The Movement of Nitrogen Species Through Three Soils Below Septic Fields1
- K. W. Brown,
- K. C. Donnelly,
- J. C. Thomas and
- J. F. Slowey2
Much effort has been directed toward designing and installing septic leach fields to prevent the contamination of both surface and groundwater. Nitrogen is a very mobile element, abundant in septic effluent, and may pose a serious threat to groundwater below septic fields.
This 2-yr field study on movement of N species below simulated septic lines in three soils encased in large undisturbed monolith lysimeters was designed to access the potential N pollution of groundwater below septic fields.
Effluent applications of 81.8, 32.7, and 16.4 L/m2 of trench bottom per day were made to sandy loam, sandy clay, and clay soils, respectively. Leachate was collected continuously from suction cups distributed across the lysimeter bottom and occasionally from cups in the soil adjacent to and below the septic line and analyzed for N fractions. At the end of the study, soil samples were collected in an array around the septic line and analyzed for NH4+-N and total N.
Accumulation of NH4+-N in the soil profile and leachate losses of NH4+-N and NO3−-N were greatest in the sandy loam soil which received the largest volume of effluent and had the lowest CEC. Sixteen months after the initiation of effluent application, NH4+-N concentrations in the leachate 120 cm below the septic line began to increase above background values, and by 23 months these concentrations were in the order of 6 µg/mL. Vertical movement of NH4+-N averaged 100 cm/yr in the sandy loam and 25 cm/yr in the sandy clay and clay soils.
Concentrations of NH4+-N in the leachate samples collected 120 cm below the septic line were > 10µg/mL during and immediately following periods when the soil profile was oxidized.
Soil samples taken at the end of 2 yr of application showed a build-up of total N under the septic line to concentrations as great as 1000 µg/g. Total N losses summed over the entire period range from 0.45 to 2.2% of that applied.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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