Journal article
Divergent drivers of carbon dioxide and methane dynamics in an agricultural coastal floodplain: post-flood hydrological and biological drivers
Chemical Geology, Vol.440, pp.313-325
2016
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Abstract
<p>Many coastal floodplains have been artificially drained for agriculture, altering hydrological connectivity and the delivery of groundwater-derived solutes including carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) to surface waters. Here, we investigated the drivers of CO<sub>2</sub> and CH<sub>4</sub> within the artificial drains of a coastal floodplain under sugarcane plantation and quantify the contribution of groundwater discharge to CO<sub>2</sub> and CH<sub>4</sub>dynamics over a flood event (290 mm of rainfall). High temporal resolution, in situ observations of dissolved CO<sub>2</sub> and CH<sub>4</sub>, carbon stable isotopes of CH<sub>4</sub> (δ<sup>13</sup>C-CH<sub>4</sub>), and the natural groundwater tracer radon (<sup>222</sup>Rn) allowed us to quantify CO<sub>2</sub>, CH<sub>4</sub> and groundwater dynamics during the rapid recession of a flood over a five day period. Extreme super-saturation of free CO<sub>2</sub> ([CO<sub>2</sub>*]) up to 2,951 μM (25,480% of atmospheric equilibrium) was driven by large groundwater input into the drains (maximum 87 cm day<sup>− 1</sup>), caused by a steep hydraulic head in the adjacent water table. Groundwater input sustained between 95 and 124% of the surface [CO<sub>2</sub>*] flux during the flood recession by delivering high carbonate alkalinity groundwater (DIC = 10,533 μM, ~ pH = 7.05) to acidic surface water (pH < 4), consequently transforming all groundwater-derived DIC to [CO<sub>2</sub>*]. In contrast, groundwater was not a major direct driver of CH<sub>4</sub> contributing only 14% of total CH<sub>4</sub> fluxes. A progressive increase in CH<sub>4</sub> concentrations of up to ~ 2400 nM day<sup>− 1</sup> occurred as a combination of increased substrate availability delivered by post-flood drainage water and longer residence times, which allowed for a biogenic CH<sub>4</sub> signal to develop. The progressive enrichment in δ<sup>13</sup>C-CH<sub>4</sub> values (− 70‰ to − 48‰) and increase in CH<sub>4</sub> concentrations (46–2460 nM) support coupled production-oxidation, with concentrations and δ<sup>13</sup>C values remaining higher (2,798 nM and -47‰) than pre-flood conditions (534 nM and -55‰) three weeks after the flood. Our findings demonstrate how separate processes can drive the aquatic CO<sub>2</sub> and CH<sub>4</sub> response to a flood event in a drained coastal floodplain, and the key role groundwater had in post-flood [CO<sub>2</sub>*] evasion to the atmosphere, but not CH<sub>4</sub>.</p>
Details
- Title
- Divergent drivers of carbon dioxide and methane dynamics in an agricultural coastal floodplain: post-flood hydrological and biological drivers
- Creators
- Jackie R Webb - Southern Cross UniversityIsaac R Santos - Southern Cross UniversityDouglas R Tait - Southern Cross UniversityJames Z Sippo - Southern Cross UniversityBen CT Macdonald - CSIRO Agriculture, CanberraBarbara Robson - CSIRO Land and Water, CanberraDamien T Maher - Southern Cross University
- Publication Details
- Chemical Geology, Vol.440, pp.313-325
- Identifiers
- 3994; 991012821890602368
- Academic Unit
- National Marine Science Centre; School of Environment, Science and Engineering; Science; Faculty of Science and Engineering
- Resource Type
- Journal article