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Spatial Variability and Transport of Nitrate in a Deep Alluvial Vadose Zone

Department of Land, Air, and Water Resources, One Shields Avenue, University of California, Davis, CA 95616-8628

View larger version (28K): [in a new window]   Fig. 1. The field experiment design showing the locations of five different fertilizer treatments at the project site. Three subplots with the 0, 110, and 365 kg N ha–1 yr–1 treatments were selected for vadose zone sampling. The three subplots were named "control," "standard," and "high," respectively.

View larger version (28K): [in a new window]   Fig. 2. A schematic representation of the components of the N budget and water mass balance.

View larger version (44K): [in a new window]   Fig. 3. Scatter plots of water content with depth for each subplot. Line represents linear regression models for the trend defined as a function of depth. Equations for the trends are C = 0.069 + 0.019z for the control subplot (a correlation coefficient of r2 = 0.48), S = 0.127 + 0.013z for the standard subplot (r2 = 0.25), and H = 0.087 + 0.017z for the high subplot (r2 = 0.36).

View larger version (34K): [in a new window]   Fig. 4. (a) Frequency distribution of NO3–Naq (g m–3 = µg mL–1) (includes 224 nondetects in the left-most class). (b) Logtransformed NO3–Naq data fitted to a normal distribution with a mean of 1.0 and variance of 1.095 (does not include nondetects).

View larger version (25K): [in a new window]   Fig. 5. (a) Facies X subplot group means and 95% confidence intervals for natural logtransformed NO3–Naq (g m–3), not including nondetect samples. Facies are sorted in vertical sequence followed by their average depth interval in meters given in parentheses: SL1, sandy loam (0–2.75); C, clay (2.75–3); Var1, thin, predominantly sand (2.75–3); HP1, hardpan-like paleosol (3–3.65); Var2, various sandy loam to clay loam (3.65– 6.7); S, medium sand (6.7–9); Var3, various textures (9–9.75); C-Si-L, fine-textured floodplain deposits (9–12); SL2, sandy loam (12–15); and HP2, a hardpan-like paleosol (15–15.85). Lithofacies symbols followed by the same numbers (1, 2) indicate no significant differences between lithofacies groups at the 95% significance level (C and Var3 had less than four samples and those were from only one subplot and were therefore not included in the ANOVA). Fertilizer treatments (control, standard, high) followed by the same Roman numerals (I, II) indicate no significant difference between the fertilizer treatments at the 95% significance level. (b) Facies X subplot group means of the detect–nondetect indicator variable, that is, the fraction of nondetects in each group. No means were computed for C and Var3.

View larger version (18K): [in a new window]   Fig. 6. Experimental and spherical model semivariograms for water content (trend residuals) in the horizontal direction for the (a) control, (b) standard, and (c) high subplot; and in the vertical direction for (d) the control, (e) standard, and (f) high subplot. Diamonds denote experimental values and lines denote spherical semivariogram models.

View larger version (44K): [in a new window]   Fig. 7. Contour maps of the kriged (a) water content at the control, standard, and high fertilizer treatment sites; (b) various depth profiles of water content at each subplot.

View larger version (16K): [in a new window]   Fig. 8. Normalized experimental and spherical model semivariograms for lnNO3–Naq in the horizontal direction for the (a) control, (b) standard, and (c) high subplot and in the vertical direction for the (d) control, (e) standard, and (f) high subplot. Diamonds represent the experimental semivariograms for the complete NO3–Naq dataset; triangles denote experimental semivariograms computed from the dataset without nondetect samples. Solid lines are the spherical semivariogram models fitted to the experimental variograms of the complete data set.

View larger version (39K): [in a new window]   Fig. 9. Contour maps of the kriged (a) lnNO3–Naq (g m–3 = µg mL–1) at the control, standard, and high fertilizer treatment sites; (b) various depth profiles of lnNO3–Naq at each subplot.