Determining hydrologic pathways of streamflow using geochemical tracers in a claypan watershed

Authors: LUI, FENGJING - Michigan Technological University; Lerch, Robert; YANG, JOHN - Lincoln University Of Missouri; PETERS, GREG - Ohio University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/2/2020
Publication Date: 3/9/2020
Citation: Lui, F., Lerch, R.N., Yang, J., Peters, G. 2020. Determining hydrologic pathways of streamflow using geochemical tracers in a claypan watershed. Hydrological Processes. 34(11):2494-2509. https://doi.org/10.1002/hyp.13743.
DOI: https://doi.org/10.1002/hyp.13743

Interpretive Summary: Streamflow is comprised of various sources of water, such as surface runoff, groundwater recharge, and, in some watersheds, interflow. Interflow is a type of shallow groundwater that flows at the top of a restrictive soil layer. Watersheds in northeastern Missouri have a naturally formed restrictive soil layer known as a claypan, but the amounts of runoff, groundwater recharge, and interflow in these streams has not been determined. An improved understanding of these major flow paths will support the development of more efffective land management practices that better protect stream water quality. The objective of this research was to identify the primary sources of water contributing to streams in a claypan watershed and estimate the proportion of streamflow coming from each. The research applied a technique that identifies and quantifies the sources based on their chemical make-up. The study was conducted in Goodwater Creek watershed, a 28 sq. mi. drainage area located in northeastern Missouri and well established to be representative of the Central Claypan Region of the US Midwest. Just over 1,000 water samples were collected for six years (2011-2017) from various sources, including the stream, rainfall, interflow, and groundwater. Results showed that streamflow in Goodwater Creek was dominated by surface runoff which accounted for 59% of the flow followed by interflow (25% of flow) and groundwater (16% of flow). Despite their poor drainage, claypan soil watersheds have significant nitrate contamination of groundwater, and the results showed it to be the primary source of nitrate in the stream. Although runoff contributes seasonally high levels of nitrate to the stream, it was a secondary source, overall, compared to groundwater contributions. The importance of interflow contributions provided an explanation for earlier observations of persistent, high levels of herbicides in Goodwater Creek under low flow conditions in late spring and early summer. In most streams, herbicide concentrations typically are high during and immediately following spring runoff events, then quickly fall to low levels as streamflow decreases. In claypan watersheds like Goodwater Creek, herbicide concentrations increase during runoff events, but then remain at high levels for weeks to months under low flow. Findings of this study provided strong evidence that interflow, apparently contaminated with high herbicide levels, is the cause for the prolonged high levels in the stream. Findings of this study will be used to improve existing computer simulation models used to predict movement of contaminants in fields and watersheds. These improvements will benefit scientists and land management agency personnel that utilize simulation models to predict the affects of crop management and conservation practices on stream and ground water quality.

Technical Abstract: Despite slowly permeable soils, groundwater has been heavily contaminated by nitrate in claypan-dominated agricultural watersheds, but it is unclear how nitrate concentrations in groundwater affect stream water quality. This limitation hinders the development of sound farm management plans for improving water quality in claypan watersheds. To tackle this problem, the sources of streamflow were investigated using natural geochemical tracers. The study was conducted in the 73 sq. km Goodwater Creek Experimental Watershed in northeastern Missouri. Samples were collected from 2011 to 2017 from stream water (weekly-biweekly), precipitation (event-based), groundwater in 25 piezometers with screened depths varying from 3 to 16 m (monthly-seasonal), and interflow above the claypan in 7 shallow piezometers (weekly-monthly). The results of end-member mixing analysis using major ions indicate that streamflow was dominated by near-surface runoff (59%), followed by interflow (25%), and groundwater (16%). Analysis of end-member distances using the mixing space defined by stream water chemistry suggests that groundwater contributed to streamflow came primarily from deep glacial aquifers near and below 8 m. Near-surface runoff was persistent and dominant even during prolonged dry period after isolated precipitation events. Both interflow and groundwater were responsive to precipitation, leading to a hypothesis that alluvial aquifer acts as a mixing zone to rapidly mobilized groundwater from within the glacial till aquifer. Groundwater, though its contribution was limited, plays a significant role in regulating streamflow nitrate concentrations. This information is useful for configuration of hydrologic models in simulating streamflow quantity and quality and improve our understanding of contaminant transport and controls in claypan watersheds.