Critical fluctuations in the arsenic concentration of tube-well water in Nepal: Influences of monsoon and groundwater irrigation

Arsenic contamination has remained a serious health concern in Nepal. The water quality of 24 tube-wells and the hydraulic head were continuously monitored. The fluctuations in arsenic concentrations in individual tube wells reached a maximum of 1463 μg/L. Groundwater irrigation was applied twice a year from April to early June for rice and October to January for wheat. The rapid decline and rise in the hydraulic heads of the observation tube-wells coincided with the timing of groundwater irrigation. Arsenic concentrations also peak twice a year, ca. May and November. Notably, large fluctuations in arsenic concentrations in tube-wells persisted even during the dry season, suggesting the influence of anthropogenic activities other than natural rainfall, such as groundwater irrigation. Arsenic concentrations in a tube-well increased by 1252 μg/L over one month period as the hydraulic head decreased and increased rapidly within two weeks. The observed fluctuations in groundwater arsenic levels may be caused by leaching of arsenic and dissolved organic carbon from the bounding clay layers into the adjacent aquifer. This is due to the rapid fluctuations in hydraulic head within the aquifers caused by groundwater irrigation. This study uncovered the possibility that groundwater irrigation plays a significant role in the fluctuations of arsenic concentrations in tube-wells. Further detailed and large-scale studies on arsenic in tube-well water and groundwater irrigation are recommended.