Investigating groundwater dissolved organic carbon on global, regional and local scales

Projected changes in climate and urbanisation will not only impact on groundwater availability, but also its quality. Natural dissolved organic matter (DOM) character and concentration are major controls of groundwater quality, potability and treatment costs. Whilst the factors impacting DOM in surface waters are increasingly well understood, the sources, character and fate of groundwater DOM remains unclear. This thesis assesses global groundwater dissolved organic carbon (DOC) concentrations (average = 2.7 mg C / L, median = 1.0 mg C / L and σ = 15.1 mg C / L) and then quantifies variables impacting DOC concentrations on a continental scale to determine future changes. Ultra-high resolution DOM characterisation techniques combined with size exclusion chromatography and isotopic (14CDIC, 14CDOC, 18O, 2H, 3H) analyses are then used to identify groundwater DOM end-members in geographically contrasting groundwater environments in New South Wales, Australia. One of these sites was further sampled at multiple time points to determine the effects of extreme wet and dry periods on groundwater DOM character. This thesis identifies water chemistry, climate, aquifer age and land use as significant drivers of groundwater DOC concentrations. Hotspots in the United States are identified where groundwater DOC is projected to increase by almost half its current concentration by 2050 due to changes in temperature and precipitation alone. In-depth groundwater DOM analysis utilising carbon isotopes and DOM characterisation techniques in three contrasting locations in Australia revealed three major groundwater DOM end-members. These include a young terrestrial aliphatic DOM source identified at all sites, a low aromaticity, low molecular weight, aged recalcitrant by-product DOM identified at inland sites, and an aged, aromatic sedimentary DOM source identified at a coastal site. This work highlights the importance of DOM source rather than age in determining groundwater DOM character