Delaware Geological Survey Issues Report on Groundwater Monitoring and Water-Quality Impacts of Rapid Infiltration Basin Systems
B21C Groundwater Quality and Monitoring of Rapid Infiltration Basin Systems (RIBS), Theory and Field Experiments at Cape Henlopen State Park, Delaware
A rapid infiltration basin system (RIBS) consists of several simple and relatively standard technologies; collection and conveyance of wastewater, treatment, and discharge to an unlined excavated or constructed basin. By design, the effluent quickly infiltrates through the unsaturated or vadose zone to the water table. During infiltration, some contaminants may be treated by biological and/or geochemical processes and diluted by dispersion and diffusion. The combination of contaminant attenuation and dilution processes that may occur during infiltration and flow through the aquifer are termed soil-aquifer-treatment, or SAT. In the past decade, RIBS have been proposed more frequently for use in Delaware because they stop the direct discharge of treated effluent to surface water, can accommodate significant flow volumes typical of residential subdivisions, yet require much less land than options such as spray irrigation or sub-surface disposal systems.
Decades of research on the shallow Columbia aquifer of the Delmarva Peninsula have clearly identified the high susceptibility of the aquifer from land- and water-use practices, and the processes that control the fate and transport of contaminants from their origin at or near land surface to points of discharge in creeks, estuaries, and wells. The risk of aquifer contamination is great because it is highly permeable, has little organic matter in the aquifer matrix, and the depth to groundwater is very commonly less than 10 ft below land surface. USEPA guidance documents and several engineering texts that cover RIBS design clearly identify these same factors as increasing risk for groundwater contamination but do not provide much information on means to monitor and mitigate those risks. Further, design criteria are based on a small group of experiments conducted in the 1970s prior to development of current understanding of the processes that control groundwater contaminant transport.
Field and laboratory experiments to characterize the physical, chemical, and biological controls and processes associated with the rapid infiltration of treated sewage effluent through infiltration beds and the vadose zone were undertaken at a RIBS located at Cape Henlopen State Park (CHSP), Delaware. Field experiments to understand the geochemical effects of the long-term operation of a RIBS on ground and surface waters, and to evaluate monitoring systems were also conducted at the site. The CHSP RIBS has been in operation since the early 1980s.
Significant concentrations of nitrogen and phosphorus occur in groundwater from the point of effluent entry at the water table to distances greater than 150 ft from the infiltration beds. The high hydraulic, nitrogen (N), phosporus (P), and organic loading rates associated with the operation of RIBS overwhelm natural attenuation (e.g., sorption and precipitation) processes. Data are not sufficient to indicate whether denitrification is occurring. If there is denitrification, the rate is insufficient to remediate RIBS effluent at the site — despite a 25-ft thick vadose zone, an effluent with enough organic carbon to facilitate anaerobic conditions that permit abiotic denitrification and feed microorganism-driven denitrification processes, and hypoxic to anoxic groundwater.
Significant horizontal and vertical variability of contaminant concentrations were observed within the portion of the aquifer most impacted by effluent disposal. Despite the relatively small spatial extent of the disposal area in our study area, identification of the preferential flow zone and characterization of the vertical and temporal variability in the concentrations of contaminants required a multi-phase subsurface investigation program that included an analysis of data from samples collected at bi-monthly intervals from dozens of monitoring points and high frequency temperature monitoring in several wells. A well-designed monitoring system should be based on experimentally determined site specific evidence collected under conditions that duplicate the flow rates that are expected during full-scale operation of the RIBS. Conservative tracers should be used to determine if the monitoring wells are in locations that intercept flow from the infiltration beds.
The hydrogeologic framework of Cape Henlopen State Park (CHSP), Delaware was characterized to document the hydrologic effects of treated wastewater disposal on a rapid infiltration basin system (RIBS). Characterization efforts included installation of test borings and monitoring wells; collection of core samples, geophysical logs, hydraulic test data, groundwater levels and temperatures; testing of grain size distribution; and interpretation of stratigraphic lithofacies, hydraulic test data, groundwater levels, and temperature data. This work was part of a larger effort to assess the potential benefits and risks of using RIBS in Delaware.
The infiltration basins at CHSP are constructed on the Great Dune, an aeolian dune feature composed of relatively uniform, medium-grained quartz sand. The age of the dune, determined by carbon-14 dating of woody material in swamp deposits under the dune, is less than 800 years. Underlying the dune deposits are relatively heterogeneous, areally continuous, coarse-grained spit deposits of the proto-Cape Henlopen spit with interbedded and relatively fine-grained, discontinuous swamp and marsh deposits, and beneath, relatively fine-grained, continuous, near-shore marine deposits. The dune deposits can be 45 ft thick under the crest of the dune and nonexistent at the surface. Spit deposits range from 5 to 15 ft thick. Test drilling determined that the near-shore marine deposits are at least 10 ft thick in the vicinity of the infiltration basins. The complete thickness of these deposits was not determined in this study.
Hydraulic testing and grain-size data indicate that the dune and spit deposits are relatively permeable, with average hydraulic conductivities of 140 ft/day and that the swamp and marsh deposits are more than one order of magnitude less permeable, with average hydraulic conductivity of 25 to 10 ft/day. The water-table aquifer is present in the sandier dune and spit deposits. The swamp, marsh, and near-shore marine deposits form a leaky confining unit. The water-table aquifer is 15 to 20 ft thick under the thickest section of the Great Dune and nonexistent where the dune deposits are absent. The vadose zone is greater than 25 ft thick under the infiltration basins.
High-frequency groundwater level and temperature monitoring during periods of maximum wastewater disposal rates indicates that wastewater disposal causes increases in water-table elevations on the order of 1 ft. Groundwater elevations indicate that the water-table elevation is greatest under the infiltration basins and that most flow is directed southward toward a swampy discharge area.
Maximum disposal rates typically occur in summer months when the numbers of park users and water use are greatest. Coincident with greater disposal rates are higher wastewater temperatures. These higher wastewater temperatures are observed in groundwater and provide a means to track the flow of water from beneath the infiltration beds towards a nearby discharge area. Tracking of the warmer groundwater and modeling two-dimensional particle tracking both indicate that wastewater discharged to the infiltration basins reaches the nearby discharge area within 180 days.