Presence of dissolved substances in the liquid:

The presence of any foreign substances in the form of solutes in a liquid, changes or affects the surface tension of that liquid differently. Most organic substances when dissolved reduce the surface tension, on the other hand, inorganic substances when dissolved increase the surface tension, increasing linearly with concentration.

contamination of liquid surface:

The surface tension of a liquid reacts immediately and more or less sharply, to any contamination of the liquid surface. The presence of impurities of any kind on the surface of a liquid may bring a marked change in the value of its surface tension, depending on the degree of contamination thus produced. For example, oil, grease, etc can easily spread over liquid and thereby reduce the surface tension.