Assessing Potential Impacts of a Wastewater Rapid Infiltration Basin System on Groundwater Quality: A Delaware Case Study

Rapid infiltration basin systems (RIBS) are receiving increased interest for domestic wastewater disposal in rural areas. They rely on natural treatment processes to filter pollutants and use extremely high effluent loading rates, much greater than natural precipitation, applied to a small geographic area instead of disposal to surface water. Concerns exist today that adopting RIBS in areas with shallow groundwater and sandy soils may increase ground and surface water pollution. We conducted a field study of RIBS effects on N and P concentrations in soils and groundwater at Cape Henlopen State Park, Delaware, where a RIBS designed and operated following USEPA guidance has been used for > 25 yr. Site and wastewater characteristics (water table of 8 m, Fe- and Al-oxide coatings on soils, organic-rich effl uent) were favorable for denitrifi cation and P sorption; however, we found high P saturation, reduced soil P sorption capacity, and signifi cant total P accumulation at depths > 1.5 m, factors that could lead to dissolved P leaching. Very low soil inorganic N levels suggest that wastewater N was converted rapidly to NO3-N and leached from the RIBS. Extensive groundwater monitoring supported these concerns and showed rapid off site transport of N and P at concentrations similar to the effluent. Results suggest that high hydraulic loads and preferential flow led to flow velocities that were too large, and contact times between effluent and soils that were too short, foreff ective N and P attenuation processes. These fi ndings indicate the need for better site characterization and facility designs to reduce and monitor contaminant loss from RIBS in similar settings.