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The legacy and drivers of groundwater nutrients and pesticides in an agriculturally impacted Quaternary aquifer system
Journal article   Peer reviewed

The legacy and drivers of groundwater nutrients and pesticides in an agriculturally impacted Quaternary aquifer system

Haile A Shishaye, Douglas R Tait, Damien T Maher, Kevin M Befus, Dirk Erler, Luke Jeffrey, Michael J Reading, Uwe Morgenstern, Sarit Kaserzon, Jochen Mueller, …
The Science of the Total Environment, Vol.753, 142010
2021
DOI: https://doi.org/10.1016/j.scitotenv.2020.142010
PMID: 32890880
url
https://doi.org/10.1016/j.scitotenv.2020.142010View
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Abstract

3D geologic modelling MODFLOW Lower Burdekin aquifer contamination Stable isotopes Groundwater age Great Barrier Reef
Nutrient and pesticide pollution are among the major threats to groundwater quality in agriculturally impacted aquifers. Understanding their legacy effects and drivers are important to protect aquifers from exposures to contamination. However, the complexities of groundwater flowpaths make it difficult to predict the time-scales of groundwater flow and contaminant transport. To determine these controls of groundwater nutrient and pesticides in an aquifer system underlying an intensive agricultural area in the Great Barrier Reef catchment, Australia, we sampled tritium (3H) to estimate groundwater-age, nutrient and pesticide concentrations to investigate groundwater contamination, and nitrogen (ẟ15N-NO3−) and oxygen (ẟ18O-NO3−) isotopes to determine groundwater nitrate dynamics. We, then, constructed high-resolution 3D geological and groundwater flow models of the aquifer system to determine the role of the geologic heterogeneity on the observed nutrient and pesticide concentrations. Groundwater 3H derived ages, and nutrient and pesticide concentrations did not follow distinct spatial trends. ẟ15N-NO3− and ẟ18O-NO3− values indicated that nitrification and denitrification processes influenced nitrate dynamics in the aquifer system; however, they were not solely able to explain the entire 3D variability. The 3D geologic modelling identified possible preferential flowpaths and perched systems, which helped to explain the observed groundwater-age, nutrient and pesticide variabilities. Old-groundwater (~100-years) was found in shallow depths (<15 m) where perched systems were identified. In areas with preferential flowpaths, young-groundwater (⁓1-year) with significant nitrate (~12 mg-N/L) and pesticides (up to 315 ng/L) concentrations were detected at deeper depths (>25 m), below perched and locally confined systems. Downward increasing groundwater-age, and decreasing nutrient and pesticide concentrations were detected in the unconfined aquifer, while old-groundwater (~160-years) and lower nitrate (<3 mg-N/L) and pesticides (<2 ng/L) concentrations were detected in the confined systems. This study demonstrates the importance of understanding both the geology and the hydrogeology of an area before deploying monitoring studies and/or making conclusions from tritium, nutrient and pesticide data alone. [Display omitted] •Multiple aquifer settings were identified including; perched, confined, unconfined and preferential flowpaths.•Groundwater tritium derived ages did not follow distinct spatial trends.•Where preferential flowpaths exist, younger groundwater was found below older groundwater.•ẟ15N and ẟ18O showed active denitrification, but could not solely explain the 3D variability of groundwater nutrient.•Nutrient and pesticide dynamics were largely driven by geologic heterogeneity and groundwater mixing.

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