An investigation on groundwater geochemistry changes after 17 years: a case study from the west of Iran

Abstract

The application of chemical and organic fertilizers to agricultural lands increases nutrient pools and affects soil and water quality. Understanding changes in groundwater quality due to the anthropogenic activities over time is important to human and ecosystem health. In 2017, we resampled 58 wells monitored in the year 2000 to evaluate the rate of changes in groundwater quality and water quality indices over time in response to agricultural and industrial activities and climate changes. The groundwater in two sampling years was dominated by Ca–HCO₃ water type. The mean groundwater pH, electrical conductivity (EC), calcium (${\text{Ca}}^{2 + }$), magnesium (${\text{Mg}}^{2 + }$), and sodium (${\text{Na}}^{ + }$) concentrations did not significantly change over time, while a significant buildup in bicarbonate $({\text{HCO}}_{3}^{ - } )$ (189–305 mg l⁻¹), nitrate (${\text{NO}}_{3}^{ - }$) (41–56 mg l⁻¹), chloride (${\text{Cl}}^{ - }$) (57–77 mg l⁻¹) and a significant decrease in sulfate (${\text{SO}}_{4}^{2 - }$) (159–91 mg l⁻¹) and potassium (${\text{K}}^{ + }$) (3–2 mg l⁻¹) was observed. In 2000, 45% of the water samples were classified as high salinity hazard, and this value increased to 52% by 2017, indicating that salinity of the water samples increased over 17 years. In 2000, only 25% of the total area had a ${\text{NO}}_{3}^{ - }$ value greater than 50 mg l⁻¹; and this value increased sharply to 62% by 2017, indicating that ${\text{NO}}_{3}^{ - }$ concentrations significantly increased with approximately 2.2% annually in groundwater, over 17 years. Approximately, 55% of the groundwaters with ${\text{NO}}_{3}^{ - }$ concentration values of less than 40 mg l⁻¹ in 2000 shifts one to four categories upward in 2017. The percentage of water samples which was shifted to the next category with ${\text{NO}}_{3}^{ - }$ concentration higher than 50 mg l⁻¹ and approaching the limit of the World Health Organization was doubled in 2017 compared with 2000. High application of chemical and manure fertilizers in excess of crop needs should be avoided in the studied area to prevent continued increases in groundwater ${\text{NO}}_{3}^{ - }$ concentrations.