We study the transport properties of single-atom-thick Cu wires submerged in an electrochemical solvent containing HCl. As a first step, we investigate the stability of hydrogen coadsorbing with chlorine on the Cu(111) surface in an implicit electrochemical environment. We find that adding hydrogen to a Cl-covered Cu surface is energetically unfavorable. The result serves as an estimate for the number of Cl atoms that adsorb near the single-atom wire. We use it to construct model junctions (Cu wire plus adsorbates), the electron transport properties of which we investigate with density functional theory. We find that the Cl and H adsorbates tend to deplete the density of states of the Cu wire near the Fermi energy. As a consequence, the transmission is reduced. Interestingly, we observe that in the case of H-adsorption, the amount of depletion is quite sensitive to the wire geometry (relaxed vs unrelaxed), but this is not the case with Cl.