Hydrochemical apportioning of irrigation groundwater sources in an alluvial aquifer.

River floodplains sustain irrigated agriculture worldwide. Despite generalised groundwater level falls, limited hard data are available to apportion groundwater sources in many irrigated regions. In this paper, we propose a workflow based on: hydrochemical analysis, water stable isotopes, radiocarbon contents and multivariate statistical analysis to facilitate the quantification of groundwater source attribution at regional scales. Irrigation water supply wells and groundwater monitoring wells sampled in the alluvial aquifer of the Condamine River (Queensland, Australia) are used to test this approach that can easily be implemented in catchments worldwide. The methodology identified four groundwater sources: 1) river/flood water; 2) modified river/flood water; 3) groundwater recharged through regional volcanic materials and 4) groundwater recharged predominantly through sands and/or sandstone materials. The first two sources are characterised by fresh water, dominant sodium bicarbonate chemistry, short residence time and depleted water stable isotope signatures. Groundwater sources 3 and 4 are characterised by saline groundwater, sodium chloride chemistries, enriched water stable isotopes and very low radiocarbon contents, inferred to correspond to long residence times. The majority of wells assessed are dominated by flood water recharge, linked to decadal >300 mm rainfall events and associated flooding in the region. The approach presented here provides a groundwater source fingerprint, reinforcing the importance of floodwater recharge in the regional water budgets. This apportioning of groundwater sources will allow irrigators, modelers and managers to assess the long-term sustainability of groundwater use in alluvial catchments.

Combining environmental isotopes with Contaminants of Emerging Concern (CECs) to characterise wastewater derived impacts on groundwater quality.

The potential for Wastewater Treatment Plants (WWTPs) to cause adverse impacts to groundwater quality is a major global environmental challenge. Robust and sensitive techniques are required to characterise these impacts, particularly in settings with multiple potential contaminant sources (e.g. agricultural vs. site-derived). Stable (δ2HH2O, δ18OH2O, δ15NNO3, δ18ONO3 and δ13CDIC) and radioactive (3H and 14C) isotopes were used in conjunction with three Contaminants of Emerging Concern (CECs) - carbamazepine, simazine and sulfamethoxazole - to discriminate between multiple potential contamination sources at an Australian WWTP. The radioactive isotope tritium provided a sensitive indicator of recent (post-1990s) leakage, with groundwater activities between 0.68 and 1.83 TU, suggesting WWTP infrastructure (activities between 1.65 and 2.41) acted as a recharge 'window', inputting treated or partially treated effluent to the underlying groundwater system. This was corroborated by water stable isotopes, which showed clear demarcation between δ18OH2O and δ2HH2O in background groundwater (δ18OH2O and δ2HH2O values of approximately -5 and -28‰, respectively) and those associated with on-site wastewater (median δ18OH2O and δ2HH2O values of -1.2 and -7.6‰, respectively), with groundwater down-gradient of the plant plotting on a mixing line between these values. The CECs, particularly the carbamazepine:simazine ratio, provided a means to further distinguish wastewater impacts from other sources, with groundwater down-gradient of the plant reporting elevated ratios (median of 0.98) compared to those up-gradient (median of 0.11). Distinctive CEC ratios in impacted groundwater close to the WWTP (∼3.0) and further down-gradient (2.7-9.3) are interpreted to represent a change in composition over time (i.e., recent vs. legacy contamination), consistent with the site development timeline and possible changes in effluent composition resulting from infrastructure upgrades over time. The data indicate a complex set of co-mingled plumes, reflecting different inputs (in terms of both quantity and concentration) over time. Our approach provides a means to better characterise the nature and timing of wastewater derived impacts on groundwater systems, with significant global implications for site management, potentially allowing more targeted monitoring, management and remedial actions to be undertaken.

Movement of a tritium plume in shallow groundwater at a legacy low-level radioactive waste disposal site in eastern Australia.

Between 1960 and 1968 low-level radioactive waste was buried in a series of shallow trenches near the Lucas Heights facility, south of Sydney, Australia. Groundwater monitoring carried out since the mid 1970s indicates that with the exception of tritium, no radioactivity above typical background levels has been detected outside the immediate vicinity of the trenches. The maximum tritium level detected in ground water was 390 kBq/L and the median value was 5400 Bq/L, decay corrected to the time of disposal. Since 1968, a plume of tritiated water has migrated from the disposal trenches and extends at least 100 m from the source area. Tritium in rainfall is negligible, however leachate from an adjacent and fill represents a significant additional tritium source. Study data indicate variation in concentration levels and plume distribution in response to wet and dry climatic periods and have been used to determine pathways for tritium migration through the subsurface.