Tracing anthropogenically-driven groundwater discharge into a coastal lagoon from southern Brazil

摘要: Summary We investigated the distribution of naturally occurring geochemical tracers ( 222 Rn, 223 Ra, 224 226 CH 4 , δ 18 O, and 2 H) in water column adjacent groundwater Mangueira Lagoon as proxies discharge. is a large (90 km long), shallow (∼4–5 m deep), fresh, non-tidal coastal lagoon southern Brazil surrounded by extensively irrigated rice plantations numerous irrigation canals. hypothesized that annual, intense for agriculture creates extreme conditions seasonally change discharge patterns lagoon. further supposed dredging canals alters fluxes. While activities Rn were 2–3 orders magnitude higher than surface water, radium isotopes only ∼1 order higher. Therefore, appears to be preferred tracer this system. Radon concentrations conductivities dramatically near pump house canals, consistent with source. Modeling radon inventories accounting total inputs (groundwater advection, diffusion from sediments, decay Ra) losses (atmospheric evasion, horizontal mixing decay) indicated advection rates (∼25 cm/d) are over along shoreline (∼0.1 cm/d). Nearly 75% area found half lagoon, where seem also methane stable isotope trends. In spite relatively small we estimate they contribute nearly 70% (∼57,000 m 3 /d) input into entire Lagoon. suggest these cut through aquitards which previously restricted upward underlying permeable strata. The channels may therefore represent an important but overlooked source nutrients other dissolved chemicals derived agricultural practices