Characterizing Groundwater Chemistry and Recharge in the Critical Zone of an Agricultural Claypan Watershed. Issue 10 (12th October 2022) - Hofmeister, Kathryn
Abstract: Soils with low permeability horizons (e.g., claypans) are vulnerable to loss of nutrients through surface runoff along with preferential flow paths through the restrictive horizon to deeper aquifers. Partitioning between these hydrologic pathways is 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 during 2011–2017 from 32 piezometers ranging from 0.13 to 16 m deep along with stream water and precipitation. Diagnostic tools of mixing models indicated that chemistry of perched water directly above the claypan and shallow groundwater immediately below was controlled primarily by chemical equilibrium. Five solutes behaved conservatively in most deep piezometers (>5 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 perched water, highlighting the importance of both horizontal and vertical preferential recharge pathways. Vertical pathways are likely dictated by soil heterogeneity throughout the critical zone and do not occur synchronously with precipitation events or simultaneously 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 spatially and temporally heterogeneous preferential flow pathways, including the Mississippi River Basin. Plain Language Summary: The transport of nutrients and herbicides to streams is a concern in the U.S. Midwest, where soils with low permeability layers, such as claypans, are extensive. These layers restrict water movement through deeper soil except when cracks form during dry periods. Determining how contaminants move through soil into surface and ground water is important to develop strategies to mitigate water contamination. Using natural geochemical tracers, we determined that deep groundwater originated primarily from groundwater at similar depths with some contributions from water above the claypan. Deep groundwater chemistry resulted from the mixing of these different sources, with little chemical transformations of the contaminants. In contrast, water chemistry above the claypan and shallow groundwater immediately below was controlled primarily by chemical transformations. Claypan cracks provide conduits for the transport of nutrients and herbicides from the surface to deeper groundwater where they accumulate. Soils with restrictive horizons and a high likelihood of cracking challenge water quality management, as deep groundwater provides a major source of high nitrate concentrations in streams. This information can help water managers and water resource agencies evaluate the effectiveness of nutrient management and mitigation practices. Key Points: Groundwater chemistry was dominated by chemical equilibrium in perched and shallow aquifers (<4 m) and mixing in the deeper aquifers Groundwater in the deeper aquifers was recharged by groundwater from similar depths and above strata but via heterogeneous pathways Complex recharge and nitrate transport routes pose a challenge for legacy nitrogen mitigation in regions with restrictive soil horizons