Jaya K. Dharmasiri1,Lidia Morawska1 and John Hillier. ICentre for Medical and ... of weirs have been built across Lockyer and Laidley creeks with the intention ...
Murray-DarlinK
Basin Workshop
'97, Toowoomba
26-28 AUKust 1997
Application of Stable Isotopes to Identify Sources of Recharge to an Alluvial Aquifer in Gatton, Queensland Jaya K. Dharmasiri1, Lidia Morawska1 and John Hillier ICentre for Medical and Health Physics, QUT, Brisbane, QLD 4001 2Department of Natural Resources, 80 Meiers Road, Indooroopilly, QLD 4068 INTRODUCTION Gatton is an agriculturally important area producing about 40% of the vegetable needs of Queensland. The area is located within the valley of Lockyer creek, a tributary of Brisbane river (Figure 1). Groundwater is extracted from the alluvial aquifer at depths of about 30 m and is virtually the sole source of irrigation water. Since groundwater development started about 50 years ago, shortage of supply has been experienced in the Lockyer Valley during drought periods in the 1980s and 1990s (Qld. Water Resources Commission, 1982). The Crowley Vale Irrigation area (20 km2) has shown consistently declining groundwater levels over the last 25 years and most of the bores are nearly dry in 1997. Groundwater in the rest of the area generally responds to creek flows, especially during major floods, A number of weirs have been built across Lockyer and Laidley creeks with the intention of boosting natural recharge through creek beds. This action has not helped to increase the recharge to Crowley Vale irrigation area. The sources of recharge to groundwater in the Crowley Vale area have not been fully understood. The present investigation was started in 1993 to understand the mechanisms of recharge to the Crowley Vale area specifically, and the rest of the area in general, to enable the Department of Natural Resources (DNR) to consider ways of artificially recharging the depleted resource.
Hence a network of three monthly rain water collection stations was set up in 1993 in Brisbane, Gatton and Toowoomba, though it is not possible to cover long term rainfall patterns in a period of three years. Fortunately, Brisbane has been one of IAENWMO Global Network (lAEA, 1983) of more than 100 stations and a valuable data base exists from 1961 to 1993. The present study added three more years of monthly data to this data base. The long term trends in isotopic variations for Brisbane can be applied to Gatton which is only 80 km away. ::..':.~~-;:::-
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+ 10 (Craig 1961) 249
Murrav-Darlin{:
Basin Workshop
'97, Toowoomba
26-28 Au{:usr 1997
This relationship is generally known as Meteoric Water Line. The gradient and the intercept vary depending on the geographical location and climatic conditions. Globally, the relationship hold very well with a gradient of 8.17 and an intercept of 10.56 (!AEA 1983). The stable isotope data base for Brisbane produces a line given by;
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The smaller gradient. is typical for Australian situation caused by evaporation taking place on rain drops during fall through dry air masses, Groundwater originated from this rain water falls on a line deviating from a line of gradient 8, towards 0180 axis. DISCUSSION A total of 51 bores were sampled within the study area for 02H and 0180 during June-July 1993. A set of 18 bores selected from the original 51 bores were sampled again during July-August 1994 to understand any short term changes. The consistency was excellent, suggesting no short term variations, even though the two sets were measured at the University of Queensland and CSIRO in Adelaide respectively. A set of 12 bores carefully selected from outside the study area to represent sandstone outcrops in Gatton, Helidon spa area, sandstone in Withcott and basalt bores in Toowoomba. The period 1993-95 consisted of dry years and runoff in Lockyer and Laidley creeks was rare and intermittent. The summer of 1995-96 was wet causing good flows in the creeks. A total of 16 samples were collected from Lockyer and Laidley creeks. A total of 34 monthly rain water samples were taken from the Gatton Research Station during 1993-96. Figure 2 shows the plot of 02H against 0180 for groundwater sampled within the study area.
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studying the stable isotope ratios and the locations of bores they belong to.
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Group 1 The most depleted stable isotope results belonged to bores located in Crowley Vale area. The average 0180 value for this group of bores is -5.4°/00. This group represents groundwater recharged through infiltration of rainwater over a long period of time. One can use long term records of stable isotopes in monthly rain water for Gatton for predictive purposes but the three years of data available do not represent long term trends in isotope data. Hence the data base for Brisbane was used here as predictive tool for Gatton. The long term average stable isotope ratios weighted for the amount of rainfall (e.g. 0180w ) can be considered as closest to the stable isotope contents of groundwater recharged by infiltration of rain water over a long period of time. The 0180w for Brisbane was calculated to be -4.9°/00 using data for 1961-96. Gatton being located about 80 km inland and about 150 m above sea level, -5.4°/00for 0180wis a very reasonable value. Infiltration of rain water through the top soil or sandstone outcrops will result in a similar stable isotope composition. It has been found that direct infiltration of rain water through the top soil layers (Dharmasiri, 1997) was very small (less than 50 mmlyear). Group 3 - This group had an average 0180 of -4.4°/00' These bores are located outside the Crowley Vale area. It is known that this group of bores respond to creek flows and the stable isotope ratio is typical for summer floods in Gatton. The Lockyer creek catchment extends up to the Great Dividing Range around Toowoomba, which is about 350 m above sea level. Hence the creek flow of Lockyer near Gatton will show a stable isotope composition depending on rainfall received by that part of the catchment above Gatton. Therefore, the stable isotope composition of creek flow becomes important now. Figure 3 shows the stable isotope relationship of creek water in Lockyer and Laidley creeks to the meteoric water line.
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Figure 2. Plot of 02H against 0180 for groundwater from Gatton study area. The World Meteoric Water Line is also shown.
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The most outstanding feature of this set of data is the spread observed for such a small area. Four categories or groups of data can be recognised by carefully
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for creek waters
Major floods after heavy rain have an average stable isotope composition of -4.3°/00' which is very close to that found for groundwater in this group of bores.
Murrav-Darlin!(
Basin Workshop
'97. Toowoomba
26-28 Au!(ust 1997
Another study by Dixon and Chiswell (1994) carried out in an area upstream of Gatton has shown similar stable isotope compositions for bore water recharged by the creeks. This is a strong indication that creeks are the main source of recharge to this group of bores. Further, it must be stressed that major floods are the significant recharge source by infiltration through creek beds. There are a few highly enriched creek water samples on Figure 3, indicating the presence of evaporated water in creeks after the flood events, yet this Group of bores showed no evaporation effect. A two-day flood event that measured a highly depleted stable isotope composition (February 1995) of -7.8%0 may not have much significance in recharging groundwater. Group 2 - These bores are located in the transition zone between Group 2 and 3 bores. The stable isotope composition are typical of a mixture between the two source waters. For instance, 0180 value of -4.7%0 means a mixture of Group 1 water having a 0180 value of -5.4%0 and Group 3 water having a 0180 value of -4.4%0 in the ratio of 3:7.
-
Group 4 This group consists of only two bores located very close to the Lockyer and Laidley creeks indicating groundwater having a contribution that has undergone evaporation prior to recharge. The most likely source again is creek water undergoing evaporation as result of weirs built across the creeks. In fact, A few creek water samples collected in early 1993 showed marked evaporation enrichment as shown in Figure 3. The absence of any evaporation in almost all the bores in Group 3 raises the question as to how effective weirs are for recharge in the short term. It is important to know if there are outside sources of recharge to groundwater in the study area. A selected set of bores were also sampled for groundwater from sandstone outcrops to the north and south of the study area, one sandstone bore in Withcott, spa bores in Helidon and shallow basalt bores in Toowoomba. The sandstone bores to the north and south of the study area had 0180 values of -5.4%0 (Figure 4) suggesting a strong relationship with ground water in the Crowley Vale area. The predicted weighted mean stable isotope composition for Gatton based on the altitude effect matches well with this stable isotope composition suggesting rain water infiltration averaged over long time as the source origin of recharge. The chemical quality of the sandstone water was much better than groundwater in the Crowley Vale area. Hence the stable isotopes strongly indicate the sandstone outcrops outside the study area as the source area for recharging groundwater in Crowley Vale.
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FigUre 4. Stable isotope relationship for groundwater outside the study area in Gatton. The sandstone bore in Withcott had a 0180 values of -5.8%0 in agreement with depletion expected based on altitude effect. The three artesian spa bores from Iredale area near Helidon had stable isotopes that were too depleted to be of any local origin. The 0180 values were -6.7, -6.7 and -6.5%0 giving a very strong indication that these groundwaters originated from the Great Artesian Basin. The average 0180 value for GAB ground water has been reported as -6.9%0 by Airey et al. (1979). Four basalt bores sampled in Toowoomba had 0180 values of -4.2, -4.7, -4.8 and +1.3%0 suggesting some similarity with the stable isotope composition in Group 3 groundwater on Figure 2. As Toowoomba is located at around 350 m above sea level, a long term weighted average 0180 value of -6.0%0 is expected. The shallow basalt groundwater does not support the idea that this groundwater has been recharged from long term infiltration of rain water. In fact, there seems to be selective recharge taking place from summer rainfall which has a similar stable isotope composition measured for groundwater. The likely explanation for this selective summer rain recharge would be the characteristics of top soil in Toowoomba. This soil has a tendency to develop deep cracks during the dry season. With the onset of the summer wet season, the rain water infiltrates straight into the deep cracks. When the top soil is sufficiently wet, it swells up sealing all cracks. This prevents further rain water infiltration through top soil. Yet, the rain falling just to the east of Toowoomba ends up as runoff in Lockyer creek and its tributaries and recharges the ground water. This is the only connection between Toowoomba and Gatton with respect to recharge. Both areas are recharged from summer rains. Hydrogeologically, groundwater in Toowoomba has no connection with that in Gatton. Stable isotope composition in monthly rain water in Toowoomba seems to behave like any other station in Australia with seasonal variations. Yet, Hartley (1980) could not find expected relationships between stable isotope composition and meteorological parameters.
251
Murray-Darlin!l
Basin Workshop
'97, Toowoomba
26-28 AU!lust 1997
CONCLUSIONS
REFERENCES
The findings of this study based on the stable isotope results are summarised below: The stable isotope compositions of recharge through sandstone outcrops and creek beds are distinctly different. The groundwater in Crowley Vale area of Gatton is recharged by infiltrating rain water through sandstone outcrops to the north and south of the area, The direct infiltration through top soil in the local area is very limited. There is apparently no connection of groundwater in Crowley Vale area to the creek recharge. . The groundwater in the alluvial aquifer in the rest of Gatton is mainly recharged by the infiltration of creek flows through beds of Lockyer and Laidley creeks.
Airey, P.L., Bentley, H., Calf, G.E., Davies, S.N., Elmore, D., Gove, H., Habermehl, M.A., Phillips, E, Smith, J., and Torgersen, T. (1983) 'Isotope Hydrology of the Great Artesian Basin, Australia', In Proceedings of International Conference of Groundwater and Man, 1-11. Craig, H. (1961) 'Isotope Variations in Meteoric Waters, Science, 133, 1702. Dharmasiri, J.K., Morawska, L., and Hillier, J. (1997)
