Seasonal Trailer Park with Suspected Groundwater Mounding
A 60-site seasonal trailer park uses a communal septic system: a 9,000 L tank and a 45 m × 12 m leaching bed in a shallow sandy soil. The water table is 1.2 m below the bed in spring, and the bed is 25 m from a wetland. Residents report soggy ground and odour near the bed in June, which clears by August. The owner wants to add 10 sites and asks whether the existing bed can handle the additional flow. The soil profile shows 1.5 m of sand over a clay layer, and the percolation rate is 6 min/cm in the sand but the clay layer is much tighter.
1. Explain the likely cause of the spring ponding and why the sand's percolation rate alone does not determine the bed's capacity.
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The spring ponding is consistent with a seasonal high water table and groundwater mounding. The water table at 1.2 m below the bed in spring leaves too little unsaturated depth for effluent to infiltrate and be treated before it reaches the saturated zone. Effluent added to the bed raises the water table locally, creating a mound that can reach the surface, causing soggy ground and odour. The fact that it clears by August suggests the mound dissipates as the water table drops, but the system is still overloaded during the wet season.
The sand's percolation rate of 6 min/cm describes the sand, but the clay layer at 1.5 m controls the long-term capacity. If the clay is much tighter, the sand above it can become saturated and the effective loading rate is limited by the clay. The design must use the most restrictive layer and must maintain vertical separation to the seasonal high water table, not just to the sand surface.
The wetland at 25 m is also relevant, because a raised water table and effluent plume can migrate toward it. The owner should commission a site evaluation that includes monitoring wells or test pits to establish the seasonal high water table and the depth and permeability of the clay layer.
2. Evaluate the request to add 10 sites and recommend what the owner must do before any expansion is approved.
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The expansion should not be approved on the existing bed. The system is already showing signs of failure during the wet season, and adding 10 sites increases the design flow by roughly 17% if the per-site flow is unchanged. That additional load would increase the mound and extend the period of saturation, raising the risk of surfacing effluent and contamination of the wetland.
Before expansion, the owner must complete a site evaluation to establish the seasonal high water table, the depth and permeability of the clay layer, and the direction of groundwater flow. The design must then be recalculated using the most restrictive layer, with adequate vertical separation and a loading rate that accounts for the wet-season conditions. Options may include a raised bed with imported fill, a mound system, a secondary treatment unit to reduce the organic load, or a larger dispersal area. If none of these can be accommodated, the expansion should be refused or conditioned on a new system.
The approval should also require a maintenance plan, including tank pump-outs, bed inspection, and monitoring of the water table and the wetland. The owner should be told that the existing ponding is a warning sign, not a seasonal nuisance.