Stream-aquifer and in-stream processes affecting nitrogen along a major river and contributing tributary.

This study assesses the spatio-temporal patterns of water and nutrient mass exchange in a stream-riparian system of a major river and a contributing tributary in an irrigated semi-arid region. Field monitoring is performed along reaches of the Arkansas River (4.7km) and Timpas Creek (2.0km) in southeastern Colorado during the 2014 growing season, with water quantity and water quality data collected using a network of in-stream sampling sites and groundwater monitoring wells. Mass balance approaches were used to identify temporal and spatial trends in flow, nitrogen (N), and salinity in stream-aquifer exchange. In the Arkansas River, percent decrease of N concentration along the study reach averaged 36% over the period, with results from a stochastic mass balance simulation indicating a 90% probability that 44% to 50% of NO-N mass in the study reach (109-124kg/day/km) was removed by in-stream processes between 1 September and 8 November. Results suggest that contact with organic-rich river bed sediments has a strong impact on N removal. A greater decrease in concentrations of NO-N along the reach during the low flow period suggests the effect of both in-stream processes and dilution by inflowing groundwater that undergoes denitrification as it flows through the riparian and hyporheic zones into the river. In contrast, N concentration decreases in the smaller Timpas Creek were negligible. Results for the Arkansas River also are in contrast with other large agriculturally-influenced rivers, which have not exhibited capacity to remove N at significant rates. Results provide important insights across spatial and temporal scales and point to the need for investigating nutrient dynamics in large streams draining agriculturally-dominated watersheds.