In this study, a three-dimensional surface enhanced Raman scattering (SERS) substrate comprised of silver coated gold nanorods (Ag/AuNRs) decorated on electrospun polycaprolactone (PCL) fibers has been applied, for the first time, to quantitative analytical measurements on various arsenic species: p-arsanilic acid (pAsA), roxarsone (Rox), and arsenate (As-V), with a demonstrated sensitivity below 5 ppb. As-V detection in a solution of common salt ions has been demonstrated, showing the tolerance of the substrate to more complex environments. pAsA adsorption behavior on the substrate surface has been investigated in detail using these unique SERS substrates. Calculations based on density functional theory (DFT) support the spectral observation for pAsA. This substrate also has been shown to serve as a platform for in situ studies of arsenic desorption and reduction. This SERS substrate is potentially an excellent environmental sensor for both fundamental studies and practical applications.