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Seasonal fluctuation in delta 15N of groundwater nitrate in a mantled karst aquifer due to macropore transport of fertilizer-derived nitrate


Journal of hydrology 112(1-2): 191-201
Seasonal fluctuation in delta 15N of groundwater nitrate in a mantled karst aquifer due to macropore transport of fertilizer-derived nitrate
Twenty wells were sampled and analyzed for nitrate content and nitrogen isotopes during late September, 1986. The groundwater nitrate delta 15N values ranged from + 6.8 to + 17.6 per thousand with a mean of + 10.3 per thousand. Eleven samples had delta 15N values greater than + 9.5 per thousand indicating an animal waste origin. Septic tanks are probably the predominant source of waste-derived nitrate. Macropores draining septic tank filter fields may provide a nearly continuous flow of effluent to the aquifer. The delta 15N values of the other nine samples ranged from + 6.8 to 8.4 per thousand suggesting a possible mixing of waste-derived nitrate and that derived from isotopically lighter cultivation sources, i.e. inorganic fertilizer and/or mineralized soil organic nitrogen. The twenty wells were resampled during late May, 1987 shortly after inorganic fertilizers had been applied to the cropland throughout the study area. The groundwater nitrate delta 15N values ranged from + 3.6 to + 14.7 per thousand with a mean of + 8.3 per thousand. Nineteen of the wells showed a shift towards the lighter delta 15N values expected from fertilizer-derived nitrate. The delta 15N decreases ranged from 0.6 to 4.8 per thousand with a mean of 2.2 per thousand. Despite the apparent influx of fertilizer-derived nitrate, eleven samples (with delta 15N + 8.0 to + 14.7 per thousand) still were composed of predominantly waste-derived nitrate. Only nine samples (with delta 15N + 3.6 to + 5.8 per thousand) may have had a fertilizer-derived component approaching in magnitude that of waste-derived nitrate. Regional groundwater nitrate concentration did not substantially increase. Macropore (cracks, root channels, etc.) flow appears to contribute significantly to the recharge of the aquifer. Rapid response of the groundwater nitrate delta 15N values to fertilization demonstrates the ability of macropore flow to transport fertilizer-derived nitrate which has been flushed from the tilled soil layer. Other agriculture chemicals (pesticides and herbicides) may also be transported to the aquifer in this manner.


Accession: 001940697



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