Advances in Water Treatment and Enviromental Management

206 WATER TREATMENTAn additional 14 auger holes were drilled to a depth of up to 10 m, with the objective ofproviding an early warning of aviation fuel contaminant migration towards the PWS borehole.The shallow depth minimised creation of fast flow paths to the high yielding horizons. One ofthe auger boreholes was drilled on the side of PWS borehole away from the known contaminatedareas, to provide a background control for water quality. Fig. 5 and Fig.6 show the distributionof the auger boreholes and a cross section of the site respectively.In order to determine the nature, degree and extent of the contamination, a major pump testexercise was carried out on the PWS borehole using the auger holes and boreholes asobservation wells, and monitoring the flow in the drainage ditches over a low weir. Geophysicallogging was carried out on all available boreholes to identify the major flow horizons in theaquifer. Groundwater quality monitoring was conducted in conjunction with test pumping ofthe PWS borehole. Samples were collected primarily by depth sampling at the major flowhorizons in the aquifer, and analysed by GC/MS for organic constituents.ResultsThe JP4 aviation fuel was not found to contaminate the PWS borehole at any stage, despitethe quantities in the aquifer nearby.However, sampling of the auger holes and PWS borehole indicated the presence in thegroundwater of PCE and TCE at maximum concentrations of 85 and 20 µg/1 respectively.The PWS borehole was removed from supply immediately after discovery of the pollution.As well as VOCs, other hydrocarbons were also detected in the auger holes. The hydrocarbonswere traced to a source at a nearby petrol filling station. The auger holes delineated a “pancake”of petrol of limited areal extent floating on the water table and leading from the petrol station.The source of the TCE and PCE was never identified but the data suggested that the input ofTCE and PCE to the aquifer had occured from one of the drains, in the form of a slug. A sumpwhich drained into the ditch contained TCE and PCE. This was emptied to a recognised wastedisposal facility. Depth samples indicated that both total volatile organic compounds andTCE/PCE increased with depth, even though geophysical logging demonstrated clear horizontallayering of flow in the aquifer. In 1985, routine monitoring of the auger boreholes indicatedan increase of TCE/PCE.DiscussionAlthough the PCE and TCE were major contaminants of the PWS borehole, they were onlyidentified as a result of another contamination incident. Today, the substances are routinelylooked for in all PWS sources. Therefore, where a source has a history of, or is vulnerable to,industrial pollution, a wide range of potential contaminants should be analysed for in anymonitoring programme.The variable levels of PCE and TCE in the groundwater, due either to continuing pollution, orto remnant concentrations of PCE/TCE in the aquifer, must be appreciated when predictingworst case scenarios for treatment specification.3. TREATMENT FOR THE REMOVAL OF VOLATILE ORGANIC CONTAMINANTS3.1 IntroductionVOC pollutants can be removed by activated carbon but a much cheaper process, particularlyas a first stage treatment is removal by air stripping. The design characteristics for removalare described in Section 3.2. This is followed by a discussion of the operating results for the