Characterizing groundwater chemistry and recharge in the critical zone of an agricultural claypan watershed

Authors: HOFMEISTER, KATHRYN - MICHIGAN TECHNOLOGICAL UNIVERSITY; LERCH, ROBERT - RETIRED ARS EMPLOYEE; Baffaut, Claire; YANG, JOHN - LINCOLN UNIVERSITY OF MISSOURI; LIU, FENGJING - MICHIGAN TECHNOLOGICAL UNIVERSITY

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/27/2022
Publication Date: 10/3/2022
Citation: Hofmeister, K., Lerch, R., Baffaut, C., Yang, J., Liu, F. 2022. Characterizing groundwater chemistry and recharge in the critical zone of an agricultural claypan watershed. Water Resources Research. 58(10). Article e2021WR031797. https://doi.org/10.1029/2021WR031797.
DOI: https://doi.org/10.1029/2021WR031797

Interpretive Summary: Nutrients and herbicides stream loads are significant concerns in the Central Claypan Region of northeastern Missouri, where restrictive layer (claypan) soils are dominant. This claypan restricts percolation through the soil profile and causes high surface and subsurface flow. However, the clay is subject to cracking during extended dry periods and vertical movement of water and contaminants can occur via these cracks during events that follow these dry periods. Quantifying how much is lost through surface runoff, subsurface flow, and deep percolation to groundwater (a.k.a. groundwater recharge) is difficult but important to develop strategies that mitigate stream contamination. Our objective was to investigate what controls groundwater recharge and its chemistry in the Goodwater Creek Experimental Watershed in Missouri, U.S. using natural geochemical tracers. Precipitation, surface runoff, subsurface flow, and groundwater have characteristic concentrations of these tracers, which allows determination of the pathways contributing to groundwater. Results showed that deeper groundwater originated primarily from groundwater at similar depths, often upslope or from strata directly above, with some contributions from subsurface water, highlighting the importance of both horizontal and vertical pathways. Deep groundwater chemistry resulted from the mixing of these different sources, with little chemical transformations of the contaminants, including a lack of denitrification processes. In contrast, water chemistry in subsurface water directly above the claypan and shallow groundwater immediately below was controlled primarily by chemical transformations. Vertical recharge pathways are likely dictated by cracking through the soil profile and provide conduits for the transport of nitrates to deeper groundwater where they accumulate. Soils with restrictive soil horizons and a high likelihood of cracking present a significant challenge for water quality management as deep groundwater provides a major source of high nitrate concentrations in stream water. This information is useful for water managers and water resource agencies to evaluate the effectiveness of nutrient management and mitigation practices.

Technical Abstract: Soils with restrictive horizons (e.g., claypans) are vulnerable to loss of nutrients, particularly nitrate, through shallow and deep hydrologic pathways. Partitioning between these pathways is difficult but important to determine transport processes and develop strategies that mitigate stream contamination. Our objective was to investigate controls on groundwater chemistry and recharge pathways using natural geochemical tracers in the Goodwater Creek Experimental Watershed in Missouri, U.S. Groundwater samples were collected from 2011-2017 in 32 piezometers ranging from 0.13-16 m deep along with stream water and precipitation samples. Diagnostic tools of mixing models indicated that water chemistry in subsurface water directly above the claypan and shallow groundwater immediately below was controlled primarily by chemical equilibrium. Five solutes behaved conservatively in most deep piezometers (>4 m), reflecting mixing of two end members and the lack of significant denitrification processes. End member mixing analysis showed that the deeper groundwater originated primarily from groundwater at similar depths, often upslope or from strata directly above, with small contributions from subsurface water, highlighting the importance of both horizontal and vertical preferential recharge pathways. Vertical recharge pathways are likely dictated by soil heterogeneity throughout the critical zone and do not occur spontaneously with precipitation events or synchronously over all piezometer locations. The complex recharge pathways provide stochastic conduits for nitrate transport to deeper aquifers where legacy stores accumulate, presenting a significant challenge for water quality management in watersheds with restrictive soil horizons and a high likelihood of preferential flow pathways, including the Mississippi River Basin.