Analysis of Trend in Groundwater-Quality Parameters: A Case Study

Abstract

In the 21st century, groundwater has a pivotal role in ensuring water, food, and environmental securities worldwide. Systematic observation, protection and restoration are essential for sustainable management of water resources. Regular monitoring is key to investigate temporal changes in groundwater quality, and statistical trend tests define whether these changes are significant or not. This study focuses on investigating trend in seasonal groundwater quality in an alluvial coastal basin of West Bengal, India. The seasonal groundwater-quality data (pH, TH, TDS, Fe2+ and HCO3ˉ) of pre-monsoon and post-monsoon seasons were collected for 2011–2018 period and analyzed using three non-parametric statistical trend detection tests, namely: (i) Original Mann-Kendall (M-K) test, (ii) Modified Mann-Kendall (mM-K) test, and (iii) Spearman Rank Order Correlation (SROC) test. The trend magnitudes were estimated by using the Sen’s slope estimation test. Statistical analyses revealed that seasonal concentrations of all five groundwater-quality parameters have large spatial (block-wise) variation within the study area. The results of trend analyses indicated that seasonal TH and TDS concentrations mainly have significant decreasing trends (α = 5% or 1%), whereas seasonal HCO3ˉ and Fe2+ concentrations mostly show significant increasing trends (α = 5% or 1%) in different blocks. However, seasonal pH concentrations exhibited no trend. The mM-K test was found to be over-sensitive in finding trends than M-K and SROC tests. The SROC test was found to be less sensitive in detecting trends than M-K and mM-K tests. Trend magnitudes of seasonal pH, TH, TDS, HCO3ˉ and Fe2+ concentrations varied from –0.03/year to 0.23/year, –57.44 mg/L/year to 25.88 mg/L/year, –172.98 mg/L/year to 92.58 mg/L/year, –15.81 mg/L/year to 27.88 mg/L/year, and –0.05 mg/L/year to 0.61 mg/L/year, respectively. Continuous and proper groundwater-quality monitoring is critically required in all aquifer systems. The outcomes of this study will aid policy-makers in appropriately monitoring and managing groundwater quality.