The contribution of groundwater discharge to nutrient exports from a coastal catchment: post flood seepage increases estuarine N/P ratios

Four months of daily nutrient and radon (a natural groundwater tracer) observations at the outlet of a heavily drained coastal wetland illustrated how episodic floods and diffuse groundwater seepage influence the biogeochemistry of a sub-tropical estuary (Richmond River, New South Wales, Australia). Our observations downstream of the Tuckean Swamp (an acid sulphate soil floodplain) covered a dry stage, a flood triggered by a 213-mm rain event and a post-flood stage when surface water chemistry was dominated by groundwater discharge. Significant correlations were found between radon and ammonium and N/P ratios and between radon and dissolved organic nitrogen (DON) during the post-flood stage. While the flood lasted for 14 % of the time of the surface water time series, it accounted for 18 % of NH4, 32 % of NO x , 66 % of DON, 58 % of PO4 and 55 % of dissolved organic phosphorus (DOP) catchment exports. Over the 4-month study period, groundwater fluxes of 35.0, 3.6, 36.3, 0.5 and 0.7 mmol m−2 day−1 for NH4, NO x , DON, PO4 and DOP, respectively, were estimated. The groundwater contribution to the total surface water catchment exports was nearly 100 % for ammonium, and% for the other nutrients. Post-flood groundwater seepage shifted the system from a DON to a dissolved inorganic N-dominated system and doubled N/P ratios in surface waters. We hypothesise that the Richmond River Estuary N/P ratios may reflect a widespread trend of tidal rivers and estuaries becoming more groundwater-dominated and phosphorus-limited as coastal wetlands are drained for agriculture, grazing and development

Coupling automated radon and carbon dioxide measurements in coastal waters

Groundwater discharge could be a major, but as yet poorly constrained, source of carbon dioxide to lakes, wetlands, rivers, estuaries, and coastal waters. We demonstrate how coupled radon (222Rn, a natural groundwater tracer) and pCO2 measurements in water can be easily performed using commercially available gas analysers. Portable, automated radon and pCO2 gas analysers were connected in series and a closed air loop was established with gas equilibration devices (GED). We experimentally assessed the advantages and disadvantages of six GED. Response times shorter than 30 min for 222Rn and 5 min for pCO2 were achieved. Field trials revealed significant positive correlations between 222Rn and pCO2 in estuarine waterways and in a mangrove tidal creek, implying that submarine groundwater discharge was a source of CO2 to surface water. The described system can provide high resolution, high precision concentrations of both radon and pCO2 with nearly no additional effort compared to measuring only one of these gases. Coupling automated 222Rn and pCO2 measurements can provide new insights into how groundwater seepage contributes to aquatic carbon budgets