Journal article
Tracing anthropogenically-driven groundwater discharge into a coastal lagoon from southern Brazil
Journal of Hydrology, Vol.353(3-4), pp.275-293
2008
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Abstract
<p><p id="x-x-x-">We investigated the distribution of naturally occurring geochemical tracers (<sup>222</sup>Rn, <sup>223</sup>Ra, <sup>224</sup>Ra, <sup>226</sup>Ra, CH<sub>4</sub>, <em>δ</em><sup>18</sup>O, and <em>δ</em><sup>2</sup>H) in the water column and adjacent groundwater of Mangueira Lagoon as proxies of groundwater discharge. Mangueira Lagoon is a large (90 km long), shallow (∼4–5 m deep), fresh, and non-tidal coastal lagoon in southern Brazil surrounded by extensively irrigated rice plantations and numerous irrigation canals. We hypothesized that the annual, intense irrigation for rice agriculture creates extreme conditions that seasonally change groundwater discharge patterns in the adjacent lagoon. We further supposed that dredging of irrigation canals alters groundwater fluxes. <p id="x-x-x-">While the activities of <sup>222</sup>Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH<sub>4</sub> and radium isotopes were only ∼1 order of magnitude higher. Therefore, <sup>222</sup>Rn appears to be the preferred groundwater tracer in this system. Radon concentrations and conductivities were dramatically higher near the pump house of rice irrigation canals, consistent with a groundwater source. Modeling of radon inventories accounting for total inputs (groundwater advection, diffusion from sediments, and decay of <sup>226</sup>Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicated that groundwater advection rates in the irrigation canals (∼25 cm/d) are over 2 orders of magnitude higher than along the shoreline (∼0.1 cm/d). Nearly 75% of the total area of the canals is found in the southern half of the lagoon, where groundwater inputs seem to be higher as also indicated by methane and stable isotope trends. In spite of the relatively small area of the canals, we estimate that they contribute nearly 70% of the total (∼57,000 m<sup>3</sup>/d) groundwater input into the entire Mangueira Lagoon. We suggest that the dredging of these canals cut through aquitards which previously restricted upward advection from the underlying permeable strata. The irrigation channels may therefore represent an important but previously overlooked source of nutrients and other dissolved chemicals derived from agricultural practices into the lagoon.</p>
Details
- Title
- Tracing anthropogenically-driven groundwater discharge into a coastal lagoon from southern Brazil
- Creators
- Isaac R Santos - Florida State UniversityFelipe Niencheski - Fundação Universidade do Rio GrandeWilliam Burnett - Florida State UniversityRichard Peterson - Florida State UniversityJeffrey Chanton - Florida State UniversityCarlos FF Andrade - Fundação Universidade do Rio GrandeIdel B Milani - Fundação Universidade do Rio GrandeAxel Schmidt - Helmholtz Centre for Environmental ResearchKay Knoeller - Helmholtz Centre for Environmental Research
- Publication Details
- Journal of Hydrology, Vol.353(3-4), pp.275-293
- Identifiers
- 2752; 991012820401102368
- Academic Unit
- Faculty of Science and Engineering; Centre for Coastal Biogeochemistry; School of Environment, Science and Engineering
- Resource Type
- Journal article