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Tracing Nitrate Transport and Environmental Impact from Intensive Swine Farming using Delta Nitrogen-15

^a Dep. of Biology, Phytotron Building, Duke University, Durham, NC 27708
^b Dep. of Marine, Earth and Atmospheric Sciences, North Carolina State Univ., Raleigh, NC 27695-8208
^c Dep. of Soil Science, North Carolina State University, Raleigh, NC 27695
^d Delaware Geological Survey, Univ. of Delaware, Newark, DE 19716-7501

View larger version (49K): [in a new window]   Fig. 1. Study site map with monitoring well transects and stream sampling sites. Swine houses and lagoons are near the northern border of each farm. Well transects traverse sprayfields into riparian buffers. The farms are located within the Stewarts Creek watershed, Black River subbasin, Cape Fear River basin, North Carolina. Elevations in meters above sea level. Average water table contours follow the topography very closely.

View larger version (84K): [in a new window]   Fig. 2a. Geologic profiles of Well Transect 1. Nests consist of three individual wells each. Water table depth is generally 0 to 1 m at Nest D and E wells and 1 to 3 m at Nest B and C wells. Transport across confining clay layers within the Black Creek Fm (formation) is unknown, but upward flow of deep ground water may be significant near stream beds. The Black Creek Fm probably isolates Nests 1D and 1E from the sprayfields. The rest of Transect 1 probably receives some ground water flow from north of the sprayfields.

View larger version (60K): [in a new window]   Fig. 2b. Geologic profiles of Well Transects 3 and 6. Nests consist of three individual wells each. Water table depth is generally 0 to 1 m at Nest D and E wells and 1 to 3 m at Nest B and C wells. Transport across confining clay layers within the Black Creek Fm (formation) is probably minor, as evidenced by nitrate-free water collected within the Black Creek, but upward flow of deep ground water may be significant near stream beds. The riparian zone between Transects 3 and 6 is very narrow (ca. 10 m max.).

View larger version (35K): [in a new window]   Fig. 3. Swine lagoon 15N-total N and monthly mean air temperature. Multiple samples from different points in lagoons were taken during three of the sampling events. Increased ammonia volatilization in warmer months should increase 15N enrichment in residual lagoon ammonium.

View larger version (28K): [in a new window]   Fig. 4. Chloride and potassium vs. ammonium N in lagoons and nitrate N in wells. Swine waste lagoons were enriched in potassium and chloride.

View larger version (14K): [in a new window]   Fig. 5. Left panel: Boxplots of 15N in lagoon ammonium and in well and stream (onsite, adjacent, and downstream) nitrate. Right panel: Boxplots of 15N in depth-integrated nests for combined transects. Boxes enclose the upper and lower quartile (25%) of data about the median, which is represented by a horizontal line within each box. Whiskers above and below boxes represent the upper quartile plus 1.5 times the interquartile distance (IQD) and the lower quartile minus 1.5 times the IQD. Outliers are represented by circles. Since samples from Transect 1 Nest E contained insufficient nitrate for isotopic analysis, these were not included in the Nest E plot.

View larger version (19K): [in a new window]   Fig. 6. Mean nitrate N concentrations and 15N-NO3 with standard error (no error shown for stations with only one sample) for on-site stream stations and Stewarts Creek stations upstream, adjacent to, and downstream of the farms.

View larger version (26K): [in a new window]   Fig. 7. Transects 1 and 3 log-linear relationships of nitrate N concentration vs. 15N-NO3. Slopes are indicative of denitrification enrichment of 15N of residual nitrate in some samples.

View larger version (25K): [in a new window]   Fig. 8. 15N-NO3 vs. nitrate N concentrations, dissolved oxygen (DO) levels, and pH. The 15N well above the median is exhibited only in samples with DO < 2 mg/L and pH > 5, supporting the general trend of denitrification enrichment of 15N in a small number of samples.

View larger version (27K): [in a new window]   Fig. 9. 15N-NO3 values vs. depth (m). Well 3E-S had the most positive shallow well 15N value (in August, 1996). Well 1E-I had the least positive 15N value (in April, 1997). Well 3B-D showed several very positive 15N values, with dissolved oxygen < 2 mg/L and pH > 5. This well is within an organic silt layer (see Fig. 2b).

View larger version (23K): [in a new window]   Fig. 10. Boxplots of 15N-NO3 and nitrate N concentrations in stream-edge (Nest E) wells of Transects 3 and 6, showing trends with depth. The 15N enrichment is pronounced only in shallow wells, indicating little denitrification in deeper stream-edge wells. This limited nitrate attenuation allows the off-site export of sprayfield nitrate via discharge of high-nitrate ground water to the stream that drains the sprayfields and then enters Stewarts Creek. Boxes enclose the upper and lower quartile (25%) of data about the median, which is represented by a horizontal line within each box. Whiskers above and below boxes represent the upper quartile plus 1.5 times the interquartile distance (IQD) and the lower quartile minus 1.5 times the IQD. Outliers are represented by circles.