  The 2009 Lunar CRater Observation and Sensing Satellite (LCROSS) impact
mission detected water ice absorption using spectroscopic observations of the
impact-generated debris plume taken by the Shepherding Spacecraft, confirming
an existing hypothesis regarding the existence of water ice in permanently
shadowed regions within Cabeus crater. Ground-based observations in support of
the mission were able to further constrain the mass of the debris plume and the
concentration of the water ice ejected during the impact. In this work, we
explore additional constraints on the initial conditions of the pre-impact
lunar sediment required in order to produce a plume model that is consistent
with the ground-based observations. We match the observed debris plume
lightcurve using a layer of dirty ice with an ice concentration that increases
with depth, a layer of pure regolith, and a layer of material at about 6 meters
below the lunar surface that would otherwise have been visible in the plume but
has a high enough tensile strength to resist excavation. Among a few possible
materials, a mixture of regolith and ice with a sufficiently high ice
concentration could plausibly produce such a behavior. The vertical albedo
profiles used in the best fit model allows us to calculate a pre-impact mass of
water ice within Cabeus crater of $5 \pm 3.0 \times 10^{11}$ kg and a mass
concentration of water in the lunar sediment of $8.2 \pm 0.001$ %wt, assuming a
water ice albedo of 0.8 and a lunar regolith density of 1.5 g cm$^{-3}$, or a
mass concentration of water of $4.3 \pm 0.01$ %wt, assuming a lunar regolith
density of 3.0. These models fit to ground-based observations result in derived
masses of regolith and water ice within the debris plume that are consistent
with \emph{in situ} measurements, with a model debris plume ice mass of 108 kg.
