A study of the aquatic humic substances and hydrogeology in a prairie watershed: Use of humic material as a tracer of recharge through soils

Throughfall collectors, soil water samplers, and observation wells were installed at the Konza Prairie Research Natural Area, to characterize DOC and humic material along a flow path from plant source to groundwater and to understand the origin and degradation of humic material in groundwater. The Konza Prairie, a 3,487-hectare tract of dissected tall grass prairie near Manhattan, Kansas, was selected as a field site because the {dollar}deltasp{lcub}13{rcub}{dollar}C signal of big bluestem (Andropogon gerardii) could be followed along a flow path from plant source to groundwater. Samples were collected for the study of DOC and humic material at points along the flow path: plant source (leachate and throughfall), soil water, soil organic matter, and groundwater. Humic material was intensively characterized evaluate its fate during the transition from plant source to groundwater.; Hydrogeologic data show the combination of fractures and the potential for flow between vertically-perched limestone beds provides a favorable framework for the washing of DOC leached from plants through soils into deeper aquifers. Pulses are recognized in groundwater when DOC peaks coincide with decreased specific conductance following rainstorms.; Characterization data clearly reveal that DOC and humic material from grass and woody vegetation sources evolves along different pathways in which changes occur in nitrogen, carbohydrate, carboxyl and phenolic contents; color, and {dollar}deltasp{lcub}13{rcub}{dollar}C values during the transition from plant source to groundwater. Changes in these parameters are progressive, but equivalent changes occur at shallower depths for woody vegetation than grass. Nitrogen and {dollar}deltasp{lcub}13{dollar}C data show that humic material in soil water incorporates some water-soluble soil organic matter. However, the observation of {dollar}deltasp{lcub}13{rcub}{dollar}C ratios for soil water beneath bushes consistent with C{dollar}sb3{dollar} sources despite a C{dollar}sb4{dollar} source for soil organic matter shows that surface vegetation influences the composition of soil water more so than soil organic matter. Based on characterization data, fulvic acid in groundwater originates from either woody or grass sources, and indicates transport through the soil. Both titration and {dollar}deltasp{lcub}13{rcub}{dollar}C for groundwater humic material denote a mixed vegetation source. Consequently, woody and grass sources contribute humic material to groundwater along pathways that include recharge through the soil.