Long Term Effectiveness of Wellhead Protection Areas

Abstract

A preventive instrument to ensure the protection of groundwater is the establishment of wellhead protect areas (WPA) for public supply wells. The shape of the WPA depends on the rate of pumping and aquifer characteristics, such as the transmissivity, porosity, hydraulic gradient, and aquifer thickness. If any parameter changes after the design of the WPA, it will no longer be effective in protecting the aquifer and its catchment. With population growth in urban areas, the pressure on groundwater abstraction increases. Changes in flow, drawdowns and hydraulic gradients often occur. The purpose of this work is to evaluate the effectiveness of the WPA after a long period of establishment, in public wells with continuous pumping, located in densely populated urban area of the municipality of Montijo (Portugal). Considering the aquifer scenario in 2019, new extended WPAs were calculated using the combined results of three analytical methods and numerical modelling. In 2009 the aquifer presented hydraulic gradients varying between 0.0005 and 0.002, giving rise to a protection area with essentially circular shape. Although there was no increase in extraction flow, in 2019 the hydraulic gradients vary from 0.0008 to 0.008, and the flow directions have changed because of the water level decline. The shape of the WPA in this case is essentially elliptical and longer upstream and it can pose difficulties in the protection of public water catchments, in an urban area with already defined and consolidated land use. The best protection of the public supply wells in disturbed aquifers is obtained through numerical modeling.