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

Abstract

We investigated the distribution of naturally occurring geochemical tracers (222Rn, 223Ra, 224Ra, 226Ra, CH4, d18O, and d2H) 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. While the activities of 222Rn in shallow groundwater were 2–3 orders of magnitude higher than in surface water, CH4 and radium isotopes were only _1 order of magnitude higher. Therefore, 222Rn 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 226Ra) and losses (atmospheric evasion, horizontal mixing and decay) indicate 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 m3/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.