Professor Isaac R Santos

Intermittently Closed and Open Lakes and Lagoons (ICOLLs) are brackish coastal water bodies with an intermittent connection to the ocean that is closed periodically due to the accumulation of marine sediment forming an entrance berm. ICOLLs have dynamic coastal systems that may be vulnerable to minor changes in catchment hydrology. However, little is known regarding the impacts of groundwater on the hydrological cycles of ICOLLs. The relative contribution of rainfall versus groundwater discharge in two ICOLLs (Welsby, and Mermaid Lagoon) and a nearby wetland (South Welsby Lagoon) located on Bribie Island (Australia) were investigated using radon ( (super 222) Rn) as natural geochemical groundwater tracer. Four seasonal surveys were undertaken to quantify the temporal and spatial groundwater dynamics of the ICOLLs. Radon contour maps revealed temporal and spatial changes over the study period. The estimated groundwater discharge rates from a radon-mass balance were 3.4+ or -3.1, 7.3+ or -9.8 and 2.6+ or -3.8 cm d (super -1) in Weslby, South Weslby and Mermaid Lagoons, respectively. These values are at least 8-fold greater than rainfall (1420 mm per year, or 0.4 cm d (super -1) ). Assuming very minor surface water flows (not perceived during field surveys), this demonstrates that these systems are groundwater-dominated and their hydrology can be influenced by regional changes in groundwater level.

Atmospheric radon ( (super 222) Rn), carbon dioxide (CO (sub 2) ), and methane concentrations (CH (sub 4) ) as well as carbon stable isotope ratios (delta (super 13) C) were used to gain insight into atmospheric chemistry within an Australian coal seam gas (CSG) field (Surat Basin, Tara region, Queensland). A approximately 3 fold increase in maximum (super 222) Rn concentration was observed inside the gas field compared to outside of it. There was a significant relationship between maximum and average (super 222) Rn concentrations and the number of gas wells within a 2 km to 4 km radius of the sampling sites (n = 5 stations; p < 0.05). We hypothesize that the radon relationship was a response to enhanced emissions within the gas field related to point sources (well heads, pipelines, etc.) and diffuse soil sources due to changes in the soil structural and hydrological characteristics. A rapid qualitative assessment of CH (sub 4) and CO (sub 2) concentration, and carbon isotopes using a mobile cavity ring-down spectrometer system showed a widespread enrichment of both CH (sub 4) and CO (sub 2) within the production gas field. Concentrations of CH (sub 4) and CO (sub 2) were as high as 6.89 ppm and 541 ppm respectively compared average concentrations of 1.78 ppm (CH (sub 4) ) and 388 ppm (CO (sub 2) ) outside the gas field. The delta (super 13) C values showed distinct differences between areas inside and outside the production field with the delta (super 13) C value of the CH (sub 4) source within the field matching that of the methane in the CSG.

The carbon chemistry of coral reef lagoons can be highly variable over short time scales. While much of the diel variability in seawater carbon chemistry is explained by biological processes, external sources such as river and groundwater seepage may deliver large amounts of organic and inorganic carbon to coral reefs and represent a poorly understood feedback to ocean acidification. Here, we assess the impact of submarine groundwater discharge (SGD) on pCO2 variability in two coral reef lagoons with distinct SGD driving mechanisms. Diel variability of pCO2 in the two ecosystems was explained by a combination of biological drivers and SGD inputs. In Rarotonga, a South Pacific volcanic island, SGD was driven primarily by a steep terrestrial hydraulic gradient, and the lagoon was influenced by the high pCO2 (5,501 mu atm) of the fresh groundwater. In Heron Island, a Great Barrier Reef coral cay, SGD was dominated by seawater recirculation through sediments (i.e. tidal pumping) and pCO2 was mainly impacted through the stimulation of biological processes. The Rarotonga water column had a relatively higher average pCO2 (549 mu atm) than Heron Island (471 mu atm). However, pCO2 exhibited a greater diel range in Heron Island (778 mu atm) than in Rarotonga (507 mu atm). The Rarotonga lagoon received 31.2 mmol CO2 m-2 d-1 from SGD, while the Heron Island lagoon received 12.3 mmol CO2 m-2 d-1. Over the course of this study both systems were sources of CO2 to the atmosphere (3.00 to 9.67 mmol CO2 m-2 d-1), with SGD-derived CO2 contributing a large portion to the air-sea CO2 flux. The relationship between both water column pH and aragonite saturation state (Omega Ar) and radon (222Rn) concentrations indicate that SGD may enhance the local acidification of some coral reef lagoons. Studies measuring the carbon chemistry of coral reefs (e.g. community metabolism, calcification rates) may need to consider SGD-derived CO2.

Surface water (SW) and groundwater (GW) interact across multiple spatial and temporal scales and their interaction is important for ecological and biogeochemical functions. The mixing of GW and SW has been challenging to simultaneously map with sufficient detail and coverage. Fortunately, ambient differences in salinity of waters occupying geologic formations and sediment are an ideal target for electrical resistivity imaging (ERI). We present examples of the application of ERI for mapping GW discharge and for understanding GW-SW interactions at: (1) a large regulated river, (2) neighboring lakes with differing salinity, (3) fringing coral reefs and lagoons, (4) beaches, and (5) estuaries. In all these cases, the ER tomograms were critical for improving conceptual understanding of GW-SW interactions. At the Lower Colorado River in Austin, Texas (USA), time-lapse ERI was conducted across a 12-hour dam-release cycle when the river stage varied by 0.7 m. Using temporal variability in electrical resistivity (ER) signatures, we identified a shallow well-flushed hyporheic zone, a transition zone where SW and GW mix, and a stable deep zone hosting only GW. In alkaline lakes in the Nebraska Sand Hills (Nebraska, USA), ER surveys using boat-towed cables allowed for mapping the 3D electrical structure underneath the lake. The tomograms were used to distinguish flow-through lakes, which have decreasing subsurface ER from GW inflow to outflow area, from pure GW discharge lakes, which have uniformly stratified increasing-with-depth ER profiles. Moreover, GW plumes in both discharge and recharge zones were clearly outlined underneath the lake. More than 30 km of ER profiles collected via boat-towed surveys over a fringing coral reef in the Philippines identified areas of high ER within the reef that coincide with resistive zones in the seawater. Analysis of (super 222) Rn of bottom waters and vertical conductivity-temperature-depth measurements show the persistence of fresh GW input into the ocean where low salinity and high (super 222) Rn areas coincided with high ER areas. In Muri Lagoon in Rarotonga Island of the Cook Islands, boat-towed ER surveys similarly showed areas underneath the lagoon that have groundwater that is fresher than seawater. Likewise, there was (super 222) Rn high concentrations throughout the lagoon. Closer to shore, ER surveys using fixed electrodes showed complex 3D mixing processes between seawater and terrestrial-sourced fresh groundwater in beach sediment. Lastly, both boat-towed and fixed surveys across the salt wedge of Werribee Estuary west of Melbourne, Australia, outlined the estuarine salt wedge and its relationship with and effect on fresh and nutrient-laden groundwater discharging to the estuary. The examples discussed illustrate that ERI is a powerful and convenient tool for mapping GW discharge and GW-SW interactions across different scales and diverse environments.