The role of seepage in constructed wetlands receiving agricultural tile drainage

Constructed wetlands positioned in the landscape between row crop agriculture and surface waters can be used to intercept tile drainage and serve as agricultural waste water detention basins. A potential exit pathway in constructed wetlands for detained water and possibly NO3 -N is via seepage through and under an earthen berm. The objective of this study was to determine if seepage was an important pathway for NO3 -N transport from two constructed wetlands receiving tile drainage from adjacent agricultural land (wetland A, surface area of 0.6 ha; wetland D, 0.78 ha). A mean apparent hydraulic conductivity (K) was calculated (10.8 cm h, range 8.2–14.3 cm h) using empirical water budgets. Using Darcy’s law, which included the apparent hydraulic conductivity, effective seepage area and daily hydraulic gradient measurements, daily seepage volumes for both wetlands were calculated for the 1997 water year. Total seepage volumes for wetlands A and D were 26 and 22 million liters, respectively, for the 1997 water year, which represented 47 and 27% of the total inlet flow. The amount of NO3 -N exiting wetlands A and D in seepage water was estimated to be 61 and 25 kg N, respectively, and represented 10 and 4% of the total inlet NO3 -N load. Seepage connected the wetland with the riparian buffer strip and transported NO3 -N to populations of denitrifiers deeper in the sediment profile and outside the wetland perimeter, thereby enhancing overall NO3 -N removal efficiencies. © 2000 Elsevier Science B.V. All rights reserved.