Which should NOT be allowed over an OSSF treatment unit or drainfield?

Rainfall runoff should not be allowed over an OSSF treatment unit or drainfield because it can interfere with the proper functioning of the system. The primary concern with runoff is that it can lead to over-saturation of the drainfield area, affecting the natural absorption and treatment processes of the effluent. When a drainfield becomes too saturated, it may cause the treated effluent to surface, potentially contaminating the surrounding area and groundwater and posing a health risk.

In contrast, vegetation is beneficial as it helps with soil stability and can absorb excess moisture, while mowing is typically acceptable to maintain access and manage plant growth without compromising the system. Earth fill is not permitted because it can add weight and restrict aeration, leading to system failure. Each of these considerations is vital in maintaining the effectiveness and longevity of an OSSF system.