A higher hydronium ion concentration is represented by-

pH is a number that represents whether a solution is acidic or not.

It is represented as-

$$\begin{gathered} \mathrm{pH} = -\log \left[{\mathrm{H}}^{+}\right] \\ \mathrm{or} \\ \mathrm{pH} = - \log \left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right] \end{gathered}$$

A very high hydronium (${\mathrm{H}}_3{\mathrm{O}}^{+}$) concentration is represented by a low pH indicating that the solution is highly acidic in nature.

Hence, a higher hydronium ion concentration is represented by a low pH.