Cape Henlopen study
Delaware Geological Survey issues report on modeling of impacts of rapid infiltration basin systems
8:51 a.m., May 6, 2016--DGS Bulletin 21D documents comparisons of the results of numerical model simulations with those from detailed study of groundwater flow and quality at a rapid infiltration basin system.
This is the fourth of four reports that evaluate rapid infiltration basin systems (RIBS), which are one of several land-based wastewater disposal methods used in Delaware. The study was prompted by concerns that RIBS may pose a threat to groundwater quality that would cause problems with the quality of water in aquifers and in bodies of surface water. Other study components and reports evaluate performance of wastewater treatment plants that discharge effluent to RIBS, hydraulic performance of RIBS and groundwater quality impacts of RIBS.
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This study conducted detailed three-dimensional, time-dependent numerical simulations of groundwater flow and contaminant transport at the Cape Henlopen State Park RIBS and compared simulation results to field measured hydraulic and chemical characteristics. Numerical simulations are routinely used to evaluate how aquifers respond to changes caused by wastewater disposal, pumping for water supply and variations in recharge.
The study found that simulation results match the averages and general tendencies of groundwater flow velocity and nitrogen and phosphorus concentrations observed in monitoring well data, but smooth out the temporal and spatial variations observed in those data. The need to document how simulations can and cannot represent real-world situations has become important to Delaware as simulation studies have become more frequently used to support permit applications for wastewater disposal.
Model results support conclusions reached in previous reports in this series. Treated wastewater flows rapidly from the RIBS and first-order decay of nitrate and sorption of phosphorus can simulate reductions in concentrations as the effluent flows through the aquifer. Continued use of RIBS leads to oversaturation of phosphorus sorption sites on the aquifer matrix and buildup of phosphorus concentrations in groundwater. The cyclic nature of RIBS operation and aquifer heterogeneity causes considerable variability and complexity in the vertical and lateral distributions of nitrogen and phosphorus in the groundwater over short periods of time. Monitoring systems designed with standard single wells and quarterly or less frequent sampling may not provide necessary early warning of impacts of RIBS on groundwater. The authors note that the relatively short two-year time frame of the simulation is not adequate to address long-term impacts of RIBS operation.
Disposal of inadequately treated wastewater through RIBS presents a risk for Delaware’s hydrogeologic setting as many previous studies have documented that the shallow Columbia aquifer is especially vulnerable to contamination by nitrate from wastewater disposal. The Columbia aquifer is an important source of potable water and is the source of water for fair-weather streamflow.
The report fulfills part of the DGS’s mission to understand hydrologic systems and to advise, inform and educate Delawareans about the results of such investigations for use in such topics as water supply and pollution, agriculture, public health, economic development, land-use planning, geologic hazards, environmental protection, energy and mineral resources, emergency management and recreation.
Bulletin 21D is available in pdf format from the DGS website. For additional information, contact the Survey at (302) 831-2833 or at delgeosurvey@udel.edu.