Using a 20-year continuous broadband record and two independent single-station techniques -ambient noise autocorrelation and receiver functions-we document a relationship between subsurface seismic response and groundwater levels (GWLs) in the Gulf Coast Aquifer System of southern Texas. We find that a surge of GWL following three consecutive hurricanes and documented at an adjacent monitoring well is accompanied with changes in receiver function power spectra and ambient noise autocorrelations. Using a simple physical model, we show that GWL changes should affect P-(V P ) more strongly than S-wave (V S ) velocities, consistent with our observations and previous ones based on inter-station correlations. Agreement between receiver function and ambient noise analyses shows that both can be used to reliably estimate temporal changes in subsurface properties on long timescales. Due to their sensitivity to V P , single-station techniques respond more strongly to GWL changes, making them useful for characterizing and monitoring aquifer systems.Plain Language Summary Even though groundwater in large aquifer systems is often relied upon as a water source, it is one of the least well-known components of the hydrologic cycle. Here, we document a relationship between ground vibrations recorded by a seismometer and the groundwater levels in the Gulf Coast Aquifer System of southern Texas. We find that a surge of groundwater level following three consecutive hurricanes is accompanied with changes in the characteristics of seismic vibrations set-up in the shallow subsurface. Characteristics of vibrations due to both ambient noise and from earthquake show similar signals. Using a simple physical model, we show how groundwater level changes the properties of the subsurface to explain the seismic observations. Our work illustrates how even a single seismometer can be used to monitor groundwater level changes, which is particularly useful in aquifer systems lacking monitoring wells, and can complement other geophysical measurements from gravity or uplift/subsidence sensed data from satellites.