Groundwater is important to the city of Perth for public water supply, irrigation of parks and gardens and horticulture and about one in four houses has its own bore for garden watering. The superficial aquifer receives stormwater drainage and is easily contaminated from accidental spills or waste disposal. It supports perennial wetlands, and has been drained in areas of urban development with shallow water tables. The confined aquifers are used mainly for public water supply and are used conjunctively with the unconfined groundwater and surface water. The Gnangara Mound, which is the main resource in the superficial aquifer and also recharges the confined aquifers, is protected from urban development and is covered by native woodland or pine forest. Groundwater in the crystalline rocks of the Darling Range is limited to low yielding bores and wells for gardens or orchards.