Controls on nitrate‐N concentrations in groundwater in a Missourian claypan watershed

Abstract

Nitrogen (N) fertilizer applications have resulted in widespread groundwater nitrate-N (NO 3 -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) inspect the spatial and temporal variations of NO 3 -N concentrations in GCEW's groundwater, particularly with well depth at scales ranging from individual well, well nest, and field to the entire watershed during the period 1991 to 2004; (2) understand the processes controlling the variability of NO 3 -N concentrations in groundwater at various scales within GCEW; and (3) compare groundwater NO 3 -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 NO 3 -N contamination of the glacial till aquifer occurred, with 38% of the wells exceeding 10 mg L À1 . Groundwater recharge by preferential pathways through the claypan appeared to be the primary mechanism for NO 3 -N movement to the aquifer. Changes in concentration with depth steadily increased to 8.5-10 m and 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 À1 in nonclaypan agricultural watersheds over the continental U.S., suggesting that groundwater in GCEW was more susceptible to NO 3 -N contamination than nonclaypan watersheds. These results demonstrated that preferential flow through the soil and hydraulic conductivity of the subsurface strata controlled NO 3 -N transport in this claypan watershed.

Determining Hydrologic Pathways of Streamflow Using Geochemical Tracers in a Claypan Watershed

“…Though groundwater contributed only 15.9(±13.1)% on average to streamflow in this claypan watershed, its effect on streamflow quality appears to be extremely important. Despite the claypan soils, NO 3 − leaching was a major fate for fertiliser N and significant contamination of the glacial till aquifer has occurred when long-term fertiliser and manure N inputs exceeded crop N requirements (Al-Qudah et al, 2016;Kitchen et al, 2015;Lerch, Kitchen, et al, 2015). For example, median NO 3 − concentration in deep groundwater was more than 30-fold that of both near-surface runoff (not measured but approximated by its concentration in precipitation) and interflow (PZ#7) (…”

“…However, results of this study suggest that the alluvial aquifer may be a mixing zone for the various flow sources just before they reach the stream. Geological deposits are stratified with varying hydraulic conductivities for large portions of the watershed (Al‐Qudah et al, ; Kitchen et al, ). Interflow moves horizontally due to the impediment of smectitic clay in the claypan as suggested by Mudgal, Baffaut, Anderson and Sadler (Mudgal et al, ) and so does groundwater in the lower glacial till by the same logic.…”

“…However, the medians are close to means for SC and all ions, except for Na + , NO 3 − and SO 4 2− in piezometers (all seven together) and Cl − in groundwater (skewness was not explicitly shown but can be inferred from the difference between median and 25 and 75% quartiles). Shallow vs deep groundwater were divided by 8 m screen‐depth following Al‐Qudah et al ().…”

“…Classified in hydrologic soil groups C and D, claypan soils have long been recognised to restrict air and water movement, retard the development of plant roots and promote surface runoff (aka, overland flow) (e.g., Blanchard & Donald, 1997;Blevins, Wilkison, Kelly, & Silva, 1996;Hjelmfelt, Blanchard, Donald, & Alberts, 1999;Jamison et al, 1968;Sadler et al, 2015). However, several studies have shown that groundwater in claypan watersheds was heavily contaminated by NO3 − ─N, derived from nitrogen fertiliser and manure applications (Al-Qudah, Liu, Lerch, Kitchen, & Yang, 2016;Blanchard & Lerch, 2000;Donald, Hjelmfelt, & Alberts, 1998;Kelly & Pomes, 1998;Kitchen, Blanchard, & Lerch, 2015;Wilkinson & Maley, 1996). In these watersheds, water content of the plant root zone decreases during summer because evapotranspiration from rapidly growing and maturing crops exceeds rainfall amount, resulting in significant cracks and macro-pores in the claypan layer (Alberts, Jaynes, & Lerch, 2003;Baer & Anderson, 1997;Donald et al, 1998).…”

Characterizing Groundwater Chemistry and Recharge in the Critical Zone of an Agricultural Claypan Watershed

“…high, and generally remained so even after reduced manure and fertilizer applications in the 1990s-early 2000s (Al-Qudah et al, 2016;Kitchen et al, 1997). 3 (Liu et al, 2020).…”