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Limited Occurrence of Denitrification in Four Shallow Aquifers in Agricultural Areas of the United States

^a USGS, 345 Middlefield Rd., Menlo Park, CA 94025
^b USGS, 12201 Sunrise Valley Dr., Reston, VA 20192
^c USGS, 6000 J St., Placer Hall, Sacramento, CA 95819
^d USGS, 810 Bear Tavern Rd., West Trenton, NJ 08628
^e J.M. Denver, USGS, 1289 McD Dr., Dover, Delaware 19901
^f USGS, 10615 SE Cherry Blossom Dr., Portland, OR 97216

View larger version (95K): [in this window] [in a new window]   Fig. 1. Plan view maps showing well nest locations and names, simulated backward particle tracking paths, and simulated source locations for each particle for (A) Maryland, (B) Washington, (C) California, and (D) Nebraska.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Excess N2 estimates compared to dissolved O2 concentrations. Two samples from MD (M20q and M20r) were excluded due to apparent mixing of aerobic and denitrified water.

View larger version (13K): [in this window] [in a new window]   Fig. 3. Distributions in all ground water samples from each site of (A) average age with mixing (piston flow ages shown for Maryland, where mixed ages were not calculated), (B) piston flow age, (C) dissolved O2, (D) denitrification progress (proportion of initial nitrate in ground water subsequently converted to N2, denit in Eq. [4]), and (E) NO3–. Error bars show the minimum and maximum values, boxes show the interquartile range, and horizontal lines inside the boxes show the median.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Trends of stable isotopes in ground water. Values of 15N and 18O are given with respect to air N2 and Vienna standard mean ocean water, respectively, in parts per thousand. (A) 15N[NO3–] versus 18O [NO3–]. Lines are least squares regressions of aerobic and anaerobic ground water data. (B) Change in 15N[NO3–] during transport (15N[NO3–]- 15N[NO3–]0) versus denitrification progress, denit. For comparison, curves show hypothetical Rayleigh fractionation based on apparent fractionation factors () of –5 and –20. (C) 18O values of coexisting NO3– and H2O in aerobic ground water and near stream samples. Dotted lines are given for reference and are defined by the isotopic composition of the major potential O sources during nitrification: 18O[NO3–] = a 18O[H2O] + (1–a) 18O[air O2], with a = 0, 0.67, and 1. For (A), (B), and (C), "aerobic" samples (gray symbols) are defined by O2 > 0.016 mmol L–1 and water age < 25 yr. "Near stream" samples (unfilled symbols shown here for comparison and not included in other analyses) are ground water from near-river piezometers (Puckett et al., 2007) that show minimal interaction with surface water.

View larger version (12K): [in this window] [in a new window]   Fig. 5. Distributions of reconstructed 15N[NO3–]0 of ground water (including undenitrified samples) compared to points (diamonds) showing 15N for surface soils, and box plots of 15N values for aquifer sediments at all study sites. Values of 15N are given with respect to air N2, in parts per thousand. For the box plots, error bars show the minimum and maximum values, boxes show the interquartile range, and horizontal lines show the median.

View larger version (13K): [in this window] [in a new window]   Fig. 6. (A) Average water age with mixing compared to reconstructed 15N[NO3–]0. Values of 15N are given with respect to air N2, in parts per thousand. (B) Average water age with mixing compared to [NO3–]0. (C) Average water age with mixing compared to denitrification progress, denit. Lines are least squares regressions to the data from the California transect, and the N23 and N21 well clusters at Nebraska. (D) Average water age with mixing versus dissolved O2.

View larger version (29K): [in this window] [in a new window]   Fig. 7. Cross sections of well locations, river locations, and denitrification progress for (A) California and (B) Nebraska. The light gray shading shows approximate zones of oxygen reduction below the water table, the moderate gray shading shows the approximate zones of denitrification, and the dark gray shading shows the zones of complete denitrification.

View larger version (15K): [in this window] [in a new window]   Fig. 8. Maximum, in situ, zero-order denitrification rates from previous studies and from this study, sorted from lowest to highest values. The rate from Singleton et al. (2007) is inferred from excess N2 and apparent age values from well W-16.

View larger version (13K): [in this window] [in a new window]   Fig. 9. Distributions of solid phase constituents including (A) organic C, (B) Fe(II), and (C) acid-volatile sulfides (AVS) + pyrite-S. Error bars show the minimum and maximum values, boxes show the interquartile range, and horizontal lines inside the boxes show the median.