Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2002
Publication Date: 8/19/2002
Citation: Gish, T., Kung, K.S., Posner, J., Bubenzer, G. Helling, C.S. Kladivko, E.J. Steenhuis, T.S. 2002. Impact of preferential flow at varying irrigation rates by quantifying mass fluxes. Journal of the Environment. 33:1033-1040. Interpretive Summary: Fertilizers and pesticides play a critical role in meeting the food and fiber demands of a growing world population. However, some of these agricultural chemicals have resulted in non-point groundwater contamination. The general public demands not only high-quality, low-cost food and fiber but also clean environments. During the past three decades soil scientists, ecologists, and agricultural engineers have exerted tremendous research effort with the goal of alleviating the undesirable trade-off between production of food/fiber and deterioration of water quality. Unfortunately, there is substantial scatter or variability in the water quality research results-making scientific interpretation and policy development difficult. As a result, ground and surface water contamination represents one of our nation's most significant, long-term problems. Although poorly understood, preferential flow is thought to be the flow mechanism responsible for much of the unexpected and rapid movement of agricultural chemicals to groundwater. In this multi-disciplinary study, the impact and relevance of preferential flow as influenced by two different irrigation intensities was evaluated with a unique tile drain system. Results indicate that preferential flow played a significant role on contaminant transport with an irrigation rate of 3.1mm h-1, with almost 20% of the bromide applied being leached beyond the root zone with less than 80mm of irrigation. The impact of preferential flow was negligible at the 0.89mm h-1 irrigation rate as less than 0.16% of the applied tracer had moved beyond the root zone with 80 mm of applied irrigation. This study indicates that irrigation intensity can have a dramatic impact on contaminant transport through preferential flow pathways.
Technical Abstract: To accurately determine the transit times of agricultural chemicals under near steady state field conditions on a no-tilled field, a unique tile drain monitoring facility was developed. A small shed 3.5m x 24m was placed 0.3m off center of established tile lines, the soil underneath the shed was irrigated at either 3.1mm h-1 or 0.89mm h-1; while outside the shed, the fields were always subjected to an irrigation rate of 4mm h-1. Tile drains were located about 1m below the soil surface and were continuously monitored. After steady state flow conditions had been achieved one of two mobile tracers, bromide (Br; 280kg ha-1) or penta-fluorobenzoic acid (PFBA; 121kg ha-1) was applied. Bromide was subjected to an irrigation rate of 3.1 mm h-1, while PFBA was subjected to a 0.89mm h-1 irrigation rate. Bromide first appeared in the tile drain within 16 minutes after application. Over 19.2% of the surface-applied Br had been leached from the soil after less than 81mm of irrigation, while over 33% had been leached through the root zone with less than 124mm of irrigation. The rapid appearance of Br and the large quantities leached with minimal water inputs indicate a flow process dominated by preferential flow. On the other hand, on the same soil but at an irrigation rate of 0.89mm h-1, PFBA breakthrough did not occur until 90 hours after application, and less than 0.16% of the applied PFBA had been lost through leaching after 80.1mm of irrigation. The main breakthrough pattern associated with PFBA coincided with an analytical solution based on 1-D convective-dispersive transport, suggesting that the PFBA transport was dominated by matrix flow. This study indicates that irrigation intensity can have a dramatic impact on contaminant transport through preferential flow pathways.