Nitrate Retention and Removal in Sub-Tropical Riparian Zones Author Newham, Michael John Published 2011 Thesis Type Thesis (PhD Doctorate) School Griffith School of Environment DOI https://doi.org/10.25904/1912/2383 Copyright Statement The author owns the copyright in this thesis, unless stated otherwise. Downloaded from http://hdl.handle.net/10072/366141 Griffith Research Online https://research-repository.griffith.edu.au Nitrate retention and removal in sub-tropical riparian zones Michael John Newham Bachelor of Environmental Management (Hons) Australian Rivers Institute Griffith School of Environment Science, Environment, Engineering and Technology Griffith University Submitted in fulfilment of the requirements of the degree of Doctor of Philosophy July 2011 Abstract Worldwide, contamination of streams and groundwater with excess nitrate has been linked to agricultural land use and particularly to the application of nitrogen fertilisers to increase agricultural production. Nitrate is an effective contaminant in agricultural areas; it is highly mobile, having a low affinity for soil sorption, and so moves with runoff and sub-surface flows. Excess nitrate can cause ecological impacts on waterways and coastal receiving water through eutrophication and, in some cases, contributes to coastal ‘dead zones’. Nitrate also has toxicological effects on aquatic organisms and those using contaminated water as a drinking source. Riparian zones, those zones where interaction of aquatic and terrestrial environments occurs, are identified as areas of intense biogeochemical cycling and can act as buffers against excess nitrate by reducing the amount of nitrate reaching stream channels. Nitrate retention processes of biotic uptake and transformation to less mobile forms can increase the residence time of nitrate within the riparian zone, while removal processes of denitrification can permanently remove nitrate-nitrogen in gaseous forms. Nitrate removal in riparian zones is most effective where nitrate-laden sub-surface flow interacts with shallow soils that are rich in organic carbon. At places in the riparian zone where these conditions occur, almost complete nitrate removal from inflow can be realised before it reaches the stream channel. This classical view of riparian nitrate removal has largely developed from studies conducted in temperate climates where riparian zones often appear effective in reducing nitrate contamination of waterways. However, the efficacy of riparian zones for reducing nitrate contamination of streams in sub-tropical environments has largely not been assessed, and the conceptual views of riparian function developed in temperate regions may not be applicable to the sub- tropical climate due to the vastly different hydrology of sub-tropical regions and the associated differences in channel morphology and groundwater interactions. In this thesis I assess and expand upon a proposed conceptual model of riparian nitrate removal in sub-tropical catchments which was developed based on observations from sub-tropical southeast Queensland, Australia. The conceptual model highlights channel morphology as a determining factor in the capacity for nitrate removal. In sub-tropical catchments, stream channels may be deep, with steep banks; a morphology which is i derived from the seasonal and intense nature of rainfall and associated large ‘flashy’ hydrographs. The conceptual model suggests that channel morphology may restrict the interaction of shallow soils, which have high denitrification potential, with nitrate-laden water, which may discharge to streams from deeper, carbon poor soil layers, where nitrate removal processes are minimal. Denitrification potential of shallow soils will only be activated when high flows results in inundation of these soil layers. To assess the conceptual model, the sub-tropical catchments of the Maroochy and Mooloolah Rivers, southeast Queensland, Australia, were chosen for investigation. Streamwater in these catchment is naturally very low in nitrate, yet monitoring of water quality shows that excess nitrate occurs in these rivers, particularly following rain events. At twelve study reaches across the catchments I measured metrics of channel morphology and distribution of total nitrogen, carbon, nitrate and ammonium in riparian soils. Across the catchments, with few exceptions, there was limited potential for interaction between shallow soils and sub-surface or streamwater. I also found that carbon, nitrate and ammonium all decreased drastically with depth in soil, also indicating a likely decrease in denitrification potentials with depth. However, in deep soils saturated with sub-surface water, there was an increased content of ammonium, showing a large retention of nitrogen as ammonium in these saturated, anoxic environments. These results highlight the importance of the processes that may occur at depth in soils where much more interaction with water occurs. Experimental anoxic assays of soil from one riparian site in the Maroochy Catchment measured nitrate reduction and denitrification rates of soils, with a focus on the deep, saturated layer. The assays revealed very low rates of denitrification occur in deep soils. The assays also assessed the potential for inorganic substances, ferrous iron and sulphide, to act as electron donors for nitrate reduction in the low-carbon, saturated soils. The assays showed that addition of ferrous iron to deep, saturated soils slightly, though significantly, raised denitrification rates, with an associated increase in nitrate consumption. However, denitrification rate in shallow soil was unaffected by addition of ferrous iron. Sulphide addition raised denitrification rates in shallow soils, though did not affect rates in deep soils. These results show that inorganic electron donors, particularly ferrous iron, may contribute to nitrate reduction through autotrophic denitrification in deep, saturated soils of the catchment. Saturated soils occur across the ii width of the alluvial floodplains of the catchment, as floodplain alluvial aquifers, which store water that is discharged to streams as baseflow. A low-rate process of nitrate reduction occurring in the alluvial aquifers may be controlling the low concentration of streamwater nitrate observed during times of baseflow. During rainfall events, when most of the observed nitrate loads may be transported to stream systems, I found that nitrate removal processes in the riparian zone may have negligible affect on the overall export of nitrate. This was found through the measurement of denitrification potential of riparian soils at a site, and detailed surveying of the stream channel and riparian zone. I then calculated the total amount of nitrate that may be removed in the riparian soils during a simulated high flow event, finding that less than 0.001% of the nitrate transported during the event was removed through riparian denitrification. From this simulation, I suggest that for nitrate management strategies to be effective, they need to limit the transport of nitrate from its source areas, before it can reach the riparian zone or stream channel. The results from this thesis lead to the amendment of the conceptual model for nitrate removal in sub-tropical riparian zones. The new conceptual model suggests that management and nitrate removal strategies should be considered at a larger scale in sub- tropical areas. Riparian zones represent only a small area of a catchment, and in the intense rainfall events of this sub-tropical climate, the nitrate removal processes are overwhelmed by the volume and velocity of water passing through them. Therefore, nitrate management needs to be viewed on a larger scale, and I suggest that considering nitrate control at the scale of the alluvial floodplains may be more effective. Encouraging infiltration of runoff, and therefore also its nitrate load into the alluvial floodplain, closer to its source areas, may enhance nitrate load reduction. Infiltration can be improved through the use of detention basins, swales or wetlands. The increased infiltration from these structures improves engagement of low-rate nitrate reduction processes in the alluvial aquifer, and reduces the flow of water overland through riparian zones. Specific areas of investigation to further improve the conceptual model are also proposed. iii Statement of Originality This work has not previously been submitted for a degree or diploma in any university. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made in the thesis itself. ________________________ Michael Newham July 2011 iv Table of Contents Abstract....................................................................................................................................................i Statement of Originality........................................................................................................................iv Table of Contents....................................................................................................................................v List of Tables........................................................................................................................................viii List of Figures ........................................................................................................................................xi Acknowledgements..............................................................................................................................xvi