INS- R - 227
PRELIMINARY MEASUREMENTS OF TRITIUM, DEUTERIUM AND OXYGEN-IS IN LAKES AND GROUNDWATER OF VOLCANIC ROTORUA REGION, NEW ZEALAND
by
C.B. Taylor, H.J. Fractions and I.A. Nairn
j»M 1*77 INSTITUTE OF NUCLEAR SCIENCES Department of Scientific tnd industrial Research LOWER HUTT, NEW ZEALAND
SIR (INS) 4 INS-R-227
PRELIMINARY MEASUREMENTS OF TRITIUM, DEUTERIUM AND OXYGEN-18 IN LAKES AND GROUNDWATER OF VOLCANIC ROTOPUA REGION, NEW ZEALAND. by C.B. Taylor Institute of Nuclear Sciences, Department of Scientific and Industrial Research, Lower Hutt.
H.J. Freestone Hydrological Survey, Ministry of Works and Development, Rotorua I.A. Nairn New Zealand Geological Survey, Department of Scientific and Industrial Research, Rotorua
INS contribution no, lk2 Preliminary Measurements of Tritium, Deuterium and 0xygen-l8 in Lakes and Groundwater of Volcanic Hotorua Region, New Zealand.
by C.B. Taylor Institute of Nuclear Sciences, Department of Scientific and Industrial Research, Lower *?utt
H.J. Freestone Hydrological Survey, Ministry of Works and Devebpment, Rotorua I. A. Nairn New Zealand Geological Survey, Department of Scientific and Industrial Research, Rotorua
INTRODUCTION
Geology of Rotorua District
The lakes of the Rotorua district (Fig. 1) lie within the diverse
volcanic terrain of the Central Volcanic Region, North Island, New
Zealand. Most of the volcanic rocks of the region have been erupted
from the Taupo Volcanic Zone, which extends in an axial belt north
eastwards from Mt Ruapehu to White Island (Fig. 2). The Jurassic
greywacke basement (Healy et al., 1964) is down-faulted thousands of
metres below the Taupo Volcanic Zone, and is overlain by volcanic rocks
which lap several tens of km over the higher basement areas to east and
west (Fig. 2). The most widely-distributed volcanic rocks are several
plateau ignimbrites of Pleistocene age, welded in their lower parts,
extending up to 100 km from source,and having volumes commonly exceeding
200 km (Healy, 1963). Later volcamem has extruded rhyolite domes and
loosely compacted pumice breccias of pyroclastic flow origin. The whole
region is mantled by up to 30 m of late Quaternary pumiceous tephras. The hydrology of Rotorua district is dominated by a group of lakes of volcanic origin; by catchaents in which perennial surface runoff is minimal; by large clear-water springs; uid by many centres of hydrothermal activity. Many of the lakes are associated, with the rhyolitic Okataina Volcanic Centre and the associated Haroharo Caldera
(Pig. 1) (Healy, 1964). Lakes Okataina, Rotoehu, Rctoma, and the eastern end of Lake Rotoiti are lakes on the margins of Tferoharo Caldera, formed by damming against rhyolite lava flows on the caldera floor.
Lake Tarawera occupies the southern part of Haroharo Caldera, blocked off at its eastern end by coalescing lava flows from Tarawera and Haroharo volcanic complexes. Lakes Rotokakahi, Tikitapu and Okareka occupy valleys dammed by lava flows. Lake Rotcrua occupies a caldera resulting from subsidence following voluminous ignimbrite eruption; flights of terraces up to 100 m above the present lake indicate progressive decline in lake levels. The lake sediments underlying these terraces (Pig. 1) contain some of the least permeable beds in the entire Rotorua region.
Throughout much of the region, the highly porous nature of the surface ash cover and the frequently jointed nature of the underlying volcanic rock result in a faidy deep water table, particularly on the Namaku ignimbrite plateau. Surface drainage in the lake catchments is mainly by small streams which have their origin as springs. In many cases these springs occur at the outcrop of geological contacts over an under lying less permeable bed. Other springs occur at the intersection of near-surface water tables with the dissected ground surface. Many of the lakes have no surface outlets. Springs emerging near to and below these lakes have been considered to represent sub-surface lake drainage.
The lake surfaces lie within a narrow altitude rang* (Lake Rotoiti
279 • t° Lake Hgapouri, 477 n above msl). In general the catchments 3.
are small relative to lake area (Table 1), and also cover a fairly
narrow altitude range.
Soope of this Study.
This paper presents an initial survey of the isotopic characteristics of non-hydrothermal waters of the Rotorua region. Measurements of
deuterium (D), oxygen-l8 ( 0) and tritium (T) have been performed on
samples collected between 1970 and 1973. Some major springs emerging close to land-locked lakes have been identified as containing mixtures of lake-derived water and precipitation-derived recharge in the catchments between lakes and the springs. The stable isotope composition of precipitation-recharged groundwaters occupies a fairly narrow range, but is nevertheless separable into two distinct isotopic families. The circumstances leading to these isotopic differences are not yet fully understood, and will require further investigation.
Tritium concentrations are interpreted by reference to the fallout history of thermonuclear HTO, as depicted in the annual means for
Kaitoke (41.1°S, 175°E) in Pig. 3.
This preliminary assessment suggests further problems for study, and provides reliable isotopic data which are essential background material for any future isotopic investigation of the hydrothermal waters.
Isotopic Measurements and Units.
Waters undergo three successive stages of batch electrolytic enrichment from 4500 ml starting volume to 1 ml final sample. The tritium enrichment is ca. x 2000. After conversion to hydrogen over a granulated zinc-sand mixture (Taylor, 1973a) at 450 C, the enriched samples are counted in cylindrical stainless-steel Ueiger counttrs
(volume350ml, filling 435 torr hydrogen 15 torr ethylamine, background about i cpm) 4.
using additional electronic quenching and standard shielding and ring anti-coincidence techniques. Pour counters can be counted simultaneously within the same ring. After full enrichment the net count rate is about
1.4 cpm per TU of original sample.
Tritium concentrations are expressed in the usual Tritium Units —18 (TU). 1 TU represents a T/H ratio of 10~ , corresponding to an activity of 7.2 dpm per litre of water.
0xygen-l8 measurements are performed by the conventional Epstein-
Maveda technique (Epstein and Mayeda, 1953) ir which the waters, together with a number of laboratory working standards as controls, are equilibrated with CO., gas at 30 C. Small fractions of the equilibrated C0„ gases are then analysed in a double focussing mass spectrometer of the Nier type. Pull details of the techniques are given in Taylor and Rulston
',1972) and Taylor (1973b).
Deuterium measurements are performed on hydrogen gas prepared by the same Zn-sand technique used for tritium measurements, the mass spectrometry being performed on another mass spectrometer using separate collectors for masses 2 and 3* For the small deuterium samples (ca. 25 mg water), a slight memory effect is inherent in the chemical preparation of the hydrogen gas.
Both deuterium and oxygen-18 concentrations are expressed as
6-valuee,representing the difference in parts per thousand between the
1H 1 ft D/H or 0/ 0 ratio of the sample and that of the internationally- accepted Standard Mean Ocean Water (SMOW) (Craig, 1961a). Thus if R 1 ft l ft represents the D/H (or 0/ 0)ratio of the sample and R,,,,.,, that of SHUN SMOW
6 « {-— 1) x 1000 SMOW 5-
•JATCHMEN. ;:TDH0L03Y OF LAKE HOTORUA.
Stream Drainage.
A r.'unber of streams dram the Mamaku ignimbrite plateau, which surrounds .-nuch of the lake, and flow across the belt of lake terrace sediments (Fig. 4). Major springs emerge close to the north-western shore. Drainage from the prominent rhyolite dome Mt. Ngongotaha (737 m) occurs from springs on the lower slopes which feed the streams at its base. Some streams are appreciably influenced by drainage from hydro- thermal areas, e.g. Puarenga Stream passes through Whakarewarewa thermal area, and Waiohewa stream receives some of its flow from Tikitere thermal area; both streams exhibit enriched stable isotcpic composition due to admixture of thermal waters.
Ieotopic Data for Lake Rotorua Catchment.
Isotopic data for the Lake Rotorua catchment are presented fully in Table 2. Springs on the sub-catchments adjacent to Lake Rotokawau
(Fig. 4) are excluded for the time being, because they might include some component of water from that lake, and therefore bhow stable isotope enrichment characteristic of evaporated waters. This part of i;he catchment will be discussed in a later section.
Mean Stable Isotope Composition for Lake Rotorua Catchment.
By selecting springs and the base flows of streams of appreciable tritium age (arbitrarily token as tritium concentration less than 50/fe of mean ambient level) and without influence of thermal waters, a mean
•I Q stable isotopic composition 6( 0) - -6.08, 6(D) - -34-2 was deduced for the groundwater drainage of Lake Rotorua catchment (Table 3). The spread of Isotopic composition within this set of samples is not significantly greater than experimental error, indicating that these groundwaters may be oonsidered to be a distinct lsotopio family, characteristic of mean 6.
groundwaier recharge in this part of the Roto ma region. They are certainly of the class of meteoric waters which have not been subjected to significant isotopic enrichment by evaporation, and should therefore lfi adhere to the world-wide correlation between 6( 0) and 6(D)
6(D) = 8 6(l80) + d
18 (Craig, 1961b; Dansgaard, 1964). In a linear 6( 0), 6(B) - diagram, the gradient 8 of this so-called meteoric water line reflects the relative changes of the two isotopes during progressive condensation processes in which atmospheric vapour and condensate are always in lBotopic equilibrium, and applies over a wide temperature range. A similar relationship exists for atmospheric vapours of widely differing origin and isotopic depletion (Taylor, 1972). The intercept d is about
+10 on the SNOW scale, and reflects a number of processes in the inter mediate stages between ocean and meteoric water, but the positive sign basically results from the extra isotopic separation involved in evaporation at the ocean surface.which is a non-equilibrium process
(Craig and Gordon, 19^5)•
Stable Isotope Composition of Meteoric Waters in Rotorua Region. 18 For the Rotorua region, an area of the 6( 0), 5(D)-diagram iray now be designated for meteoric waters, by extending a line of gradient 8 from the mean value (-6.08, -34.2) deduced above for the Lake Rotorua catchment springs and Btream flows (Pig. 5). In this case the intercept 18 is +14.4. The meteoric water field in the 6( ), 6(D) diagram is then defined more broadly by extending a pencil of gradient 8 from the extremes of the set of isotopic values used for the calculation of the mean value of the Rotorua catchment (Table 4). The breadth of this I ft pencil is Q.jJcQ in 6( 0) and 4^0 in 6(D), corresponding roughly to the range of experiaental error in both isotopes. 7.
Other Meteoric Waters measured in this Study.
Four other isotopic compositions from this investigation definitely represent meteoric waters. These are the rainwater from the storm of o May 1970, and the three values for Ohanui Stream (Table 7) which
•zhibit ambient tritium concentration, and therefore represent drainage of recent precipitation. All four stable isotope compositions lie within the proposed meteoric pencil (Fig. 5)i which will be used in the following sections for the assessment of the data of other waters in the Rotorua region.
Surface Runoff in Lake Rotorua Catchment; Data from Storm of 8 May 1970.
Ii. general, apart from the lake terraces surrounding Lake Rotorua, the porous and permeable nature of the volcanic terrain are not conducive to surface runoff. On 8 May 1970, a storm of sufficient magnitude occurred which raised the stream flows in the Lake Rotorua catchment to an abnormally high level. A comparison of isotopic compositions of stream base flow, flood flow and precipitation can in principle provide an assessment of the contribution of surface runoff to the increased flows (Mook et ml., 1974). In this method the isotopic composition
(stable isotopes and tritium) of the flood flow should be intermediate between those of the base flow and the contributing precipitation.
The limited isotopic data obtained for the Rotorua storm of 8 May
1970, serve to illustrate that the assessment of surface runoff by this method may produce completely incorrect results if the sampling of precipitation and stream flow is not performed in sufficient detail.
In principle two causes for increased stream flov from the catchment can be envisaged. Firstly, penetration of newly-precipitated water to the water table can cause increased groundwater discharge to the
streams at lower levels by a simple hydraulic effect. This effect is manifested isotopically by the maintenance of the base flow isotopic composition despite the increased flow. Secondly, surface runoff can occur, either after the surface cover becomes Saturated or else because of its low transmissivity. In this case isotopic changes in the stream flow are to be expected, provided sufficient isotopic contrast exists between the contributing precipitation and the stream base flow. In the investigation at Rotorua the streams were sampled close to their peak flow, the total flow being such that the base flow was a very ainor component^'data in Table 2). The tritium concentrations adjusted in most cases to that of the total precipitation, in two cases not lft quite so high. However, whereas the 6( 0) of the total rainwater was more negative (-6.^/!o) than the stream base flows (mean -6.1,£o), the flood flows in every case exhibited displacement in the positive direction.
Tf. .; apparent contradiction is undoubtedly a consequence of the variation of stable isotopic composition of the precipitation during the course of the storm (Dansgaard 1561, Ehhalt et al. 1963), the surface runoff originating from the later stages of the storm, after the early precipitation has been taken up by the initially under-saturated surface cover.
The example suggests that this isotopic method of measuring surface runoff requires closely spaced sampling of precipitation and streams throughout the storm period, supplemented by parallel precipitation quantity and stream hydrograph measurements. Such detailed sampling must be performed before isotopic balance equations for the system can be solved with any degree of certainty to determine the relative contribution of surface runoff and groundwater to the increased stream flow. I
Streams draining Thermal Regions.
fwo streams feeding Lake Rotorua-Puarenga Stream and Waiohewa Stream
derive significant proportions (about 8/£ and 10>J respectively) of their
waters from thermal regions. Their isotopic compositions are discussed
in Table 3. These enriched contributions to the total stream input
flow to the Lake are insufficient to offset the overall isotopic
composition significantly away from the value deduced from Table 3.
ISOTOPIC COMPOSITION OF THE ROTORUA DISTRICT LAKES.
Physical Features.
Table 1 summarises the known physical information of the lakes.
Other information can be found in Pit tarns (1966), Jolly (1968) and
Pish (1970). Most of the lakes are sufficiently deep to develop
stratification during the summer months. They belong to the class
termed "warm monomictic" by Hutchinson (1957), i.e., water never below
4 C at any level, freely circulating in winter at or above 4 C, directly
stratified in summer. Lake temperatures were recorded by Jolly (1968):
summer maxima are typically about 20 C and winter minima about 6-11 C.
Lakes Rotoehu and Rotorua are shallow and homothermous. Only two
lakes show complex structure: Lake Rotoiti has a main basin of type
warm monomictic, whereas the shallow western basin is homothermous and
may be considered to represent a portion of the Lake Rotorua outlet
channel (Ohau Channel - Lake Rotoiti western basin - Kaituna R.): the
main shallow portion ot Lake Rerewhakaaitu is homothermous, but the
Crater Bay western arm has depth ca. 30 m and is warm monomiciic.
Stable Isotopic Composition.
All New Zealand lakes so far measured exhibit no marked isotopic
gradients with depth. This means that measurable increments of isotopic 10
lb change (T, D or C) do not accrue in either hypolimnion or epilimnion during summer stratification, and the lakes turn over completely during
1 H winter. It appears that summer changes of 6( 0) and o(D) in the epilimnion are less than 0.7«o and 4 ;0 respectively; no gradients of tritium concentration with depth have been recorded.
All the Rotorua lake? are enriched in 0 and D relative to mean input water (full details in Table 4). For all water molecules in a lake there is a certain probability that evaporation rather than outflow will occur. Due to vapour pressure differences, the probability that l ft i ft
HD C and H, 0 will be evaporated from the water surface is lower than for H_ 0,wha»s for condensation the reverse is true. Thus in the net process of evaporation the lake may become enriched or depleted in D 18 and 0 depending on the isotopic composition of the atmospheric vapour lative to that of the lake. In almost all cases enrichment results. i ft In the linear 5( 0), 5(B) formulation the process of evaporation into a non-saturated atmosphere causes lakes to occupy enriched positions which may be connected to their mean input waters along lines of slope
4-6. This situation contrasts with condensation and evaporation in equilibrium conditions in which the changes in isotopic composition occur along lines of slope ti. The evaporation phenomenon has been extensively
reported in the literature (e.g. Craig et al., 1963; Craig and
Gordon, 1965; Ehhalt and Knott, 19&5; Clat, 1970 and 1971), and receives further confirmation in the o-values measured for th* Rotorua district
lakes. In ?ig. 0, :,-values for 13 lakes are displayed relative to the mean groundwater recharge composition deduced by Lake Rotorua catchment.
12of the lakes lie within a pencil of gradient 4-6 extending from the mean recharge. The maximum j( 0) enrichment is ypo (Lake Tikitapu).
Each lake isotopic composition is the end result of a complex interplay
of hydrologioal, meteorological and isotopic factors. No simple 11
empirical relationships appear to exist between the stable isotopic composition and the known parameters. At the present time it seems doubtful whether a Btudy of the isotopic balances of these lakes is feasible. One difficulty specific to Rotorua dirtrict lies in the fact that geographical catchment area estimates in regions containing such depths of permeable volcanic rock are unlikely to be accurate, so that total input of water to the lakes cannot be estimated. For the purposes of the present paper the usefulness of the stable isotopic compositions of the lakes lies in their possible contribution to groundwater and springs through sub-surface drainage.
Tritium Concentrations.
Prom the standpoint of tritium concentration the lakes may be considered to be of two basic types. Lake Rotorua and the western cism of Lake Rotoiti exhibit relatively fast throughput where inflow
rivers, precipitation and vapour exchange between atmosphere and lake surface combine to create the measured tritium concentration. In general the rivers contribute water of low tritium concentration, so
that the tritium concentration is intermediate between the rivers and
the mean ambient precipitation. The other lakes have longer residence times (input rate/volume) of many years, and are responding slowly to
the input of thermonuclear tritium. During the period 1970-73 all the
lakes exhibited tritium concentrations lower than mean ambient
precipitation (see Table 4 and Pig. 3). As would be reasonably expected,
the deepest lakes (Rotomahana, Tarawera, Okataina and Rotokawau) exhibit
the lowest tritium concentrations, with the shallower lakes ranging up
to Lake Herewhakaaitu which has highest tritium concentration and least mean depth. 12.
SPRING.: COM A; N.N J SUb-SI'^'Ai^ l,ftKE DRAINAGE.
Amongst the major springs so far measured m the Kotorua region, four exhib: ted slide isotopic conposition which indicated measurable components of lake drainage i,Table ^).
Lake i-otoruj. drains to Laxe aotoehu via Western Outlet Spring, ani to Tarawera ;aver via Wr.i kanapiti Spring. Lake tierewhakaaitu drains to Laxe notomahana via Te Kuye Spring, and Lake Okareka to Lake
Tarawera via Waitangi '.Spring.
Principles of the Method.
In such porous volcanic country, it is to be expected that groundwater drainage from the catchments between lakes and springs will also contribute to the spring flows. It will be assumed that the stable isotopic composition of the groundwater drainage (o( 0),, 5(D)_)
.lows the meteoric relationship
lfa 6(D),, = b :A oj + d [l)
-'or both D and 0, the isotopic balance equation relating spring, lake and groundwater is
jc = fo, + (L-f)6„ {2) wnere the 3ubscnpts Z and L refer to spring and lake water respectively, and f is the fractional contribution of lake water to the spring flow.
Equations \ 2) can be solved to give the isotopic composition of the
groundwater contribution in terms of measured values of 6„ and 6T• Thus > L
o( d)r. - ooi ^Uj - d <- . " . 16 . ^3) XD). - «-A Oj, - d 13.
l8 6 6( 0)s <6(D)L - d} -i|\ {6(I»s-d} o( 0)„ * ' V—775 7E \ U) J 10 ltt 6(D)L - 6(D)S - 8|6( 0)L - 6( 0)sJ with 6(D)„ being given by (1).
In practice, the measurement errors in the 6-values are somewhat too high to enable the determination of 6„ with adequate accuracy. u However, f may be estimated graphically from the position of the point
(of, 0) , 6(D)_) on the line joining the isotopic composition of the lake to the region of the 6-diagram occupied by the Lake Rotorua catchment groundwaters; this estimated f may then be applied to the tritium balance equation.
Ts « f T, + (l-f)Tc O^ where the symbols T represent tritium concentrations at the time of emergence as spring water, and the subscripts have the same meaning already applied to 6-values. The values of both T, and T_ are subject
to constraints. Tr has been steadily increasing in these lakes since the advent of thermonuclear HTO fallout in 1955» and must therefore lie between zero and the tritium concentration of the lake (measured values in Table 1) at the time of sampling of the spring. T„ must lie between u zero and the maximum post-thermonuclear values encountered in New Zealand groundwaters, which are about 30 TU. Vfhen these constraints are applied, equation (3) gives a permitted range for T, and T„, which can be graphically presented, and which may reveal dynamic features of the drainage to the spring. The procedure will be illustrated for the four springs already mentioned.
To take account of experimental errors, (5) may be rewritten T. - (l-f)T. 3 C T, - f (6) 14.
:"he standard error <*_, of T. may then be expressed as a finction of tKe standard errors a of T and o of f !„ 5 t
2 a2 l «2 ^TS ~ V Jl ,,. T = ~~2 °T + 4 f (7) XL f S f^
To establish op for fixed T f take a = 0.06 T (routine experimental "L S
standard error), and a conservatively-applied ship ?L + 2oT , against T„ is then portrayed as the permitted field Li (95;o confidence) of T, and T-. The method will now be illustrated for the four examples already cited. Te Kaue Spring; Lake Rerewhakaaitu. This spring issues from the base of a vertical-walled gully, about 120 m to the north-west of Crater Bay, Lake Rerewhakaaitu, and drains towards Lake Hotomahana. Plow is about 6 l.sec- . The divide is composed mainly of pumice, and appears to consist of thick (ca. 15 m) tephra deposits overlying ignimbrite. The isotopic data give f » 0.88 (Pig. 7A), T = 17.4 + 1.0 TU on 21 September, 1972, Lake Rerewhakaaitu tritium concentration « 18.2 TU (mean of 3 values) on the same date. The permitted field for T. and T_ L G (Fig. 7A) shows that the lake drainage does not suffer measurable radioactive decay along the short path to the spring. Waitangi Spring; Lake Okareka. rfuitangi Spring emerges from rhyolite in a gully about 700 m east of the south-east tip of Lake Okareka, (. ig. 1). In this case f * O.bO, (Fig. 7B). Spring and lake were sampled twice in December 1971, and September 1972. Because the 6-values were quite similar for the two sampling dates, mean o-values were taken to insert in Fig. 7B. In this case, no definite conclusions can be reached from the allowable field of T and T„, except that T. cannot be less than 5 TU, implying a maximum travel time of 8 years, deduced from intersection of the tritium decay line from a spring water of 5 TU in 1972 with an assumed linear rise of lake tritium concentration since 195S Waikanapiti Spring; Lake Rotoma. The small group of springs feeding Waikanapiti Stream (tributary of Tarawa River) emerges from rhyolite about 1.6 ka south-east of the south end of Lake Rotoma, f = 0.73 (Fig. 7C). As with Waitangi Spring, similar isotopic values obtained in December 1971 and September 1972 were u6ed to obtain mean values for the diagrams. The allowed T., T„ - field indicates appreciable age for both components of drainage. In this case the stable isotope balance was confirmed by chloride concentrations. Chloride analyses of samples taken on 22 September 1972 are given in Table 5» Most lake and spring samples showed low concentrations of 3-5 mg.l~ . Exceptions were Lake Rotoma (36 mg.l~ ) and Waikanapiti and Western Outlet springs (both 28 mg.l~ ), corresponding to the same mixture deduced from the stable isotopic values for both springs. Wastern Outlet Spring. Western Outlet Spring emerges over the area of a long eroding alluvial gully about 0.6 km from the north-west bay of Lake Rotoma, and drains as a stream into Lake Rotoehu. The flow of the spring responds appreciably to changes in lake level (Pig.8 )• The Spring was sampled at times of low (April 1970 and September 1972) and high (December 1971) flow. Considerable difference exists between the isotopic balances in these three cases (Pig. 9 ). In April 1970, f . 0.39; in December 1971, f - 0.68; in September 1972, f - 0.88. The data is consistent with the addition of catchment drainage of extremely lc low tritium concentration with lake drainage not appreciably aged. In December 1571, the isotopic balan-" implies that the increase in flow relative to April 1570 is entirely lake water. However, by September 1972 the flow had again dropped severely (Fig. 8), but the stable isotope composition showed no sign of reverting to the April 1970 values. Apparently the increased spring flow resulted from flow through channel of relatively high transmissivity, whose entrance is accessible only at higher lake levels. It cannot be the result of a purely hydraulic effect through the normal drainage channels, because the increase in lake height is only of order 10% of the overall height difference between lake and spring, whereas the flow response is much greater. During the period of increased flow, water drains from the high transmissivity channel into the groundwater reservoir between lake and spring. The '-.ter isotopic composition of the spring cannot therefore revert immediately to the pre-flood values after the high transmissivity flow path is shut off by lowering of the lake level. This information is of considerable practical interest because Lake Rotoma shows higher variation of level than many of the other large lakes in this region. At times the road and houses at the south-east corner of the Lake appear to be threatened by further rise of water level. The isotope and flow data for Western Outlet Spring prove that a natural safety drainage valve exists which is capable of carrying away at least 1 m sec" of low. This rate is equivalent to a precipitation rate of -1 2 2 m.yr over the total lake catchment of 16 km . Since an excess precipitation rate of this magnitude is unlikely to be maintained over long periods, it is apparent that the level of the lake can never rise above the values attained during late 1971 and early 1972. 17. SPRIHSS SHOWIKG NO EVIDENCE OF SUB-SURFACE LAKE DRAINAGE. Pongakawa Spring. Pongakawa Spring is one of a series of small perennial springs emerging from a gully about 4«5 ton north-west of Omarupoto Bay, Lake Rotoehu, about 23 m below lake level. The springs apparently represent the drainage of shallow groundwater from Rotoiti Breccia, and feed Pongakawa Stream, which flows into the Bay of Plenty via Waihi Estuary. The springs have been reputed to represent sub-surface drainage from Lake Rotoehu. However, three samples taken from the main spring at different times Bhowed no evidence of any isotopic component from the lake (details in Table 6, Fig. 10). These samples are apparently meteoric waters, but do not correspond in stable isotope composition to the springs representing drainage from the Rotorua catchment (Table 3), i ft being enriched along the meteoric pencil by about 0.65^0 in 5( 0) and 5.5jk> in 6(D) on average relative to the mean value from Table 3. The possible reasons for this enrichment will be discussed in connection with the samples representing drainage from catchments close to Lake Rotokawau which show a similar shift. One tritium concentration in the set of three from Pongakawa Spring appears anomalously high. It is doubtful whether the high value can be ascribed to recent storm drainage, because the perennial spring flow does not respond much to recent rain. It is probable that the high value results from an undetected contamination of this sample during processing, although no evidence for any experimental irregularity was apparent. The other two tritium concentrations show that this apparently quite shallow groundwater has a long drainage time, a fact already noticed for the catchment drainage in the mixture of lake and catchment water in Western Outlet Spring, and also in the springs close to Rotokawau. 10. Groundwater Drainage from Catchments close to Lake Rotokawau. Lake Rotokawau occupies a steep-sided crater with a small catchment mainly to the south and has no surface outlet. Beyond the crater rim to north and west of the lake, the catchments drain towards Lakes Rotoiti and Rotorua, and contain active hydrothermal areas, notably Tikitere 'Hell's Gate). Springs emerge both above and below the level of Lake Rotokawau. Some of these are apparently at geological contacts; in this area rhyolite, ignimbrite and Rotoiti Breccia are mapped below the surface ash and terrace cover. Others represent the discharge of shallow groundwater through the surface cover. Some small springs emerge as warm water, depositing iron at the point of emergence. A number of these cold and warm springs were sampled for isotopic analysis, to detect possible sub-surface drainage from Lake Rotokawau (Table b). The tritium concentrations of these waters all indicate predominantly older components of drainage, the lowest concentrations (0.37 to 3.4 TU) being found in the three warm springs, and the highest '12 TU) in a stream draining catchment lying mainly above the lake and emerging in a swampy region. The stable isotope compositions of all the waters sampled were completely uniform, being enriched an average 0.8#o in 6( 0) and 4;^o in 6(D) (Pig. 11), relative to the mean value deduced for the remainder of the Lake Rotorua catchment. The points lie at the edge of the meteoric region defined earlier, but it seems probable that there is no component of drainage from Lake Rotokawau since all the points lie above the tie-line joining the isotopic composition of the lake to the mean Lake Rotorua value. Moreover the isotopic composition of the samples draining catchment above lake level is no different from the springs emerging below lake level. However, in view of the uncertainties in the positions of the tie-line extremities and the exact meteoric line, a very small component of lake drainage cannot be strictly excluded from some of these springs. As wita Pongakawa Spring, the evidence for this set of samples again points to the existence of a family of meteoric waters with stable isotope composition differing from that found for other parts of the Lake Rotorua catchment (Table 3). In principle such families may derive from two causes: 1. Altitude effect, with more negative 6-values in precipitation falling at higher, and therefore colder altitudes (see e.g. Gat, 1971); 2. Seasonal selection of groundwater recharge. There is absolutely no reason to invoice the altitude effect in this case; the precipitating clouds are passing here over a plateau where minor variations in altitude within the general trend of the topography cannot be expected to produce marked differences in the mean stable isotope composition of the precipitation. In support of this, it should be noted that the Pongakawa Spring catchment lies ca. 200 • an average below the Lake Rotorua catchments, whereas the catchments near Lake Rotokawau form parts of the Lake Rotorua catchment, and are probably not very significantly different in altitude from these supplying the water to the springs m Table 3. The isotopic compositions do not fit to this altitude pattern. Seasonal recharge of groundwaters is a well-known phenomenon, which has been confirmed in some isotopic investigations in Europe, where the stable isotope composition in the saturated zone ma.ches the mean composition of winter precipitation which is considerably depleted relative to summer precipitation (see Fig. 12). The summer precipitation is removed from the unsaturated zone by evapo-transpiration. The effect is a complex interplay of various factors. Firstly, there must be an appreciable difference between the isotopic composition of summer and winter precipitation. Although confirmatory measurements do not exist 20. for the Hotorua region, it seems likely that about 3 Jo for 6( u) and about 24/oo for 5(D) represents the differences between the summer and winter nodes of the seasonal variation. This leaves enough room for 18 differences of order l;\>o(6 0) in groundwaters if seasonal selection plays a role. Secondly, the seasonal distribution or rainfall must supply sufficient water during both winter and summer to ensure that any potential of the surface lithology for seasonal selection can be exploited. This condition is satisfied in the Rotorua region. A fairly even seasonal distribution is exhibited,with a slight summer minimum, although the variation relative to this mean pattern can be quite extreme from year to year. Mean annual precipitation at Rotorua itself is about 2 m. Third criterion for seasonal selection is the conditions in the s-.l above the groundwater table. In the Rotorua region surface ash cover is of varying thickness (up to 30 m), but must be considered to be generally permeable. Varying quantities of water are therefore required to saturate the surface cover. Below this surface cover lie the ignimbntes, rhyolites or Rotoiti Breccia. It is a notable feature of the hydrology that the water table in the ignimbrites and rhyolites lies generally at considerable depth, whereas the catchments where Rotoiti Breccia underlies the ash appear to have a much shallower groundwater circulation. This distinction also applies to the isotopic families under present discussion. The family of Table 3 represents discharge from rhyolite and ignimbrite, whereas the Pongakawa Spring/ Lake Rotokawau waters are all discharged from regions underlying sections of the geological map assigned to Rotoiti Breccia. If the Rotoiti Breccia acts to keep the water-table level within the upper ash layer, then this water-table may receive precipitation at all seasons and provide a reservoir for evapotranspiration. Phis water may also be transmitted vertically and horizontally to rhyolites and lgmmbrites in contact with the Rotoiti Breccia. Where lgnimbntes and rhyolites underlie the surface ash, the water-table generally lies at some depth, so recharge will depend on the degree of saturation of the surface ash cover, which will be at maximum during winter, thus favouring recharge by winter precipitation. Obiously this seasonal recharge hypothesis requires further detailed investigation covering all geological, hydrological and isotopic aspects before it can be accepted. Prom the isotopic standpoint, however, such a mechanism seems the only reasonable explanation for the present observations. i REFERENCES CRAIG, H. "Standard for reporting concentrations of deuterium and oxygen-lb in natural waters", Science 133, (1961a), 1833-1334. ORAIG, H. "Isotopic variations in meteoric waters", Science 133, (1961b), 1702-1703. CRAIG, H., GORDOI, I., HORIBE, Y., "Isotopic exchange effects in the evaporation of water, 1. Low temperature experimental results", J. Geophys. Research 68, (1963), 5079-5087. CRAIG, H., GORDON, I..J., "Deuterium and oxygen-18 variations in the ocean and the marine atmosphere", Proc. Conf. Stable Isotopes in Oceanographic Studies and Paleotemperatures, Spoleto, (1965), 122 pp. DANSGAARD, W., "The isotopic composition of natural waters", Meddelelser om Gronland 165, (1961), 120 pp. DANSGAARD, W., "Stable isotopes in precipitation", Tellus 16, (1964), 436-468. EHHALT, D.f KNOTT, K., NAGEL, J.P., VOGEL, J.C., "Deuterium and oxygen-18 in rainwater", J. Geophys. Research 68, (1963), 3775-3780. EHHALT, D., KHOTT, K., "Kinetische Isotopentrennung bei der Verdampfung von Wasser", Tellus 17, (1965), 118-130. EPSTEIN, S., MAYEDA T., "Variations of the 0 content of wa.ers from natural sources", Geochim. et Cosmochim. Acta 4, (1353)t 213-224. PISH, G.R., "A limnological study of four lakes near Rotorua", New Zealand J. Marine and Pishwater Research 4, (1970), 165-194. GAT, .T.R., "Environmental isotope balances of Lake Tiberias", Isotope Hydrology 1970, I.A.E.A. Vienna (1970), 109-129. GAT, J.R., "Comments on the stable isotope method of regional groundwater investigations", Water Resources Research 2i 0971), 960-993. HEALY, J., "Geology of the Rotorua District", Proc. New Zealand Ecological Society 10, (1963), 53-58. HEALY, J., "Volcanic mechanisms in the Taupo Volcanic Zone, New Zealand", New Zealand J. Geology and Geophysics 7, (1964)» 6-23. HEALY, J., SCHOPIELD, J.C., THOMPSON, B.N., "Sheet 5 - Rotorua (1st Ed ) Geological Map of New Zealand 1:250.000", Dept. Scientific and Industrial Research, Wellington, (1964). JOLLT, V.H.,"The comparative limnology of some New Zealand lakes, I Physical and chemical", New Zealand J. Marine and Pishwater Research 2, (1966), 214-259- REFERENCES (cont.) HOOK, W.G., GROENVELD, D.J., BROWN, A.E., VAH GANSWIJK, A.J., "Analysis of a run-off hydrograph by means of natural ^0", Symposium on Isotope Techniques in Groundwater Hydrology, I.A.E.A. Vienna, (1974), in print. PITTANS, R.J., "Preliair\ry water balance studies of the Rotorua lakes", New Zealand J. Hydrology ±, (1968), 24-37. SIEGQiTHALER, U.C., "Sauerstoff^B, Deuterium und Tritium im Wasserkreislauf Beitrage zurNesstechnik, Modellrechnung und Anwendungen", Dissertation, Univ. of Berne,(1971), 109 pp. TAYLOR, C.B., "A comparison of tritium and Sr-90 fallout in the Southern Hemisphere", Tellus 20, (I968), 559-576. TAYLOR, C.B., "The vertical variations of the isotopic concentrations of tropospheric water vapour over Continental Europe, and their relationship to tropospheric structure". New Zealand Institute of Nuclear Sciences, Report R-107, (1972), 45 pp. TAYLOR, C.B., KJLSTON, J.R., "Measurement of oxygen-18 ratios in environ mental waters using the Epstein-Mayeda technique. Part II: mathematical aspects of the mass spectrometry measurements, the equilibration process, correction terms and computer calculation of 6-values w.r.t. Standard Mean Ocean Water (SNOW)", New Zealand Institute of Nuclear Sciences, Report LN-36 (1972), 29 pp. TAYLOR, C.B., "Precision enrichment of tritium in waters using electrolysis" New Zealand Institute of Nuclear Sciences, Report R-135t (1973a), 19 pp. TAYLOR, C.B., "Measurement of oxygen-18 ratios in environmental waters using the Epstein-Mayeda technique. Part I: theory and details of the equilibration technique", New Zealand Institute of Nuclear Sciences, Report LN-37, (1973b), 24 pp. A= UNDIFFERENTIATED DEPOSITS. —*~" RIVER OUTLETS LAKES 3= ROTOITI BRECCIA R = RHYOLITE N D= DAC17E T = BRECCIAS, TUFFS. ALLUVIUM. 1. ROTORUA I = IG.MMBRITE 2.ROTOITI L= LAKE TERRACES 3. ROTOEHU ' A.ROTOMA 5. OKATAINA 6. OKAREKA 7. TARAWERA 8. TIKITAPU 9. ROTOKAKAHI 10.ROTOMAHANA 11.REREWHAKAAITU 12.0KARO 13ROTOKAWAU SPRINGS x 14. WESTERN OUTLET 15.WAIKANAPITI 16.P0NGAKAWA 17.TE KAUAE 18.WAITANGI 19. NGONGOTAHA Pig. 1 Nap of Rotorua region showing major lakes, springs, river outlets, and di "ribution of rock and sediment types. Almost the entire region is mantled by volcanic ash, which does not appear in this diagram. Lake boundaries are thicker lines; dashed lines represent approximate boundaries between rock types. Other springs mentioned in this paper appear on Fig. 4. u • \ 1 • t 1 ° BAY OF PLENTY f *^ • • WHITE ISLAND / ROTORUA^?* \ v* WHALE ISLAND l.TAUPo/VWA,RAKE' -w ^ • MT.RUAPEHU / / />-S N 100km. *-S~m~*~" i 1 r LOCATION MAP TAUPO VOLCANIC ZONE 10 km AREA MAINLY OVERLAIN BY RECENT VOLCANIC ASH TYPICAL GEOLOGICAI SECTION ACROS3 VOLCANIC ZONE. Fig. 2 Location map and typical geologic-l section of Taupo Volcanic Zone. £!9io, 30 C4K 20 ROTORUA LAKES 10 • i J i ROTORUA SPRINGS & STRFAM BASE FLOWS -^Stcr/r/j ^ 0-5 - L i l I i i i L I I I -LJ_J L 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 ' VEAR — MEAN ANNUAL (JULY-JUNE) TRITIUM RATIOS AT KAlTOKE, WELLINGTON* RAINWATER ) Pig. 3 Tritium fallout recorded at Kaitoke, near Wellington, shown as annua1, me-.n trituni concentration:-, on a logarithmic scale. In the South Pacific region a "epular seasonal pattern running from July to June is always recorded vith a maximum during August - OCober and minimum ("arch - Kay (Taylor, I968). The differences between mean concentrations at Rotorua and Kai.oke are not more than 10/o. As an aid to interpretation, the dfcay lines Cor tritium are shown for pre-lhermon'jclcar water ca. V/jA ,and for water from the year of rcaximuTi concent j-.iti on. Kangeo oT tritium concentration exhibited by Rotorua region ).%Kes and enritvrr. during )r/IQ-1? arc also ahown. Fig. 5 Linear 6( 0), 6(D) - diagram delineating meteoric waters for the Hotorua region. • Pig. 6 6( 0), .^(D) - diagram showing measured isotopic compositions of 13 lakee in the Ftotorua region. Numbers are assigned as in Pig. 1. All these lakes exhibit marked displacement from the meteoric region as the result of evaporation. --20 T = 19.8-0.14TG I ^-40 —r— —I— 10 20 30 TU D/180 (A) LAKE REREWHAKAAITU & TE KAUE SPRING. 21.9.72 -20 1--40 30 TU D/180 (8) LAKE OKAREKA &WAITANGI SPRING, 1971-"^ -20 L-40 30TU D/1flO fC) LAKE ROTOMA * WAIKANAPITI SPRING , 1971 - 72 ?\%. 7 Stable isotopic relationships and permitted (T , T ) - fields for A. . e Kaue spring, 8. Wai tan^i Spring, C, Waitcanapiti Spring. 4m LAKE ROTOMA LEVEL V o°o • «^ o"°° 3H H.5 o' ,* ' WESTERN OUTLET / \ I SPRING FLOW / m3.sec"1 * / 1-1.0 2H W ^o1 x vV X ^x, * *x *x h0.5 X---X i i i i i I i r i i i I i i i i i I i i i i i i i i i i j i • i ,i » 1970 1971 1972 Pig. 8 Lake Hotorr,a level and Western Outlet Spring measurements, 1970-72. 20-| TU T, =12.4-0.7017 --20 GROUND WATER L-40 30TU D/,0180, 24.4.70 20 TU ^/•SPRINd .20 1ROUND fL=11.4-0.14TG i WATER I--40 -i r- —I 30TU ,B18, D/ O 10 20 6.12.71 20 n TU OV /SPRING -20 GROUND T, =13.6-0.1477 WATER 1--40 i 10 20 30 TU D/180 21.9.72 r\?. ') Stable isotypic relationships and permitted (T(jf T[J - fields for Western Outlet Spring on 3 separate sampling dates. Fig.10 stable iaotopio composition of pong&icawa Spring relative to meteoric region and Lake Rotoehu shows no measurable indication of sub-surface drainage frosi lake to spring. -5 -4 -2 -10 SPRINGS NEAR LAKE ROTOKAWAU LAKE ROTOKAWAU --20 8(D) --30 .0 MEAN L, ROTORUA CATCHMENT ORAINAQE • MEASURED VALUES 6 SPRINGS NEAR I, ROTOKAWAU LI Stable isotopic composition for npringa in catchmenta near Lake Rotokawau. Contribution of aub-9urfR.ee drainage of Lake Rotokawau to theae epringa setae unlikely on the baaia of theae measurements. 18- Seasonal Variation of 0 in Precipitation &'SMO. W -4 . -5 -6 - -7 - Valentla -8 • .-9 • -10- -11" -12- Stuttgart -13- -14- -15- Vienna T T T" -r -T- T" T M A M A s N \ Fig.12 Seasonal variation of 6(rl8 „0 ) in precipitation at 3 European stations, from Siegenthaler (1971)• The amplitude of the seasonal variation increases with increasing continentality, paralleling the amplitude of seasonal temperature variations. The mean 6-value becomes more negative with increasing continentality, paralleling the decrease in mean annual temperature. In the Southern Hemisphere the seasonal variation will be 6 months out of phase with that shown in this diagra.ii. • It.fji* J.:: COMjrnl (ortfin, it»tt..i.:*t (•) <.i«!v.-- j flu-tristion •:r*t >f;c»:i *oioru» 21*0 SI. I V~ 6 Detain 279 MO 33-4 89.6 93.5/15-0 11*6 0-7/2* fttratd by lava barrier »t east am end.(C-**..ei*,j»*nt detail* do aot include L.Kctcrua, waien -trains into L.Rotoiti via Orou channel}. Mam part of Sake as vara oeaooictic. Western baetn is anal low and aowotharaons. Ohau channel flows into and Kaituca H-(c.i-_l«t of L.3ofnt;. f .aw 22 »Jsoc~l) flows out of th* mittni basin, which avay or considered to be a cnar-nei fcr th* outlet • of both L.Satjrua an! ^.lotoiti. If Ea.it*** E. flow 10 conaidered to derive 17. J ejieec-* froo Otav ch*nr.*l and tha rrwaimr^ 4.) •* froo oaia boa in of l.Hataitx, then ttaa anowal drainafw froo ;ho o*ia beam ia a^jroa. 1# of it. voluee, Koto.kii 295 450 7.8 41.4 12/3.3 6*.5 1.6/l»* PaMMttf by lava barriers. Horrotaersoue. So surface drainage. Sua-oarfoce drain**;* r»ported oy visual observation at north and of later, b>,t oo known •pn.na; aatryinca (a** laotoptc data far Pttn^akawa Sario**). Koton Mi 420 11.0 >6.0 Bo/41.7 459 2.3/5.1 Poroation due to lava barriers and evplssian cnter^ Warn oononictic. So surface dmnt^t. Sob-oar face drainage to L.totoehu via Western Oat 1 at Spring and te Tarawer* S. via ttaifcAnap;ti Serine; (ate isotopic a-slyac*)- Eatiaaied meanf-jw' oft be a* tr outlete »a 1.0 a>*see~ . corresponding to 7% of tb* lake volua* per year. T.runn 298 J20 40.7 110.1 87.J/56.5 2300 0.4/1* larahnro caldera. Cauhsant area ie nair.ly in rayolit* lava flow* and dc*«s, ainor '.rr.i-^rut anwj ayroclasties. tolas* not 'cr.fxrsed. (n*l^hmem •r«a doc a not laclad* araa of Lake a Ckareka and ftotafcakaai, both af vbich dram to U.?arai>»ra. Van* awnoaictic. Drain*5* :./ Taravera ::. aef.n flow ia 7.5 a^aec"*; yearly i.-a;nac«. i» ULCraforc ahoat loji of total vclue*. Ckank* 353 553 3.4 16.7 31/16.4 62.5 0-9/5* Fonaation by lava barrier. w*r» aonov.c'.ic. Dralaaf* to L, Tarawera via Waitanfi Spring (COM/1 need by laotopic analyaas) and by ojt'.at •loo to ooiat or caarr^nce of Waitan^i Spring, fo flow «*!«*« available. lotokakakt )96 5C0 4-4 15.1 32/17.5 77-1 0.3/2* fonaatioo by lava barriers, vara •oooaicttc. (Or.«n l»k.) Dtaacaca by Hairoa Streatn ',0 U.Taravera ea. 0-)o>^»cc~*; yoarly drai«a*« oaout 20^ of \*%9l vol una. Tikitspu 41S 560 1.45 4.5 26/19-1 27.7 1-1/6* Poraatiori by lava barnara Vara oonoa.ctic. (Blu. l«k«) la fcaovn drainaga, Okattiu 111 500 10.9 55-4 78.J/43.8 477 2-9/7* Foroation by lava barrier a Vam ffiosoaictic. lo fcaowD drainage. lotakano .1)4 50P 0.5) 22. > 70/43.7 22.3 1.6/7* Crotor lake, wane *4rumctic, Vo known outlet (a«* aectior. in Um:oinm Strean catcMient>r 4*0 8 5 74.) 125/? 7 2.4/? Laao aavir. largely forted by fihrtMie *«->laiicr.a Rotcaakana 34) d»/ia« 1226 Tarawtra erupt.;r. *I«»n< r.fv- K*r> •oiMouctic. lo Mrfac* ou*.let- Catchment area do«o not include L. Ckaro. 460 0.33 3.3 15/U.5 3.8 -/- Loose in phreatte eiploiion crater forvel d*irmc Oi.ro 412 aveaation of Ht- Tarovora ?\, c>30 year* 3.P, Warn •vsataatettc. Sorfaco outlet to ...fiotomahana, no flow tnfonwtlon available. 490 7.5 49.7 31/6.7 50.4 1.4/21* rtlla U.OC0 yea»-old exploe.on crater %r.i lapa tar*»B*kM>ta 436 oato ..rroundin< countfyaide jaaated by .fl'.aro froa ftt.Taraworm durirnf eruption ea. SJC>t*r* n.T. Oat 1 at blocked in 19V* 4ue to atUtnic ur ard ovorsrovtn during ;enoia of low lake level. Ovtlet cleaned durinf early l&oC'e ard rrj* Howe lAtaroiicently vhen level of ,«• .e M^rr »"..*.ir-n# RlA^oeel recorded flow 0.^>t Wc"'. KoTotF.".moua in Mm onallov part of la«.« but vara oono^icivc in orator, r »,-»pouri 477 VJO 0.20 4-8 iV'12.9 -/- Ceeopie* phreatlc trploeion rrnter , o:i*i d'lrir.g eroot,on of Mt. Tarawera fn,;(j<) yf*r.T P..'. w.irn •efMOirt)r. 3rainv tc Wai/e'.-j 3. via H/nn-*.,.) p, Ro flow detatl*. \ -:-:. v;f :.A.-:F : lrt loc-.*.i>>-i an 1 gr;4 Collecti'? r*^. M o)* 6{D}»« refo-v^e •"••VJ'-r O ac. I Hfw t -%.*» f I awe: Cr> Rt.5.?,>r.*-\>t*r.». ll/i/.'O 11.6 -^-TJO.lJ -Jl.^l-0 l.rj^O.l 3r«tft.-vff<- fro* !*te PLei*t*rr:\e rhyalite above *r\>ut ha-.uhory doae c Awahou Strew Bridge* on Egongotaa* U/4/70 12.3 -6.03*0.13 -33.5*1.0 3.4*0.3 Baa* flow fed largely frost sen** of Kaeturan* road cold •prints draining KJ«*JH* igniabnte (*7tt:od5U9) •ad lit* terraces. Baa* flow about 1 .3o>3 o-^-l Awaho>i Spring County w*t*r supply 11/4/TO 11.6 -6.30*0.13 -31.4*1.0 J.0*0.2 Rain spring source for has* flow of intake (Xlo-.6dnij) AMOJUTI St re**) n/s/i* U-5 -5-9>0-07 -33.0*1.0 4.1*0.3 Easturafu. Stress. At road bridge 11/4/70 11.6 -6.12*0.13 -34.6*1.0 3.3*0.2 Sires* io fed predominantly by cold (•76:72316?) spring* draining Rasaku i£ni:?>>riie which retained vatera for 15-20 year* Imuran* Spring Rain spring 6/12/7- 11-5 -6.13fO.lJ -34- 3*1-0 2.4*0.2 Spring emer,jea with flow 1.3 «3B#C-1 («7o: 721171) from Raaak-j 1,-nimbrtte 21/9/72 -6.57*0.03 -)4.7*.i-0 2.5*0.2 •aingaebe At State Ei.cbwa/ 11/4/70 14-5 -6.08*0.13 -36.1*1.0 2.3*0.2 Stream base flow draining eas:ern part Streaa •rid** (»7o:776059) of Raxaku igniabrit* sad lake terraces. Spring at 7/3/73 -5.B7>0.0<> 35.2*--0 )2.«r*0.8 Contain* significantly aorr new water Vaingaehe than ha** flow of stream. Spring «n*ta •am lake terrace* and »*y dram relatively •hallow recent water. Otuhin* StrvJB At State Highway 11 A/70 13.2 -5-66*0.07 -33.4*1-0 6*9*0.4 Stream fea^s flow from Raaaku igniabrite •ridge (576:710052) and lake terraces with max* contribution* from cold spring* Otunina Spring City water supply 2l/972 12.4 -6.42*0.13 -35--?*.l-0 0.46^*0.04 Lowest tr.tim cjftcentrattcn z' far (176:606014) recorded ir. tht* r*£ion. Proo.iMy drain* adjacent rnyolite done ana pri-nap* alao contribution froa igniaorite \o west. Plow absut Q.5 •i3flee~l •) Stream* in flood. 8/^/TC: Rainwater Hinistry of Agri 8/5/70 -4.86*0.13 -J9.0*1.0 8.3»0.6 stoim producing flood conditions In culture % Piahcne* •troMa Research l»b.t Rgapun* (76:749C23) Vgongotana Sase a* ») above 8/5/TO -5.}l»p.l7 7.6 S* reaet tritium conrtntration of at9r3(but 6l^0 Vaiteti Strews 8/5/70 -}.17»0.2o 8.2»0.5 novoa in opposite diroction, probably due to ourfacr runoff coning only from later Awahou Slrea* 8/5/70 -5.5?iO.20 t.7»c.4 part* of •tonnlwitB lcoo ne^itvr 6(^C) Basra ran* Stress 8/5/70 than trholr •lorrn valuo Otuhina Strea* 0/5/7C 5.7q»_o.i2 Vam^aeh* 8/5/70 -5.56»0.11 t.J^O.4 Stream , Puarenga "treaa Sane as c) beiow 8/V70 -5.3O*0.?0 8.6i0. v Wasoehw* Streaj* 8/5/70 -4.15.0.20 9.0«0.6 c) Strg.-w 1 containing ar.jr»-t.atlf cci?on*nts tf Ar.Mrage fr^T. fcf3ro*.h»*m%l »T»>jii 19 Puarvnira Strvan At Format Rr^e.irch U/4/70 JO.5 -5-5410.11 -12.5*1.0 4.^0.) Slicnt 0 enrlcturcnt but Hcut«riin not InatilTit*! inrth tnricbed, cirgavt ttyiroittnr.i! r*-mpor.«?nl •rl4«o (K76:7J1017) i# not auc;ect«d to ouch a'/a^ration orfora and after ai? la aMCteJ i^''..lt •O.l-'o in 'n'".J)t h-.»t not at all in Mt) re'.ativ- • •, i>ir,t»r f ro-in'*.«*-i*.« r« cl«i*»* to Tiki'.rr.-.i'r moaro Titlt t nr.; Vt*. . i ) Kari.Al t rf r't Culvr: ly -.-.K-.t l-.*o- ir/'j'r 0.1'v''.l'l r;.Wl.y V'^'.^-'' Ir.te^rilr.l t:.i-,n-tr-* fft "i^i'T" lh#t»-;\l ..v (••""• .'si;-•) • rt:i. fnn-f.'l :n f-U. d»*ii • »• r. , n nr,1 l''.", vhlrU i,', >ui tr^i-.e i,'»»r-.,al wi'Tn t.nvr etperier.' • ! r .-i;oral ion. Fable 3 STABLE ISOTOPE DATA USED TO ESTABLISH MEAN I SOTOPIC COMPOSITION t\)R LAKE ROTORUA CATCHMENT. Water (6(0), 6(D)) Mean Ngongotaha Spring (-5.73+0.13, -33.511.0) (-5.82, -34.1) (-5.90+0.12, -34.7+1.0) Ngongotaba Stream (-5.97+0.13, -32.5+1.0) (-5.97, -32.5) Haaurana Stream/Spring (-6.12+0.13, -34.8+1.0) (-6.38, -34.6) (-6.13+0.13, -34.3+1.0) (-6.57+0.08, -34.7+1.0) Waingaeae Stream (-6.08+0.13, -36.3+1.0) (-6.08, -36.3) Utuhina Spring (-6.42+0.13, -35.2+1.0) (-6.42, -35.2) Utuhina Stream (-5.86+0.07, -33.4+1.0) (-5.86, -33.4) Awahou Stream/Spring (-6.20+0.09, -33.1+0.7) (-6.04, -33.1) (-5.94+0.07, -33.0+0.7) Mean composition (-6.08, -34.1) I i "X r"." A\A„«-'.rr. Or' . Akf San^lin^ date Depth^m) i^v*r*turr, C (grid reference) ntrattf lcu'.ian 11/4/70 17.3 -3.54.0.13 -19..5_»0.7 l3.l_.0-8 Lake hoaoihermouo. Kean motopic C0T.pQ3ii*.un on n/4/70 15 ^, — i. T3P -21.1. 11/4/70 40 • -3.13.0.13 -21.6.0.7 11.9.0.8 l?-?™). Lake 1. obviously -.ll-m.x.d (»7b:7330oO) (bottom) - — - xn lsotopic Dens*. Tritium conc«ntra*-tfn reflects the mixture of recent precipl- 11/4/70 17.4 -3.79*0.21 13.3*0.8 tution with the low concentrations of (H76:732137) ~ ths ferdstreams. 11/4/70 14 n 17.1 -3.56*0.21 -21.8.1.0 12.4.0.7 (»76:73.-H37) (bottom) 20/12/72 17.0 -4.06.0.10 -23.5^1.0 9.0^1.6 Sample taxen in shallow western basin Stratified "~ "~ which is hoMotharmous. ^oth intet from L.Rotorua (Ohau channel) and outlet from L.Rotoiti (Kaituna R.) are situ ated in this basin. Main part of laks is nonoauctic. 3/12/71 10 1 Koaothermoue -3.36_jO.12 -18.1.1.0 14.3_.0-9 (K77--5591S1) 24/4/70 Surface Stratified -2.2_i_i_0.21 -11.5*1.0 10.6___0.7 (1(77:140043) 24/4/70 58 a Stratified -1.71.0.21 -").4.1.0 n.6_.0.7 (K77:140043) 3/12/71 CO a Stratified -2.04.0.12 -13.2.1.0 13.CM).8 (.77:140043) Surface 10.9 2.14J_0.12 - 12.6.1.0 10.6*0.7 21/9/72 lon-etratified 21/9/72 30 a 10.1 -2.27___0.07 -14.3.1.0 10.9*0.7 Hon-stratified 21/12/72 Surface 17-5 -3.38___0.09 -22.2.1.0 7.6___p.5 First sampling point is close to inflow (.76:839955) Stratified point of Vairoa Strcaa which is outflow of L.Rotokakehi. Second sampling point 29/8/73 Surface 11.3 -3.0V0.08 is at L.Tarawsra outlet (177:959934) lon-etratified 6/12/71 30 a 11.7 -2.33+0.12 -18.2_.1.0 15.8.1.0 (176:825005) Stratified Surface 10.7 -2.86_jp.07 -17.3.1.0 13.8___0.3 21/9/72 Ion-it ratified 25 a 9.8 -2.32_t0.20 -18.8.1.0 13.4___0.8 21/9/72 •on-stratifisd Rotokakahi 26/9//2 Outlet Ion-stratified -3.67j_0.08 -23.2_>1.0 10.7.0.6 (176:811954) 21/12/72 Outlet Stratified -3.25+0.08 -20.1.1.0 (1176:805954) 30/8/73 Outlet Ion-stratified -3.45*0.17 (176:811954) Tikitapu 21/12/72 Surface Stratified -0.98*0.06 -11.1.1.0 1_>.6.1.0 Ssmplin« point at edge of laks. (176:795979) 30/8/73 Surfaca Hon-.(ratified -1. Syo. 17 (176:795979) 20/12/72 Surface Stratified -3.68.0.11 -21.9.I.O 7-9*0.5 Sampling point near edge of laks. (K76:893084) Rotokawao 26/9/72 Surface 14-3 -2.10___0.12.10___0.i1l -17.6.1.0< 8.8.0.5 first sample taken at sdge of lake. (176:837123) lot known if Samples of 1/10/73 froa centre of lake. stratified 1/10/73 Surfaca 13.8 -2.80*0.13 Sttattfled 1/10/73 71.> 9.7 -2.54.0.07 (bottom) Stratified potoff.ahar.a 21/12/72 3urr oe •* -2.43*0.07 -21.3*1.0 7-3___0.5 Sampling point at edgs of lake. (177:932905) Stratified OkAro 21/12/72 Surface 7 -3.98_lO.09 -26.00.0 Sampling point at edge of lake. (I85!85ie45) 7 fter«whak»aitu 2l/9/'2 Surface 10.6 -2.20j_0.07 -i5.9j.J-O 13.3.1.1 Firel two samples above crater. 7lnr Stratified sample in sain portion of lake. 21/9/72 30 a 8.3 -2.53.0.14 -li.OO.O 19.5.1.1 21/7/7? 3urfece 10.8 -2.57iO.10 -17.3'LO l~. .1.1 >*:VSirirTIS C? :s WITH rv:s:v •^-t;'"i:^cs LKAJ: rs l Sprm* Ontv Ipprox. H*oh 6(D)* « (nearby laka) •arepled flow Cr *d reference (o'sec-1) (alt*tuds(»)} (*.e»p.) Te Kane 21 A>/?2 -2.73*0.11 -19.4^1.0 17.4>1.0 Source of tributary stream to LuXe RotoaahanA. situated ( L. Re rewhakaattu) in vertical w;»tlad gully, divid* between eprir.3 and »86:944361 L.Rerewhaxaaitu (about 120 c) consisting of thick tcpbra deposit* ov#rlyin£ ignimbnte. Flow ca.Q.OOo m^see-l. Waitanfti 6/i2/7i ? -2.9951.12 -JO.2*0.1 u-5i0.a Flows through young rhyolite. Isotopic data shows that (L.Okareita) 21/9/72 ? -i.44^0.07 ^22.0*1.0 10.4*0.8 lake water 1a the major contribution to thia spring. K76:S47W2 (290 •) Vaikanapiti 6/12/71 -3.2310.13 -17.7*0.7 6.6*0.4 One of a group of small closely-epraad springs. Total (L.Rototcs) flow about 0.4 m3sec"I, Isotopic data consistent with a 25/9/72 5.9*0.3 ; 1177:071113 -3.16*0.12 -18.3*1.0 7!>:25 Mixture of water derived froa L.Rotona wita so- topically-unaltered catch/sent recharge, see text. Vestem Outlet 24/4/70 0.4 -3.83*0.13 -20.1*1.0 7-3KI.6 At the !«W end of L.P.otoaa, cub-surface drainage occurs K77iOl6l63 through a bank pyroclastic alluvium, consisting of 6/12/71 1.3 -2.63+0.12 -13.9*1.0 10.1*0.6 shower-beddid lapilli and coarse ash, passing upward into 21/9/72 0.5 -2.69*0.16 -15.9*1.0 12.0*0.7 coarse angular breccia containing expanded obsidian blodox, being an explosion breccia associated with eruption of Rotoaa Ash from a nearby crater in L. Rotona, ca. 7*000 - 8pOO years B.P. The receding d-icbarge race 1a aituated only about 0.5 km from, the nearest approach of the lake. At the time of first sampling, a mixture of about equal parts of lake water with catchment water of low tritium concentration is indicated* Sy December \¥l\, the spring flow had increased due to rise in lake level. Plow and isotopic data are consistent with an 63:12 mixture lake/catchmant water. This mixture pertains also on 21/9/72 Table b CHLORIDE CONCENTRATIONS OP WATERS IN ROTORUA REGION (SAMPLES COLLECTED 22 SEPTEMBER, 1972). -1, Sample Chloride (mg.l ) Lake Rotoma (surface) 36.3 Lake Rotoma (bottom) 36.3 Lake Okareka (surface) 4«7 Lake Okareka (bottom) 4*6 Lake Rerewhakaaitu (surface) 4*4 Lake Rerewhakaaitu (Crater Basin surface) 3.9 Lake Re-ewhakaaitu (Crater Basin bottom) 3.5 Kaikanapiti Spring 28.0 Western Outlet Spring 28.2 Awahou Spring 5*3 Hamurana Spring 4«8 Te Kaue Spring 5-0 Waitangi Spring 5*2 -'HlNr* A.ND ira?Ay; • :rr;i:r or- nm-st's^tT L»KK D-^.S^E. ",-vnpl in#( Altitude-) T* 6tl8o). 6(D). Ceaoant (Cri4 referenci.} relnt. to ^° lake level a) PonK*«a»a Sarins fLftV*> Koio.hj) Pongaka*. Sprir^ 6/12/71 -28 • -5.65*0.12 -23.3.0.7 18.6^1.2 Plowa out of Rotoitl Sraccia anj ta (*6S:936221) probably ralativaly .hallow groundwater. 5/10/72 -5-59.0.09 -23.5.0.7 3.3.0.2 Flrat tritiw reaulc ta eonatdere.1 auapact. 29/8/7) -5-19»0.1l -29.4.0.7 4.9*0.2 a) iaotoptc csjoattiort of atrcaga and aprin/ra on catchments cloaa to l^nkt Hotolcanau. Oauantii Straaa 2/10/72 •7 • 8.6 -J.3U0.10 -29.7*1.0 17.8*1.1 Tht chances of temp-rature and stable (>T.:a»109) isotope composition indicate that this 7/3/7J 14-5 -4.WiO.U -21.6*1.0 16.6*1.0 stream drains recent precipitation. 10/8/7) 6.8 -5.67*0.17 -32.4*1.0 17.0*1.0 Tritium concentration is also close to ambient vslus. Spring 2/10/72 *4 • 11.3 -5-24i0.l0 -30.3*1.0 4-9*0.3 Spring lies higher than lake surface, and (»76:831113) therefore cannot contain any component of lake drainage. Isotopic compositeen matches exactly those of springs lying below level of lake surface. Spring emerges fro* bank of pyroclaetic alluvium .and appears to be discharge of shallow-lying water table, but tritium concentration indicates at coat a very minor fraction of recent precipitation. Spring 23/9/72 -20ii 11.7 -5-29iO.il -26.^1.0 5.8+0,4 One of a series of springs discharging into (.76:824115) Ohuanui Stream just above its lower gorge. This spring emerges on left-hand bank looking upstream. Straaaj 2/10/72 -7 a (176:828124) 12.8 -5.46*0.14 -30.8*1.0 12.0*0.7 Stress* drains swampy area on Wiggin'a farr.v Mostly above lake level. Sample was taken just above confluence with strra* coming from Kiw\ Ranch. Tritium concentration suggests sort contribution from recent precipitation than the other samples in this senegas might be expected from swampy reg.«i, but there is still an appreciable fraction of elder water. Vara apnng 2/10/72 -19 31.3 -5-",2*0.12 -29.8*1.0 3.4*0.2 All three warn springs have isotopic (1176:82)12)) composition siirilar to the coM springs. Iron precipitated at easrgenccs. Vara apring 2/10/72 -21 17.4 -5.19iO.10 -29.6*1.0 1.320-1 (176:82)119) Vara apring 2/10/72 -14 16.9 -5.31*0.10 -29*7+1.0 0.37*0.06 (176:825119) Spring at 20/12/72 -8 s I5.5 -5.14*0.14 -30.6*1.0 9.320*° Thim spring is in catchment of Lake Ta Taroa Rotoitl and flows through Rotoiti Breccia. (•76:85313)) *) Values only approximate because of lake level vsnation.