An instrument for near real-time, in situ detection and measurement of tritium in the aqueous phase has been developed and tested using standards and real groundwater samples obtained from UK nuclear licensed sites. The instrument allows for selective, electrochemically driven, pre-concentration of tritium into a palladium layer that has been electrodeposited onto a solid scintillator crystal and subsequent photomultiplier tube (PMT)-based counting of its associated emitted β particles. The Pd may be electrodeposited as either non- or nano-porous layers. As such, the complete device is comprised of Pd-modified solid scintillator coupled to both an electrochemical cell with a standard, three electrode configuration and a photon counting PMT with associated counting electronics. One side of the solid scintillator is modified with a sample solution-facing Pd film whereas the other was coupled to the PMT. CASINO simulations confirm both the benefits to detection of pre-concentrating the tritium into the Pd layer at the scintillator surface and the detriment to counting efficiency arising from self-shielding by the same layer - and confirm that the latter can be mitigated against by rendering the layer nano-porous. Tritium counting experiments have been conducted with and without the application of electrolytic pre-concentration of tritium into the Pd layer, which can be either non- or nano-porous. Both with and without electrolytic pre-concentration, a higher count rate was recorded using solid scintillator crystals modified with nano-porous Pd than those with non-porous Pd. Similarly, higher count rates were observed with electrolytic pre-concentration than without for both types of layer. It was found that electrolytically pre-concentrated tritium is retained in the Pd layer both upon removal of the infusing potential and upon flushing the (electrochemical) sample cell, hence allowing interference-free tritium measurement in samples containing tritium and other radionuclides.