastal area especially in Kalaroa Thana of the area. A paramount relation was found among the salinity and EC, TDS, Na+, er' sol·, K+, where these parameters are increased with the increasing of salinity, and decreased with the decreasing of salinity. CEV result indicates that most of the constituents in groundwater are inland origin and the Gibbs diagram result suggested that these chemical constituents are rock dominance. The bivariate plot of Na+-cr (meq/1) versus Ca2++ Mg2+- HCo/·-so/· (meq/1) indicates that ion exchange was taking place in most of the groundwater samples in the area. The potential source of groundwater salinity in the area due to sea water intrusion which was identified by using the various methods i.e. the correlation matrix, where Na+ and er ions were positively correlated with TDS (R2 = 0.927 for Na+ with TDS, R2 = 0.986 for Cr with TDS 0.986); by the plot of Na+ versus er which result shows the concentration of Na+ is greater than Cl- in most of the samples indicting presence of brine or seawater that occurred due to the sea water intrusion; by the equitation of Appelo & Postma ( 1993) result shows about 1 % to 15% sea water er fraction present in the 27 groundwater samples which indicates sea water intrusion occurred in the aquifers of the area; based on Ionic ratios of er with EC of groundwater result shows most of the ionic ratios value are higher in fresh water and consequently lower in brackish and in saline water, which indicates that er is comparatively low in fresh and high in brackish and saline water that compare with MIRS indicates sea intrusion occurred to the aquifer of the area ; by the frequency plot of the major groundwater constituents and their threshold values especially er (583.21 mg/I) and TDS (1588.37 mg/I) strongly support the sea water intrusion occurred in the area; From the Piper plot, Na-Cl type of water which also indicates presence of sea water in groundwater of the area. Salinity affects the local environment of the area; field observation result shows that soils, infrastructures such as houses, buildings, tube wells, polls and other concrete structure were damaged due to the influence of salinity. Various diseases such as diarrhea, dysentery, typhoid, jaundice etc. were also found in overall salinity prone area. Many kinds of trees such as mango, papaya, banana, guava, berry-berry, jackfruit, palm, coconut etc. and various crops such as paddy, wheat, vegetables and jute etc. are decreasing due to high magnitude of salinity in the area. The study of rice, wheat, vegetables and Jute production secondary data for 2010 and 2011 shows that production of each crop gradually deceasing to the coastal adjoining area where salinity is comparatively high. To protect the salinity in the study area some structural and non­structural management techniques are important. So, the study brings out an overall scenario about the groundwater and its quality, salinity and its management and impacts on the local environment of the area.