  We present ALMA Cycle~2 observations at $0.5^{\prime\prime}$ resolution of TW
Hya of CS $J=5-4$ emission. The radial profile of the integrated line emission
displays oscillatory features outwards of $1.5^{\prime\prime}$ ($\approx 90$
au). A dip-like feature at $1.6^{\prime\prime}$ is coincident in location,
depth and width with features observed in dust scattered light at near-infrared
wavelengths. Using a thermochemical model indicative of TW Hya, gas-grain
chemical modelling and non-LTE radiative transfer, we demonstrate that such a
feature can be reproduced with a surface density depression, consistent with
the modelling performed for scattered light observations of TW Hya. We further
demonstrate that a gap in the dust distribution and dust opacity only cannot
reproduce the observed CS feature. The outer enhancement at
$3.1^{\prime\prime}$ is identified as a region of intensified desorption due to
enhanced penetration of the interstellar FUV radiation at the exponential edge
of the disk surface density, which intensifies the photochemical processing of
gas and ices.
