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Is the neutral ISM in the Galaxy clumpyquestionMark Does pristine gas infalling on the disk immediately mix with the metal-enriched gasquestionMark Recent medium-resolution STIS studies found frequent sub-solar metallicities in the neutral ISM comma integrated along the line of sight comma suggesting the contribution of low-metallicity gas. Intriguingly comma in some cases deviations from the expected depletion patterns were observed for volatile elements. One possible explanation for both effects is a mix between metal-dust-rich and pristine gas (low metallicity and zero depletion) in the ISM. Indeed comma an inhomogeneous ISM mix has been recently observed in high-resolution STIS observations of HD 62542. Here we propose to target 8 lines of sight that show potential contribution of pristine gas in the following waysdoublePoint 1) low metallicity integrated along the line of sight comma 2) deviations in the depletion of the volatile elements comma and 3) complex-asymmetric kinematics comma as measured from lower-resolution studies. The STIS high-resolution will enable a component-by-component analysis to characterize the depletion properties of individual clumps. Our primary goal is to constrain dust-rich and dust-free ISM clumps towards our targets. In addition comma we aim at roughly constraining the metallicity of these ISM clumps by exploring the parameter space that can reproduce the observed deviations of the volatile elements. This way we will characterize the inhomogeneities in the ISM and unveil the potential presence of clumps of pristine gas. While chemical evolution models show that infalling pristine gas is necessary to sustain star formation and reproduce the observed abundances comma its presence in the ISM is yet to be confirmed.