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ORIGINAL RESEARCH

Chloride Mass Balance

Cautions in Predicting Increased Recharge Rates

^a Pacific Northwest National Laboratory, Richland, WA
^b University of Nevada and Desert Research Institute, Reno, NV
^c Lawrence Berkeley Laboratory, Berkeley, CA
^* Corresponding author (glendon.gee{at}pnl.gov)

Received 21 February 2004.

The chloride mass balance (CMB) method has been used extensively to estimate recharge in arid and semiarid environments. Required data include estimates of annual precipitation, total Cl^– input (from dry fallout and precipitation), and pore water Cl^– concentrations. Typically, CMB has been used to estimate ancient recharge, but recharge from recent land-use change has also been documented. Recharge rates below a few millimeters per year are reliably detected with CMB; however, estimates above a few millimeters per year appear to be less reliable. We tested the CMB method against 26 yr of drainage from a 7.6-m-deep lysimeter at a simulated waste burial ground located on the Department of Energy's Hanford Site in southeastern Washington, USA where removal of vegetation has increased recharge rates. Measured drainage from the lysimeter for the past 26 yr averaged 62 mm yr^–1. Precipitation averaged 190 mm yr^–1 with an estimated Cl^– input of 0.22 mg L^–1. Initial pore water Cl^– concentration was 88 mg L^–1 and decreased to about 6 mg L^–1 after 26 yr, while the drainage water Cl^– concentration decreased to <1 mg L^–1. A recharge estimate made using Cl^– concentrations in drain water was within 26% of the measured drainage rate. In contrast, recharge estimates using 1:1 (water/soil) extracts were lower than actual values by factors ranging from 2 to 8 or more. The results suggest that when recharge is above a few millimeters per year, soil water extracts can lead to unreliable estimates of recharge.

Abbreviations: CMB, chloride mass balance • HMS, Hanford Meteorological Station

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