Environmental Study on a Coastal River of Bangladesh with Reference to Irrigation Water Quality Assessment: A Case Study on Shailmari River, Khulna

Water quality is the critical environmental determinant that influences theagricultural production and therefore, the economy that solely depends on itsagricultural productions. Batiaghata Upazilla is one of the major cropproductive areas of Khulna region and the agricultural production here largelydepends on the natural water of the Shailmari River system around it. Thepresent study was conducted to assess the suitability of this coastal riverwater for irrigational use. 66 water samples were collected during this studyin three consecutive agricultural seasons, viz., pre-monsoon (22), monsoon (22)and post-monsoon (22) from 11 sampling stations within the river system consideringthe high (11) and ebb (11) tides for each station. Standard methods were followedthroughout the study period for the collection of the samples and analysis ofmajor physicochemical parameters (pH, EC, TDS, salinity, Na, K, Ca, Mg, Cl, HCO3,SO4, NO3 and PO4). Hydrocehmical characterizationusing Piper trilinear diagram shows that the water of the river system is Na-Cldominated saline water type in pre-monsoon, whilst most of the samples of theriver and the connected channel are characterized as fresh water of Ca-Mg-Na-HCO3 and Ca-Na-Mg-HCO3 types in monsoon and post-monsoon respectively. Thecalculated values of chemical indices like SAR, %Na, KI, PI and MH using theresults of the analysed parameters indicate that the river water is chemicallyunsuitable for use in irrigation during pre-monsoon, while in the monsoon andpost-monsoon the river water is within good to permissible limit for the use inthe agricultural fields. However, high %Na, KI and MH values with high salinityof the channel water limit its use in agricultural applications in monsoon andpost-monsoon. The USSL (United States Salinity Laboratory) diagram depicts thatthe river water is C4-S4 type with very high salinity and sodium content inpre-monsoon, while in other seasons the water is mostly C1-S1, C2-S1 or C2-S2 types(low to moderate salinity and sodium) in the river. Wilcox diagram shows thatthe river water is “suitable” for irrigation during monsoon and post-monsoon asthe most of the samples range within “Excellent to Good” and “Good topermissible” categories, while it is “unsuitable” for crop irrigation duringpre-monsoon when SAR (Sodium Adsorption Ratio) and EC (Electrical Conductivity)values of the water reach to their peaks.