Nitrate attenuation in agricultural catchments: Shifting balances between transport and reaction

Abstract

[1] Simultaneous observations, across multiple spatial and temporal scales, are needed to understand the complex interactions between hydrological and biogeochemical processes in catchments and the primary controls on NO 3 À availability and mobility. This paper addresses these issues by using data collected from a detailed field experiment, carried out on two topographically different hillslopes (one steep and the other flat) located within an agricultural catchment in Western Australia. Continuous hydrometric data and measurements of chemical tracers, geochemical parameters, and NO 3 À concentrations taken from the shallow perched aquifer across riparian, midslope, and upland locations were analyzed and interpreted through a simple process-based numerical model of transport and reaction. The NO 3 À concentration data indicated that the temporal and spatial patterns of NO 3 À concentrations within the hillslopes are linked to the state of hydrological connectivity of the three landscape units as the shallow perched aquifer developed during the winter. Significant NO 3 À attenuation occurs within the riparian zones after the transport of NO 3 À from midslope sources begins. Application of a mixing model, which partitions the riparian zones into three water source components, and the numerical model of NO 3 À transport and reaction indicates that different mechanisms, dilution in the steep hillslope and denitrification in the flat hillslope, are responsible for much of the observed NO 3 À attenuation in the riparian zones. In this way, this work highlights the importance of hillslope topography in determining the relative roles of transport and reaction in NO 3 À attenuation and export from riparian zones. The experimental results also supported the use of the Damköhler number, a simple dimensionless number that is a measure of the competition between transport and reaction processes, which allowed a favorable comparison of our findings with previous results published in the literature for different geographical settings.Citation: Ocampo, C. J., C. E. Oldham, and M. Sivapalan (2006), Nitrate attenuation in agricultural catchments: Shifting balances between transport and reaction, Water Resour. Res., 42, W01408,

Biogeochemical hotspots: Role of small water bodies in landscape nutrient processing

“…Harvey et al [2013] similarly found that the hyporheic zone was largely reaction limited in their study site. Ocampo et al [2006] applied the Da framework to hillslopes and riparian zones and found that the slope of the system acted as a major control on nutrient attenuation. The flatter hillslope, which had longer transport times, would consequently be reaction limited whereas the nitrate would behave similarly to a conservative tracer in the steep hillslope due to insufficient reaction time.…”

Influence of stream‐groundwater interactions in the streambed sediments on NO3− flux to a low‐relief coastal stream

“…Denitrification, the biological conversion of NO 3 À to N 2 , is of importance in riparian zones and particularly in streambed sediments Hedin et al, 1998;Hill, 1996;Hill et al, 2000]. The extent to which biogeochemical processes influence contaminant transport depends on the relative rates of water transport and biological reactions [Brusseau et al, 1999;Gu et al, 2007;Ocampo et al, 2006]. Groundwater discharges during base flow and storms can have different residence times that can influence the extent of biological reactions .…”

Constitution of a catchment virtual observatory for sharing flow and transport models outputs

“…These hydrological parameters influence catchment biogeochemistry (Ocampo et al, 2006;Oldham et al, 2013;Pinay et al, 2015;Tetzlaff et al, 2007), further increasing their value as 4 indicators and predictors of catchment-scale water quality and chemistry. Because these parameters are of great general interest they feature prominently in the inputs and outputs of many models (e.g.…”

“…The mean transit time and whole stream residence time distribution are powerful metrics of catchment functioning, providing synoptic hydrological information such as water renewal time, heterogeneity of flowpaths, and overall water volume (Godsey et al, 2010;Hrachowitz et al, 2010; Marçais et al, 2015;McGuire and McDonnell, 2006;Van der Velde et al, 2012) . These hydrological parameters influence catchment biogeochemistry (Ocampo et al, 2006;Oldham et al, 2013;Pinay et al, 2015;Tetzlaff et al, 2007), further increasing their value as …”