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B21D Using Numerical Models to Assess a Rapid Infiltration Basin System (RIBS), Cape Henlopen State Park, Delaware

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 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.

B21C Groundwater Quality and Monitoring of Rapid Infiltration Basin Systems (RIBS), Theory and Field Experiments at Cape Henlopen State Park, Delaware

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.

B21A Evaluation of Wastewater Treatment Options Used in Rapid Infiltration Basin Systems (RIBS)

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.

Delaware Geological Survey issues report on groundwater modeling in eastern Sussex County

The Delaware Geological Survey (DGS) has released a new technical report titled Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware with Emphasis on Impacts of Spray Irrigation of Treated Wastewater, which was prepared by Changming He and A. Scott Andres of DGS.

DGS Report of Investigations No. 79 documents development of a detailed study of subsurface hydrogeology, interactions between aquifers and streams, and the effects of spray irrigation of treated wastewater on groundwater beneath southern eastern Sussex County.

RI79 Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware With Emphasis on Impacts of Spray Irrigation of Treated Wastewater

Simulation of Groundwater Flow and Contaminant Transport in Eastern Sussex County, Delaware With Emphasis on Impacts of Spray Irrigation of Treated Wastewater

This report presents a conceptual model of groundwater flow and the effects of nitrate (NO3-) loading and transport on shallow groundwater quality in a portion of the Indian River watershed, eastern Sussex County, Delaware. Three-dimensional, numerical simulations of groundwater flow, particle tracking, and contaminant transport were constructed and tested against data collected in previous hydrogeological and water-quality studies.

The simulations show a bimodal distribution of groundwater residence time in the study area, with the largest grouping at less than 10 years, the second largest grouping at more than 100 years, and a median of approximately 29 years.

Historically, the principal source of nitrate to the shallow groundwater in the study area has been from the chemical- and manure-based fertilizers used in agriculture. A total mass of NO3- -nitrogen (N) of about 169 kg/day is currently simulated to discharge to surface water. As the result of improved N-management practices, after 45 years a 20 percent decrease in the mass of NO3- -N reaching the water table would result in an approximately 4 percent decrease in the mass of simulated N discharge to streams. The disproportionally smaller decrease in N discharge reflects the large mass of N in the aquifer coupled with long groundwater residence times.

Currently, there are two large wastewater spray irrigation facilities located in the study domain: the Mountaire Wastewater Treatment Facility and Inland Bays Wastewater Facility. The effects of wastewater application through spray irrigation were simulated with a two-step process. First, under different operations and soil conditions, evaporation and water flux, NO3- -N uptake by plants, and NO3- -N leaching were simulated using an unsaturated flow model, Hydrus-1D. Next, the range of simulated NO3- -N loads were input into the flow and transport model to study the impacts on groundwater elevation and NO3- -N conditions.

Over the long term, the spray irrigation of wastewater may increase water-table elevations up to 2.5m and impact large volumes of groundwater with NO3-. Reducing the concentration of NO3- in effluent and increasing the irrigation rate may reduce the volumes of water impacted by high concentrations of NO3-, but may facilitate the lateral and vertical migration of NO3-. Simulations indicate that NO3- will eventually impact deeper aquifers. An optimal practice of wastewater irrigation can be achieved by adjusting irrigation rate and effluent concentration. Further work is needed to determine these optimum application rates and concentrations.

Presentation on RIBS at Fall 2012 AGU meeting

A poster "Modeling Engineered Approaches to Enhance Denitrification under Rapid Infiltration Basins" resulting from a collaborative research project between Paul Imhoff, Maryam Akhavan (UD Civil&Environmental Engineering), A. Scott Andres (DGS), and Stefan Finsterle (Lawrence Berkley National Lab) was presented at the Fall 2012 AGU meeting in San Francisco, CA on Dec. 3.

Scientists study flow of groundwater into bays

Scientists study flow of groundwater into bays

On a small, homemade barge, built from the skeleton of an old ship, a gray slurry of bay bottom sand flows out, of a pipe into a bucket. Two scientists, a well driller and two student interns drill a hole in the floor of the Indian River Bay. They'll install a very long pipe into the hole and use it to monitor groundwater - how much flows 'into the bay, how salty it is and how many nutrients it carries with it.