New Zealand Journal of Marine and Freshwater Research abstracts
M99020Received 7 April 1999; accepted 15 March 2000
Geochemical signatures of discharge waters, Macraes mine flotation
tailings, east Otago, New Zealand
D. CRAW
Geology Department
University of Otago
P. O. Box 56
Dunedin, New Zealand
dave.craw@stonebow.otago.ac.nz
M. NELSON*
Environmental Science Programme
University of Otago
P. O. Box 56
Dunedin, New Zealand
*Present address: Fish and Game Otago, P. O. Box 76, Dunedin,
New Zealand.
Abstract The stream catchment containing the tailings dam
complex at the Macraes mine, east Otago, New Zealand has three chemically
distinct water types. (1) Natural groundwater has pH beween 6 and 8, and
sulphate levels locally elevated to 30 ppm. The total carbonate/calcium (Ca)
ratio is c. 2, consistent with carbonic acid dissolution of basement schist
calcite. Sodium (Na) is strongly elevated over chloride, and these levels are
dominated by water-rock interaction. (2) Decant pond waters lie on two tailings
dams, one of which was inactive between 1993 and 1998. Their pH is strongly
alkaline (8-9). Sulphate, Na, chloride, and Ca contents are higher than
groundwater and these levels have risen steadily over time. Carbonate content
is lower than groundwater. Arsenic (As) and iron (Fe) contents are high and
variable; arsenic in the active tailings pile is commonly 5-15 ppm. (3)
Mixtures of tailings water and natural groundwater (c. 1:1) form from seepage
from the upper part of the main tailings pile only and mix beneath the main
tailings dam to emerge down stream of the dam. The mixture pH (6.3) is slightly
lower than that of the groundwater (mainly >7) whereas the sulphate content
is high (>1500 ppm) because of the water component of the tailings. There is
no resolvable time lag on the 1-month scale between discharge of tailings water
into the dam and emergence at the foot of the dam. Two distinct water types are
identifiable in this setting. Chimney drain discharge is collected in pipes and
discharged from those pipes. The tailings component of this water is chemically
little different from that which left the tailings. Subsurface flow travels
unconfined beneath the tailings dam and interacts chemically with the dam
aggregate. Attenuation of Na and chloride, and addition of Ca, carbonate, and
magnesium (Mg) occurs. Nearly all dissolved As and copper (Cu) was extracted
from the subsurface flow during passage through the dam, whereas some As and
all Cu is extracted from the chimney drain discharge. A small volume of a
mixture of tailings water and groundwater has continued down stream of the
tailings dam complex for c. 50 m in the basement schist as a
contaminant plume which took c. 2 years to travel that distance in
fractures within the schist. This contaminated water is chemically similar to
that of the subsurface flow, and has high sulphate content but no detectable As
or Cu.
Keywords water chemistry; environmental chemistry; mine
tailings; metal attenuation; Otago Schist
New Zealand Journal of Marine and Freshwater Research, 2000, Vol. 34:
615-627
0028-8330/00/3404-0615 $7.00 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1272K); (scanned from paper original: notes about this process)
This year's abstracts |
Journal home page |
All abstracts |
Publishing home page
