Unravelling the sources and sinks of nitrate in contaminated groundwater and estuarine systems

2017-02-15T04:46:08Z (GMT) by Wong, Wei Wen
Coastal eutrophication is widely recognised as a pervasive environmental problem due to increasing urbanisation along the coastline and intensive agricultural activities near to coastal areas. Estuaries in particular are often threatened by increased nitrogen loading from anthropogenic activities. This thesis aims to identify and quantify the diffuse sources and transformation processes of nitrate in a subsurface environment and a eutrophic temperate estuary in the south west of Melbourne, Australia. To achieve these aims, combinations of hydrological and geochemical tracers were used with the main focus on the application of stable (δ15N-NO3- and δ18O-NO3-) and radiogenic (222Rn) isotopes. The monthly longitudinal surveys of the Werribee River estuary over 14 months provided strong evidence for the occurrence of a hotspot of submarine groundwater discharge (SGD). The rate of SGD into the estuary was quantified based on two time series surveys by manipulating a two layer 222Rn mass balance model. Comparison between the salinity mass balance and the 222Rn-derived SGD rate inferred that 80 % of the total SGD is from terrestrial sources, driven by a hydraulic gradient between the groundwater level and the estuary. The modelled rate accounted for a total of 15 × 109 µmol d-1 NO3- and 170 µmol m-2 d 1 N2O and exceeded the NO3- and N2O load from riverine discharge by ~30 and ~20 fold, respectively during baseflow conditions. The average NO3- flux is the highest groundwater-derived NO3- load relative to surface water loading reported in the literature. The predominance of groundwater-derived NO3- in the estuary was also indicated by the estuarine δ15N-NO3- and δ18O-NO3- values. Distinctive isotopic values between shallow and deep groundwater have also allowed estimation of the relative contribution of the two groundwater parcels. During dry periods, shallow groundwater was the primary source of NO3- to the estuary while deep groundwater was predicted to be more significant during wet periods when local shallow lateral subsurface flow was inhibited. This groundwater-derived NO3- was primarily removed in the surface water via phytoplankton assimilation rather than by benthic denitrification. While denitrification in groundwater was evident from measurements of dissolved gasses (excess N2) in addition to the δ15N-NO3- and δ18O-NO3- measurements, it was not the dominant process controlling isotopic composition or the concentrations of residual NO3- in both the silt-clay (shallow) and sandy-gravel (deep) aquifers. In fact, the characteristics of the remaining NO3- were heavily modulated by conservative mixing between groundwater parcels from different locations. Using a two end member mixing model, NO3- in shallow groundwater originated entirely from agricultural sources (mixture of fertiliser and recycled water) while deep groundwater NO3- constituted ~ 69 % sewage and ~31 % fertiliser. This research demonstrates that the dual isotopic composition of NO3- (δ15N-NO3- and δ18O-NO3-) can be an effective determinant of the role of groundwater-surface water interactions in the biogeochemistry of NO3- in an estuarine environment. This study highlights that: (1) Submarine groundwater discharge (SGD) should be considered as a potentially substantial component of the nitrogen budget of estuaries near contaminated aquifers, and (2) SGD should be well constrained as one of the most important NO3- end members when δ15N-NO3- and δ18O-NO3- values are used to evaluate sources of NO3- to estuaries.