Impact of groundwater-surface water interactions on groundwater ecosystems

Groundwater is a large and vital component of the hydrological cycle and sustains societies and ecosystems globally. The microbes and invertebrates that live in groundwater ecosystems provide important services that contribute to the maintenance of groundwater quality and quantity, making it fit for human uses. Human disturbances threaten the integrity of groundwater ecosystems, their ability to undertake critical ecosystem functions, and ultimately compromise future groundwater availability. Threats to groundwater ecosystems include abstraction, contamination, agricultural activities and alterations of groundwater-surface water dynamics through river regulation. Such threats are not routinely addressed in water management policies, in which the state of groundwater ecosystems is typically neglected. This is partly a consequence of the currently limited knowledge of how groundwater ecosystems respond to such changes. The aim of this thesis was to understand how human activities that influence groundwater hydrology influence water quality and biota of shallow alluvial aquifers of the Murray-Darling Basin. This thesis addresses gaps in current knowledge of how the management of surface and groundwater resources alters the characteristics of the groundwater ecosystems, focusing on the alluvial aquifers in the Macquarie and Namoi catchments (New South Wales, Australia). Using three case studies, this thesis investigates the impacts of dam releases (Chapter 2), groundwater abstraction (Chapter 3) and agricultural practices (Chapter 4) on hydrological changes and the consequential impacts on diversity and distribution of both prokaryotic and eukaryotic organisms that inhabit the aquifers. This was achieved by combining isotopic and chemical analysis of river and groundwater with biotic sampling, using environmental DNA, flow cytometry and traditional “count and collect” methods of invertebrates to characterise the ecosystems. All the human activities investigated influenced the hydrology and water quality of the aquifers, with flow on effects to the biology, however, the relative magnitude and scale of the impacts observed differed between studies. The relatively long-term studies of dam release and agricultural effects showed a lagged response of biota relative to the hydrological changes. Under prevailing drought conditions, relatively high releases of water from Burrendong Dam (Chapter 2) affected groundwater levels, but there was little evidence of surface water entering the aquifer, except in wells closest to the river. The biota varied between the unconfined and semi-confined layers of the aquifer but varied little in response to changes in river flow, reflecting the limited influx of river water to the aquifer. Groundwater abstraction (Chapter 3) can alter flow direction and dynamics within an aquifer and as groundwater flow and water levels change. In a short-term Before-After-Control-Impact study, groundwater pumping for 48 h caused localised drawdown in the shallow alluvial aquifer. Pumping caused a shift in eukaryotic and prokaryotic communities between before and after pumping, but spatial heterogeneity of assemblages within the aquifer were typically greater than the changes due to pumping. Irrigation of broad-acre crops caused seasonal changes in groundwater quality in wells adjacent to the irrigated field (Chapter 4). The influx of irrigation water to the aquifer was also evident in the groundwater biota, with increased abundance of common surface water taxa in the aquifer during irrigation periods. The influx was also evident by the detection of DNA of the crops. This study has furthered current knowledge of how human activities and water management practices impact groundwater ecosystems. This new knowledge will inform the improvements to the sustainable management of shallow aquifers and, highlights the complexity of issues surrounding ground and surface water usage. In doing so, the necessity for a holistic water management regime, which incorporates hydrological and biological monitoring of both surface and groundwaters, is highlighted.