Nitrogen recovery from simulated sand-filter effluent by grass

Graduation date: 1984The effluent from on-site sewage disposal systems has been\ud found to be a source of nitrates (NO\ud ₃⁻\ud ) in groundwater. Vegetation\ud has the ability to remove some of the nitrogen (N) from sewage\ud effluent as the solution moves through the soil around a disposal\ud trench, before it reaches groundwater.\ud Shallow aquifers are particularly susceptible to NO₃⁻\ud contamination under on-site sewage disposal systems in permeable,\ud coarse-textured soils. This is due to the relatively rapid downward\ud movement of leachate from a disposal trench, and to the shorter\ud trench length allowed in these soils.\ud The objective of this study was to determine if reducing the\ud effluent application rate, in a coarse-textured soil, would increase\ud the percent of N that grass could recover from the effluent. The\ud potential NO\ud ₃⁻\ud toxicity of the grass was also considered.\ud A solution with a NO\ud ₃\ud ⁻N concentration of 30 mg/L was introduced\ud through shallow disposal trenches into a sandy soil vegetated with\ud tall fescue (Festuca arundinacea Schreb.). The NO\ud ₃⁻\ud solution\ud simulated sand-filter effluent, since the primary contaminant of\ud sand -f liter effluent is NO\ud ₃⁻.\ud Four treatments were used with three replications of each\ud treatment. Three treatments received the NO\ud ₃⁻\ud solution. The daily\ud application rates of this solution were 25.2, 12.6 and 6.3 L/m of\ud disposal trench for the high-, medium-, and low-rate treatments,\ud respectively. The fourth treatment received water only, as a\ud control. The application rate of solution for the high-rate\ud treatment matched the mean application rate for sand-filter effluent\ud in a sandy soil.\ud The grass that grew in response to the NO₃⁻ solution seeping\ud out from the trenches was harvested 3 times during the 134 day study\ud period. The three cuttings from each plot were mixed and a\ud subsample from each was tested for total-N, NO₃⁻N and other\ud nutrients.\ud Reducing the effluent application rate did not significantly\ud change the percent of N recovered from the effluent. The mean\ud (± S.E.) percent N recovery ranged from 25.1 ± 1.8% for the low-rate\ud treatment to 31.7 ± 0.6% for the medium-rate treatment.\ud The mean NO\ud ₃⁻\ud concentration of the grass from the high-rate\ud treatment was 3100 ± 265 ppm. At this NO₃-N concentration the\ud percent NO\ud ₃⁻\ud in the grass is 1.36%. If this grass made up the\ud entire ration of an animal, NO₃⁻ poisoning could occur