Isotope contribution to geochemical investigations in aquifer storage and recovery

Aquifer Storage and Recovery (ASR) is an important resource management tool. An available water source; such as surface water, seasonal rainfall, or sewage effluent, is stored in a suitable aquifer for reuse when required to balance the seasonal demand for irrigation supply, thus relieving the pressure on groundwater resources. In ASR schemes, geochemical and biogeochemical reactions play an important role, impacting on both the aquifer matrix integrity and the recovered water quality. Understanding the driving processes that trigger these reactions is essential for determining the feasibility of new schemes and to adequately manage operating schemes. However the resulting geochemical signature is often due to a complex suite of reactions that is difficult to unravel. Few studies analyse the potential of isotopic tracers to contribute to the understanding of the biogeochemical reactions induced by ASR. This paper investigates the potential of the stable isotopes of the water molecule, carbon-13 and carbon-14 and sulfur-34 to contribute to our understanding of the geochemical processes involved in reclaimed water ASR. The field trial at Bolivar, South Australia is investigating the viability of reclaimed water i.e. nutrient rich water, as an injectant. The stable isotopes of the water molecule, can be used as conservative tracers to calculate the extent of mixing as the stable isotopic signature of the reclaimed water and the native groundwater of the carbonate aquifer are significantly different. The δ2H and δ18O of native groundwater are fairly constant at -26±1 and -4.4±0.1 per mille vs. SMOW respectively, while the injectant signature is more enriched and more variable, with δ2H ranging from -10.6 to -3.6 per mille and δ18O from -1.74 to -0.21 per mille. The variation in the injectant signature is attributed to seasonal variation in the degree of evaporation occurring in storage lagoons. The seasonal signature variation is maintained as injectant penetrates observation wells 4m and 50m from the point of injection. This can be utilised in mixing calculations to constrain the portion of injected end-member that is penetrating an observation well and reduces uncertainty with using an average representation of the variable injectant quality. The isotopes of carbon, carbon-13 and carbon-14, can help to characterise the source of oxidised organic matter and dissolved inorganic carbon. In this study, carbon-13 and carbon-14 of TDIC are used to gain insight into two of the important processes involved with ASR, organic matter oxidation and calcite dissolution. The native groundwater δ13C signature is -11 ± 3 per mille vs. PDB and the 14C activity ranges from 3-10 pMC. The injectant δ13C signature is more enriched, ranging from -7.0 to +0.1 per mille, and has a modern 14C activity of 100 ± 9 pmC. Upon injection, both organic matter oxidation and calcite dissolution are evident within 4m from the ASR well. This is reflected by a lowering of the carbon-14 activity in the 4m groundwater consistent with calcite dissolution. This suggests the carbon-14 signature is sensitive to small additions of TDIC through reaction processes. The carbon isotopes behave differently upon breakthrough of injectant to the 50m radius, where the ambient signature dominates until the groundwater is 100% injectant. The final signature at 50m after full breakthrough, δ13C -8.1 ± 0.2 per mille and 14C activity 58 ± 1 pmC, is somewhat lower than the injectant signature and may be attributed to additional reaction processes. Sulfate isotopes, sulfur-34 and oxygen-18, can provide insight on sulfate reduction and pyrite oxidation reactions. Sulfate reduction up to 1.5 mmol L-1 is evident in groundwater sampled from the ASR well during a period of aquifer storage, while sulfate concentrations 4m from the ASR well remain unchanged. Enrichment in residual sulfate, of around 12 per mille vs SMOW, accompanied the decline in sulfate concentration, is typical of biologically mediated sulfate reduction. Stable sulfate and sulfur-34 signatures at the 4m observation well, indicate the sulfate reducing zone does not extend far from the ASR well