Distribution and Movement of Nitrate in Soils from Snowpack in a Stream Riparian Zone, Waterloo, Ontario

Abstract

Stream riparian zones are landscape features that retain nutrients and enhance water quality. However, little is known about winter controls on nutrient transport in riparian zones. In this study we examine the distribution and movement of nitrate (NO3−) between snowpack, underlying soils and groundwater in a riparian zone to quantify processes which control NO3− transport through seasonally-frozen surface soils. Soils and vegetation in buffer zones attenuate nitrate during the growing season. However, this ability is uncertain with vegetation senescence, and when soils freeze and seasonal snowcover can contain an important nitrate load and later release it to surface waters during melt. At our study site snowcover reached a maximum depth of 32 cm following the major snowfall event from Julian Day (JD) 41 to 47, 2000. During this event, the snow water equivalent (SWE) increased twofold to 4.7 cm. A melt event starting on JD 53 resulted in a SWE loss of 2 cm. Snowpack NO3− concentrations reached a maximum value of 1.4 mg L−1 and a maximum loading of 51.7 mg m−2. During the main melt event, snowpack loading reduced to 36.9 mg m−2 and concentrations of NO3− in the snow decreased to 0.55 mg L−1. Over the study period, groundwater NO3− concentrations were relatively constant near 0.25 mg L−1. However, evidence of mixing of groundwater with stream water is strongly suggested by higher NO3− concentrations in near-stream groundwater (0.75 mg L−1), which was under hydrostatic pressure caused by the stream ice-cover. No identifiable NO3− pulse was observed during the main snowmelt period because persistent soil frost promoted depression storage, overland flow and shallow throughflow. Our results indicate that overland flow and shallow throughflow were the likely pathways for NO3− export from this system. Clearly these processes cannot be ignored when quantifying snowpack NO3− through riparian buffers with frozen soils. © 2002 Taylor & Francis Group, LLC.