tHe iNtRoDuCtIoN oF cHeMiCaL, pHySiCaL, Or bIoLoGiCaL mAtErIaL iNtO wAtEr tHaT dEgRaDeS tHe oF tHe wAtEr aNd aFfEcTs tHe oRgAnIsMs tHaT dRiNk iT aNd LiVe iN iT.
