Which of the following factors can reduce the effectiveness of a mound system?

Surface runoff can significantly reduce the effectiveness of a mound system because it can lead to the saturation of the soil and overwhelm the designed absorption area. When rainwater or melting snow runs off over the surface, it can carry pollutants into the mound system, increase soil moisture levels, and hinder the proper infiltration and treatment of wastewater. If the mound becomes oversaturated, the ability of the system to absorb and treat wastewater decreases, which can result in system failure or pollution of surrounding areas.

In contrast, regular maintenance is essential for optimal performance and helps to prevent issues. Adequate soil aeration facilitates the necessary microbial activity for treating wastewater. Deep-rooted vegetation can even help with nutrient uptake and prevent erosion when managed correctly, thus supporting the mound system's overall effectiveness. Therefore, surface runoff stands out as a specific factor that disrupts the delicate balance needed for a properly functioning mound system.