Farm Nitrogen Balance for CA

I am working on an article for the upcoming Nitrate issue of Southwest Hydrology. One of the big picture numbers that I have been following is the statewide farm nitrogen balance in California. Here is the back-of-the-envelope computation that I am coming up with - somebody correct me if I am way wrong:

Nitrogen Fertilizer Centennial

One hundred years of nitrogen fertilizer - and what an incredible green revolution it has been! The article below from Scientific American highlights some of the current issues with global fertilizer use in the context of biofuels. Funny that the article should leave out any mention of groundwater. Nitrate is only the most common groundwater contaminant worldwide next to salt. In California nearly 10% of water supply wells are at levels above the maximum contaminant level (MCL) of 45 mg/L (Nitrate) or 10 mg/L (Nitrate-Nitrogen). In a recent survey of domestic wells in Tulare County, California, over 40% of domestic wells exceeded the nitrate MCL. The county is one of the largest agricultural producers in the country with large acreages of citrus, vineyards, tree crops, and forage crops. It is also the largest dairy producer in the United States. Dr. Erik Ekdahl from the California SWRCB recently gave an illustrative presentation with maps showing 1980-2007 groundwater nitrate trends in California based on results from nearly 10,000 wells. If you are a GRA member, you can download his presentation from their website.