Impact of a real-time controlled wastewater subsurface drip disposal system on the selected chemical properties of a vertisol

Abstract

The operation of onsite septic effluent disposal without considering seasonal moisture changes in drain field conditions can be a major cause of the failure of conventional septic systems. This study addressed this issue from a soil hydraulic perspective by using real-time drain field soil moisture levels to limit septic effluent disposal in a vertisol via subsurface drip irrigation. A prototype system was field-tested in a Houston clay soil and results describe the subsequent impact on selected soil chemical properties. After one year of hydraulic dosing with a synthetic wastewater, soil total carbon and nitrogen concentrations increased, but no increase in soil total phosphorus concentration was observed. Soil NO₃-N leaching potential was noted, but soil NH₄-N concentrations decreased, which could be ascribed to NH₄-N nitrification, fixation within clay sheets and NH₃ volatilization. Soil K⁺, Mg²⁺ and Na⁺ concentrations increased in soil layers above the drip lines, but decreased in soil layers below drip lines. Soil electrical conductivity accordingly increased in soil layers above drip lines, but the range was significantly lower than the threshold for soil salinity. Although the moisture-controlled effluent disposal strategy successfully avoided hydraulic dosing during unfavourable wet drain field conditions and prevented accumulation of soil salts in the soil profile beneath the drip lines, soil salts tended to accumulate in top soil layers. These adverse effects warrant system corrections before large-scale implementation of subsurface drip irrigation of effluent in similar vertisols.

Keywords:

• septic effluent
• soil chemical properties
• subsurface drip disposal
• vertisols
• wastewater

Acknowledgements

This research was funded by Alabama Agricultural Land Grant Alliance (AALGA) and Alabama Water Resources Research Institute (AWRRI). The National Soil Dynamics Laboratory of the United States Department of Agriculture Agricultural Research Service also partially aided in soil sampling analysis. The authors wish to thank James Holliman at the Alabama Black Belt Research and Extension Center for his dedicated field support. The Departments of Biosystems Engineering, Agronomy and Soils and Civil Engineering at Auburn University also provided support for this research.