Spatio-hydrogeochemical assessment of groundwater in Saharsa, Bihar: Insight from HH-WQI and multivariate statistics

The quality of groundwater in floodplain environments is determined by a complex interrelationship of geogenic processes, geomorphological settings, soil texture and land-use practices. The current study assesses the physico-chemical properties of ground water in Saharsa District, Bihar, an area in the Koshi megafan and one that is extremely vulnerable to floods and agricultural intensification. The study area is mainly comprised of Quaternary alluvium with geomorphic features including active floodplains, paleochannels and oxbow lakes, and is therefore hydrogeologically sensitive. Ground water samples were taken across spatial units and analyzed to major physico-chemical parameters. A Hydro-Health Water Quality Index (HH-WQI) was calculated with respect to the health-related impact and canonical correlation analysis (CCA) and two-way ANOVA (Analysis of Variance) was used to evaluate the association between groundwater chemistry and geomorphology, soil texture, and land-use/land-cover (LULC). Results show that there is considerable spatial variation in the quality of ground water. High levels of total hardness, TDS, and phosphate ions were found in southern Saharsa, which is affected by the use of fertilizers and saturation of floodplains. The levels of arsenic and iron were significantly higher in younger alluvial plains and calcareous aquifers due to the significant geogenic input in reducing conditions. Oxbow lakes were characterized by low water quality because of nutrient buildup and limited natural attenuation, and paleochannels were characterized by relatively high water quality with less anthropogenic stress. The analysis of soil texture revealed that fine loamy soils have a high retention of contaminants and thus the WQI is highly variable, whereas sandy soils have low retention and a dilution effect. CCA established a moderate and significant relationship between LULC classes (particularly agricultural and sand-dominated areas) and the groundwater chemistry. The paper concludes that the groundwater quality of Saharsa is both controlled by natural hydro-geological processes and human-induced pressures, especially intensive agriculture. These observations emphasize the importance of comprehensive land and water management policies, which should be based on sustainable agricultural practices and periodic surveillance in vulnerable floodplain areas.