  The stability of the low-frequency peaks (< 1 Hz) obtained in the passive
seismic survey of Campo de Dal\'ias basin (CDB) by applying the
horizontal-to-vertical spectral ratio (HVSR) method was investigated. Three
temporary seismic stations were installed in remote sites that enabled studying
the stationarity of their characteristic microtremor HVSR (MHVSR) shapes. All
stations began to operate in mid-2016 and recorded at least one year of
continuous seismic ambient noise data, having up to two years in some. Each
seismic station counted with a monitored borehole in their vicinity,
registering the groundwater level every 30 minutes. The MHVSR curves were
calculated for time windows of 150 s and averaged hourly. Four parameters have
been defined to characterize the shape of the MHVSR around the main peak and to
compare them with several environmental variables. Correlations between MHVSR
characteristics and the groundwater level showed to be the most persistent. The
robustness of MHVSR method for applications to seismic engineering was not
found to be compromised since the observed variations were within the margins
of acceptable deviations. Our results widen the possibilities of the MHVSR
method from being a reliable predictor for seismic resonance to also being an
autonomous monitoring tool, especially sensitive to the S-wave modifications.
