This paper presents a methodology for determining the optimum sampling frequency using long-term monitoring data, with a high sampling frequency, from a real field case in an industrial site. The initial data used in the development of the methodology using Fast Fourier Transformer (FFT) was obtained from an already established continuous surface water monitoring location for conductivity variable. The methodology was later applied to the laboratory test data of different Reference TUBERs provided by two industrial companies. The suggested methodology aims at providing a guideline for the suitable sampling frequency for some water quality parameters such as: (temperature, pH, electrical conductivity, dissolved oxygen, nitrate, and chloride, as they are being monitored around mining and landfill sites. Results showed that the current sampling interval could be increased by 4 times for some parameters and more for others without much change in the main information recorded in the final output signal.