  Sulfur-bearing molecules are highly reactive in the gas phase of the ISM.
However, the form in which most of the sulfur is locked onto interstellar dust
grains is unknown. By taking advantage of the short time-scales of shocks in
young molecular outflows, one could track back the main form of sulfur in the
ices. In this paper, six transitions of H$_2$S and its isotopologues in the
L1157-B1 bowshock have been detected using data from the Herschel-CHESS survey
and the IRAM-30m ASAI large program. These detections are used to calculate the
properties of H$_2$S gas in L1157-B1 through use of a rotation diagram and to
explore the possible carriers of sulfur on the grains. The isotopologue
detections allow the first calculation of the H$_2$S deuteration fraction in an
outflow from a low mass protostar. The fractional abundance of H$_2$S in the
region is found to be 6.0$\times$10$^{-7}$ and the deuteration fraction is
2$\times$10$^{-2}$. In order to investigate the form of sulfur on the grains, a
chemical model is run with four different networks, each with different
branching ratios for the freeze out of sulfur bearing species into molecules
such as OCS and H$_2$S. It is found that the model best fits the data when at
least half of each sulfur bearing species hydrogenates when freezing. We
therefore conclude that a significant fraction of sulfur in L1157-B1 is likely
to be locked in H$_2$S on the grains.
