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Hong Kong Geologist, 2002, 8, 14-20 Preliminary conceptual study on impact of land reclamation on groundwater flow and contaminant migration in Penny’s Bay J.J. Jiao Department of Earth Sciences, University of Hong Kong, Hong Kong Abstract A large-scale land reclamation project is now being carried out at Penny’s Bay, Lantau Island, Hong 2 Kong. The completed reclamation will provide 2.8 km of land for the construction of the new Hong Kong Disneyland and other essential infrastructure developm
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   Hong Kong Geologist, 2002, 8, 14-20 14 Background In November 1999, the HKSAR Governmentannounced that agreement had been reached withthe Walt Disney Company to build a Disney theme park in Hong Kong. Penny’s Bay, Lantau Island, Preliminary conceptual study on impact of land reclamation ongroundwater flow and contaminant migration in Penny’s Bay J.J. Jiao  Department of Earth Sciences, University of Hong Kong, Hong Kong  Abstract A large-scale land reclamation project is now being carried out at Penny’s Bay, Lantau Island, HongKong. The completed reclamation will provide 2.8 km 2 of land for the construction of the new HongKong Disneyland and other essential infrastructure developments. The impact of this project on variousaspects, such as ecology and environment has been widely discussed. This paper studies change in thegroundwater system around the bay area in response to the land reclamation. This paper also predictsthe possible flow pathway of the contaminated groundwater due to the Cheoy Lee Shipyard, whichwas located on the north and eastern shores of Penny’s Bay and had operated for almost 40 years.Findings observed on basis of this preliminary study include: 1) After reclamation, the total groundwater head in the entire model area increased, and the slopes immediately behind the srcinal coast of theBay have the most significant buildup in total head. 2) Seepage along the coastlines beyond Penny’sBay will increase in response to the reclamation, with Yam O Wan seepage increased being especiallysignificant. 3) If contaminated soil near the Cheoy Lee Shipyard is not removed, the contaminatedgroundwater will not spread within Penny’s Bay, but will migrate northeast toward Yam O Wan.FEMWATER, a three-dimensional finite element ground water model, is used in this study. It should be noted that this paper is entirely based on desk computer simulation and all the basic data such asaquifer properties and the infiltration coefficient are assumed values. The overall qualitative conclusionsof the study are believed to be reasonable, but quantitative values of the computer output such asgroundwater head or contaminant travel time may be very uncertain due to lack of actual data andfield hydrogeological study from this reclamation site.was selected as the best site for constructing such a park. The topography of Penny’s Bay is shown inFig. 1. This project involves several major workselements, including the reclamation of about 2.8  15  J. J. Jiao km 2 of land in Penny’s Bay. The project started inMay 2000 and the entire works are expected tocomplete by the end of 2008.The reclamation methods and fill materialsvary over the site. Marine mud in the seawall areawill be dredged. The mud in other areas will belargely left in place. Over 70 million m 3 of fillmaterial (including surcharging) will be placed inthe reclamation site. The fill materials vary andinclude marine sand from offshore sites, river sandfrom Mainland China, decomposed igneous soil,construction waste, and public fill, which wouldotherwise be disposed of at strategic landfills or fill banks. The elevation of the final ground surfaceafter reclamation will be about 11 mPD.The bedrock around Penny’s Bay is largelyfeldsparphyric rhyolite. The coastal area near YamO Wan consists of tuff. The stratigraphy of theQuaternary deposits at the site is primarily a two-fold succession of soft mud of the Hang HauFormation overlying a complex mixture of firm tostiff silty clay with some sand and silt, which formsthe Chek Lap Kok Formation.A former ship-building site called the CheoyLee Shipyard (CLS) was located on the north andeastern shores of Penny’s Bay with an area of about0.19 km 2 . It commenced operation in 1964 and wasused for boat manufacture, repair and maintenance.The shipyard was decommissioned in 2001 to makeway for roads and other infrastructure linkingPenny’s Bay to the rest of Hong Kong. In earlyJanuary 2000, it was reported that the soil on theCLS site had been seriously polluted over the years by oils, heavy metals, dyes and organic solvents brought about by ship-breaking activities and thedisposal and burning of wastes on site. A number of substances require specialized forms of treatmentto ensure their eradication from the site. Notableamong these is dioxin-contaminated soil, a cancer-causing chemical produced by burning plastic or  polyvinyl-chloride (PVC) materials.The contract for the excavation anddemolition of the CLS was awarded in July 2002with the work commencing in September 2002,involving the removal of around 87,000 m 3 of contaminated soil from the site. However, it isdifficult to remove all the contaminated soil entirely.Even if the soil can be removed, the contaminatedgroundwater around the site may still cause problems.Large-scale land reclamation may modify theregional groundwater flow system and theoreticallysuch modification usually has adverse engineeringand environmental consequences (Jiao, 2000; Jiao et al  , 2001). For the Penny’s Bay reclamation project, although there are studies on various issuessuch as marine environment and coastal ecologyrelated to this reclamation project, there seems to be no study directed twoards understanding changeof the groundwater flow system due to thereclamation.This paper involves a theoretical desk studyof the groundwater flow systems near Penny’s Bay before and after the reclamation, and attempts to predict the possible flow pathway for thecontaminated groundwater srcinating from theCLS. The potential engineering problems andenvironmental effects caused by any change in thegroundwater system is also discussed. Theinvestigation in this paper is largely conceptual and  Fig. 1 Topography of Penny’s Bay (modified from free mapdownloaded from http://www.info.gov.hk/landsd/mapping/mp/ html/index_fmd.htm)   Hong Kong Geologist, 2002, 8, 14-20 16  by no means comprehensive due to lack of hydrogeological data from the site. Assumptions and parameters used inthis study This conceptual numerical study is based onmany assumptions. All the coastlines are treatedas fixed-head boundaries, with constant head of 1.23mPD. The west boundary is chosen to be over 3kmfrom Penny’s Bay, and is represented by a no-flow boundary. The bottom boundary of the model isselected to be -20 mPD and impermeable. Thesoil above the impermeable bottom is dividedinto two layers. The boundary between the lower and upper layers is chosen to be the middle point between the ground elevation and -20 mPD. Thegeological material in each layer is treated ashomogeneous and isotropic. It is assumed thatin the upper layer, K  x =K  y = 10 -5 m/s, and in thelower layer, K  x =K  y = 10 -6 m/s (Table 1). Thesevalues are within the typical permeability rangeof decomposed granite in Hong Kong (GEO,1993).A man-made reclamation area is produced by dumping fill materials. The nature of the soil ina reclamation is usually heterogeneous, and may beeven more heterogeneous than soils or sedimentsdeveloped naturally from geological materials. The permeability in a reclamation is extremelyunpredictable and varies with the fill materials used,and the method of placement. On the basis of laboratory testing, the typical range of  permeabilities for compacted fill materials of completely decomposed granite and volcanics are10 -6 to 10 -7 and 10 -6 to 10 -8 m/s, respectively (GEO,1993). Marine sand is probably the most permeablematerial among the commonly used fills.Shen and Lee (1995) investigation carried outstudies of hydraulic fill performance in Hong Kong.They note that the typical permeability of marinesand used in the generalized soil profile in Tseung  ParameterTop layerBottom layer  Permeability 10 -5 m/s10 -6 m/sLongitudinal dispersivity10 m10 mLateral dispersivity1 m1 mMolecular dispersion coefficient0.0001 m 2 /s0.0001 m 2 /s Table 1 Key aquifer hydraulic and transport parameters used in the model  Kwan O was 1.0 x 10 -4 m/s, and that in WestKowloon the permeability was between 6.0 x 10 -5 and 8.0 x 10 -5 m/s. The permeability value in thefield is usually much lower than that estimated inthe laboratory. While the quality and nature of fillat a site can be extremely variable, there is usuallya layer of soft marine mud at the seabed beneathreclamation sites. The permeability of the marinemud is usually extremely low, with a range of 10 -7 to 10 -9 m/s (Kwong, 1996). This layer of mud isusually troublesome. It gradually becomes less andless permeable as consolidation continues. In thisstudy, the reclaimed site is represented by twolayers. The permeability of these two layers istreated as the same as the background soil.Rainfall is the dominant recharge to thegroundwater system. The recharge rate is chosento be 1.27 x 10 -8 m/s, which is about 20% of theaverage annual rainfall of 2000 mm in Hong Kong.Since this study concerns long-term change of thegroundwater flow system, only steady state issimulated. A steady state flow model is run beforeland reclamation, then another steady state modelis run after land reclamation.FEMWATER, a three-dimensional finiteelement ground water model (Lin et al  , 1997), ischosen for this modeling study. This code is runvia GMS (Groundwater Modeling System)developed by Brigham Young University under thedirection of the U.S. Army Corps of Engineers.Before reclamation, the study area is discretised into3542 nodes and 5124 elements, after reclamation,the model area is discretised into 4382 nodes and6618 elements. Groundwater system before and afterreclamation Fig. 2 shows the total groundwater headdistributions in the groundwater system near Penny’s Bay before and after reclamation. Beforereclamation, the total groundwater head is high at  17  J. J. Jiao two topographical centers near Tsing Chau Tsai andYam O, which are located to the east and west of Penny’s Bay. There is a groundwater divide betweenPenny’s Bay and Yam O Wan.After reclamation, there is a regionalincrease in total groundwater head. The groundwater divide srcinally between Penny’s Bay and Yam OWan has moved to a position within Penny’s Bay.The high total head srcinally located at the hilltopof Tsing Chau Tsai has moved to areas immediately behind Mong Tung Hang, which is on the east coastof Penny’s Bay.As can be seen from Figure 2b, a steephydraulic gradient, which indicates high seepage,toward the coastal slopes can also be observed near Mong Tung Hang. Table 2 lists the percentageincrease of total head in a few selected locationsshown in Figure 2b. The slopes (Locations A-C)around the Bay experience a great increase in totalhead. The total head at Location A has been almosttripled. There is also a significant increase in totalhead in areas far from Penny’s Bay (Location D-F).The head at Location E near Sze Pak has increased by almost 60%.As can be seen from the 16 m contour near the western model boundary before and after reclamation, the head there has the smallestincrease in total head, which indicates that thereclamation has limited impact on areas that arefar from the Bay. This also shows that the choicefor the location of the western boundary of themodel is reasonable. Table 2 Percentage increase of total groundwater head in selective points after reclamation Location A B C D E FIncreasein head196%178%153%16%58%35%Fig. 3 shows the groundwater flowdirections before and after reclamation. Thereclamation has led to a change in the flow pattern not only near the Bay, but also to areas beyond it. For example, before reclamation, thecoastal area near Sze Pak does not show anobvious groundwater catchment, but after reclamation, there is a well-defined groundwater catchment behind the slopes near Sze Pak. Beforereclamation, the coastal area behind Fa Pengindicates an obvious groundwater catchment, butafter reclamation, the catchment is much larger withstrong convergent flow toward the coastline.Figure 3b also shows that towards thecenterline of Penny’s Bay, there is a stronggroundwater convergent flow (see the shadow stripinside the Bay). It is expected that the groundwater level may be reduced if a zone of very permeablefill materials is placed along this centerline. Thisdemonstrates that, if the possible impact of landreclamation on groundwater regimes isinvestigated in advance of the reclamation project, there are certain measures that canreduce the buildup of the water levels and the possible environmental and engineeringconsequences of land reclamation.FEMWATER can output the boundary fluxwhich provides information on the potential seepagerate along the coastline. Three portions of thecoastline A-A’, B-B’ and C-C’ (see Figure 3b) arechosen for detailed discussion. The results indicate  Fig. 2 Total groundwater head distributions in the land areaaround Penny’s Bay before (a) and after (b) land reclamation
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