Assessment of submarine groundwater discharge and seasonal oscillations in seawater sup(222)Rn inventores of Ubatuba, Brazil

Abstract

In order to assess submarine groundwater discharge (SGD) and evaluate seasonal oscillations of such fluxes, measurements of the naturally occurring 222Rn in Ubatuba embayments were carried out from March/03 through July/2005. In this area, the main geologic/geomorphologic feature is the presence of pre-Cambrian granites and migmatites of Serra do Mar. The coastal aquifer is a fractured rock aquifer, covered by Pleistocene and Holocene sediments. The discharge pattern of this kind of aquifer is spatially heterogeneous, with preferential flow paths along rock fractures. During this investigation 222Rn in excess inventories obtained in 24 vertical profiles varied from 345 ± 24 dpm m-2 to 18,700 ± 4,900 dpm m-2. The highest inventories of 222Rn in excess were observed both in Flamengo and Fortaleza embayments, during summer campaigns. The estimated total fluxes required to support inventories measured varied from 62 ± 4 to 3,385 ± 880 dpm m-2 d-1. SGD fluxes calculated in Ubatuba embayments ranged from 0.1x10-1 to 1.9 cm d-1. Taking into account all fluxes estimated, the percentual variability was 89% (seasonal variation in 3 years period, n = 24 measurements). Although, if we consider each year of study separately, the respective percentual variabilities estimated are 72% in 2003 (n = 10 measurements), 127% in 2004 (n = 6 measurements) and 97% in 2005 (n = 8 measurements).