Delaware Geological Survey Issues Report on Groundwater Monitoring and Water-Quality Impacts of Rapid Infiltration Basin Systems

The Delaware Geological Survey released a new technical report entitled “Groundwater Quality and Monitoring of Rapid Infiltration Basin Systems, Theory and Field Experiments at Cape Henlopen State Park, Delaware” which was prepared by A. Scott Andres and Changming He of the Delaware Geological Survey, Edward Walther of the South Water Management District, Florida, Müserref Türkmen of the Izmir Water and Sewerage Administration, Turkey, and Anastasia Chirnside and William Ritter of the University of Delaware. DGS Bulletin 21C documents the results of a detailed study of groundwater quality at a rapid infiltration basin system.

DGS Bulletin 21C is the third 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 our 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 the use of simulation techniques to predict hydraulic and water-quality impacts of RIBS.

This study conducted detailed evaluations of the groundwater at the Cape Henlopen State Park RIBS to document the changes to the chemistry of treated wastewater as it infiltrates to the water table and flows away from the RIBS and changes to the chemistry of groundwater over time where. The work found that the chemical composition of treated wastewater varies significantly from month to month and once in the ground flows rapidly from the RIBS to the water table and undergoes minimal chemical changes as it flows through the aquifer. Sampling results from multi-level and standard monitoring wells found considerable variability and complexity in the vertical and lateral distributions of nitrogen and phosphorus in the groundwater over periods of two to three months, a result of weekly and monthly changes in wastewater chemistry and disposal rates and the relatively rapid flow of groundwater. 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.

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 21C is available in pdf format from the DGS web site at For additional information, contact the Survey at (302) 831-2833 or via email at

For questions and information, contact DGS at, 302-831-2833