Power Station Background
The coal-fired power plant, located in Gallia County, Ohio, has been in operation for over 50 years. During the course of plant operations, Coal Combustion Residuals (CCRs) have been managed in three (3) CCR landfills at the station.
CCR Landfill Project Work
The landfill is a lined landfill constructed in an existing stream valley. The base of the landfill is separated from the uppermost aquifer by more than 100 feet of low permeability shale and sandstone. The landfill is being constructed in phases and has been the subject of an ongoing state groundwater monitoring program in which numerous wells have been installed around the planned final limits of the landfill. To evaluate if the existing wells screened in the uppermost aquifer would be suitable for use under the CCR program, a preliminary groundwater sampling event was conducted under attorney-client privilege using the same groundwater sampling methods to be used under the CCR program. Based on the results of the preliminary sampling, the existing wells were deemed to be suitable for use in the CCR groundwater monitoring network.
Groundwater flow in the uppermost aquifer was evaluated and it was determined that two (2) additional monitoring wells were required to satisfy the CCR requirement of one (1) upgradient and three (3) downgradient monitoring wells. The new wells were installed close to the limit of the active phase of the landfill using the same design and construction as the existing landfill wells.
Two (2) surface impoundments were to be monitored under the CCR program. Neither impoundment had an existing groundwater monitoring network in place. Several wells and piezometers installed around each impoundment had been used to collect water levels but had never been sampled and most of these existing wells and piezometers were not installed in the uppermost aquifer. The existing wells and piezometers were sampled during the preliminary groundwater sampling event conducted under attorney-client privilege to evaluate groundwater quality around the surface impoundments.
Available water level data and boring logs from the installation of the existing wells and piezometers were reviewed to evaluate groundwater level and flow. Very few of the existing wells and piezometers had been installed into the uppermost aquifer beneath each impoundment. Because there was limited subsurface data for the impoundments, two (2) soil borings were conducted at each impoundment to better define the location of the uppermost aquifer and to collect a sample of the aquifer formation for grain size analysis.
Monitoring Well Design
The CCR rule assumes that the CCR network monitoring wells be designed and installed so as to produce a turbidity of less than 5 NTUs without field filtering. The proper design of the sand pack and well screen is therefore critical to obtaining representative groundwater samples. To support CCR well design, representative samples were collected of material from the uppermost aquifer beneath each impoundment and submitted to a geotechnical laboratory for grain-size analysis. The results of the grain-size analysis were used to select the grain size for the sand packs and the slot size for the well screens.
To reduce turbidity as much as possible, AGES used pre-packed well screens consisting of an inner filter pack containing clean quartz sand and an outer layer of food-grade nylon mesh. The pre-packed well screens were constructed without metal components, thus eliminating potential interferences with metals analysis.
Over the course of approximately three (3) months, 40 new CCR wells were installed around the surface impoundments and at the landfill. Following installation, the new monitoring wells around the impoundments were developed until turbidities in all wells were below 5 NTUs. The uppermost aquifer beneath the landfill is a low-yielding sandstone in which most of the wells are purged dry before sampling. The landfill wells were developed over a period of several weeks until the development parameters stabilized.
AGES began the required eight (8) rounds of background samples to be collected before August 2017. All of the monitoring wells are sampled using low-flow sampling techniques in accordance with US EPA guidelines to minimize the turbidity of the unfiltered samples. After collection of the first three or four rounds of data, AGES will run preliminary statistical analyses to evaluate the effectiveness of the groundwater monitoring network.
This utility was pro-active in addressing the requirements of the CCR rule and all of the required CCR groundwater monitoring networks were installed before the end of 2015. This gives the utility the ability to spread out the eight (8) required groundwater sampling events evenly over the next year-and-a-half with enough time to address unforeseen issues with the groundwater monitoring network.