The Remediation of Chemical Groundwater Contamination at Harwell

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The Remediation of Chemical Groundwater Contamination at Harwell© Crown Copyright 2005. All rights reserved. OrdnanceSurvey Licence Number 10047376.Figure 1Location of the Western and Southern Storage Areas at HarwellFigure 2Geological map of the Harwell AreaLSG Groundwater Report 2 March 2009
The Remediation of Chemical Groundwater Contamination at Harwell2 GROUNDWATER LEVELSThe variation in groundwater levels (the “water table”) with time in the Chalk aquifer underHarwell site is shown in Figure 3. These data relate to borehole HWS9 (Figure 1), whichhas been monitored weekly since 1991. Groundwater levels are relevant to themanagement of groundwater contamination for a number of reasons:1. Historically, as groundwater levels have risen (usually in the late autumn and winter)contamination has been washed out from the Chalk beneath the waste pits at theWSA, resulting in higher contaminant concentrations in the groundwater captured bythe containment plant.2. Away from the WSA, the relationship between groundwater levels and contaminantconcentrations is the opposite. At times of high groundwater levels, concentrations arelowest, because of dilution of the contaminants by rainfall entering the aquifer. Whengroundwater levels are low, contaminant concentrations rise.3. In the Chalk aquifer beneath the northern part of the Harwell site, the direction ofgroundwater flow changes depending on the groundwater levels. At times of highgroundwater levels, water flows from the springs at the top of the Lydebank Brook,resulting in groundwater flows to the north from much of the site. As the groundwaterlevels fall flow from the springs stops and the dominant groundwater flow directionchanges to the east and southeast. Groundwater flows beneath the southern part ofthe Harwell site do not show the same seasonal change in direction.The peak groundwater level recorded in borehole HWS9 during 2008 was 110.8 m AOD,recorded on 25 March (Figure 3). Groundwater levels then dropped until late May/EarlyJune, when groundwater levels rose in response to heavy rainfall. Met Office data forOxford show that the rainfall total for May and June 2008 was 60% above average 1 . Thiswas the second successive year in which summer recharge of groundwater had occurred,something not seen since collection of groundwater level data began at Harwell in 1990.Figure 4 shows groundwater level data for borehole HWS9, re-plotted to allow comparisonof data for each year. During the summer, groundwater recharge usually does not occurbecause rainfall either evaporates from the surface or is utilised by plants. However,Figure 4 shows that groundwater levels increased by 3.2 metres in response to heavyrainfall in July 2007 and by 1.3 metres in June 2008.From July 2008 onwards, groundwater levels fell until early November, when groundwaterbegan to increase.The maps in Figure 5 show groundwater levels in the Chalk in April and October 2008.The solid lines are contours of equal groundwater level. The groundwater flow direction isapproximated by the arrows. These plots coincide with the major six-monthly groundwatersampling events undertaken by RSRL on site. April and October were selected as themain sampling events because they generally coincide with the highest and lowestgroundwater levels respectively.The patterns of groundwater flow seen in 2008 (Figure 5) were similar to those seen in2007. The groundwater level data show that the groundwater plant at the WSA maintainedeffective containment during 2008, continuing to prevent contaminated groundwatermoving away from the WSA.1Based on data for the period 1971-2000LSG Groundwater Report 3 March 2009
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The Remediation of Chemical Groundwater Contamination at Harwell

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