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Preferential flow and transport of nitrate and bromide in claypan soil

Ground Water

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Abstract

The in situ measurement of water flow and chemical transport through claypan soils is crucial to understanding potential water contamination from agricultural sources. It is important due to the large areal extent of these soils in agricultural regions of the midwestern United States and because of preferential flowpaths caused by desiccation cracks, worm burrowing, and root development. A study plot at the Missouri Management Systems Evaluation Area near Centralia, Missouri, was instrumented to determine the rate of preferential flow of water and transport of NO3-1 fertilizer in the unsaturated zone through a claypan soil using 15N-NO3-1 and Br-1 tracers. The areal distribution of preferential flowpaths was between 2 and 20% in the topsoil. Gravity lysimeter flow caused by preferential flow through the claypan was as much as 150 times greater than the estimated average rate of vertical recharge. As much as 2.4% of the volume of the sod below the claypan may be occupied by preferential flowpaths. The 15N-NO3-1 concentrations in ground water indicate that substantial quantifies of fertilizer-derived NO3-1 were transported to ground water through the claypan during the first recharge event following fertilizer application even though that event occurred six months after application. Hydraulic conductivity, measured at three scales, ranged from 6.2 x 10-8 to 7.5 x 10-3 cm/s. The observed increase of calculated hydraulic conductivity with each increase in scale was attributed to the inclusion of more and larger preferential flowpaths within the volume over which the measurement was made, indicating hydraulic conductivity measured at one scale may not describe flow and transport at another scale.The in situ measurement of water flow and chemical transport through claypan soils is crucial to understanding potential water contamination from agricultural sources. It is important due to the large areal extent of these soils in agricultural regions of the midwestern United States and because of preferential flowpaths caused by desiccation cracks, worm burrowing, and root development. A study plot at the Missouri Management Systems Evaluation Area near Centralia, Missouri, was instrumented to determine the rate of preferential flow of water and transport of NO3-1 fertilizer in the unsaturated zone through a claypan soil using 15N-NO3-1 and Br-1 tracers. The areal distribution of preferential flowpaths was between 2 and 20% in the topsoil. Gravity lysimeter flow caused by preferential flow through the claypan was as much as 150 times greater than the estimated average rate of vertical recharge. As much as 2.4% of the volume of the soil below the claypan may be occupied by preferential flowpaths. The 15N-NO3-1 concentrations in ground water indicate that substantial quantities of fertilizer-derived NO3-1 were transported to ground water through the claypan during the first recharge event following fertilizer application even though that event occurred six months after application. Hydraulic conductivity, measured at three scales, ranged from 6.2??10-8 to 7.5??10-3 cm/s. The observed increase of calculated hydraulic conductivity with each increase in scale was attributed to the inclusion of more and larger preferential flowpaths within the volume over which the measurement was made, indicating hydraulic conductivity measured at one scale may not describe flow and transport at another scale.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Preferential flow and transport of nitrate and bromide in claypan soil
Series title:
Ground Water
Volume
36
Issue:
3
Year Published:
1998
Language:
English
Publisher:
Ground Water Publ Co
Publisher location:
Westerville, OH, United States
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Ground Water
First page:
484
Last page:
494