Nitrate Leaching During Long‐term Spray Irrigation for Treatment of Secondary Sewage Effluent on Woodland Sites

Two waste water‐treated sites at The Pennsylvania State University Wastewater Renovation Project were monitored to measure nitrate and total N in soil water. Both long‐term waste water irrigation and varied irrigation rates were examined. Soil water was sampled by porous cup samplers. The concentration of N at the 120‐cm depth was taken as a measure of N which escaped the root zone and which would leach to the ground water. Recharge volume was calculated from irrigation, rainfall, and potential evapotranspiration. A hardwood forest site located on a well‐drained sandy loam soil was irrigated year round with secondary municipal effluent at a rate of 5 cm/week. In the last 6 of 9 years of effluent treatment 83% of the 4,954 kg N/ha added leached from the site. Nitrate concentration in soil water at the 120‐cm depth was generally > 15 mg N/liter. The hardwood forest was also ineffective in keeping nitrate concentration below 10 mg N/liter at the 120‐cm depth when the 5‐cm weekly application was split into two 2.5‐cm irrigations or when it was lowered to 2.5 cm/week year round. An abandoned old field site planted with white spruce (Picea glauca Moench) and located on a well‐drained clay loam soil was irrigated with effluent at 5 cm/week from Apr. through Nov. each year, beginning in 1963. Nitrate concentration at the 120‐cm depth rarely exceeded 10 mg N/liter and only 36% of the 1,246 kg N/ha applied leached during the 6th through the 9th years of this effluent treatment. In the 10th and 11th years, when the application rate was increased to 7.5 cm/week, nitrate concentration exceeded 10 mg N/liter at the 120‐cm depth, and the amount of N which leached increased to 75% of the amount applied.