AU0221643

Investigation of Groundwater-Streamflow interactions in the Bega alluvial aquifer using Tritium and Stable Isotope ratios.

DAVID J. M. STONE1, MEREDITH THOMAS2 and GREG RUSSELL3

1. Environment Division, Australian Nuclear Science and Technology Organisation, 2. School of Geosciences, The University of Wollongong 3. NSW Department of Land and Water Conservation, Sydney, South Coast Region

SUMMARY

An isotope hydrology study of the Bega Valley groundwater system has been made. The investigation which focussed on environmental tritium and stable isotope ratios confirms that that the resource is sustainable at the current usage rate.

analysed for stable isotopes (2H/H, I8O/I6O) 3 INTRODUCTION ratios, tritium ( H), and major and minor chemical species. Rainwaters were collected and The sustainability of Groundwater analysed for stable isotopes only. extraction from the alluvial flats that extend along Ion Chromatography was used for the the Bega and Brogo on the far south coast analysis of the anions while either ICP-MS or of NSW has been investigated using stable ICP-AES was used for cations. The tritium isotopes of water and tritium. Extraction from analysis was carried out by standard procedures these aquifers industrial, agricultural and of electrolytic concentration and liquid domestic purposes is high and the aquifer is scintillation counting (Calf, Seatonbury, and regarded to be at moderate risk of depletion and Smith, 1975). Analysis of the water samples for degradation. An improved understanding of how deuterium was conducted by CSIRO, Isotope the groundwater system interacts with surface Analysis laboratory using the zinc reduction water will allow this resource to be managed and method and a VG Isogas mass spectrometer sustainably utilised. (error; ± 0.8 per mil). The determination of Of the two major rivers in the Bega oxygen-18 was conducted at the University of valley, a catchment of about 1955 km2, the Wollongong using the CO2 gas equilibration flows east to where it is joined by method, purified using a Micromass Multiprep the flowing from the north (Fig 7). Unit and measured on a Micromass Prism III ( From the , where the town of Bega is error; ±0.1 per mil). located, the Bega River flows east to the coast 20 km away. A small dam, with a capacity of 9900 RESULTS megalitres in the upper Brogo catchment, regulates the flow of the Brogo River. Nearly all samples had low Total Dissolved Solids and EC, and analysis of major METHODS ions revealed low concentrations in general, most groundwaters only slightly more salty than the Groundwater and streamflow samples streamflow samples. In addition Piper diagrams were collected from sites, including transects (Fig 1) revealed that with the exception of a across the alluvium, in both valleys upstream of groundwater samples approaching the tidal limit the confluence and downstream to the tidal limit (bores 75049, and 39012) and several in April and July, 2000. Streamflow was sampled groundwaters at moderate distances from the using a plastic bailer while groundwaters were Brogo River (39003, 39004), all the samples withdrawn with the use of a Grundfos MP1 cluster in a tight group, typical of meteoric water. environmental sampling pump. They were These samples, and particularly those near the

191 confluence of the rivers all show a linear indicating the aquifer and streams are a well relationship between total ions TDI and each mixed system of two end members, possibly major ion in composition diagrams (Fig 2), rainfall and evaporated rainfall.

3t- Groundwater • stream flow •^ 39003 •w 39004 © 39012 • 75049

80 60 40 20 20 40 60 80 Ca Na+K HCO3 Ci

Figure 1: A piper plot of the analysis of water samples collected in April 2000

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0- —1— 1—1— 1—1— 6 12 18 2430 6 12 18 24 30 TDI (meq/l) TDI (meq/l) Figure 2: Composition diagrams of the concentrations of the major ions plotted against Total Dissolved Ions for samples in April 2000

192 Stable isotope analysis for April, when the 2.0 C difference in groundwater temperature the aquifer waters averaged 18.5 C and may be better explained by significant mixing of streamflow 21.1 C, indicated that the waters were the substantially colder seasonal streamwater. meteoric. All samples plot close to or above the This is supported by the trend toward global meteoric water line derived from the convergence of the stream and groundwater equation 52H = 8.13 51SO + 10.8 (Craig, 1961a), isotopic values in samples taken progressively with an O18 range from -4.5 to -6.0 per mil (Fig downstream. 3). The stream waters were more depleted than all Tritium activity in the samples collected but a few groundwaters, notably those in April range between 3.6 TU and 0.2 TU. These downstream of the confluence. The isotopic values need to be interpreted in light of the difference between streamwater and groundwater, current long-term stability of tritium in which can be taken as an indicator of temperature precipitation over Southern where of recharge, was greatest in the upstream parts of rainfall in the period 1990 -2000 has been the two rivers, while the difference was very consistently in the range 3.0-3.5 TU, (Fig 4). small below the confluence. The samples taken in Natural stratospheric input of tritium to the July, when the aquifer was 16.5 C and streamflow atmosphere is affected by latitude and altitude, was 12.5 C, were consistently more depleted than and a considerable input of tritium into the those of April, (Fig 3). The strong correlation atmosphere via thermonuclear weapons testing between temperature and stable isotopes in (Clark and Fritz, 1997) occurred between 1952 meteoric waters provides a seasonal signal that and 1963 resulting in dramatic increases in tritium can be used to date groundwaters. The amplitude levels. The peak in the concentrations occurring of seasonal variations in delta !SO and 2H is in 1963 (Fontes, 1980), when values of 10,000 attenuated during groundwater recharge and tritium units were measured in North America. preservation of seasonal variations implies short The distribution of tritium levels in precipitation mean residence times. averaged over the period from 1963 to 1968 for Australia indicate that the highest levels were The average shift in O18 was -0.16 per found in the southeast over Victoria and reached mil for groundwaters and -0.17 for stream 50 tritium units (Fontes, 1980), and had stabilised samples, with one or two values nearly double the at about 4 TU by 1980, (Fig 5). Whereas it is not averages. Rainfall in this period was light and in clear that tritium in Australian has ceased the range -5 to -8 per mil. The shift in O18 is declining, it has stabilised at levels useful for consistent with a temperature of recharge the study of modern water age. difference of 0.6 C, a value comparable with the difference in groundwater temperature. However

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-34.0- / / I 1 1—1 11 1 1 -6.0 -S.0 -4.0 -3.0 180

Figure 3. Delta 2H v 18O analysis for samples collected in April (left) and July (right) 2000; (streamflow; blue diamonds), vs global meteoric water line.precipitation has ceased declining, it has stabilised at levels useful for study of modern water age.

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Figure 4. Tritium in Precipitation, selected stations in Southern Australia, 1979-2000.

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1962 1965 1970 1975 1980 1985 Figure 5. Weighted mean of Tritium activity in Australian Rainwater, 1962 -1985

194 StreamflbWL* •_— LJ | Downstream Bega River

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Figure 6. Plot of Tritium activity versus depth of sample in metres for groups of bores

Tritium in Groundwater reflects the conditions of recharge, and the history of water CONCLUSION movement in the aquifer. The tritium activity of streams make apparent whether the stream is In conclusion the isotopic evidence suggests that recharging groundwater or whether the the groundwater in the alluvial aquifers of the groundwater is recharging the stream (Ferronsky Bega valley is generally well mixed and of low and Polyakov, 1982). The stream samples varied residence time. The groundwater is recharged from 3.6 TU in the Brogo river, whilst having a from upstream sources in some areas with large dam on its upper reaches provides regulated localised infiltration of surface runoff and stream streamflow which contains a substantial amount flow. The groundwater is largely modern of surface water, to 2.7 TU in the unregulated indicating a relatively constant and rapid recharge Bega river, with a composite value of 3.2 TU regime for this, probably streamflow- found below the confluence. All the low EC groundwater interaction. The current use of the groundwaters noted to cluster tightly on the basis Bega Valley groundwater system will not put the of major ion composition had tritium activities in quantity of the groundwater at risk. the range 2.3 to 3.2 TU, which would indicate mean residence times for the aquifer waters between 1-5 years. The other groundwaters had activity typically less than 1.0 TU indicating REFERENCES much older waters consistent with their Calf, G.E, Seatonbury, B.W, and Smith, L.W. exceptional major ion compositions. A plot of (1975). The Measurement of Natural Levels of tritium activity versus borehole location and Tritium in Water. AAEC /E373. depth (Fig 6), indicates that the Brogo river is not Clark, I. and Fritz, P. (1997). Environmental recharging the adjacent alluvium since the tritium Isotopes in Hydrogeology. Lewis Publishers, New is much lower than in the streamwater. The Bega York. river however is recharging the adjacent alluvium, Craig, H. (1961a). Isotopic Variations in Meteoric both above and below its confluence with the Waters. Science 133, 1702-1703. Brogo river, since tritium values are similar in Craig, H (1961b). Standard for Reporting stream and alluvium. This conclusion is supported Concentrations of Deuterium and Oxygen-18 in by hydraulic data for the various transects. Natural Waters. Science, 133, 1833-1834.

195 Calf, G.E. (1988). Tritium Activity in Australian Ferronsky, V.I, and Polyakov, V.A. (1982). Rainwater 1962-1986. Australian Nuclear Environmental Isotopes in the Hydrosphere. John Science and Technology Organisation, Lucas Wiley and Sons, New York. Heights Research Laboratories.

Figure 7. A plan of the Bega area showing the town, rivers, alluvial deposits and borehole locations.

196