Metals in groundwater

1: Potential concerns for groundwater quality in the lower South Island 2: Margin of Dunstan Range, central Otago, with broad alluvial fans made of schist gravel. Slow-moving groundwater in this arid environment can potentially leach arsenic from the gravel.

Groundwater is extracted from young sediments in basins, valleys, and coastal plains throughout New Zealand.

• Most groundwater is of high quality and is readily replaced by rain and river recharge.
• Some groundwater resources are being extracted at the limits of sustainability, and deeper resources are being tapped.
• Deeper waters move more slowly, are older, and can interact with more with rocks, and have potential for containing elevated metal contents.
• Groundwater from coal-bearing sediments also has the potential to become acidified with enhanced metal dissolving properties (e.g., Southland, North Otago; see map to right).

Arsenic in groundwater: the Bangladesh analogy

• Groundwater in many areas of Bangladesh has elevated arsenic concentrations, leading to what has been called the largest case of mass poisoning in human history.
• Key geological features which have contributed to high arsenic in Bangladesh are:
  • Water is in young sediments derived from active mountains.
  • The sediments contain only background arsenic concentrations.
  • Because the sediments were transported and deposited rapidly, they still contain minerals which are susceptible to chemical interaction with groundwater.
  • This groundwater interaction can dissolve arsenic at low levels.
  • Water levels in sediments fluctuate in seasonal wet/dry climate.
  • This water level fluctuation can induce chemical processes which concentrate the arsenic.
• Many of these geological features apply for New Zealand groundwater (map above).
• Generally, groundwater sources tapped in New Zealand have sufficient water movement to ensure that any dissolved arsenic is diluted to low levels.
• Some slope gravels in Central Otago have all of the above characteristics, and arsenic levels are slightly elevated locally (marginally above drinking water limits).
• Arsenic in groundwater in Otago Schist