B21D Using Numerical Models to Assess a Rapid Infiltration Basin System (RIBS), Cape Henlopen State Park, Delaware
The long-term performance of rapid infiltration basin systems (RIBS) and their potential impacts on the receiving environment have been previously unknown for Delaware. A variety of field experiments were conducted to characterize the geology and hydrogeology of a RIBS facility that has been in operation for more than 20 years at Cape Henlopen State Park. Pairs of standard monitoring wells and short-screened multi-level wells were used to evaluate the significance of small-scale vertical variability in water quality. A three-dimensional transient groundwater flow and contaminant transport model was constructed to simulate the groundwater mounding and the movements of nitrate-nitrogen (NO3--N) and orthophosphorus (OP) in the groundwater. In the numerical model, NO3--N was treated as a reactive species and denitrification was simulated with a first-order degradation rate constant. The major mechanism affecting OP transport in groundwater is sorption/desorption, which was simulated using a linear sorption isotherm. Simulated concentrations reasonably fit observed concentrations of NO3--N and OP in both standard wells and multi-level wells. The calibrated model predicts that with a denitrification rate of 0.006/day and a distribution coefficient of 4×10-7 L/mg, 63 percent of the reduction in the mass of NO3--N is attributable to denitrification, and more than 99 percent of OP is detained in the aquifer due to sorption on subsurface solids. However, the long-term operation of RIBS has led to a reduction of the sorption capacity of subsurface solids for phosphorous, resulting in significant concentrations of OP in groundwater adjacent to RIBS.
Delaware Geological Survey Issues Report on Hydrogeologic Impacts of Rapid Infiltration Basin Systems
Delaware Geological Survey Issues Report on Wastewater Treatment used for Rapid Infiltration Basin Systems
This technical report evaluates several aspects of potential environmental risks, use, and regulation of rapid infiltration basin systems (RIBS) in Delaware. The report reviews and compares regulations regarding RIBS from Delaware, Florida,North Carolina, New Jersey, Maryland, and Massachusetts. Influent and effluent samples from ten advanced wastewater treatment systems that operate in conjunction with RIBS were collected and analyzed. Effluent data obtained from the Non-Hazardous Waste Sites database provided by the Delaware Department of Natural Resources and Environmental Control and other states were assessed. Performance evaluations of the treatment processes that discharge to RIBS were ascertained from the exceedance of concentrations of regulated pollutants in effluent samples.
Although RIBS technology has the potential to be a beneficial alternative to surface discharge and a means for groundwater recharge, this technology is appropriate only if the adverse environmental impacts are minimized. Overall operation and maintenance practices play important roles in the performance of treatment plants. The most common and serious problems associated with treatment plants located in Delaware and neighboring states are high nutrient and pathogen concentrations in the effluent. In Delaware, the discharge of poorly treated effluent to RIBS creates a risk of nutrient and pathogen contamination in the receiving water body, the shallow Columbia aquifer. Years of application of treated effluent with high nutrient, pathogen, and organic content to RIBS will result in significant risks for the environment and public health.