An assembly for use in a wellbore can include a plurality of sensors positioned external to a casing string. The plurality of sensors can be positioned for detecting an amount of a hydrocarbon that is present in a fluid in the wellbore and a pH of the fluid in the wellbore. The plurality of sensors can be positioned for wirelessly transmitting signals representing the amount of the hydrocarbon that is present in the fluid and the pH of the fluid to a receiving device.