Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2006
Publication Date: 3/1/2008
Citation: Wannamaker, C., Keller, K.C., Smith, J.L., Allen-King, R. 2008. Nitrate in tile drainage of the semiarid Palouse Basin. Journal of Environmental Quality. 37:353-361. Interpretive Summary: Non-point-source contamination of water bodies from agriculturally applied nitrogen fertilizer has been a source of great interest in recent years. In the United States, thousands of river systems are considered impaired by the Environmental Protection Agency; many of these impairments are attributable to non-point-source agricultural contamination. Over a 5 year period we monitored chemical concentrations in surface and shallow groundwater that drains 6 to 6000 ha agricultural watersheds near Pullman, Washington. This established the contaminant load from increasing larger scales. We found that the highest nitrate concentrations (20-30 mg-N/L) occur with the highest water discharge rates in early spring. Peak nitrate concentrations varied by water year but concentrations > 10mg-N/L occurred over all 5 years, with baseflow nitrate concentrations consistently at 4 mg-N/L. Our findings show that it may be possible to manage chemicals in streams and groundwater if the movement of chemicals can be quantified over increasing larger scales. From this record we developed a model of chemical transport to assist in developing cropping systems that will reduce chemical transport to streams.
Technical Abstract: Topographically heterogeneous landscapes can enhance fertilizer NO3 contamination of streams due to the rapid downslope transport of water to poorly drained bottom slopes and shallow groundwater. In the semi-arid dry-land wheat production area of the Palouse region of Washington State, large areas of poorly drained landscapes have been tile drained to enhance crop yield. We monitored the discharge of such a tile drain (TD), and a nearby profile of soil-water samplers, for NO3, electrical conductivity (EC), and water content levels in an effort to develop a conceptual model of the hydrological controls on NO3 loss by subsurface pathways. Increases in TD flow rate and nitrate concentrations occur together in early winter following ~150 mm of fall precipitation. The highest nitrate concentrations (20-30 mg-N/L) occur approximately contemporaneous with the highest TD discharges in winter through early spring. Peak nitrate concentrations varied by water year but concentrations > 10mg-N/L occurred in each of the four study years, with baseflow nitrate concentrations consistently at 4 mg-N/L. Electrical conductivity (EC) of TD discharge water was generally 200 - 300 uS/cm and decreased to 100 uS/cm during high volume discharge, consistent with a dilution effect and opposite the pattern of nitrate. Temporal EC patterns suggest that the residence time of water in the profile above the tile drain could be as little as 30 days during high discharge. The EC data also suggest that most streamflow in the Palouse reaches waterways via subsurface flow paths. Seasonal precipitation and fertilizer application are a primary control of N loss in the Palouse region. Cropping systems that reduce the large reservoir of soil N, along with spring-only applications of N, would be effective in reducing N loading to streams in this region.