Hydrogeologic Characterization of Ridge-side Wetlands and Installation of Passive Groundwater Supply Wells for Mitigation Wetland
Client: Morris Management, Altoona, Pennsylvania
In comparison to valley-bottom wetlands, the hydrogeology of ridge-side wetlands has been little investigated. A permitted commercial development along a ridge-side near Altoona, Pennsylvania, planned to excavate and remove some of the wetlands. PADEP and the U.S. Army Corps of Engineers required that destroyed wetlands be replaced with an equal total area of mitigation (man-made) wetlands, sourced either by natural seepage or by very gently inclined, passive groundwater supply wells (i.e., gravity flow rather than pumped). This requirement was unprecedented and had not been accomplished, to our knowledge, anywhere in the United States.Significant challenges to the project included:
- The ridge-side hydrogeology was characterized mainly during an unusually dry period, having relatively little groundwater recharge.
- The ridge slope was blanketed by a thick overburden of very hard quartzite particles of up to boulder size, which impeded the drilling of the inclined source wells.
- Very detailed hydrologic budgets had to be developed for the mitigation wetlands, in order to estimate the amount of groundwater sourcing required from seepage or passive wells.
- Areas of potential artesian discharge to the ground were discovered by exploration/monitoring wells, but were not suitably located to provide passive groundwater supply to the mitigation wetlands.
- Numeric groundwater modeling of the ridge-side was challenging, as was the inclusion of hypothetical, gently inclined passive supply wells.
- Some individuals and groups strongly opposed the commercial development project.
The Interdisciplinary Approach
The regulatory agencies required that the client hire a hydrogeologist for this project. An extensive program was conducted to characterize the overburden and bedrock compositions and the hydrogeology. This program included the following elements: summary of an extensive, historic program of geotechnical borings; backhoe digging of overburden test pits, for evaluation of the soil material properties; installation of stillwells at multiple wetlands; stream flume installation to measure groundwater seepage discharge at the head of a wetland; monitoring well installation with measurement of groundwater levels and sampling for water quality analysis; water quality analysis of both wetland and well waters for temperature, specific conductance, field pH, major ion concentrations, total dissolved solids, and aluminum and iron; pumping test analyses of two bedrock wells with dedicated observation wells; analysis of vertical hydraulic gradients and artesian groundwater pressures; downhole geophysical survey of two deep (~ 400 ft below ground) bedrock wells, comprising digital optical televiewer and borehole flowmeter results.
The Sustainable Result
- Insight into the surprisingly complex hydrogeology of ridge-side wetlands, many of which are significantly sourced by groundwater seepage.
- Though free-flowing artesian wells were not located suitably to act as passive supply wells, we were able to exploit the presence, at depth, of areas where artesian bedrock water seeped upward into the overlying overburden.
- The successful installation of two passive groundwater supply wells to source a large mitigation wetland.
- Installation of monitoring well network into bedrock and overburden, for measuring groundwater levels and classifying water chemistry.
- Development of site conceptual model for groundwater flow and sourcing to ridge-side wetlands.
- Aquifer pumping tests near heads of two wetlands.
- Numeric groundwater modeling of potential dewatering effects of planned excavations and passive groundwater supply wells for mitigation wetlands.
- Installation of very gently inclined, passive groundwater supply wells to source a large mitigation wetland.
- Negotiations with Pennsylvania Department of Environmental Protection (PADEP) and U.S. Army Corps of Engineers.