IMPACT OF MACROPORES AND SOIL PIPES ON HYPORHEIC EXCHANGE IN STREAMS

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

Streams and rivers form a vital part of the water cycle, are 'downstream of everything" and are most affected by most human actions. It is therefore important to understand how stream and rivers work so we can account for these benefits and anticipate how they may respond to trends like urbanization. The hyporheic zone is the boundary between the water in a river or stream and the water in sediment beneath or alongside the river. The hyporheic zone has unique properties including enhanced chemical reactions (many of which are good for reducing pollutant levels) and habitat for organisms that live nowhere else. The hyporheic zone is particularly active when water levels fluctuate in the stream or river, for instance due to storm events, because this forces water into or out of the riverbanks. This project will use field surveys to determine how much this storm-enhanced hyporheic exchange varies through a wide range of streams in the eastern USA. The field data will be combined with computer models to predict the presence and significance of this exchange in other streams. This will allow better understanding of this exchange on aquatic organisms and the transport of nutrients and pollutants. This project will also enhance several existing classes at Virginia Tech and increase student interest in pursuing careers in Science, Technology, Engineering, and Mathematics (STEM) fields which are critical for preserving and enhancing America's dynamic economy and quality of life. The project will reach out to under-represented undergraduates to improve their participation in the career-enhancing research opportunities provided by the project. Streams and rivers form a vital link among compartments of the hydrosphere. The hyporheic zone is a key interface where water exchanges between surface and subsurface, and is important for aquatic organisms, biogeochemical reactions, and contaminant attenuation. "Lung model" hyporheic exchange occurs where water enters and exits the subsurface during transient phenomena such as storms, hydropeaking, and daily snowmelt. The effect of lung model exchange on water quality has been recently shown to be substantial, yet highly dependent on exchange rate. This project will use field surveys of stream bank macropores to test whether macropores/soil pipes are common in several provinces of the eastern USA and a range of stream sizes and land uses. Field surveys will include detailed measurements of macropore geometry, connectivity, sedimentation, and plugging. These data will be used to compare the Darcy-based SUTRA model of reaction and transport to the computational fluid dynamics code Ansys CFX. The modeling will determine the relationships between controlling factors (e.g., macropore geometry, head gradients, sediment hydraulic conductivity) and the degree that macropores enhance lung model exchange. These results will then be upscaled to estimate the degree to which the presence of macropores increases the percent of stream length where lung model hyporheic exchange is significant.