Development and validation of a low-cost modular in-line filtration apparatus for high-volume microplastic sampling in groundwater wells

Microplastics are increasingly detected across environmental matrices, yet their presence and characterization in groundwater, particularly unconsolidated sediment aquifers, remains critically understudied. Sampling these systems is uniquely challenging due to recommended high extraction volumes (> 100 L), costly commercial filtration systems, and contamination risks associated with plastic-based sampling equipment. We developed a low-cost (~ USD $120), modular in-line filtration apparatus constructed from all-metal water-contact components, with silicone gaskets used only as sealing elements. The system is assembled from readily available hardware-store parts, compatible with multiple pump types, and configured as three sequential filtration modules fitted with stainless-steel meshes (200–270 μm, 110 μm, and ~ 30 μm) for size fractionation. Laboratory spike experiments were performed in triplicate using a peristaltic pump and a modified stainless-steel submersible pump. The apparatus achieved an average recovery of 96% with the peristaltic pump, including 98–100% recovery for fibers, foams, and films, and 75% for fragments. With the submersible pump, apparatus recovery was up to 76%, with particle losses attributed to retention within the pump housing rather than the in-line filtration system. Field deployment in three groundwater monitoring wells (11.7–39 m total depth) demonstrated effective operation while filtering ~ 500 L per well, with minimal leakage and no clogging. The apparatus can also be adapted for surface-water sampling using a peristaltic pump. This modular in-line filtration system provides an affordable and scalable approach for high-volume microplastic sampling in groundwater wells, addressing a critical gap in subsurface monitoring while minimizing contamination and enabling the collection of size-fractionated samples for subsequent analysis.