Groundwater dilution and residence times, and constraints on chloride source, in the Mokai geothermal system, New Zealand, from chemical, stable isotope, tritium, and sup 36 Cl data

Journal Article · Sat Nov 10 04:00:00 EST 1990 · Journal of Geophysical Research; (United States)

OSTI ID:5359272

Hedenquist, J W ^[1]; Goff, F ^[2]; Phillips, F M ^[3]; Elmore, D ^[4]; Stewart, M K ^[5]

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1. Geological Survey of Japan, Higashi (Japan)
2. Los Alamos National Lab., NM (USA)
3. New Mexico Inst. of Mines and Technology, Socorro (USA)
4. Univ. of Rochester, NY (USA)
5. Inst. of Nuclear Sciences, Lower Hutt (New Zealand)

The maximum measured temperature of the production wells is near 323C at 2500 m depth, whereas temperatures in two wells on the northern margin of the upflow are <200C, with thermal inversions at 350 and 1,000 m indicating lateral flow. These two marginal wells have about 50% of the chloride of the deep fluid, indicating dilution by cold groundwater. Chloride-bearing springs discharge 6 km north of the zone of upflow and are even further diluted by cold groundwater than the wells on the margin of the system. The tritium data in this hydrologically rather simple system indicate that there is a mean residence time of greater than 100 years for the groundwater diluent; also, older water is mixed with <25% of young (thermonuclear, post-1955) water before the mixture dilutes outflowing chloride fluid from the deep system. The deep chloride fluid has a {sup 36}Cl/Cl ratio of 2.6 {times} 10{sup {minus}15} and a {sup 36}Cl concentration of 106 atoms per 10{sup {minus}6} kg. This fluid has a lower ration (by a factor of 90) and a higher absolute concentration (by a factor of 7) than present-day stream water. However, the marginal wells have been diluted by groundwater with at least 500 {sup 36}Cl atoms per 10{sup {minus}6} kg, 30 times richer in {sup 36}Cl than present-day stream waters. This spike is most likely due to circulation of precipitation formed during thermonuclear testing to depths of 1,000 m. The amount of {sup 36}Cl in the deep fluid at Mokai is 5-10 times less than that predicted from a calculation of the neutron fluxes in the silicic reservoir and underlying greywacke basement rocks. This indicates that the fluids have interacted with more primitive rocks below the drilled depths of the Taupo Volcanic Zone, with the chloride possibly derived from a magmatic brine.

• OSTI ID: 5359272
• Journal Information: Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 95:B12; ISSN JGREA; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
150200* -- Geology &amp; Hydrology of Geothermal Systems
54 ENVIRONMENTAL SCIENCES
540330 -- Environment
Aquatic-- Radioactive Materials Monitoring &amp; Transport-- (1990-)
AGE ESTIMATION
AUSTRALASIA
BETA DECAY RADIOISOTOPES
BETA-MINUS DECAY RADIOISOTOPES
BETA-PLUS DECAY RADIOISOTOPES
BRINES
CHEMICAL ANALYSIS
CHEMISTRY
CHLORIDES
CHLORINE 36
CHLORINE COMPOUNDS
CHLORINE ISOTOPES
DEPTH
DEVELOPED COUNTRIES
DILUTION
DIMENSIONS
ELECTRON CAPTURE RADIOISOTOPES
ENVIRONMENTAL EFFECTS
EXPLOSIONS
FLUIDS
GEOCHEMISTRY
GEOTHERMAL FLUIDS
GEOTHERMAL RESOURCES
GEOTHERMAL WELLS
GROUND WATER
HALIDES
HALOGEN COMPOUNDS
HYDROGEN COMPOUNDS
HYDROGEN ISOTOPES
HYDROLOGY
ISOTOPE RATIO
ISOTOPES
LIGHT NUCLEI
MASS SPECTROSCOPY
NEW ZEALAND
NUCLEAR EXPLOSIONS
NUCLEI
ODD-EVEN NUCLEI
ODD-ODD NUCLEI
OXYGEN COMPOUNDS
RADIOISOTOPES
RESERVOIR TEMPERATURE
RESOURCES
SPECTROSCOPY
THERMONUCLEAR EXPLOSIONS
TIME DEPENDENCE
TRITIUM
VOLCANIC REGIONS
WATER
WELLS
YEARS LIVING RADIOISOTOPES