Salinization processes in paleowaters in coastal sediments of Suriname: evidence from δ37Cl analysis and diffusion modelling

Abstract

In the coastal plain of Suriname a stagnant body of fresh (<250 mg/l Cl) and moderately brackish (250–1000 mg/l Cl) groundwater of meteoric origin is found in permeable Tertiary formations. This groundwater body, formed during the last Wisconsin regression, extends offshore into deposits on the continental shelf. The authors found that this paleowater is slowly being salinized by downward solute transport from overlying Holocene marine clays and upward transport from saline Cretaceous sediments. The downward transport has been studied at two sites by relating analyses of chloride contents and 37Cl/ 35Cl isotope ratios in pore water with computer simulations of diffusive transport. Optimization of process parameters yielded an effective diffusion coefficient of 7×10 −10 m 2/s and a fractionation factor of 1.0027, which agrees well with earlier studies. With the computer model the following periods of marine influence were found: 6000–4,100 yr BP and 10,000–500 yr BP at the sites located 20 and 3 km from the coastline, respectively. According to the diffusion model the downward flux of salts has not penetrated deeper than 100 m. Salts below this depth are thought to originate from the Cretaceous sediments. However, groundwater chloride concentration and chlorine isotope data are scarce below this depth, while uncertainty regarding the various regressions and transgressions precludes solute transport modelling. Clay layers at the base of the Tertiary appear to inhibit the upward transport by lateral dispersive mixing at the Tertiary/Cretaceous boundary. Oil reservoirs are also related to these clay seals. The presence of the seal, and perhaps the oil below, can therefore be predicted by the chloride profile in the upper 250 m.