Groundwater Nitrate-Nitrite Modeling in a Grazed Hillslope with Agroforestry and Grass Buffers

Groundwater pollution negatively impacts aquatic ecosystems and human health. On the other hand, conservation practices can help reduce groundwater and surface water pollution. Baseflow from agricultural fields can be an important source of nitrate-nitrite (NN) loads in lakes and other surface water bodies. Riparian agroforestry buffers can be an effective barrier between groundwater NN and surface water bodies. The study aimed to determine the effects of agroforestry buffers and widths on groundwater nitrate-nitrite (NN) exports from an agricultural grazing area into a farm lake using flow and solute transport models. The flow and solute models were calibrated and validated for the weather and land use (grazing) conditions observed during the monitoring period, and these conditions were repeated throughout the 10-year projection. The calibration and validation of the flow and solute transport models were satisfactory, yielding determination coefficients R2 > 0.95 and Nash-Sutcliffe coefficients > 0.94. The area of study was modeled under four scenarios: tree-only buffers [cottonwood (Populus deltoides Bortr. ex Marsh.)]; grass-only buffers ([Tall fescue Schedonorus phoenix (Scop.) Holub, Red clover (Trifolium pretense L.), and Lespedeza (Lespedeza Michx)]); tree + grass buffers (a combination of the same tree and grass species of the other two scenarios; and a no-buffer scenario. The tree-only, grass-only, and tree + grass buffers reduced the total mass of NN discharged from the study unit to the lake by 98%, 97%, and 99%, respectively, compared to the no-buffer scenario. Doubling the buffer width from 15 m to 30 m decreased the NN discharge to the lake by 16-fold. Moreover, 7.5 m wide buffers had up to nine times greater NN discharge than 15 m buffers. Results show that agroforestry buffers with trees and grasses in riparian areas significantly remove NN exports in groundwater from agricultural fields, protecting the environment and human health.