For dosimetry of beta -radionuclides in surface and potable water, an impregnated porous scintillator based on p-terphenyl is proposed, which is a matrix with through pores with a sorbent selectively absorbing radionuclides on the pore surface. Synthetic hydroxyapatite is used as sorbent for strontium-90. High measurement sensitivity is ensured by concentration of the emitter from large volume of water and realization of 4 pi -geometry of radiation detection.