AbstractChanges of groundwater chemistry have long been observed. We review some studies of the earthquake-induced changes of groundwater and streamflow composition. When data are relatively abundant and the hydrogeology is relatively simple, the observed changes may provide valuable insight into earthquake-induced changes of hydrogeological processes. Progress in this aspect, however, has been slow not only because systematic measurements are scare but also because of the distribution of chemical sources and sinks in the crust are often complex and unknown. Most changes are consistent with the model of earthquake-enhanced groundwater transport through basin-wide or local enhanced permeability caused by earthquake-induced breaching of hydrologic barriers such as aquitards, connecting otherwise isolated aquifers or other fluid sources, leading to fluid source switching and/or mixing. Because the interpretation of earthquake-induced groundwater and stream compositions is often under-constrained, multi-disciplinary approaches may be needed to provide a better constrained interpretation of the observed changes.