Shawn Barndt
ABSTRACT
| Perched water tables are common in soils of the Palouse region dense, very slowly permeable subsoil layers. Perched water tables within sloping Palouse landscapes may cause rapid downslope transport of applied agrichemicals resulting in contamination of surface and subsurface waters. The first objective of this research was to study the transport of Br- and NO3- through a perched water table on different slope configurations in a small watershed near Troy, Idaho. A second objective was to complete an agricultural survey of producers in the Troy area. The goal of the survey was to obtain a qualitative understanding of the agricultural practices in the Troy area and how these agricultural practices may relate to the potential transport of applied agrichemicals such as nitrogen fertilizers via perched water tables. In the fall, bromide (Br-) and nitrate-nitrogen (NO3--N) were applied in trenches located near the top of two hillslopes (convex and concave) comprising the watershed. Perched water samples were collected downslope and analyzed for tracer concentrations approximately every two weeks. Bromide was also applied in an isolated hillslope plot and sampled every two hours to obtain transport rates over a short time period. A flow path model was used to indicate the probable flow paths from each application trench and to compare with observed tracer distributions. On the convex slope, some of the applied Br- was transported 66 m from the point of application during the first 44 days that perched water table was present, representing an average movement of 1.5 m/d. On the concave slope, some of the applied NO3--N moved 24 m during the first 9 days the perched water table was formed, an average movement of 2.7 m/d. Unlike Br-, which continued to move downslope to 66 m, NO3--N was not found beyond 27 m. The shallow depth to fragipan of the concave slope and denitrification are two possible explanations for NO3--N not being found beyond 27 m. In the isolated hillslope plot experiment the tracer moved 7 m in 9 hours, or 18.7 m/d. Results of flow path analysis on the concave slope predicted actual tracer distribution. Results of the flow path analysis on the convex slope predicted tracer distribution in a direction different from the actual tracer distribution. The agricultural survey was conducted by personal interviews during the spring of 2000. Based on the survey of producers in the Troy area, seven of the 10 producers applied at least 50% of their fertilizer in the fall. Two out of 10 producers applied 20% of their fertilizer in the fall and 80% in the spring. Seven of 10 producers were aware of perched water tables, but the majority do not believe their fertilizers were being transported during the winter and therefore apply a majority of fertilizer in the fall. The results of this study show fall applied tracers are transported at considerable rates and distances downslope in perched water tables during the winter months. Transport rates of tracers are directly related to the height of the perched water table and permeability of saturated soil horizons. High transport rates of tracers are the result of preferential flow via macropores in saturated surface soil horizons. Because producers currently apply the majority of their fertilizer in the fall it can be transported considerable distances from point of application during the winter months. Based on the results, re-evaluation of fertilizer application practices in the Palouse region may be warranted to reduce fertilizer loss and possible contamination of surface and subsurface waters. |