Isotope evolution and contribution to geochemical investigations in aquifer storage and recovery: a case study using reclaimed water at Bolivar, South Australia

摘要

Aquifer storage and recovery (ASR) is an important resource management tool whereby water from an available source is stored in a suitable aquifer for later use in periods of higher demand. Important issues in ASR include maintaining the injection rate and recovering water of suitable quality. Both of these depend on subsurface biogeochemical processes. This paper investigates the use of deuterium, ~(18)O, ~(13)C, ~(14)C and ~(34)S in understanding the reactions induced by reclaimed water injection in a carbonate aquifer at Bolivar, South Australia. Additionally, the injection scheme provides a natural laboratory to observe the process of carbon isotope exchange. The injectant deuterium (-64 ± 2.9per thousand versus Vienna standard mean ocean water (V-SMOW)) and ~(18)O (—0.9 ± 0.5per tousand V-SMOW) signature is more enriched and variable than the native groundwater signature of —26 ± 1per thousand and —44 ± 0.lper thousand respectively. The variability of the injectant signature is maintained with injectant migration and is useful in constraining the portion of the injected end-member reaching observation wells. Effluent treatment results in total dissolved inorganic carbon (TDIC) enriched in ~(13)C (—3.3±2.5per thousand versus Pee Dee belemnite) and modern carbon (100 ±7 per cent modern carbon (pmC)), which is distinct from the native groundwater comparatively depleted in ~(13)C (—11 ± lper thousand) and ~(14)C (5.6 ±2.1 pmC). The carbon isotopic signature in groundwater 4 m from the ASR well is dominated by the injectant signature modified by some organic matter oxidation and calcite dissolution. However, with further migration to the 50 m radius, both ~(13)C and ~(14)C signatures are dominated by isotopic exchange with the matrix surface (initially in equilibrium with the ambient groundwater) and little overall dissolution. During storage, biogeochemical processes, including sulphate reduction and methanogenesis, are dominant near the ASR well. These are indicated by a sulphate decline of up to 1.5 mmol l~(-1) with around 12per thousand enrichment in the residual sulphate (versus Canyon Diablo troilite) and the addition of TDIC enriched in ~(13)C, while maintaining a modern ~(14)C activity.