Seasonal variation of redox species and redox potentials in shallow groundwater: A comparison of measured and calculated redox potentials

摘要

The seasonal variation of redox potential (Eh) and redox species such as As(V)/As(III), Cr(VI)/Cr(III), Fe(III)/Fe(II), NO3-/NO2-, and Se(VI)/Se(IV) were studied in a shallow groundwater for a period of three years (May, 2004-January, 2007). The study area was Chrompet area of Chennai city, India. Groundwater samples from 65 wells were monitored for pH, electrical conductivity, dissolved oxygen (DO), and major ions during pre-(May) and post-monsoon (January) seasons. The objective of the study was to gain insight into the temporal variation of the redox species due to groundwater recharge and to identify the redox reactions controlling the measured Eh of the groundwater. The study revealed that the shallow groundwater was " oxic" with DO ranging between 0.25 and 5.00mgL ~(-1), and between 0.38 and 5.05mgL ~(-1) during pre-(May, 2004) and post-monsoon (January, 2005) seasons, respectively. The measured Eh (with respect to standard hydrogen electrode, SHE) ranged between 65 and 322mV, and between 110 and 330mV during pre- and post-monsoon seasons, respectively. During post-monsoon seasons, DO and Eh increased in most of the wells due to groundwater recharge. The calculated Eh using the redox couples As(V)/As(III), NO3-/NO2-, O _2/H _2O and Se(VI)/Se(IV) neither agreed among themselves nor with the measured Eh during all the seasons. It shows that in the shallow groundwater, the various redox couples are in disequilibrium among themselves and with the Pt electrode. However, 41% (n=122) of the Eh values calculated from Fe(III)/Fe(II) couple agreed with the measured Eh within ±30mV, the uncertainty of Pt-electrode measurement. The post-monsoon seasons showed higher values of As(V)/As(III) and Se(VI)/Se(IV) compared to the pre-monsoon seasons, whereas Fe(III)/Fe(II) behaved in the opposite manner. This pattern of variation is consistent with the increased oxidizing nature, as shown by the higher DO and Eh values observed during post-monsoon seasons. The results showed that the Fe(III)/Fe(II) is the dominant redox couple to equilibrate with Pt electrode. However, the measured Eh can only be used in a semi-qualitative way and can be interpreted with other redox indicating parameters. The measured Eh though represent 'mixed potential', is a useful indicator for characterizing the speciation and temporal variation of redox sensitive species.