title: Agriculture's Contribution to Nitrate Contamination of Californian Groundwater (1945-2005).
abstract: Nitrogen (N) use in intensive agriculture can degrade groundwater resources. However, considerable time lags between groundwater recharge and extraction complicate source attribution and remedial responses. We construct a historic N mass balance of two agricultural regions of California to understand trends and drivers of past and present N loading to groundwater (1945-2005). Changes in groundwater N loading result from historic changes in three factors: the extent of agriculture (cropland area and livestock herd increased 120 and 800%, respectively), the intensity of agriculture (synthetic and manure waste effluent N input rates increased by 525 and 1500%, respectively), and the efficiency of agriculture (crop and milk production per unit of N input increased by 25 and 19%, respectively). The net consequence has been a greater-than-order-of-magnitude increase in nitrate (NO) loading over the time period, with 163 Gg N yr now being leached to groundwater from approximately 1.3 million ha of farmland (not including alfalfa [ L.]). Meeting safe drinking water standards would require NO leaching reductions of over 70% from current levels through reductions in excess manure applications, which accounts for nearly half of all groundwater N loading, and through synthetic N management improvements. This represents a broad challenge given current economic and technical conditions of California farming if farm productivity is to be maintained. The findings illustrate the growing tension-characteristic of agricultural regions globally-between intensifying food, feed, fiber, and biofuel production and preserving clean water.
authors.forename: Todd S
authors.initials: TS
authors.lastname: Rosenstock
authors.forename: Daniel
authors.initials: D
authors.lastname: Liptzin
authors.forename: Kristin
authors.initials: K
authors.lastname: Dzurella
authors.forename: Anna
authors.initials: A
authors.lastname: Fryjoff-Hung
authors.forename: Allan
authors.initials: A
authors.lastname: Hollander
authors.forename: Vivian
authors.initials: V
authors.lastname: Jensen
authors.forename: Aaron
authors.initials: A
authors.lastname: King
authors.forename: George
authors.initials: G
authors.lastname: Kourakos
authors.forename: Alison
authors.initials: A
authors.lastname: McNally
authors.forename: G Stuart
authors.initials: GS
authors.lastname: Pettygrove
authors.forename: Jim
authors.initials: J
authors.lastname: Quinn
authors.forename: Joshua H
authors.initials: JH
authors.lastname: Viers
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Tomich
authors.forename: Thomas
authors.initials: T
authors.lastname: Harter