| The use of subsurface drip irrigation for the reuse/disposal of septic tank effluent began to gain interest in the 1990's. Using subsurface drip irrigation, effluent can be applied uniformly at a lower rate over a larger area in a more biologically active part of the soil profile than with standard deep infiltration trenches. Based on an evaluation of available models, Hydrus 2D was selected as being capable of adequately modeling the geometry and nitrogen transport, and having many specific useful features for modeling subsurface drip irrigation. Initial modeling was performed for various soils, emitter spacings, depths, and loading rates. Following the initial modeling, container tests were performed with packed loamy sand, sandy loam, and silt loam soils. Nitrogen removal rates in the container tests ranged from 63 to 95% despite relatively low levels of available carbon.; Based on initial and revised modeling results it was found that a system designed for efficient irrigation generally minimized annual nitrate percolation. The greatest nitrogen losses (30% to 70%) were predicted for medium to fine texture soils. Restrictive layers or capillary breaks were necessary in coarse soils to achieve substantial nitrogen losses. The slow nitrogen transport with subsurface drip enhanced nitrogen losses and plant uptake opportunity. Simplified applications of model results are presented. |
