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General surface characteristics of local-scale sampling areas used to evaluate distribution differences in shallow-soil (0–30 cm) perchlorate with respect to shrub proximity in three geomorphic settings.

 
Geomorphic setting Slope gradient Soil interspace pavement and shallow profile description Soil carbonate stage† Geomorphic surface age
Shoulder slope well-developed pavement with embedded clasts; vesicular A horizon underlain by dense caliche IV Plio-Pleistocene
Footslope poorly developed pavement with free-lying clasts; reworked sand with little horizonation, little caliche at depth I Holocene
Valley floor <1° well-developed pavement with embedded clasts; vesicular A and thin B horizons, little caliche at depth I to II Pleistocene
Soil carbonate stage and geomorphic surface age from Swadley and Parrish (1988). Stages represent the morphogenetic sequence of soil carbonate accumulation that increases with surface age and range from stage I (youngest surface) to IV (oldest surface) (Gile et al., 1966).



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Local-scale distribution differences in shallow-soil (0–30 cm) perchlorate and relevant variables with respect to shrub proximity (far and near) within and among three geomorphic settings (shoulder slope, footslope, and valley floor).†

 
Proximity ClO4 Cl NO3–N Cl/ClO4 NO3/ClO4 Particle size‡
TOC§ CO3–C
Gravel Sand Silt Clay
μg kg−1 —— mg kg−1 —— ——— mol mol−1 ——— % of total ——— % <2 mm ——— —— g kg−1 ——
Shoulder slope
Far 1.9 (0.2) 9.9 (4.8) 2.7 (0.8) 14,400 (5,200) 10,000 (1,800) 30.2 (13.9) 71.3 (2.5) 15.5 (1.0) 13.2 (1.9) 1.07 (0.80) 8.12 (4.67)
Near 2.9 (1.7) 5.7 (3.7) 0.7 (0.3) 6,300 (2,800) 1,800 (800) 30.0 (7.5) 70.5 (4.4) 16.8 (3.9) 12.7 (1.0) 1.97 (1.58) 8.89 (2.77)
Footslope
Far 2.2 (1.6) 2.7 (1.0) 0.3 (0.1) 4,000 (1,200) 1,100 (600) 17.6 (11.2) 77.0 (1.4) 9.3 (1.0) 13.7 (1.3) 1.30 (0.38) 5.63 (0.98)
Near 4.6 (1.6) 4.4 (2.4) 1.1 (1.0) 3,300 (3,000) 1,500 (1,100) 16.7 (8.8) 78.0 (2.6) 8.5 (2.1) 13.5 (0.6) 1.48 (0.95) 6.94 (0.34)
Valley floor
Far 5.0 (2.6) 137.0 (48.6) 50.4 (22.6) 84,300 (24,800) 73,600 (12,600) 37.8 (11.3) 66.8 (1.0) 19.5 (1.7) 13.7 (1.3) 0.97 (0.77) 3.58 (0.79)
Near 0.3 (0.1) 1.3 (1.0) 0.3 (0.1) 16,900 (14,600) 8,800 (6,200) 22.8 (3.9) 66.5 (1.7) 19.8 (1.3) 13.7 (1.7) 1.21 (0.25) 1.71 (0.60)
ANOVA (probability level¶)
Source of variation df# ClO4 Cl (log)†† NO3–N(log) Cl/ClO4(log) NO3/ClO4(log) Gravel Sand Silt (log) Clay TOC CO3–C (log)
Setting (S) 2 0.410 <0.001 <0.001 <0.001 <0.001 0.025 <0.001 <0.001 0.505 0.630 <0.001
Proximity (P) 1 0.518 <0.001 <0.001 <0.001 <0.001 0.202 1.000 0.895 0.655 0.244 0.567
S × P 2 <0.001 <0.001 <0.001 0.060 <0.001 0.268 0.776 0.619 0.934 0.680 0.028
Mean and SD (in parentheses) calculated from four replications. Values reported on dry-soil mass basis.
Gravel, >2 mm diam.; sand, 0.05–0.1 mm; silt, 0.002–0.1 mm; clay, <0.002 mm.
§Total organic carbon.
Probability level of 0.05 used for testing significance of setting, proximity, and setting × proximity (S × P) interaction effects.
#Error and total degrees of freedom were 18 and 23, respectively.
††Log10(x + 1) transformation applied before ANOVA.



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Hillslope-segment effects on shallow-soil (0–30 cm) variables.†

 
Segment‡ ClO4 Cl NO3–N Cl/ClO4 NO3/ClO4 Particle size§
TOC¶ CO3–C
Gravel Sand Silt Clay
μg kg−1 —mg kg−1 — mol mol−1 % of total — % <2 mm — — g kg−1
Upper (n = 11) 1.6b# (0.6) 4.0a (1.5) 0.8ab (0.6) 7300a (2220) 4000a (2900) 29.3a (15.4) 72.4b (5.6) 13.3a (5.6) 14.3a (1.9) 2.39a (1.70) 9.69a (5.62)
Middle (n = 22) 2.5b (1.4) 4.0a (2.1) 1.0a (0.7) 4900b (2200) 2600a (1600) 26.4a (13.1) 72.2b (5.1) 13.7a (5.2) 14.1a (1.8) 1.10b (1.06) 7.40a (2.60)
Lower (n = 15) 4.9a (3.7) 3.6a (2.0) 0.4b (0.3) 2800c (1400) 1100b (1200) 20.5a (4.1) 78.2a (4.9) 9.3b (4.3) 12.5b (1.4) 1.91a (1.35) 5.96a (0.97)
ANOVA
Probability†† 0.001‡‡ 0.728‡‡ 0.049‡‡ <0.001 <0.001‡‡ 0.309‡‡ 0.003 0.033 0.010‡‡ 0.017‡‡ 0.091‡‡
Mean and SD (in parentheses) calculated from n observations. Values reported on dry-soil mass basis.
Each sample point assigned one of three segments based on its land-surface elevation (Fig. 2).
§Gravel, >2 mm diam.; sand, 0.05–0.1 mm; silt, 0.002–0.1 mm; clay, <0.002 mm.
Total organic carbon.
#Column values followed by the same letter are not significantly different at the 0.05 probability level, as determined by Fisher’s LSD test.
††Denotes probability that no differences exist within a column; 0.05 probability level used for testing significance of segment effect.
‡‡Log10(x + 1) transformation applied before ANOVA.



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Hillslope-segment effects on creosote bush variables.†

 
Segment‡ Leaf§
Sampled plant
ClO4 Cl Cl/ClO4 Height Canopy area
μg kg−1 mg kg−1 mol mol−1 m m2
Upper (n = 6) 17,400b¶ (5,590) 800a (360) 150a (80) 0.74a (0.23) 1.08a (0.61)
Middle (n = 12) 19,600b (6,600) 900a (170) 140a (50) 0.82a (0.10) 1.05a (0.36)
Lower (n = 7) 38,800a (11,500) 1100a (520) 80a (30) 0.93a (0.21) 1.56a (0.82)
ANOVA
Probability# <0.001†† 0.334 0.069 0.163 0.175
Mean and SD (in parentheses) calculated from n observations.
Each sample point assigned one of three segments based on its land-surface elevation (Fig. 2).
§Values reported on dry-leaf mass basis.
Column values followed by the same letter are not significantly different at the 0.05 probability level, as determined by Fisher’s LSD test.
#Denotes probability that no differences exist within a column; 0.05 probability level used for testing significance of segment effect.
††Log10(x + 1) transformation applied before ANOVA.



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Correlation coefficients for determining hillslope-scale associations between shallow-soil (0–30 cm) anion (perchlorate, chloride, and nitrate as nitrogen) concentrations and other variables (soil properties, nearest shrub metrics, and terrain attributes).

 
Soil anion Soil property†
Nearest shrub metric
Terrain attribute§
Gravel Sand Silt Clay TOC CO3–C Height Canopy area Distance to
Normalized distance to center‡ Zs Gs As PROFs PLANs GENs
Canopy Center
ClO4 −0.10 0.16 −0.12 −0.17 0.17 −0.14 0.17 0.09 0.11 0.16 −0.07 −0.60* −0.44* −0.32* −0.15 −0.15 −0.15
Cl 0.22 −0.08 0.08 0.02 0.13 0.16 0.09 0.00 −0.18 −0.11 −0.14 0.03 −0.01 −0.03 −0.01 −0.07 −0.04
NO3–N 0.20 −0.20 0.15 0.22 −0.05 0.18 0.16 0.21 −0.14 0.02 −0.17 0.22 0.31* 0.29* 0.27* 0.23 0.26
*Significant at the 0.05 probability level.
Gravel, >2 mm diam.; sand, 0.05–2 mm; silt, 0.002–0.05 mm; clay, <0.002 mm; TOC, total organic carbon.
Distance from soil sample to center of nearest shrub or shrub clump divided by canopy radius of that shrub or shrub clump.
§Soil pit–centered attributes are As, slope aspect; GENs, general curvature (concave negative); Gs, slope gradient; PLANs, across-slope curvature (concave negative); PROFs, downslope curvature (concave negative); Zs, elevation.



View Full Table | Close Full ViewTable 6.

Correlation coefficients for determining hillslope-scale associations between creosote bush leaf anion (perchlorate and chloride) concentrations and other variables (soil properties, sampled plant metrics, and terrain attributes).

 
Leaf anion Soil property†
Sampled plant metric
Terrain attribute‡
Gravel Sand Silt Clay TOC CO3–C Height Canopy area Zc Gc Ac PROFc PLANc GENc
ClO4 −0.16 0.28 −0.18 −0.39 0.29 −0.36 0.42* 0.30 −0.72* −0.36 −0.48* −0.26 −0.21 −0.25
Cl 0.21 0.33 −0.24 −0.38 0.11 0.18 0.61* 0.45* −0.33 −0.20 −0.30 −0.09 −0.28 −0.19
*Significant at the 0.05 probability level.
Gravel, >2 mm diam.; sand, 0.05–2 mm; silt, 0.002–0.05 mm; clay, <0.002 mm; TOC, total organic carbon.
Creosote bush–centered attributes are Ac, slope aspect; Gc, slope gradient; GENc, general curvature (concave negative); PLANc, across-slope curvature (concave negative); PROFc, downslope curvature (concave negative); Zc, elevation.



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Optimum regression models for describing the hillslope-scale distribution of shallow-soil (0–30 cm) anion (perchlorate, chloride, and nitrate as nitrogen) concentrations and creosote-bush leaf anion (perchlorate and chloride) concentrations.†

 
Regression equation (significance level of individual coefficient) R2 or r2 SE Overall F
Soil anion response variable (n = 48)§
ClO4 = 484 (<0.001) −0.57 Zs (<0.001) −1.37 Gs (0.039) 0.423 2.02 <0.001
Cl = No model with explanatory variables significant at the P = 0.05 level
NO3–N = 0.18 (0.524) + 0.39 Gs (0.034) 0.095 0.61 0.034
Creosote-bush leaf anion response variable (n = 25)
ClO4 = 2.5 × 106 (<0.001) −2888 Zc (<0.001) −1016 CO3–C (0.033) + 2600 TOC (0.034) −96 Ac (0.046) 0.736 6762 <0.001
Cl = −65.47 (0.813) + 1195 Hc (0.001) 0.375 277.9 0.001
Explanatory variables retained in a given model are significant at the 0.05 probability level.
Significance level of overall model.
§Soil pit–centered terrain attributes are Zs, elevation and Gs, slope gradient.
Creosote bush–centered terrain attributes are Zc, elevation and Ac, slope aspect; soil properties are CO3–C, carbonate carbon and TOC, total organic carbon; sampled plant metric is Hc, creosote-bush height.