Horizontal Drilling Mud Release Investigation at Detrana Fen, McHenry County, Illinois
Fens are peat-forming wetlands that receive nutrients from sources other than precipitation, usually from upslope sources through drainage from surrounding soils and from groundwater movement. Fens differ from other wetlands (such as bogs) that are fed by groundwater because they are less acidic and have higher nutrient levels. They are therefore able to support a much more diverse plant and animal community. These systems are often covered by grasses, sedges, rushes, and wildflowers. Some fens are characterized by parallel ridges of vegetation separated by less productive hollows. The ridges of these patterned fens form perpendicular to the downslope direction of water movement. Over time, peat may build up and separate the fen from its groundwater supply. When this happens, the fen receives fewer nutrients and may become a bog (modified from USEPA1).
Of all the wetland types, fens are probably the least familiar to the public. This type of wetland community contains peat deposits (partly decayed plant material) and calcareous (calcium carbonate containing) water seepage. Fens are present on or at the base of slopped areas, often occurring on hillsides where seepage and springs run from the ground. Found exclusively in the northern third of the Illinois, fens may be covered by trees and shrubs or may be dominated by herbaceous wetland communities such as sedge meadows and marshes. In Illinois, and across the United States, fens are among the rarest of wetland communities, thereby emphasizing the value of their potential restoration. Wetland restoration, in general, is of particular importance in Illinois, where over 90% of our original wetland acreage has already been lost. In northeastern Illinois, specifically, urban sprawl and development pressures of the Chicago metropolitan area are especially great (slightly modified from INHS2).
Detrana Fen is a 161 acre wetland located in McHenry County, Illinois3. Drilling mud was released under the Detrana Fen during horizontal directional drilling operations conducted during the installation of a 36-inch diameter natural gas pipeline in November of 2001. Bentonite-based drilling mud emerged at ground surface at three locations within the fen in a 50-ft by 50-ft area. The effects of the release were surveyed and bentonite was recovered. Bentonite clay expands when wet, and it is used during drilling or well sealing and also in geotechnical and environmental investigations as a sealant. A bentonite release within a wetland could affect its groundwater chemistry and also disrupt groundwater flow, which could damage or degrade the wetland. An initial assessment of the bentonite release was conducted in early 2002 that evaluated the surficial hydrogeology and makeup of the suspected bentonite-impacted soils. A follow-up investigation performed in January 2003 refined the understanding of the shallow hydrogeology, and assessed the deep hydrogeology, hydraulic responses at the fen at the study area over time, and groundwater chemistry to determine if the fen had been impacted by the bentonite release.
The hydrogeological characterization shows that the site has 10- to 20-ft of organic-rich and fine grained (silts and clay) soils at the ground surface with an intermittently present peat layer that is up to 2.5 ft thick within the fine-grained soil. Fine-grained soils and peat are underlain by over 40 feet of sand and gravel glacial outwash. High ground covered in grass, shrubs, or mature oak trees surround the Dentrana Fen area. Groundwater levels in piezometers screened above the peat and wells below the peat are occasionally artesian, and the deep well screened near the pipeline (approximately 50-ft below ground surface) is consistently artesian. Vertical groundwater gradients were consistently upward during the monitoring period. Horizontal groundwater flows were toward local streams and open surface water bodies.
Results from these investigations show that the fen had not been measurably impacted by the bentonite release. No evidence of bentonite drilling mud was identified in the 16 borings completed in the 50-ft by 50-ft study area, indicating that the areas where bentonite did emerge at ground surface were vertical features and that the extent of bentonite is not likely to be widespread. Wells inside and outside of the release area showed that hydraulic responses were consistent with upwelling groundwater that is typical of fens, and that all wells behaved in a similar manner that indicates that the hydraulic connection at the fen had not been effected by the bentonite release. Groundwater parameters and analytical results for samples collected at all wells were within expected ranges for Midwestern temperate zone fens, showing that bentonite, which does not typically impact groundwater chemistry, had not impacted the study area's groundwater character.
1. United States Environmental Protection Agency; Fens: http://water.epa.gov/type/wetlands/fen.cfm.
2. Illinois Natural History Survey, Prairie Research Institute; Fen Wetland Restoration in Northeastern Illinois: http://www.inhs.illinois.edu/inhsreports/nov-dec99/fen.html.
3. Illinois Natural History Survey, Prairie Research Institute: Development of a Natural Areas Integrity and Restorability Index and Application to Lands of the Chicago Region: http://www.inhs.uiuc.edu/research/inai/Pt1_Appendix5.pdf.