ARS | Publication request: WHAT DO WE REALLY KNOW ABOUT NITRATE IN GROUNDWATER

Submitted to: Facts Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: September 30, 1997
Publication Date: N/A

Interpretive Summary: Over two years of monitoring groundwater nitrate at five sites across the Florida peninsula has shown that nitrate levels are highly site specific. With essentially the same fertilizer application, there is a great difference in the amount of nitrate in groundwater under the groves. Detailed examination of the soils under these groves has shown that depth to water table appears to be the key factor, with higher nitrate levels in areas with deep (> 15 ft) water tables. Most of the drainage water leaving the groves has no or little nitrate in it, and even if there is nitrate in the seepage water, it immediately disappears after it enters the drainage ditches. The effect of changing the amount of fertilizer applied varies with location; in some locations there is a response after three to six months, in others there is no response after more than two years. Sharply decreasing fertilization lowers fruit production after one year and leaf sprays with urea don't seem to compensate for the lack of ground-applied nitrogen. Nowhere in this project were there nitrate levels approaching the 10 ppm NO3-N standard in deep wells (> 100 ft) or in adjoining bodies of water, such as creeks and ponds.

Technical Abstract: Over two years of monitoring groundwater nitrate, fertilizer application, and fruit production has shown that the response of groundwater nitrate levels to fertilizer application varied from location to location, very little nitrate left the groves in drainage water, and deep-aquifer water was essentially nitrate free. Groundwater nitrate was lowered in some locations by reducing N applications to 100 or 150 lbs per acre per year, but this seemed to reduce fruit production even after one year of reduced N application. Multiple, light N applications did not reduce groundwater nitrate levels; the main factor coinciding with high, persistent NO3-N levels seemed to be a deep water table.