Urban Rainwater Composition Study
Rainwater samples have been collected in New Orleans from October 1999 to October 2000. The samples were analyzed for water quality parameters: pH, total Kjeldahl nitrogen (TKN), nitrate (NO3-), ammonia (NH3) and total phosphorus (TP). Average concentrations show clear seasonal variations with higher levels in spring and summer and lower levels in winter and fall for NO3-N and TP. Comparison with Lake Pontchartrain parameters shows that the nitrogen concentration is significantly higher in rainwater than in the lake water but the TP is lower in rainwater than in the lake water. Comparison with agricultural areas (Washington Parish) shows that levels of major species of nutrients from urban areas (Orleans Parish) are higher than those of agricultural areas.
The contrasts between urban and agricultural areas can be explained by proximity to the local pollution sources, such as motor vehicles, electric utilities, and petrochemical plants. The findings suggest that for some species of nutrients such as NH4+, NO3- and PO43-, anthropogenic emissions are major sources. A strong inverse relationship generally was found between the observed nutrient concentration and the quantity of rainfall in a particular event; however, the total loading due to rainfall increased with increased amounts of rainfall but at a diminishing rate. The rainfall concentrations of ammonia, nitrate/nitrite and total phosphorus were converted to loads and compared to loads from urban storm runoff loads.
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