QUANTIFYING RIVERBANK DENITRIFICATION LOSSES IN RESPONSE TO FLOOD EVENTS ON THE BRAZOS RIVER, TEXAS

Nonpoint source nutrient contamination degrades worldwide water quality by endangering drinking water supplies and aquatic ecosystems by triggering eutrophia and causing aquatic biodiversity loss. Riverbank denitrification may reduce nutrient loads in Texas and Gulf Coast Rivers when flood events mix groundwater and flood waters. During denitrification, microbes respire flood sourced nitrate to N2 gas and dissolved organic carbon (DOC) from more labile to more recalcitrant forms.

Our objective is to measure nitrogen attenuation and accompanying DOC transformations along a segment of the Brazos River using differential gauging paired with high frequency water chemistry sampling during and after river flooding events. We hypothesize that: (H1) flooding events inject river water and nutrients into riverbank aquifers; (H2) these nutrients are denitrified and accompanied by DOC transformation from labile to recalcitrant forms; and (H3) return flow from the riverbank to the Brazos River will have lower nitrate (NO₃^-) concentrations than flood waters, accompanied by increased recalcitrant DOC

Our study site spanned a 15 km reach of the Lower Brazos main stem and tributary streams (Between US Hw 21 and FM-60 bridges). Discharge was measured at 15 min intervals at US-21 using the USGS Stream gauging monitoring station (ID# 08108700), and at FM-60 using Texas A&M’s RQ-30 (Sommer Gmbh) radar measurement system. Dissolved major ions and nutrients were measured with IC and ICP-MS. Excitation-emission matrices were measured by an Aqualog fluorometer (Horiba) to calculate fluorescence indexes to differentiate between terrestrial DOC and microbially derived (recalcitrant) DOC sourced from riverbank denitrification.

Based on differential gauging, the Brazos River is generally a gaining river during baseflow, but becomes a losing river during flood events. These flood events can replenish over 46% of bank storage. Peak flow in storm events experiences significant dilution from runoff, but at least 1% of NO₃ is lost over the reach during peak flow under losing conditions (1523 m³/s).