146-6 Quantifying the Influence of Baseflow and Water Sources on Nitrogen Export in a Large Multi-Order Watershed on the Brazos River, Texas

Session: Contaminants Near Groundwater-Surface Water Interfaces (Posters)

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Abstract:

Excess dissolved nutrients degrade water quality nationwide and cause eutrophication. Eutrophication is defined as the excessive blooms of photosynthetic algae, which impairs water quality and causes coastal “dead zones.” Nutrients are transported into rivers by different surficial and subsurface hydrological pathways, such as agricultural runoff and municipal wastewater discharge. These pathways may change over the course of successive flood events and dry seasons. Flooding can also store “event” water in permeable riverbanks, which provide baseflow for Texas Rivers. Some of the nitrogen that cycled through the riverbanks is lost via denitrification. However, the quantitative nitrate fluxes and process mediating them in large rivers during multiple successive flood events are not well known.

Thus, our objective is to investigate how nitrogen sources and sinks evolve over a sequence of flood events during an unusually wet summer and the subsequent drought along the Brazos River, Texas. We hypothesize that (H1) local groundwater and Lake Whitney water makes up the majority of baseflow with much of the nitrates (NO₃^-) being sourced from agriculturally sourced groundwater; (H2) runoff comprises most of flood event water which mobilizes significant NO₃^-; (H3) The first flush event caused by initial storms in April-June results in NO₃^- fluxes to initially be large but following storms have smaller NO₃^- fluxes.

The study area is a 24-km reach of Brazos River and three tributary stems that entered the river between US HW 21 and FM-60 bridges. Samples were collected from these locations in conjecture with groundwater wells located in the riverbank and floodplain aquifer alluvium. Floodwater samples were collected daily between April-July 2024 and later biweekly for the following drought between September and December 2024 period. Dissolved major ions, water isotopes, and NO₃^- were measured with ion chromatography (IC), and PICARRO on unacidified filtered samples (0.45-micron CA) and ICP-MS for acidified samples. We will generate a three end-member mixing model to parse groundwater and surface water contributions to baseflow. Net nutrient fluxes will be calculated with differential gauging. We expect the study to help quantify the contribution of different source waters and their role in nutrient transport during prolonged flood event sequences in larger rivers and their following droughts.

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

doi: 10.1130/abs/2025AM-10204

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