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Groundwater Age Dating & Recharge

Overview

GWRP supports applied research into the use of groundwater age information to improve our understanding of recharge in Principal Aquifers.

Purpose & Scope

Groundwater recharge is often difficult to quantify because of its spatial and temporal variability and because of the challenges of measuring it directly.  However, recharge estimates are an important component of water budgets developed to accurately assess groundwater availability.  GWRP supports efforts to evaluate the use of groundwater age data to improve our overall understanding of recharge and thereby improve groundwater availability studies through the following activities:

• Use of groundwater ages to define timescales for recharge processes that could be used as an indicator of aquifer sustainability.
• Use of groundwater ages to develop recharge estimates to constrain other recharge estimation methods.
• Characterization of climatic conditions associated with age distributions may provide a baseline for understanding how recharge might change in response to future climate scenarios.

Through this work, the USGS will:

• evaluate which age-dating study designs provide the most useful data for understanding recharge conditions,
• make recommendations as to where age-dating studies should be done in the future to support GWRP regional groundwater availability studies,
• collect additional age data to supplement the existing dataset at specific sites, and
• demonstrate techniques for using age data to understand recharge conditions.

Methods & Activities

This project will focus on synthesis of existing data and collection of new data for two general categories of recharge:

1. modern recharge (roughly representing the past 50 years) using atmospheric tracers such as CFC, SF₆, and ³H/³He and
2. old recharge (roughly 1,000 to 30,000 years before present) using carbon-14 in dissolved inorganic carbon.

This project began in early 2008 and will run through late 2009.

Synthesis of Existing Data

• Existing age data will be used to develop a hierarchical classification scheme for defining the timescale of recharge in the Principal Aquifers of the United States.
• Recharge estimates will be made using applicable groundwater age distributions and simple lumped-parameter models where appropriate.
• Climatic conditions will be characterized for time periods defined by the age distributions.  An attempt will be made to characterize recharge and climate for a range of hydrologic and climatic conditions.

Collection of New Data

Groundwater samples will be collected and analyzed to fill important data gaps in existing groundwater availability studies and to demonstrate age-dating techniques that could be used in future studies to understand recharge conditions.

• Tracers of modern recharge: Groundwater samples will be analyzed for ³H/³He, CFCs, and SF₆.
• Tracers of paleorecharge: Groundwater samples will be analyzed for ¹⁴C of dissolved inorganic carbon.

Publication(s)

McMahon, P.B., Plummer, L.N., Bohlke, J.K., Shapiro, S.D., and Hinkle, S.R., 2011, A comparison of recharge rates in aquifers of the United States based on groundwater-age data: Hydrogeology Journal, DOI 10.1007/s10040-011-0722-5

For more information:

For more information about this project, contact:
Peter B. McMahon, Research Hydrologist
(303) 236-4882 x286