Groundwater-surface water interactions : implications for nutrient transport to tropical rivers

Dixon-Jain, Prachi

Groundwater-surface water interactions : implications for nutrient transport to tropical rivers

Date

2008

Authors

Dixon-Jain, Prachi

Abstract

The interaction between groundwater and surface water systems is a key component of the hydrological cycle and an understanding of their connectivity is fundamental for sustainable water resource management. Water is a vehicle for mobilising dissolved constituents, including nutrients, between surface and subsurface waters and between terrestrial and marine systems. Therefore, knowledge of surface-subsurface linkages is critical not only for water quantity allocation, but also for water quality and its implications for ecosystem health. In particular, ascertaining the significance of groundwater fluxes for river nitrogen budgets is an important motivation for characterising river-groundwater connectivity. This overarching theme is developed through the course of the thesis. The marked seasonality of tropical river systems provides a unique opportunity to investigate groundwater contributions to surface waters, especially when there are minimal overland flows. The Herbert River in northeast Queensland represents a useful case study in the Australian tropics for assessing the potential for transport of agricultural contaminants, such as dissolved forms of nitrogen, between surface and subsurface waters, and between terrestrial and marine systems, including the ecologically significant Great Barrier Reef World Heritage Area. Whilst the lower Herbert River catchment, dominated by sugarcane production, is the focus for this thesis, the research methodology and policy implications for nutrient monitoring and management are applicable to other tropical catchments. An extensive water quality sampling program was instigated to collect river and groundwater samples during low flow conditions, for analysis of a range of conservative and nonconservative environmental tracers including major ions, stable isotopes of water, radon, and dissolved inorganic forms of nitrogen. Grab samples were collected during months representing the beginning and end of the dry season to compare connectivity relationships at contrasting stages of the stream hydrograph. Hydrochemical data at the end of the dry season is particularly useful for isolating the groundwater signal in the river and its tributaries. Existing physical and chemical datasets are also an important source of high temporal resolution information to supplement the more detailed water quality data collected specifically for this investigation. An understanding of the dynamics of water movement between river and aquifer storages is critical for assessing the mobility of dissolved nitrogen between them. A combination of hydrogeological, hydrometric, hydrological and hydrochemical tools are applied to characterise the interaction between the alluvial aquifers and the lower Herbert River at a catchment scale. Specifically, the potential for hydraulic connection and the direction of flux between the aquifer system and the river are evaluated through qualitative hydrometric approaches, including: depth relationships of the river channel with that of the underlying alluvial sediments; historical groundwater elevation-stream stage relationships; and groundwater flow patterns around the river. Hydrological techniques such as stream hydrograph and flow duration curve analysis are utilised to assess the temporal characteristics of flow in the river; the groundwater flux to the river is also quantified by hydrograph separation. Physical understanding of river-aquifer linkages is verified and enriched through analysis of surface water chemistry data, in conjunction with the conceptual hydrogeological model developed from physical and chemical assessment of the aquifers. The significance of groundwater as a vector for nitrogen is then evaluated in light of a conceptual process understanding of the river-aquifer system. This provides a platform for undertaking future catchment-scale nutrient budget studies based on detailed investigations of nitrogen sources and transformations. The research approach used in this thesis highlights the value of combining analytical techniques, not provided by any one method, to inform and verify different aspects of a complex water resource problem involving both surface and groundwater systems. The application of multiple environmental tracers, at varied spatial and temporal resolution, is particularly instructive for distinguishing between the key processes that influence the chemistry of the river in space and time. Furthermore, the spectrum of tracer techniques provides both qualitative and quantitative information regarding the flux of groundwater along the length of the lower Herbert River. Whilst the absolute groundwater fluxes determined have a degree of uncertainty, mass balances of radon and selected solutes highlight the value of quantitative estimates in combination with qualitative trends to characterise river-aquifer relationships. The analyses demonstrate that discharge of groundwater from the alluvial aquifers is a dominant influence on both the flow and chemistry of the lower Herbert River in the dry season. In particular, groundwater is a key vector for the delivery of nitrate to the river during low flow conditions. This provides a new perspective for monitoring and management of nutrients in tropical rivers where there is good connectivity with the underlying groundwater system. Key recommendations arising from this research include: (1) water quality sampling should be undertaken at recognised periods on the stream/groundwater hydrograph, with an understanding of temporal and spatial river-aquifer connectivity relationships; (2) surface and subsurface sources of water and dissolved nutrients must be considered, including identification of nutrient hotpots in both surface water and groundwater systems; (3) sampling locations should capture the longitudinal variation in river nutrient concentrations, not simply end-of-river monitoring; (4) appropriate water quality guideline values must be set to account for seasonal changes in both the sources and forms of nutrients transported to surface waters.

Keywords

groundwater, surface water, groundwater-surface water interactions, hydrogeology, hydrology, hydrochemistry, hydrometric approaches, Herbert River catchment, tropical rivers, Great Barrier Reef, terrestrial and marine systems, world heritage area, nitrogen, nutrient transport, nutrient monitoring and management, water quality sampling, radon, conservative and non-conservative environmental tracers, sugarcane production, mass balance, agricultural contaminants, low flow, seasonal variability