ARS | Publication request: Multiple Methods of Recharge Estimation at a Humid-Region Fractured Rock Site

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 11, 2006
Publication Date: January 16, 2007
Publisher's URL: http://www.springerlink.com/content/102028/
Reprint URL: http://www.springerlink.com/content/d245365625x8kj31/?p=995d058a57af4c76b336aa5021aa636f&pi=0
Citation: Hepner, C.S., Nimmo, J.R., Folmar, G.J., Gburek, W., Risser, D.W. 2007. Multiple-methods investigation of recharge at a humid-region fractured rock site, Pennsylvania, USA. Hydrogeology Journal [serial online}. Available: http://www.springerlink.com/content/102028/.

Interpretive Summary: Ground-water recharge is an important component of the hydrologic cycle under humid-climate conditions, but is one of the most difficult components to quantify, especially in fractured bedrock aquifers. Direct measurement of recharge is difficult. Consequently, recharge is usually estimated by indirect methods. This study combines lysimeter percolate data with well hydrograph analysis to evaluate ground water recharge at an experimental recharge site in east-central Pennsylvania, thereby improving our understanding of the recharge processes in humid-climate, fractured bedrock settings. A new approach for calculating recharge is demonstrated using data showing continuous water table elevation fluctuations and incorporating a characteristic of the fractured bedrock termed effective fillable porosity. However, the fillable porosity can be a transient parameter, with the seasonal evapotranspiration pattern being the primary cause of variability in the effective fillable porosity due to its effect on antecedent saturation of the fracture continuum. If the transient nature of the fillable porosity can be quantified though, results from the continuous water table fluctuation method closely match the lysimeter data. This finding allows us to take advantage of routinely available data from observation wells to estimate recharge, rather than relying on more costly, site-specific instrumentation such as percolate lysimeters.

Technical Abstract: In humid regions, aquifer recharge through an unconfined, low-porosity, fractured rock aquifer can cause large magnitude water table fluctuations over short time scales. The unsaturated hydraulic characteristics of the subsurface porous media control the magnitude and timing of these fluctuations. This study combines lysimeter percolate data with well hydrograph analysis to evaluate recharge for the Masser Recharge Site in east-central Pennsylvania and thereby improve our understanding of the recharge processes in humid-climate, fractured bedrock settings. A new approach for calculating recharge from continuous water table elevation records is demonstrated. Results indicate that the effective fillable porosity of the fractured rock medium, when used in the water table fluctuation method of recharge estimation, can be a transient parameter, with the seasonal evapotranspiration pattern being the primary cause of variability in the effective fillable porosity due to its effect on antecedent saturation of the fracture continuum. However, if the transient nature of the fillable porosity can be quantified, results from the continuous water table fluctuation method closely match the lysimeter data.