  To evaluate the impact of stellar feedback, it is critical to estimate the
mass outflow rates of galaxies. Past estimates have been plagued by uncertain
assumptions about the outflow geometry, metallicity, and ionization fraction.
Here we use Hubble Space Telescope ultraviolet spectroscopic observations of
the nearby starburst NGC 6090 to demonstrate that many of these quantities can
be constrained by the data. We use the Si~{\sc IV} absorption lines to
calculate the scaling of velocity (v), covering fraction (C$_f$), and density
with distance from the starburst (r), assuming the Sobolev optical depth and a
velocity law of the form: v$~\propto(1 -\mathrm{R}_\mathrm{i}/\mathrm{r}
)^\beta$ (where R$_\mathrm{i}$ is the inner outflow radius). We find that the
velocity ($\beta$=0.43) is consistent with an outflow driven by an r$^{-2}$
force with the outflow radially accelerated, while the scaling of the covering
fraction ($C_f \propto \mathrm{r}^{-0.82}$) suggests that cool clouds in the
outflow are in pressure equilibrium with an adiabatically expanding medium. We
use the column densities of four weak metal lines and CLOUDY photoionization
models to determine the outflow metallicity, the ionization correction, and the
initial density of the outflow. Combining these values with the profile
fitting, we find R$_\mathrm{i}$ = 63 pc, with most of the mass within 300~pc of
the starburst. Finally, we find that the maximum mass outflow rate is
2.3~M$_\odot$ yr$^{-1}$ and the mass loading factor (outflow divided by the
star formation rate) is 0.09, a factor of 10 lower than the value calculated
using common assumptions for the geometry, metallicity and ionization structure
of the outflow.
