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 δ11B signatures vary between -2.98 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 δ11B: 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 δ11B values are linked to water-rock interaction processes (e.g., dissolution/precipitation of silicates).