Transport and transformation of urea used as a de-icing agent at a regional airport, southeast Sweden

Abstract

Urea, NHi-CO-NH2, is used to de-ice runways at northern airports. In order to reduce the
nitrogen load to the coastal zone resulting from urea deicing activities at Kalmar Airport,
southeast Sweden, a wetland was constructed in 1996. In this study, the transport and
transformation of urea, from the airport runways to the constructed wetland, has been
investigated, and the wetland nitrogen reduction has been evaluated. Monthly sampling at the
wetland inlet and the wetland outlet, giving annual transports oftotal-N, have been compared
with daily transports of urea and total-N, based on high frequency sampling during urea
application and runoff events. Annual transport of total-N during 1998 shows a reduction of 8
ton (36 % of incoming tot-N load), during 1999 a reduction of 2,5 ton (6,3 %) and during
2000 a total-N reduction of 4,2 ton (14 %). During a high frequency sampling period, Jan -
March 2001, 25 % of the applied deicing urea-N were transported to the wetland inlet as ureaN, indicating that most of the urea was transformed during the transport from airport runways
to the wetland. According to calculations of cumulative urea-N loads at the wetland inlet and
the wetland outlet during February and March 2001, 40 % of the incoming urea-N was
transformed in the wetland system. The estimated average urea transformation capacity for
the wetland was during this period 3,0 mg urea-N•m·2
•h·1
• There were no indications of
nitrogen reduction in the wetland during February and March 2001, since the cumulative
total-N load at the wetland outlet was 1000 kg higher than the cumulative load at the inlet.
This was consistent with the transports for 1998, 1999 and 2000, with several occasions with
higher load of total-N at the outlet than at the inlet during Jan-March. During January and
March the urea concentration dynamics at the wetland inlet was correlated with water flow
(r2=0,55, p<0,001) air temperature (r2=0,36, p<0,001) and precipitation (r2 =0,06, p=0,02),
while during February several ice periods complicated the concentration dynamics and no
significant correlations with water flow, temperature or precipitation were found.