Impactof Urban Lawns on Nutrient Contamination of an Unconfined Aquifer

ReportNo WSAA 49

December 1992

SYNOPSIS

Experimentaland modelling approaches were developed and used to quantify water and nutrientleaching beneath urban lawns, situated on a Coastal Plain.

Fourrepresentative lawns were studied. Over a 210 day period, irrigation was themain source of water during summer, precipitation was the main source duringwinter. There was a strong seasonality in nitrogen and phosphorusconcentrations in water leaching through the soil. Fertilising is conductedalmost exclusively over spring and summer. About 50% of input water passedbelow the root zone, carrying nutrients with it. On many occasions, nitrateconcentrations in leachate exceeded the World Health Organisation (WHO)drinking water limit of 10 mg/l. Invariably groundwater concentrations,however, were lower due to dilution and denitrification.

Substantialquantities of nitrogen and phosphorous have been shown to leach below the rootzone and are a threat to many urban wetlands, especially the Swan-Canning riversystem.

Mechanisticmodelling approaches were employed to simulate chloride and nutrient leachingbelow the root zone. A reasonable comparison between measured and modelled NH₄and NO₃ values was observed. Measurements and modelling showed thatalthough some PO₄ –P passed below the root zone, it will take manyyears for the phosphate peak to reach the groundwater.

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