Mass Balance of Water and Nitrogen in the Mounded Drainfield of a Drip‐Dispersal Septic System

Quantitative assessment of nitrogen (N) loading from septic systems is needed to protect groundwater contamination. We determined the mass balance of water and N in the mounded drainfield of a drip‐dispersal septic system. Three lysimeters (152.4 cm long, 91.4 cm wide, 91.4 cm high, with 1:1 side slope) were constructed using pressure‐treated wood to mimic mounded drainfields. Of total water inputs, septic tank effluent (STE) added 57% water and natural rainfall added 43% water from January 2013 to January 2014. Outputs included leached water (46%) from the lysimeters over 67 sampling events ( n = 15 daily and n = 52 weekly flow‐weighted), potential evapotranspiration (28%), and water stored in the drainfields (26%). Over 13 mo, each drainfield received 227 g of total N (STE, 99%; rainfall, 1%), of which 33% leached, 23% accumulated in the drainfield, and 6% was taken up by grass, with the remainder (38%) estimated to be gaseous N loss. Using these data, the leaching of water from 2.5 million drip‐dispersal drainfields in the state of Florida was estimated to be 2.29 × 10 11 L yr −1 , which would transport 2.4 × 10 6 kg of total N yr −1 from the drainfields to shallow groundwater. Further reduction of N below drainfields in the soil profile could be expected before STE reaches groundwater. Our results provide quantitative information on the water and N loading and can be used to optimize drainfield conditions to attenuate N and protect groundwater quality. Core Ideas Mass balance of water and N was determined in septic system drainfields. Effluent added 57% water, and rainfall added 43% water. More water leached (46%) than evapotranspiration (28%) and soil storage (26%). Effluent added 99% of total N, and rainfall added only 1% of total N. Gaseous N loss was higher (38%) than leaching (33%) and soil storage (23%).