HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text Free Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via ISI Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by DePaolo, D. J. Articles by Gee, G. W. Search for Related Content PubMed Articles by DePaolo, D. J. Articles by Gee, G. W. GeoRef GeoRef Citation Agricola Articles by DePaolo, D. J. Articles by Gee, G. W. Related Collections Isotopes Recharge Animal Waste

Evaporation Effects on Oxygen and Hydrogen Isotopes in Deep Vadose Zone Pore Fluids at Hanford, Washington

^a Earth Sciences Division, MS 90R1116, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767
^c Pacific Northwest National Laboratory, Environmental Technology Division, MS K9-33, 3200 Q Ave., Richland, WA 99352

View larger version (27K): [in a new window]   Fig. 1. Map of the Hanford site showing the location of the study site in the southeast part of the 200 West Area.

View larger version (29K): [in a new window]   Fig. 2. Water contents of sediment samples from the 299-W22-48 core. Right side of graph shows the lithologic column, with the thickness of the column representing the relative proportions of fine and coarse material (m: mud, s: sand, g: gravel). HFGD: Hanford Formation, gravel-dominated facies; HFSD: Hanford Formation, sand-dominated facies; CCUf(lam-msv): Cold Creek unit, silt and fine-grained sand; CCUc-f(calc): Cold Creek unit, calcic paleosol; Rtf: Ringold Formation, upper fines; Rwi: Ringold Formation, sandy gravel. Data from Serne et al. (2002b) and USDOE (2002).

View larger version (33K): [in a new window]   Fig. 3. D vs. 18O values for the vadose zone pore waters extracted from the 299-W22-48 core. Also shown are the global meteoric water line, local precipitation values (Hearn et al., 1989; data from BWI-DP-061), and the range of 18O values for shallow groundwater from Hearn et al. (1989). The estimated mean value for winter precipitation, which provides the bulk of infiltrated water, is shown by the circle and corresponds to 18O = –18 ± 0.5 and D = –138 ± 4. Deep vadose zone waters have 18O values that are 2.5 to 5 higher than mean winter precipitation. The highest 18O value is from the sample at the 38-cm depth in the core. The slope of the best fit line through the vadose zone data is 4.9, which is significantly lower than the value for the meteoric water line (approximately 8), but higher than the values measured by Allison and Barnes (1993) in experiments on evaporating soil columns (close to 3).

View larger version (25K): [in a new window]   Fig. 4. 18O vs. depth for the 299-W22-48 pore water samples. The stratigraphic units given on Fig. 2 are also shown, along with the approximate groundwater level and the average O isotopic composition of winter precipitation and Columbia River water (the water used for Hanford operations).

View larger version (24K): [in a new window]   Fig. 5. Predicted relationship between (a) the maximum 18O of near surface soil water and atmospheric relative humidity and (b) the D of near surface soil water and atmospheric relative humidity for T = 20°C. The curves give the surface value in a saturated soil column using the equation given by Barnes and Allison (1983). For soil columns that dry out, the maximum value occurs below the surface and can be somewhat different from that shown (Barnes and Allison, 1988; Mathieu and Bariac, 1996). The values measured for the sample from the 38-cm depth in the W22-48 core are reasonably consistent with the local conditions (summer relative humidity of 40% and mean winter precipitation values of 18O = –18 and D –138). (c) Monthly mean humidity at the Hanford weather station (http://etd.pnl.gov:2080/HMS/) for 1999 through 2001.

View larger version (22K): [in a new window]   Fig. 6. (a) Measured evaporation factor (Ef) vs. percentage fine material (<53 µm) for Hanford soils as determined by Gee and Ward (2002). The Ef value is an estimate of annual water loss by evaporation. (b) Estimated water content range and midpoint for Hanford soils as a function of percentage of fine material in the soil. The maximum values apply to the winter wet season and the minimum values, which are reached only by the upper 50 cm or so of the soil, apply to late summer. (c) Evaporation factor vs. the midpoint water content value from Fig. 6b. The slope of the line and the x-intercept represent the values used for Eo and s (Eq. [4b]).

View larger version (18K): [in a new window]   Fig. 7. Conceptual model for the origin of heavy 18O values in deep vadose zone fluids. Initial infiltration involves water with the 18O value characteristic of local winter precipitation (=p) and results in moisture and 18O profiles shown by the dashed lines. By the end of the summer dry season (solid lines in figure), evaporation has produced isotopic enrichment of the water in the uppermost meter or so of the soil column. The enrichment is accompanied by partial dry-out of the soil to a depth Lv, and continued downward drainage to a depth Lin. Subsequent displacement of the enriched waters by the next year's infiltration, coupled with dispersion generates a relatively uniform 18O value in the deep vadose zone fluids (mean) that represents the mean isotopic composition of the water that initially resided between the surface and the depth Lin.

View larger version (30K): [in a new window]   Fig. 8. Predicted relationship between annual net infiltration and the 18O of deep vadose zone waters, for soils with different water retention characteristics. The curves shown represent the solution to Eq. [5] for soils with different contents of fine material (<53 µm), using the minimum water contents from Fig. 6b (s; given in parentheses in the legend), and for ha = 0.4, T = 20°C, and Eo = 5640 mm yr–1. The percentage of fine material corresponds approximately to the following soil types (0%: gravel; 3%: coarse sand; 10%: sand; 25%: sandy loam). The horizontal lines denote the mean and standard deviation of measured 18O values for vadose zone water at the 299-W22-48 site (Table 1 and Fig. 4; average = –14.4 ± 0.6). The data correspond to a net infiltration flux in the range 35 to 60 mm yr–1.