Abstract
The concentration and flux of organic carbon in aquifers is influenced by recharge and abstraction, and surface and subsurface processing. In this study groundwater was abstracted from a shallow fractured rock aquifer and dissolved organic carbon (DOC) was measured in observation bores at different distances from the abstraction bore. Groundwater abstraction at rates exceeding the aquifers yield resulted in increased DOC concentration up to 3,500 percent of initial concentrations. Potential sources of this increased DOC were determined using optical fluorescence and absorbance analysis. Groundwater fluorescent dissolved organic material (FDOM) were found to be a combination of terrestrial-derived humic material and microbial or protein sourced material. Relative molecular weight of FDOM within four metres of the abstraction well increased during the experiment, while the relative molecular weight of FDOM between four and ten metres from the abstraction well decreased. When the aquifer is not being pumped, DOC mobilisation in the aquifer is low. We hypothesise that the physical shear stress on aquifer materials caused by intense abstraction significantly increases the temporary release of DOC from sloughing of biofilms and release of otherwise bound colloidal and sedimentary organic carbon (SOC).
Highlights
The concentration of dissolved organic carbon (DOC) in groundwater is highly variable and sources and migration mechanisms are poorly understood
The results show that the amount of sedimentary organic carbon (SOC) leached into distilled water is generally low (~2 mg/L) within the deeper (> 7 mbgl) sediments, elevated concentrations were present within the soil profile, suggesting a potential DOC source which could be mobilised by infiltrating water
We have undertaken a study of the effects of high intensity abstraction of groundwater on OM mobilisation within a fractured rock aquifer
Summary
The concentration of DOC in groundwater is highly variable and sources and migration mechanisms are poorly understood. Research into the stabilisation and destabilisation of groundwater OM has focussed on the effects of: biofouling due to injection of nutrient rich water during bioremediation of contaminated soils or groundwater[11,12,13,14,15,16,17]; the creation of impermeable barriers around contaminated sites using enhanced biofilm growth[18,19,20]; the injection of water into oil reservoirs[21]; injection of CO2 rich brine on biofilm growth[22] These studies identify a significant accumulation of aerobic microbial populations in the immediate vicinity of the injection well, with increased concentrations of anaerobic microbial populations in areas further from the well screen[17,21]. It was noted that biofilm loss rate due to shear stress was influenced by the composition of the porous medium with losses being higher on smooth surface sands relative to activated carbon In this instance we are testing the effects of abstraction only within a fractured rock aquifer with no water or solute injection. We expect that shearing of biofilms would in this scenario play the major role in DOM mobilisation
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