Geomembranes (GMs) are widely used in hydraulic applications. Unfortunately, some additives contained in GMs are extracted upon prolonged contact with water or soil. Depending on their chemical nature, additives contained within GMs or degradation products thereof may affect natural ecosystems. To better understand this effect, the present study evaluates the potential impact of additives extracted from GMs that are commonly used in hydraulic applications. The approach consists of identifying additives contained in both unaged and aged high-density polyethylene and polypropylene GMs. Next, these GMs are immersed for various time intervals at 80, 55, and 25 °C in aqueous solutions (pH 4 and 7), and the chemical composition of the resulting solution is analyzed by ultraviolet absorbance spectroscopy. In addition, the effect of these solutions containing the GM extracts on the growth of Chlorella vulgaris and on the swimming of Daphnia sp. is studied. The first few days of exposure to the extracted molecules negatively impact C. vulgaris growth and Daphnia sp. swimming behavior. However, after prolonged exposure, the impact is reversed and microalgae growth is stimulated instead. This result may be related to the change of the nature of the molecules detected by UV spectroscopy.