Pilot-scale vadose zone biobarriers removed nitrate leaching from a cattle corral

Abstract

Agricultural activities that involve animal wastes can result in the contamination of subsurface soils by nitrates (NO₃). In saturated or nearly saturated soils, microbial biobarriers are a common method used to remove contaminants from water. This study evaluated the presence of NO₃ in the soil profile beneath a cattle corral and the use of biobarriers to prevent the movement of NO₃ through these semiarid vadose zone soils. Soils from beneath the pen contained as much as 330 mg kg⁻¹ (330 ppm) nitrate-nitrogen (NO₃-N), and a well located ~50 m (~55 yd) from the pens contained 18 mg L⁻¹ (18 ppm) NO₃-N, indicating that denitrification activity in the surface soils of the corral had not prevented NO₃ from migrating into deeper soils where denitrification activity was limited by electron donor (i.e., carbon [C] substrate) availability. Biobarriers used to intercept the percolating soil water contaminated with NO₃ included a control that contained only sand and treatment biobarriers that contained sand blended with sawdust, soybean (<i>Glycine max</i> L.) oil, or sawdust and soybean oil. A distinctive attribute of the study was that the biobarriers were installed near the surface of a semiarid soil well above the water table. Results show that all of the treatment biobarriers removed NO₃ from percolating soil water. Water collected from beneath the control barriers contained 99 mg L⁻¹ (99 ppm) NO₃-N while NO₃-N levels in water that moved through the sawdust, soybean oil, and sawdust plus soybean oil barriers averaged 4, 17, and 28 mg L⁻¹ (4, 17, and 28 ppm) NO₃-N for removal efficiencies of 96%, 83%, and 72%, respectively. In waters from the sawdust plus soybean oil barriers, 6.8 mg L⁻¹ (6.8 ppm) nitrite-N was detected. With all three substrate combinations, biobarrier life was such that barriers of this type should last for many years before substrates need to be replaced.