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Title: Field Studies to Confirm Uncertainty Estimates of Ground Water Discharge

Author
item CHINKUYU, A - MARYLAND DEPT. OF ENVIRON
item Guber, Andrey
item Gish, Timothy
item Timlin, Dennis
item Starr, James
item NICHOLSON, T - ONRR,NUCLEAR REG. COM.
item CADY, R - ONRR,NUCLEAR REG. COM.
item SCHWARTZMAN, A - ONRR,NUCLEAR REG. COM.
item McCarty, Gregory

Submitted to: Government Publication/Report
Publication Type: Government Publication
Publication Acceptance Date: 5/1/2007
Publication Date: 6/20/2007
Citation: Chinkuyu, A., Guber, A., Gish, T.J., Timlin, D.J., Starr, J.L., Nicholson, T., Cady, R., Schwartzman, A., McCarty, G.W. 2007. Field studies to confirm uncertainty estimates of Ground Water Discharge. Office of Nuclear Regulatory Commission. NUREG.CR-6946. p. 46.

Interpretive Summary: Little is known about the relevance of spatially varying soil properties and landscape position on groundwater recharge. Data requirements and model uncertainties for evaluating groundwater recharge were determined on a small watershed in Beltsville, Maryland. Soil water content observations, surface runoff, stream hydrographs and eddy covariance meteorological data were used to evaluate and model groundwater recharge in this multi-agency research effort. Five field methods with increasing complexity were used to estimate within-field short- and long-term ground-water recharge. Model estimates of groundwater recharge were compared to observed groundwater recharge values from the in-stream hydrographs. Methods that accounted for the spatial variability in soil properties and landscape position were highly correlated (R >0.92) with observed recharge while simple mass balance approaches performed poorly, often indicating no groundwater recharge while the stream observations demonstrated significant groundwater recharge. Although recharge estimates demonstrate significant spatial and temporal variability, there appear to be locations within the site that accurately reflect recharge estimates based on observed stream flow hydrographs. The location of these optimal measurement points appear to be spatially close to the subsurface pathways, as identified with ground-penetrating radar, and exist in well to excessively well drained-areas.

Technical Abstract: Little is known regarding data requirements and model uncertainties for evaluating surface and subsurface fluxes at the small watershed scale which are common to decommissioning and waste disposal sites. Over 3.3 million soil water content observations, with surface runoff, stream hydrographs and eddy covariance meteorological data were used at the USDA-ARS research site in Beltsville, MD to calculate groundwater recharge at the small watershed scale. Five field methods were used for estimating short- and long-term ground-water recharge at different spatial and temporal scales. Within field model estimates of groundwater recharge were compared to observed groundwater recharge calculated from de-convoluting in-stream hydrographs. Methods that accounted for the spatial variability in soil properties and landscape position were positively correlated (r>0.92) with “observed” stream recharge calculations while simple mass balance approaches were generally negatively correlated (R < -0.66). Although recharge estimates demonstrate significant spatial and temporal variability, there appear to be locations within the site that accurately reflect recharge estimates based on observed stream flow hydrographs. The location of these optimal measurement points appear to be spatially close to the subsurface pathways, as identified with ground-penetrating radar, and exist in well to excessively well drained-areas.