Analysis of a LEACHN-based management technique in ‘predictive mode’

A major concern in New Zealand is the potential impact on groundwater quality following the application of wastewater to land. The concern is that land application of wastes at excessive rates may cause nitrate leaching and groundwater contamination. It was learned from a field experiment [Mahmood, B., Wall, G.L., 2001. Environmental impact of effluent irrigation onto the land planted with trees and pasture. Int. Agric. Eng. J. 10 (3-4), 209-230] that land application of effluent on a regular basis, even when the nitrogen loading rate was within fertiliser application guidelines, can still pollute the groundwater. Therefore, there is a need to develop better management options for land treatment systems, which reduce the risk of groundwater contamination. Based on the findings of this field scale experiment, a management technique was developed to make the long-term (seasonal or yearly) management decisions (i.e. to check the long-term effects of land application of wastewater on groundwater quality) at a land treatment system (LTS). The purpose of this study was to undertake an analysis of the technique in predictive mode in order to check if it is reasonable to use the average rainfall data instead of actual rainfall data to do the management runs of the technique/model. Past 26 years (1975-2000) daily rainfall, temperature, and pan evaporation data were used for this analysis (using a calibrated LEACHN model). Ten scenarios of the technique/model were run on a monthly basis using the planned irrigation volume and actual rainfall data for 365 days. The average rainfall data was replaced with actual rainfall data on a month-bymonth basis. The analysis of the technique revealed that it would be reasonable to use the average rainfall data to do the management runs of the technique. The analysis of the technique also revealed that by using this technique it would be possible to identify the crucial times of the year when effluent could not be applied. It would also be possible to identify the time(s) of the year when the excess amount of effluent (which could not be applied during wet periods) could safely be applied o the land during summer time.