A Gnip Station in the Southern Hemisphere
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WATER & ENVIRONMENT NEWS XA9953202 A GNIP STATION IN THE SOUTHERN HEMISPHERE retained at Kaitoke to ensure no genic tritium supplemented by a Location discontinuity in the tritium record, smaller amount of bomb HTO re- but with the collector aperture re- evaporated from the surrounding aitoke Regional Park is a duced in size. oceans. The seasonal variation is bush-covered area (Fig.1) of K now less pronounced. Bomb tritium steep hills and river gorges at the reached the Southern Hemisphere southern tip of the Tararua moun- A Tritium Station by a high stratospheric route, de- tain range, about 40 km north of scending over the Antarctic in July- New Zealand's capital city Welling- Kaitoke is ideaiiy situated as a rep- October, during the period of ab- ton. Run-off from the highest-lying resentative Southern Hemisphere sence of the polar tropopause. Cos- sub-catchment of Kaitoke River is station, and has now become the mogenic tritium enters the tropo- diverted southwards by a tunnel "master station" against which data sphere, as in the Northern Hemi- through the hills, where it is treated from other stations are compared sphere, mainly through the mid- and distributed to supply 50% of and statistically analyzed. Fig.2 iatitude tropopause during spring- water use within the greater compares average annual mean early summer. Wellington region. concentrations at Kaitoke and Vi- enna, illustrating the markedly lower The river leaves the Park through amount of bomb-tritium fallout in the Many earlier GNIP-reported results a gorge, is joined by the Akatarawa Southern Hemisphere. Rainfall at originating from Northern Hemi- River at the head of the Upper Hutt Kaitoke (about 1 meter/year) is sea- sphere laboratories show obvious valley basin, and flows on, now sonally evenly distributed , so suffi- evidence of contamination during called Hutt River, towards the cient water is collected every month storage and analysis during the pe- Lower Hutt valley and its outlet in to provide the necessary sample riod when Northern Hemisphere lev- Wellington harbor. Hutt River quantity, this is an advantage over els were strongly enhanced by H- recharges groundwater of the the originally-selected GNIP master bomb tritium. Reliability of any Lower Hutt artesian basin, which station at Pretoria, where absence Southern Hemisphere record can be supplies a further 25% of the re- of rainfall in many months pre- established by comparing mean an- gion's water needs. This regionally vented assessment of seasonal and nual tritium concentrations (July- indispensable hydrological system year-to-year changes which might June) with those at Kaitoke. has played an important role in the be considered typical for the South- history of environmental isotope ern Hemisphere. Although situated hydrology in the Southern Hemi- at some distance from the ocean to Applications sphere. both west and east, rainfall at It is very fortunate that river water Kaitoke is truly mid-latitude oceanic. south of Kaitoke was regularly col- This ensures that the seasonal dis- Close to the supply catchment at lected and stored during the years tribution of tritium concentration re- Kaitoke, hidden behind distinctive before rainfall collections com- flects the response of the tropo- native grasses (toetoe), stands an menced. Later comparisons of tri- sphere to the input from te strato- unassuming rainwater collector, tium and deuterium in rain and river sphere, without any interference which has served for now 37 years showed that the Kaitoke catchment due to continental influence, such is as a key tritium station within the has a short hold-up time of at most apparent at Brisbane, where sea- Global Network for Isotopes in Pre- several weeks. The river data could sonal shifts of wind increase the cipitation (GNIP). The collector therefore be used to reconstruct an- amplitude of the seasonal variation was originally established for both nual averages for tritium in precipita- by bringing tritium-enriched mois- tritium and sulfur isotopes by the tion during the years before 1961. A ture from the continental interior Institute of Nuclear Sciences, De- pre-bomb average 1.5TU was esti- during the spring months and partment of Scientific and Indus- mated from groundwater data; its tritium-depleted moisture from the trial Research, Lower Hutt. Large reliability is now confirmed by the ocean during autumn. This is illus- quantities of water were required recent concentrations. trated in Fig.3, which compares the for the sulfur analyses, so a wide- average seasonal variations at mouthed collector was set up at Kaitoke and Brisbane. The earlier river water collections Kaitoke, where rainfall is much were supplemented by regular col- greater than at Lower Hutt. lections of water tapped from a river- Peak tritium concentrations in the fed, confined aquifer in the Lower At the end of every month, a staff Southern Hemisphere following H- Hutt basin. Measurement of both member drives to Kaitoke to re- bomb tests were reached in 1964- series produced what was probably cover the integrated monthly sam- 65. Those of recent years the first application of environmental ple. Following cessation of sulfur (averaging about 2TU at Kaitoke) tritium to determine travel time in a isotope collections, the station was comprise only the fallout of cosmo- confined gravel aquifer under condi- Issue No. 4, July 1998 - Page 13 WATER & ENVIRONMENT NEWS tions approximating to piston flow. During the late 1950s, tritium fall- out in New Zealand responded stepwise to United States H-bomb testing in the Pacific (Fig.2). Con- centrations at the aquifer sampling location matched those of river wa- ter delayed by about 3 years. The travel time estimate was narrowed to 40 months by the appearance in the aquifer of the jump associated with arrival of the atmosphere 6 river pulse which followed the 1956 US Castle test. Recent Developments Within the past two years this catchment six river six aquifer sys- tem has again been applied as a test case in silicon-32 and CFC investigations. The 32Si (half-life 160 years) study is especially inter- esting, because it is likely to stimu- Fig.1. The bush-clad hill catchment and river valley at Kaitoke. The collector is located close to the late renewed interest in this iso- buildings in the middle-right distance tope as a dating tool. Sample processing and measurement techniques have been greatly sim- plified and improved by U. Morgen- stern and R.G. Ditchburn. Monthly O collections at Kaitoke establish the < concentrations and seasonal varia- 01 tion in precipitation. Measurements in the river record the extent to which this catchment type (bush t cover over greywacke/argillite) transmits the precipitated tracer to the river. Comparison of concen- tration levels in the confined Lower 1970 1980 2CO0 Hutt basin aquifers with those of 1950 1960 YEAR tritium indicates whether 32Si is conserved in the aquifer environ- ment (greywacke gravels) over Fig. 2. Average annual tritium ratio in precipitation at Kaitoke and Vienna times up to several decades. (Claude Taylor, Institute of Nuclear Sciences, Department of Scientific and Industrial Re- search, Lower Hutt, New Zealand, e-mail: [email protected] 4 6 8 10 MONTH (July - Next June) Fig. 3. Seasonal variation ofHTO concentration at Kaitoke and Brisbane Issue No. 4, July 1998 - Page 14.