76-6 Tracking the Transformation of Dissolved Organic Carbon Species in the Brazos River and its Riverbanks during and after Storm Events

Session: Contaminants Near Groundwater-Surface Water Interfaces

Presenting Author:

Reid Buskirk

Authors:

Buskirk, Reid¹, Smith, Braden², Cassidy, Hunter³, McKay, Garrett⁴, Jung, Bahngmi⁵, Leonce, Burke⁶, Knappett, Peter Shawn Kuehl⁷

(1) Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA, (2) Department of Geology & Geophysics, Texas A&M University, College Station, Texas, USA, (3) Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA, (4) Department of Civil and Environmental Engineering, Texas A&M University, College Station, Texas, USA, (5) Department of Civil and Environmental Engineering, Texas A&M University, College Station, Texas, USA, (6) Department of Civil and Environmental Engineering, Texas A&M University, College Station, Texas, USA, (7) Department of Geology & Geophysics, Texas A&M University, College Station, TX, USA,

Abstract:

Dissolved organic carbon (DOC) plays a key role in mediating biogeochemical processes within rivers and their adjacent aquifers that support contaminant removal in river corridors, such as the denitrification of excess nitrate. In riverbanks and floodplain aquifers, the efficiency of denitrification is largely determined by DOC composition, which varies across aquifers, river-aquifer interfaces, and along longitudinal gradients of large rivers.

Most of the annual riverine DOC and nitrate (NO₃^-) loads (kg/yr) are transported during flood events. Yet, the sources and transformations of DOC during prolonged flood events in large, higher-order rivers remain poorly understood. Our objective was to characterize the evolution of DOC composition during a series of flood events in the Brazos River throughout a particularly wet early summer. Sampling locations included the mainstem, tributary inputs and the adjacent aquifer.

We hypothesize that (1) flood events mobilize terrestrially derived DOC which mixes during flood peaks with microbially derived DOC from baseflow, and (2) the first flood event mobilizes the most DOC concurrently with NO₃^- during the “first flush” while the following flood events co-mobilize progressively less terrestrially derived DOC alongside NO₃^- in floodwaters.

To capture DOC changes throughout the flood event series, water samples were collected daily from April to July 2024. Samples were filtered, and the filtrate was analyzed for DOC using a Shimadzu TOC analyzer. Next, DOC composition was characterized using excitation-emission matrices (EEMs) obtained from an Aqualog fluorescence spectrometer. Fluorescence indices (FI), biological indices (BIX), and the spectral E2:E3 ratio (high vs. low molecular weight moieties) were used as optical surrogates for DOC composition.

The E2:E3 ratio decreased during flood peaks, indicating an influx of heavier, terrestrial DOC. However, the E2:E3 ratio, BIX, and FI gradually increased over the course of the series of flood events. Dissolved NO₃^- fluxes increased during the flood event peaks, but their magnitude decreased over the overall series of flood events. This indicates progressive depletion of easily mobilizable terrestrially derived DOC and NO₃^- from overland runoff.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10019

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