Latest study a first step toward major groundwater carbon research project goal

Posted 24 December 2015

Measuring organic carbon concentration in a groundwater observation bore.

By Andy Baker and Rick Arena

Research published this week by the UNSW Connected Waters Initiative (CWI) provides new information about the effect of groundwater extraction on sub-surface carbon reserves.

The findings are a first step toward addressing the focus of a new research project funded by the Australian Research Council (ARC) and National Collaborative Research Infrastructure Strategy (NCRIS) that will study the role played by organic matter found in groundwater in the terrestrial carbon budget.

Currently, very little is known about how groundwater fits into the carbon cycle, its role in climate regulation and the implications of human activities such as groundwater extraction.

The new research published in the journal Scientific Reports by Peter Graham, Andy Baker and Martin Andersen, investigated whether groundwater pumping mobilises stored carbon. 

The concentration and movement of organic matter containing carbon in groundwater stored in aquifers is influenced by the movement of water into and out of those aquifers, as well as processes that occur naturally under the ground. Organic carbon is introduced to groundwater from sources of recharge including rain water and river water that contain dissolved organic carbon. This organic carbon may be taken up by minerals under the ground, and it can also be used as a food source by the sub-surface microbial community. Currently, these processes affecting organic carbon concentrations in groundwater are poorly understood.

The CWI research team worked at one of the NCRIS Groundwater Infrastructure sites at Wellington, New South Wales, Australia. At this specialist research facility, groundwater was pumped from a shallow fractured rock aquifer and dissolved organic carbon (DOC) was measured in groundwater in observation bores at different distances from the extraction bore.

Site location, regional topography and relative bore locations. Figure © Land and Property Information (date of extraction 19/02/2015).Site location, regional topography and relative bore locations. Figure © Land and Property Information (date of extraction 19/02/2015).

When groundwater was extracted at very high rates, the DOC concentration in the observation bores increased by up to 3,500 percent of the initial concentrations.

The carbon was analysed to work out where it might have come from. Optical fluorescence and absorbance analysis was used to measure the relative molecular weight of the DOC. This was found to have increased closer to the extraction point and decreased further away. The researchers hypothesised this was because substances containing the carbon that adhere to the surfaces of pore spaces in the aquifer were drawn toward the extraction bore and concentrated there by the intense rate of pumping.  

This is the first such time the effects of groundwater extraction on sub-surface organic carbon have been investigated.

Starting in 2016, the Groundwater organic matter: carbon source or sink? project, which will also include ARC Future Fellow Denis O’Carroll (CWI), will investigate the contribution made by organic matter in groundwater to the global terrestrial carbon budget.

The research will study processes that control the amount of organic carbon in groundwater by using the NCRIS-supported bore-field infrastructure to determine the concentration and character of organic carbon.

The project is designed to determine the rate and extent of both the biological and the physico-chemical processes affecting concentrations of organic carbon in groundwater.

By quantifying the environmental conditions in which groundwater acts either as a carbon source or sink, the project outcomes could help inform how groundwater management policy addresses any impacts of groundwater use on the carbon economy.

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