An improved method and apparatus for monitoring the concentration of contaminants, including volatile organic compounds, in groundwater is provided comprising a semipermeable membrane defining an inner chamber. The chamber is carried within a mesh protective sleeve of cover to prevent abrasion or puncture of the chamber. The membrane is permeable to contaminants but impermeable to a reference fluid, which is distilled water. The method of sampling comprises placing the semipermeable membrane chamber, which contains the reference fluid, in contact with contaminated groundwater, thereby allowing contaminants to diffuse through the semipermeable membrane.