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Hine's Emerald Dragonfly
Delineation of areas contributing groundwater to springs and wetlands supporting the Hine’s Emerald Dragonfly, Door County, WI
Final report to the Wisconsin Coastal Management Program
Michael K. Cobb and Kenneth R. Bradbury
Below is the Abstract and Introduction. Go here for the PDF version of the entire report - 33 pages and 2.9MB
The coastal springs and wetlands of Door County, Wisconsin, provide rich habitat for the highly endangered Hine’s emerald dragonfly. Understanding the source of groundwater discharging at the springs is critical to evaluating how local land-use decisions might impact the springs and to future efforts at groundwater and spring protection. This study delineated surface areas contributing groundwater to eleven sites understood to be critical Hine’s habitat in Door County. Delineations used a combination of soil water-balance modeling and simple groundwater flow modeling to determine contributing areas. Contributing areas ranged in size from 0.2 to 11.4 square miles. Shallow groundwater flows through a fractured dolomite aquifer. Predicted groundwater velocities are extremely high (up to 40 ft/day) and residence times can be quite short (less than two years at most sites). Geochemical and isotopic data collected at several springs are consistent with model results. The scope of the project did not allow detailed study at any one site, but instead focused on an overview study of many sites. The results represent a starting point for more refined studies at specific critical sites.
The coastal springs and wetlands of Door County, Wisconsin, provide rich habitat for the highly endangered Hine’s emerald dragonfly. The U.S. Fish and Wildlife Service, Wisconsin Department of Natural Resources, the Nature Conservancy, and biologists from the University of South Dakota are all actively engaged in research and other actions to better understand and protect the Hine’s emerald. Despite these efforts, a significant risk to the Hines emerald has remained poorly understood. Development and disturbance in upgradient recharge areas has the potential to alter groundwater flow to the springs and wetlands that provide habitat for the Hine’s emerald. Understanding, maintaining, and protecting groundwater flow to these coastal areas is essential for protection of the species. Delineating areas contributing water to the springs is the first step in this process.
This study has developed preliminary estimates of the areas contributing groundwater recharge that may affect eleven different Hine’s emerald dragonfly habitats in Door County (Figure 1). Recharge-area delineations include a combination of water-balance and groundwater-flow modeling supported with field measurements of water levels and baseflows. We estimated groundwater recharge rates using a GIS-linked soil-water budget model. Contributing-area delineations were made using a series of relatively simple groundwater flow models calibrated to field measurements of surface water and groundwater levels and surface-water discharges. Measurements of spring chemistry, temperature, and isotopic indicators assisted in verifying model results and will provide baseline data currently lacking at the Hine’s emerald sites.
The Hine’s emerald dragonfly was federally listed as an endangered species in 1995. It is currently known to exist in only four states (Illinois, Michigan, Missouri, and Wisconsin) and was recently found in Ontario. Its habitat is largely restricted to spring-fed wetlands in areas of dolomite bedrock. The survival of the species has been threatened by habitat destruction, degradation and fragmentation.
Adult female dragonflies lay eggs in water or mud. When the eggs hatch the larvae spend up to five years in small streams and wetlands. Only after this multi-year period as larvae dwelling in shallow surface water do they transform into adults that are recognizable as dragonflies. This adult stage is comparatively brief, lasting no more than six weeks in a period from June through August. They capture prey in flight, feeding actively during daylight hours. Adults require complex wetlands with a forest edge and cool shallow water for foraging, roosting, and reproducing.
This study would not have been possible without assistance and advice from a project advisory committee. The committee met periodically to develop the proposal, review progress, advise on next steps, and to facilitate the project. Members of the project advisory committee were as follows.
The project was funded by a grant from the Wisconsin Coastal Management Program. The following organizations provided in-kind matching assistance for carrying out the project:
We also thank private land owners in Door County who provided access to their land and allowed water-level measurements in private wells.
Door County’s principal aquifer is composed of fractured, solution-weathered Silurian age dolomite. Extensive research has been conducted on the hydrogeology of the aquifer (e.g., Sherrill, 1978; Bradbury, 1989; Bradbury and Muldoon, 1992; Muldoon and others, 2001). The dolomite strata dip gently to the east, thickening from just tens of feet in the extreme southwest on the Green Bay shore to as much as 500 ft along Lake Michigan in the northeast of the county. Soil cover over the dolomite is frequently very thin, particularly in upland areas, and rainfall and snowmelt can infiltrate rapidly. Soil thicknesses increase in occasional buried bedrock valleys, particularly along the Lake Michigan shoreline. North of Sturgeon Bay, springs, streams and wetlands are typically restricted to these depressions in the bedrock surface.
The dolomite is very permeable but has relatively little storage. Recharge is conducted rapidly into the aquifer by vertical joints. Groundwater moves laterally along bedding plane fractures, many of which have been enlarged by rock dissolution. Muldoon and others (2001) showed that discrete near-horizontal zones of high permeability may be continuous over distances of as much as 10 miles.
Groundwater discharge occurs in springs, wetlands and into Lake Michigan and Green Bay. The majority of springs in Door County occur as focused discharge though a loose cover of sediment into a spring pool or stream bed. The visible turbulence in the sand or peat is commonly called a boil. Door County’s springs have not been studied in detail, though it is assumed that most occur where highly permeable bedding plane fractures or joints intersects the bedrock surface. In many of the Hine’s emerald habitats, we infer that a bedding plane fracture opens to a buried depression in the bedrock surface. The nature and volume of these springs suggests that they are not regional discharge points receiving far-field recharge transported as deeply circulating groundwater. We consider it more likely that most identified springs receive relatively local recharge conveyed in the shallower intervals of the dolomite aquifer.
Above is the Abstract and Introduction. Go here for the PDF version of the entire report - 33 pages and 2.9MB
Last updated: April 14, 2015