  We explore the codimension one strata in the degree-one cohomology jumping
loci of a finitely generated group, through the prism of the multivariable
Alexander polynomial. As an application, we give new criteria that must be
satisfied by fundamental groups of smooth, quasi-projective complex varieties.
These criteria establish precisely which fundamental groups of boundary
manifolds of complex line arrangements are quasi-projective. We also give sharp
upper bounds for the twisted Betti ranks of a group, in terms of multiplicities
constructed from the Alexander polynomial. For Seifert links in homology
3-spheres, these bounds become equalities, and our formula shows explicitly how
the Alexander polynomial determines all the characteristic varieties.
