Fig. 2. Average N concentration profiles. Comparison between the conventional leachfield model (native soil, control column) and the modified leachfield models (0.3- and 0.9-m organic layer columns). Note "sawdust" refers to a sawdust–sand mix as the organic layer. Hatch marks illustrate the depths of the organic layers in the respective columns. Nitrate and nitrite N values are averaged over the sampling period of October 2001–July 2002 (15–23 samples). Ammonia N values are averaged from March–July 2002 (6 samples) and organic N values were averaged from April–July 2002 (4 samples). Synthetic wastewater was applied from August 2001–August 2002.