MODELING OF WASTEWATER INFILTRATION IN A FRESHWATER LENS SYSTEM USING ANALYTIC ELEMENTS

Subsurface sewage disposal via infiltration of treated effluent (spray irrigation or rapid infiltration beds) must meet various regulatory requirements regarding infiltration rates, soil assimilative capacity, ground water mounding, subsurface effluent flow paths, impacts on nearby surface water bodies, etc. Ground water flow models are often used to examine the required hydraulic factors.

For a sewage treatment and disposal project on Cape Cod, a modeling study was conducted to provide estimates of the effects of wastewater infiltration on the hydrologic system in the vicinity of proposed discharge areas to guide engineering design decisions and to obtain regulatory approval for the project. The twelve square mile aquifer study area consists of a freshwater lens in relatively homogeneous wind-blown deposits of medium-grained sand, bounded on three sides by ocean, and on the fourth side by a lake that represents the easternmost hydrogeologic boundary of the aquifer.

GFLOW 2000 was selected as the modeling software to evaluate the effects of the wastewater infiltration on water levels and ground water flow directions. The analytic element method was selected in part because of its short computation time and ease of setup and reconfiguration to allow for multiple model runs and efficient adaptation to evolving design criteria. The shape of the freshwater lens was defined in the model by varying the bottom elevation of the model to match the approximate depth of the freshwater/saltwater interface. The model was calibrated against a comprehensive round of water level measurements and a potentiometric surface map generated from field data, and sensitivity analyses were performed. Modeling of water level changes and particle tracking under varying climatologic and infiltration conditions was used to evaluate the potential effects of the infiltration areas on nearby wetlands and surface water bodies. The results of the modeling analyses assisted in defining infiltration areas and rates that would prevent the direct discharge of treated wastewater to surface water bodies and wetlands while providing the capacity necessary for the success of the wastewater treatment project.