Sarah Tweed

Stable isotope geochemistry of cold CO2-bearing mineral spring waters, Daylesford, Victoria, Australia: sources of gas and water and links with waning volcanism

Abstract Mineral springs in the Victorian Central Highlands, Australia, have high CO 2 contents and naturally effervesce. δ 13 C values of CO 2 gas and dissolved inorganic carbon are −10.6‰ to −7.0‰ and −5.9‰ to −0.1‰, respectively, with a net δ 13 C of −8‰ to −3‰. The carbon in these waters was derived from a mantle source associated with local Pliocene to Recent basaltic Newer Volcanic Province rocks. Previously reported 3 He/ 4 He (1.2–3.1 relative to air) and high He/Ne ratios are also consistent with the presence of magmatic volatiles. Silica contents imply that the waters were never heated above 130 °C and that the system is not hydrothermal. The occurrence of carbonated mineral springs in a relatively small region of the Newer Volcanics Province where volcanic activity ceased several thousands of years ago may be due to the presence of late intrusions combined with deep circulation of water through deeply weathered and fractured Ordovician basement. A region of low seismic velocity under the Daylesford area potentially images those intrusions. Most spring waters have δ 18 O and δ 2 H values of −8‰ to −6‰ and −45‰ to −35‰, respectively, and lie to the left of the local and global meteoric water lines. The anomalously low δ 18 O values results from CO 2 exsolution at low temperatures which strongly partitions 18 O into the gas. The lack of waters lying to the right of the local meteoric water line also implies that water–rock interaction at elevated temperatures did not occur. The δ 2 H values are lower than contemporary meteoric water, suggesting that the waters may have recharged under colder climate conditions several thousand years ago. The local Ordovician rocks are gold bearing. The present spring system is cold and would not efficiently transport Au. However, volcanic is waning and the spring systems at the time of volcanism may have been hotter and able to redistribute Au.

Groundwater depletion in the Hai River Basin, China, from in situ and GRACE observations

Abstract The Hai River Basin (HRB) is a heavily irrigated region encompassing the North China Plain (NCP) in northeast China. In the last decades, continuous lowering of groundwater levels had been reported in the NCP. This study used data from the Gravity Recovery and Climate Experiment (GRACE) and in situ measurements to quantify recent changes in groundwater storage from 2003 to 2012. The signal from GRACE observations highlight a sharp decline in the deep subsurface water stores (deep unsaturated zone and groundwater systems) up to a rate of 17.0 ± 4.3 mm year-1 between 2003 and 2012 over the HRB, equal to a volumetric loss of 5.5 ± 1.4 km3 year-1. This result shows good consistency with in situ observations of groundwater hydraulic heads compiled from monitoring bores, and emphasizes GRACE’s ability to monitor large-scale groundwater storage variations. Results from GRACE also provide an independent assessment of the effectiveness of water saving programmes that have been implemented by the government so far. Our study indicates that groundwater overdrawal is still prevalent and the dominant factor for the persistent loss in groundwater storage over the HRB/NCP; the current groundwater consumption pattern is far beyond the natural recharge ability in groundwater system. Editor D. Koutsoyiannis; Associate editor T. Wagener

O, H, C isotope geochemistry of carbonated mineral springs in central Victoria, Australia: sources of gas and water–rock interaction during dying basaltic volcanism

Waters discharging from carbonated cold mineral springs in the central Victorian Highlands of Australia have δ2H and δ18O values of −45 to −35‰ and −8 to −5.5‰, respectively, that suggest that they were derived from local meteoric water recharged under cooler climatic conditions than present. δ13C values of entrained CO2 (−10.4 to −7.0‰) and dissolved inorganic carbon (−3.9 to −0.1‰) imply that the C was probably derived from outgassing of Recent igneous rocks within the Newer Volcanic Province. The waters lie to the left of both local and global meteoric water lines due to CO2 exsolution from waters that were never involved in high-temperature water–rock interaction.