|AL-QUDAH, O - Lincoln University Of Missouri|
|LIU, F - Lincoln University Of Missouri|
|YANG, J - Lincoln University Of Missouri|
Submitted to: Earth and Space Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/22/2015
Publication Date: 10/28/2015
Citation: Al-Qudah, O.M., Liu, F., Lerch, R.N., Kitchen, N.R., Yang, J. 2015. Controls on nitrate-N concentrations in groundwater in a Missouri claypan watershed. Earth and Space Science. 3(3):90-105. doi: 10.1002/2015EA000117.
Interpretive Summary: Contamination of groundwater by nitrate-nitrogen is a major problem in agricultural watersheds because of long-term fertilizer and manure applications used for crop production. A total of 96 groundwater wells installed throughout Goodwater Creek Experimental Watershed (GCEW), located in the Central Claypan Region of northeastern Missouri, were intensively sampled from 1991 to 2004 and analyzed for nitrate concentration. The objective of this research was to: 1) summarize and assess trends in the nitrate concentrations in groundwater wells located throughout GCEW; 2) compare nitrate concentrations of GCEW to non-claypan agricultural watersheds of the US Corn Belt; and 3) describe the processes and soil properties controlling the movement of nitrate to groundwater. Despite the poorly drained claypan soils, considerable nitrate contamination of the groundwater occurred, with 38% of the wells exceeding the drinking water standard of 10 parts per million (ppm). This indicated that preferential flow through soil cracks and large pores allowed downward movement of rain water and nitrate into the groundwater. In general, nitrate concentrations showed no time trends except for a decreasing trend in wells within one field. Nitrate concentrations tended to reach a maximum at a depth of about 30 feet below the land surface, then decreased below that depth. The high nitrate concentrations occurred in a subsurface layer with very low porosity and percolation rates that restricted downward movement of water and nitrate. Studies conducted in non-claypan agricultural watersheds throughout the continental US reported that only 19-23% of sampled wells exceeded 10 ppm, suggesting that groundwater in GCEW was more susceptible to nitrate contamination than non-claypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic properties of the sub-surface strata controlled the transport of nitrate to groundwater in this claypan watershed. These results will benefit other researchers attempting to understand the processes affecting nitate transport and will lead to improved strategies for reducing nitrate contamination of groundwater in agricultural watersheds.
Technical Abstract: Nitrogen (N) fertilizer applications have resulted in widespread groundwater nitrate-N (NO3-N) contamination in the U.S. Corn Belt. Goodwater Creek Experimental Watershed (GCEW) is an agricultural watershed in the claypan soil region of northeastern Missouri with a network of 96 wells at depths of 2.7-15.7 m. The objectives of this study were to: (1) understand the processes controlling the variability of NO3-N concentrations in groundwater at various scales within GCEW; and (2) compare groundwater NO3-N concentrations in GCEW to other agricultural watersheds in the U.S.. Nitrate-N concentrations were determined in more than 2000 samples collected from 1991 to 2004. Despite the low hydraulic conductivity of the claypan soils, considerable NO3-N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10 mg/l. Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO3-N movement to the aquifer. Changes in concentration with depth steadily increased to 8.5-10 m, then decreased with further depth. This pattern was consistent with decreased hydraulic conductivity in the Paleosol layer at 8.5-10 m, denitrification below this layer, and mixing of recent contaminated water with older uncontaminated water in the lowest strata. Only 19-23% of sampled wells exceeded 10 mg/l in non-claypan agricultural watersheds over the continental U.S., suggesting that groundwater in GCEW was more susceptible to NO3-N contamination than non-claypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic conductivity of the sub-surface strata controlled NO3-N transport in this claypan watershed.