Abstract:
The Grootspruit valley bottom wetland in South Africa, due to the impact of acid mine
drainage (AMD) from an abandoned coal mine, was severely degraded before ecologically engineered
interventions, as a passive treatment process, in 2014. The surface water flow of the wetland was
redirected using concrete structures to enlarge the surface area of the wetland by 9.4 ha and to optimize
passive treatment. Although the ecologically engineered interventions showed an improvement in
water quality after the rewetting of the enlarged wetland areas, the 2016 drought had a devastating
effect on the wetland’s water quality. Limited natural removal of metals and sulfate concentrations by
the wetland occurred during the 2016 drought, when compared with the 2015 pre-drought conditions.
This period showed higher concentrations of metals, sulfate (SO4
2−), and electrical conductivity (EC)
associated with the acidic surface water. Of particular interest was an observation of a substantial
shift in pollutant-tolerant algae species in the ecologically engineered wetland outflow between
the years 2015 and 2016. During the dry spell period of 2016, the diatoms Gyrosigma rautenbachiae
(Cholnoky), Craticula buderi (Brebisson), and Klebsormidium acidophilum (Noris) were observed at the
outflow. The latter species were not observed during the wetland surveys of 2015, before the dry
spell. From late 2017 onwards, after the drought, environmental conditions started improving. In
2018, periphyton indicator species and the surface water quality were comparable to the wetland’s
recorded status pre-2016. The study revealed not only a regime shift, but also an ecological function
loss during the drought period of 2016, followed by recovery after the dry spell. A distinct reduction
in SO4
2−, sodium (Na), magnesium (Mg), EC, manganese (Mn), iron (Fe), silicon (Si), aluminum (Al),
and pH, following the 2016 drought, highlights the utilization of water quality variables to not only
assess the passive treatment responses of an ecologically engineered wetland, but also the progress
relating to ecological recovery.
