Boron isotope signatures in the coastal groundwaters of French Guiana

Abstract

[1] Drinking water used by the population of French Guiana comes mostly from rivers and can induce major health impacts. Consequently, groundwaters are being studied in terms of an alternative drinking water supply. In this context, boron isotopes were applied to the coastal aquifers to better constrain the water-rock interactions. We found that the d 11 B signatures vary between À2.98 and 49.35% in the groundwaters and between 36.35 and 42.41% in the river waters and that the local seawater gave a value of 40.76%. We also noted an inverse correlation between groundwater pH and d 11 B: this relationship, previously demonstrated experimentally, is particularly well defined in the French Guiana coastal aquifer because of the wide pH range (4.18 to 7.64) naturally present in the groundwaters. It would appear that the variations in d11 B values are linked to water-rock interaction processes (e.g., dissolution/precipitation of silicates).

Cyclic variations of sulfate and boron concentrations and isotopes in deep groundwaters in the Aquitaine Basin, France

“…J o u r n a l P r e -p r o o f Because of the low δ 11 B values identified in the Lussagnet sand aquifer (between -3.15 and +2.00 ‰), it could be inferred that boron may come from the alteration of silicates. Indeed, the alteration of silicate minerals by waters gives low δ 11 B values, ranging between -10 and +10 ‰ (Barth, 2000;Pennisi et al, 2000;Casanova et al, 2002;Négrel et al, 2002 ;Lemarchand and Gaillardet, 2006;Millot et al, 2007;Clauer et al, 2018). As shown in Figure 10, this value is also consistent with the end-member of silicates identified by Negrel et al (2009).…”

Origins and processes of groundwater salinization in the urban coastal aquifers of Recife (Pernambuco, Brazil): A multi-isotope approach

“…Water interacting with silicate rocks generally has low δ 11 B values (− 10 to + 10‰) including some Precambrian granites (− 3.2 to +6.8‰) as shown by numerous studies (Aggarwal et al, 2000;Barth, 2000;Lemarchand and Gaillardet, 2006;Négrel et al, 2002;Pennisi et al, 2000;Spivack and Edmond, 1987;Vengosh et al, 1995). All the groundwater samples from the Beberibe aquifer present quite homogeneous δ 11 B signatures (36.8-42.5‰) close to the marine value (39.1‰) except for group B1.…”

“…The basement aquifer in this low land area was previously intruded by seawater that was further flushed by a fresh recharge component. The highest value (Fis70: 41.1‰) with a moderate salinity could represent a diluted palaeo-seawater with little interaction with the basement matrix and could reflect the signature of the recharge with a δ 11 B close to the meteoric recharge (Négrel et al, 2002). Moreover, Well Fis70 is located near a river fed by the Duas Unas dam, and can easily receive a recharge component from the surface water.…”

“…7). Although we did not directly measure the δ 11 B in local rain, the ETA samples (ETA0 and ETA2 of respectively 40.3 and 42‰), which are considered as representative for recharge, display nearly unaltered seawater signatures, likely to be inherited from coastal rain, without any significant boron input from interaction with silicate minerals (Millot et al, 2010;Négrel et al, 2002). In other words, water-rock interaction does not release B in solution, probably because the regional basements and their associated weathering profiles lack of easily leachable boron-bearing mineral phases.…”