We have investigated the viability of using plasmas formed by ionization of high Z, low ionization potential element rubidium (Rb) for beam-driven plasma wakefield acceleration. The Rb vapor column confined by argon (Ar) buffer gas was used to reduce the expected limitation on the beam propagation length due to head erosion that was observed previously when a lower Z but higher ionization potential lithium vapor was used. However, injection of electrons into the wakefield due to ionization of Ar buffer gas and non-uniform ionization of Rb$^{1+}$ to Rb$^{2+}$ was a possible concern. In this paper we describe experimental results and the supporting simulations which indicate that such ionization of Ar and Rb$^{1+}$ in the presence of combined fields of the beam and the wakefield inside the wake does indeed occur. Some of this charge accumulates in the accelerating region of the wake leading to the reduction of the electric field- an effect known as beam loading. The beam-loading effect is quantified by determining the average transformer ratio $\langle R \rangle$ which is the maximum energy gained divided by the maximum energy lost by the electrons in the bunch used to produce the wake. $\langle R \rangle$ is shown to depend on the propagation length and the quantity of the accumulated charge, indicating that the distributed injection of secondary Rb electrons is the main cause of beam loading in this experiment. The average transformer ratio is reduced from 1.5 to less than 1 as the excess charge from secondary ionization increased from 100 pC to 700 pC. The simulations show Show Partial Abstract