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Groundwater Nitrate Spatial and Temporal Patterns and Correlations

Influence of Natural Controls and Nitrogen Management

^a Div. of Plant Science, Univ. of Missouri, Columbia, MO 65211
^b Dep. of Soil Science, North Carolina State Univ., Raleigh, NC 27695-7619
^c Dep. of Crop Science, North Carolina State Univ., Raleigh, NC 27695-7620
^d Dep. of Statistics, North Carolina State Univ., Raleigh, NC 27695-8203

View larger version (43K): [in this window] [in a new window]   Fig. 1. Field layout of the randomized complete block design at the Lower Coastal Plain Tobacco Research Station, Kinston, NC. Shading patterns indicate plots within the same block. The N management treatment number is in the upper left corner of each plot. Nitrogen treatments are: RYE = uniform realistic yield expectation N management; FA = remote sensing informed, in-season, uniform, field-average N management; SS = remote sensing informed, in-season, site-specific, variable-rate N management.

View larger version (34K): [in this window] [in a new window]   Fig. 2. (a) Daily precipitation from November 2000 through July 2003 and (b) actual (November 2000–July 2003, bars) and 30-yr-average (1974–2003, line) monthly precipitation at the study site. Note that the beginning of the precipitation reporting period precedes the groundwater reporting period (Fig. 3) by 8 mo to provide the meteorological context for the latter.

View larger version (32K): [in this window] [in a new window]   Fig. 3. (a) Field-mean shallow groundwater NO3–N concentration sampled over the 0.9- to 3.7-m depth from July 2001 to July 2003; the reference line is the USEPA drinking water maximum contaminant level goal, 10 mg L–1 NO3–N; (b) field-average water table depth (0 m represents the soil surface); (c) NO3–N spatial correlation range; and (d) total (field-scale) and plot-scale NO3–N spatial variances. The field-scale spatial variance is analogous to the sill and the plot-scale variance analogous to the first lag interval variance of a semivariogram. Error bars indicate ±1 SE of all estimates.

View larger version (60K): [in this window] [in a new window]   Fig. 4. Spatial patterns of groundwater NO3–N concentrations sampled over the 0.9- to 3.7-m depth on (a) 22 Feb. 2002 and (b) 8 Apr. 2003. Nitrate-N concentrations were estimated using inverse distance-squared weighted interpolation. The N management treatments were: RYE = uniform realistic yield expectation N management; FA = remote sensing informed, in-season, uniform, field-average N management; SS = remote sensing informed, in-season, site-specific, variable-rate N management.

View larger version (15K): [in this window] [in a new window]   Fig. 5. Global (isotropic) semivariograms of groundwater NO3–N concentration on 22 Feb. 2002 (spherical model: nugget = 0.57, sill = 5.1, range = 196 m; R2 = 0.99, residual sum of squares = 0.03) and 8 Apr. 2003 (exponential model: nugget = 1.13, sill = 11.040, effective range = 112 m; R2 = 0.93; residual sums of squares = 1.7).

View larger version (44K): [in this window] [in a new window]   Fig. 6. (a) Directional semivariograms of groundwater NO3–N concentrations and (b) field water table elevation gradient, both from 22 Feb. 2002. Water table elevation was estimated using inverse distance-squared weighted interpolation of measured depth to the water table at each well nest, corrected for surface elevation derived from lidar. The N management treatments were: RYE = uniform realistic yield expectation N management; FA = remote sensing informed, in-season, uniform, field-average N management; SS = remote sensing informed, in-season, site-specific, variable-rate N management.

View larger version (21K): [in this window] [in a new window]   Fig. 7. Temporal variation of the within-plot (between the two well nests in a treatment plot) variance of groundwater NO3–N concentrations by N treatment during a 2-yr study. Dates with fewer than five pairs to compute the treatment variance are not shown. The N treatments were: RYE = uniform realistic yield expectation N management; FA = remote sensing informed, in-season, uniform, field-average N management; SS = remote sensing informed, in-season, site-specific, variable-rate N management.

View larger version (27K): [in this window] [in a new window]   Fig. 8. Significance of N management treatments (Trt) and spatial natural controls (Elev, soil surface elevation; Sand, mean sand content; Silt, mean silt content; Clay, mean clay content; SOM, surficial soil organic matter content; Ksat, mean saturated hydraulic conductivity; WTE, water table elevation; and WTD, water table depth) in explaining within-field groundwater NO3–N concentrations on 54 individual dates across a 2-yr study. Asterisks (*) indicates dates when Trt was significant; + and – indicate dates when individual covariates were significant and whether the association between the covariate and groundwater NO3–N concentration was positive or negative, respectively.