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Alluvial plains are transition areas between terrestrial and aquatic environments where groundwater (GW) and surface water (SW) interactions occur. They are the subject of our present study, using hydrochemistry and environmental isotopes. Two water sampling campaigns in dry and wet seasons for studying physicochemical and natural isotope analysis provided 44 groundwater samples and watertable level measurements from monitoring wells and eight water samples from river, creek, and lake. In the dry season, lake and creek waters presented TDS (<3.64 mg L−1) and high values of TDS in groundwater and river (average 47.71 and 108.7 mg L−1, respectively), showing CaMgHCO3, NaHCO3, and NaHCO3Cl types and precipitation prevalence. However, GW–SW interaction occurred in CaMgCl and CaMgSO4 groundwater with TDS varying from 93.8 to 164.7 mg L−1 and weathering dominance close to riverbanks. The TDS, sulfate, and sodium were monitoring markers of this interaction. In the wet season, superficial waters and groundwater were more diluted, presenting CaMgHCO3, NaHCO3, and NaHCO3Cl types and precipitation dominance. Overall, the ion exchange was the main hydrochemical process. The δ18O, δ2H, and d‐excess indicated groundwater recharge with a weak evaporation process, rainwater mixture, and water–rock interaction. Maps of the distribution of ions, isotopes, and d‐excess showed GW–SW interaction along the riverbanks and depleted groundwater in a large portion of the alluvial plain. The alluvial plains are susceptible areas to precipitation changes due to fast and direct infiltration. Their monitoring can be essential to understand the climate variability and reduce impacts on the shallow aquifer that sustain forests and human life. [ABSTRACT FROM AUTHOR]