The potential of duckweed in acid mine drainage impact
assessment and detoxification
Acid mine drainage (AMD) is a major pollutant of surface waters.  Although the effects are either local
or regional in significance this is a global problem resulting in the uncontrolled discharge of heavy
metals and acidity into the hydrological cycle.  This results in the contamination of both surface and
ground waters with the effects causing significant impacts to both estuarine and coastal waters.  

The project explores two fundamental problems.  The first is to identify a reliable indicator of both AMD
strength and impact, and secondly to find a reliable method to detoxify AMD prior to discharge to
surface waters.  

Duckweed (
Lemna minor) has been widely used as a toxicity assessment tool for wastewaters, with an
ISO assay method available, while duckweed treatment ponds have been widely employed to treat
domestic wastewaters.  Under ideal conditions,
Lemna minor can double in number in just over two
days and has the ability to adsorb heavy metals.  It is these traits that allow duckweed to be
considered not only as a potentially ideal indicator but also as a detoxifier of AMD.

  • Quantify the sensitivity of duckweed to AMD and explore the relative effects of some of its key
    components (e.g. Cu, Zn, acidity, sulphate).  This includes measuring the effectiveness of
    duckweed as a bioindicator of AMD impact in the field.
  • Determine the optimal conditions for growth of duckweed on AMD.  Evaluate a variety of high
    nutrient wastewaters to find an alternative nutrient source.
  • Quantify metal uptake and removal rates.

The project can be divided into three distinct themes:
  • Toxicity of AMD to duckweed: Duckweed is widely used to measure the toxicity of wastewaters.  
    However, AMD is a multi-factor pollutant with a number of potentially toxic physico-chemical
    elements (e.g. acidity, heavy metals and increased sulphate concentrations).   As metal
    bioavailability is linked to pH, assessing the whole toxicity of AMD is more complex than most
    wastewaters.  The first part of the project was to assess the toxicity of both real and synthetic
    AMD to duckweed and the toxicity of its individual components.  Once complete, the next phase
    was to use duckweed as an indicator of AMD toxicity in surface waters.  The study was carried
    out on the AMD contaminated Avoca River, Co. Wicklow.
  • Growth of duckweed on AMD: Acid mine drainage is highly toxic and thus adversely affects
    plant growth.  For duckweed to be successful in the treatment of AMD it must first be shown that
    it can sustain healthy and active populations when it is grown with it.  However, as AMD contains
    several of the elements required for plant growth (at supra optimal concentrations), it was
    necessary to determine by dilution the point at which AMD is no longer toxic to the organism.  
    The study also looks at how this concentration known as the lowest ineffective dilution (LID) is
    effected by the addition of nutrients.  This section also looks at substitution of growth medium
    with high nutrient wastewaters and their potential impact on the LID.
  • Use of duckweed in the removal of metals:  The final part of this study is to actually test whether
    duckweed can remove or mitigate the key toxic physic-chemical elements.  Metal uptake and
    removal rates and mechanisms will be determined.  If successful, system loading and operating
    limits will be determined for pilot scale treatment.

Lisa Egan
Centre for the Environment, TCD


Funded: Trinity College Dublin/ Water Technology Research
 Water Technology Research Group
 Trinity College Dublin
   1980-2010 thirty years of research, training and consultancy
Lisa Egan is in her
second year of her PhD
An updated photo to follow!