On February 17, 2016, chief hydrogeologist for AGES, Mr. Robert King, PG, presented "Design of Monitoring Wells to Meet the CCR Requirement for Low-turbidity Without Field Filtering of Groundwater Samples" at the Utility Solid Waste Activities Group (USWAG) CCR Workshop in Washington, D.C. The presentation highlighted AGES success with a new approach to sand filter packs used for CCR groundwater monitoring.
The case study involves an electric power generation client with a 1.1 gigawatt coal-fired power station that has been in operation since the 1950s. Since operations began, the facility has utilized a 32-acre, onsite Boiler Slag Pond as a process and disposal area for the coal combustion products.
To meet the requirements of the CCR rule, three (3) background and six (6) downgradient monitoring wells were installed around the Boiler Slag Pond. Per the CCR rule, an owner/operator must measure “Total Recoverable Metals” in groundwater, as the rule assumes that “wells will be designed so that a turbidity of less than 5 NTUs (Nephelometric Turbidity Units) can be achieved without field filtering the samples.” Based on results from other wells at the site, achieving this turbidity level without filtering was not achievable with a standard well design.
To reduce groundwater sample turbidity, the wells at the Boiler Slag Pond were drilled with the Rotasonic method and designed with pre-packed screens, consisting of an inner layer of food-grade nylon mesh over a 0.01-inch slotted Schedule 40 PVC screen; an inner filter pack of 0.0165-inch sand; an outer layer of food-grade nylon mesh, and an outer filter pack of 0.0165-inch sand. The filter sand size was selected based on sieve/hydrometer analyses of the aquifer material in the area. No metal components were used in the pre-pack screen, thereby eliminating potential interference with metals analysis.
After installation, the wells were developed for 2-4 hours via over-pumping and surging. At the conclusion of development, all of the wells exhibited a turbidity level of less than 5 NTUs demonstrating the effectiveness of well design method. This prepack screen design can be applied to wells at other CCR sites, especially at sites where achieving lower turbidity levels is expected to be impractical due to geologic conditions.