Ian Cartwright

Ian's research involves the application of geochemical tracers to understand processes in groundwater and surface water systems. This includes: use of radioactive tracers such as C-14 and tritium to examine the pathways and timescales of water flow in catchments; using geochemistry to understand processes such as water-rock interaction and weathering; using Radon and major ions to determine location and fluxes of groundwater inflows to rivers; and using carbon isotopes to understand the origins and fate of riverine carbon. The work is multi-disciplinary and involves fieldwork, lab geochemistry, and numerical analysis.

Where does the water in rivers come from and how long does it take to get there?

This might seem like an obvious question, but many details of how river catchments function are poorly known. Streams are fed by several catchment stores, including recent rainfall, shallow young riparian groundwater, deeper older groundwater, water stored for shorter periods in the riverbanks and floodplains, and water from the soils and regolith. Determining the relative contributions from these stores is critical to understanding catchment functioning and to protecting rivers and their ecosystems. The time taken for waters to transit through the catchments to the rivers may range from years to centuries. Understanding the timescales of water flow is critical to predicting the resilience of rivers to environmental or climate change and for assessing contaminant transport.

We are investigating these questions using a range of environmental isotopes and major ions together with streamflow data and groundwater elevations. These data allow us to understand the fluxes of groundwater to rivers, the origin and residence time of river waters, and whether different water stores are activated under different flow conditions. The projects emphasise integration of geochemical and physical data to achieve an understanding of key processes that affect the water balance and water quality in rivers. Field areas are mainly in Victoria (southeast Australia) and include a variety of river types (ephemeral / perennial, headwater catchments, regulated / unregulated).

River geochemistry as a record of catchment processes

Rivers provide an understanding of chemical processes at the Earth’s surface. The major ion geochemistry together with the rare earth elements and isotopes of Sr, Ca, and Mg may be used to understand chemical weathering in river catchments. The concentration, speciation, and isotopic ratios of carbon in rivers constrain the origin, evolution, and fate of this critical element. Many studies have viewed rivers as largely draining surface water; however, much of the water in rivers has been retained for decades. These long timescales allow bio- and geochemical reactions to occur and a full understanding of river chemistry requires an understanding of these timescales. We are integrating river chemistry and catchment residence time estimates to better understand chemical processing in catchments.