As water grows scarcer in semiarid and arid regions around the world, new tools are needed to quantify fluxes of water and chemicals between aquifers and rivers. In this study, we quantify the volumetric flux of subsurface water to a 24 km reach of the Brazos River, a lowland river that meanders through the Brazos River Alluvium Aquifer (BRAA), with 8 months of high‐frequency differential gaging measurements using fixed gaging stations. Subsurface discharge sources were determined using natural tracers and End‐Member Mixing Analysis (EMMA). During a 4 month river stage recession following a high stage event, subsurface discharge decreased from 50 m3/s to 0, releasing a total of 1.0 × 108 m3 of water. Subsurface discharge dried up even as the groundwater table at two locations in the BRAA located 300–500 m from the river remained ∼4 m higher than the river stage. Less than 4% of the water discharged from the subsurface during the prolonged recession period resembled the chemical fingerprint of the alluvial aquifer. Instead, the chemistry of this discharged water closely resembled high stage “event” river water. Together, these findings suggest that the river is well connected to rechargeable bank storage reservoirs but disconnected from the broader alluvial aquifer. The average width of discrete bank storage zones on each side of the river, identified with Electrical Resistivity Tomography (ERT), was approximately 1.5 km. In such highly compartmentalized aquifers, groundwater pumping is unlikely to impact the exchange between the river and the alluvium.

Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH 4 +-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO 4 3were the major predictor of com

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