Tracing anthropogenically-driven groundwater discharge into a coastal lagoon from Brazil Isaac R. Santos; William Burnett; Richard Peterson; Jeffrey Chanton Department of Oceanography, Florida State University, Tallahassee, FL, USA Felipe Niencheski; Carlos F.F. Andrade; Idel B. Milani Department of Chemistry, Fundaçao Universidade do Rio Grande, Rio Grande, Brazil Axel Schmidt; Kay Knoeller Helmholtz Centre for Environmental Research, Leipzig, Germany

(1)Density driven circulation (2)Tidal pumping (3)Wave setup (4)Fresh groundwater discharge Robinson et al., 2007 Submarine groundwater discharge driving forces

Hypotheses 1 - The annual irrigation creates extreme conditions that seasonally change groundwater discharge into Mangueira Lagoon 2 - Dredging of irrigation canals alters the groundwater fluxes

Patos Mangueira Mirim Atlantic Ocean Brazil

222 Rn survey results – Aug/2006

Groundwater and Lagoon water tracer concentrations

222 Rn time series – Vitor Barbosa Canal Winter – Aug/2006 Summer – Jan/2007 Time (h) Pumps on

Modeling 222 Rn-derived groundwater discharge Approach 1: Steady state J ben I I J atm - J hor = 0 Inputs - Outputs = 0 Approach 2: Non-steady state (irrigation canals disturbed by pumping) dI 222 /dt = 222 I J ben I J atm - J hor Inventory rate of change = Inputs - Outputs

Offshore 222 Rn transects

Determining mixing losses m: slope of the log-linear curve K h : mixing coefficient (m 2 /d) : decay constant Determine mixing coefficients from 222 Rn and conductivity transects Neglecting advection and Production: Mixing coefficients K h in th order of 10 3 m 2 /day Use iterative approach to adjust 222 Rn concentrations along the transect

Estimated advection rates Mixing contributes an average of ~6% (maximum of 12%) of the total 222 Rn losses Advection rates in the irrigation canals from the south are much higher Extrapolated water fluxes (~50000 m 3 /day) represent only 2% of the direct rainfall

Major ions in Mangueira Lagoon and Groundwater GW

Groundwater-derived ion fluxes F Cl SO 4 HCO 3 Na K Ca Mg

Conclusions 1) No major seasonal changes in groundwater advection rates driven by irrigation 2) Advection rates in the canals were ~2 orders of magnitude higher than along the lagoon shoreline contribution ~80% of total groundwater inputs. 3) In spite of the small volume contribution (<2% of total water inputs), groundwater should not be neglected in dissolved species budgets 4) Anthropogenic changes in the water cycle may have a major effect on groundwater discharge in some coastal environments.