Impact of seasonal variation on the treatment response of constructed wetlands receiving acid mine drainage in a subtropical region

Acid mine drainage (AMD) emanating from coalfields is a severe form of water pollution with adverse ecological impacts. AMD from Makum Coalfield, Assam, India, is highly polluted and unregulated. The present study describes the influence of seasonality on the efficacy of constructed wetlands (CWs) applied for treating synthetic AMD, analogous to mine water characteristics from the Makum Coalfield. Rainfall is predominant in the northeast region of India. The effect of rainfall events on sheltered (SCW) and unsheltered (UCW) CWs was investigated under natural conditions from January to August. Acidic AMD containing Fe, Al, Mn, Zn, Co, Ni, Cr and sulfate concentrations as 100, 25, 6, 5, 1, 1, 1 and 1100 mg L-1 was continuously supplied to CWs at a hydraulic loading rate of 0.014 m(3) m(-2) d(-1). SCW and UCW raised the pH (>5.5) and demonstrated comparable treatment performance in the dry season. Heavy rainfall imposed a highly sensitive and poor treatment response in UCW, resulting in significantly lower sulfate removal in UCW (51.34 %) than SCW (60.29 %). SCW demonstrated superior metal removal for Fe (90 %), Al (79 %), Zn (94 %), Co (92 %), Ni (91 %) and Cr (96 %), which significantly differed from UCW (85 % Fe, 70 % Al, 93 % Zn, 82 % Co, 83 % Ni and 90 % Cr), except Mn. These findings indicated possible shortening of hydraulic retention time in UCW with minimal or insignificant dilution. This experimental study underlines the limitations of wetland systems in the rainfall-affected region and recommends necessary design control measures to manage the effects of rainfall.