Temporal variations of physical and hydrochemical ...presents the results of a 1-year comprehensive monitoring programme of the springs in the Mid-Levels area. The variations of physical

HYDROLOGICAL PROCESSESHydrol. Process. 22, 1080–1092 (2008)Published online 5 November 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/hyp.6656

Temporal variations of physical and hydrochemical propertiesof springs in the Mid-Levels area, Hong Kong: results of a

1-year comprehensive monitoring programme

Chi-Man Leung* and Jiu Jimmy JiaoDepartment of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China

Abstract:

Springs and seeps occur in the spaces around Po Hing Fong Street in the Mid-Levels area, Hong Kong. Most of the springsoccur through the drainage weepholes on retaining walls at the street. This paper first examines the geology and history ofthe springs. The paper then reports the findings from a 1-year comprehensive spring monitoring programme. The temporalvariations of flow rate, physiochemical parameters and hydrochemistry of the springs are discussed. The average temperaturesof the springs were close to the mean air temperature, although there was a systematic lag time of 40 to 50 days between thepeak air temperature and highest water temperatures. Spring waters from two rows of weepholes in the retaining wall showedsignificantly different physical and hydrochemical responses to the changes in rainfall and temperature, though their verticaldistance is only about 1 m. The results suggest that water from the upper row of weepholes may represent a recharge sourcethat is shallow or close to the spring outlets, whereas that from the lower row of weepholes may represent a recharge sourcethat is much deeper or further up the hill. Although the spring flows increased rapidly after rainstorms, analysis of the totaldissolved solids showed a delayed response to rainstorm events. The concentration of individual ions in the spring water variedin a unique way in response to rainstorm events. It is clear that the presence of underground man-made drainage systems andthe leakage from water mains in the study area may add complexity to the solute responses and transport mechanisms. Furtherstudies are required to constrain the impacts of these man-made structures on the hydrogeology of the springs. Copyright 2007 John Wiley & Sons, Ltd.

KEY WORDS spring; continuous monitoring programme; flow and chemical responses; rainstorm event; Hong Kong

Received 13 November 2005; Accepted 27 November 2006

INTRODUCTION

The lower slope of the Mid-Levels area is one of the mosthighly urbanized coastal areas in Hong Kong (Figure 1).Because of its hilly topography and long history of slopeinstability, the geology and the hydrogeology of the areagained special attention by local authorities. From 1979to 1981, the Hong Kong Government conducted the firstsystematic geological and hydrogeological survey in thestudy area. The results demonstrated that the subsurfaceconditions are extremely heterogeneous and anisotropic(GCO, 1982). Since then, a number of studies have beenundertaken by local researchers in an attempt to revealmore information about the groundwater conditions overthe area (e.g. Lerner, 1986; Jiao et al., 2006a,b; Leungand Jiao, 2005, 2006a,b; Leung et al., 2005). This paperpresents the results of a 1-year comprehensive monitoringprogramme of the springs in the Mid-Levels area. Thevariations of physical and hydrochemical properties ofsprings in response to rainstorm events are discussed.Besides providing a comprehensive dataset for springstudies in Hong Kong, this study can shed further lighton the hydrogeology of the areas near the springs.

* Correspondence to: Chi-Man Leung, Room 206, James Lee ScienceBuilding, The University of Hong Kong, Pokfulam Road, Hong Kong.E-mail: [email protected]

BACKGROUND OF THE STUDY AREA

Location of the Po Hing Fong spring

Several seepages are observed in the drainage weep-holes of the retaining walls behind a street named ‘PoHing Fong’ (PHF). The locations of seepages are shownin Figure 1. Five retaining walls, situatedg at an ele-vation of about 45–50 m above mean sea level, wereconstructed behind PHF. The walls were probably con-structed in the early 19th century and were rebuilt in 1925after being damaged in a landslide event. The walls werenumbered as 11SW-A/R36 (nos. 20–30 PHF), 11SW-A/R49 (no. 40 PHF), 11SW-A/R53 (nos. 50–56 PHF),11SW-A/R56 (nos. 50–60 PHF) and 11SW-A/R64 (nos.62–72 PHF) by the local authority for easy reference.

Historical information about the Po Hing Fong area

PHF and the nearby areas are possibly one of the majorgroundwater discharge zones in the Mid-Levels area. Inorder to draw down the high groundwater table, an under-ground tunnel with location shown in Figure 1, drainingabout 3Ð82 m3 of water per hour, was constructed in 1940(PWD, 1979). Cheung (2001) suggested that local com-munities had long been collecting spring water from theareas near PHF, as is done nowadays. According to GCO(1979a), continuous seepage could be found at some

Copyright 2007 John Wiley & Sons, Ltd.

TEMPORAL VARIATIONS OF SPRINGS IN THE MID-LEVELS AREA 1081

Figure 1. Location of the Po Hing Fong springs. In (a) the shaded area represents the natural slopes with minimum development. The area boundedby the grey line is the Mid-Levels area. Dotted lines represent the contact between granite and volcanic rock. Black lines represent the locationsof normal faults. Remarks: seepage location in 1979 was recorded by Geotechnical Control Office, Hong Kong Government (GCO, 1979b); in (b),

‘Blake Garden’ is an urban park and ‘Hong Kong Gardens’ is a residential building

places along the toe, and ponding of water occurs near themiddle of the wall at nos. 20–30 PHF. A little seepagewas found on the weepholes of the retaining wall fromnos. 50–60 PHF in January 1976. Before urban develop-ment, the area around what is now Hong Kong Garden(Figure 1) was frequently flooded after heavy rains. Thesprings in the PHF area have existed for a long time anddid not run dry even in 1963, the driest year in the historyof Hong Kong. According to the South China MorningPost (1963):

Hundreds of residents around a lane in WesternDistrict have found a 24-hour water supply—anatural spring that went almost unnoticed until thecurrent water crisis. Long queues of people now

line the lane, near Po Hing Fong, day and night,to draw supplies under the vigilant eye of policeconstables stationed there to keep order. . .

Besides collecting spring information from the litera-ture, we conducted several site visits and interviews withlocal communities in October 2001. It is reported by localresidents that there was a natural pond fed by seepagefrom the weepholes of the retaining wall in PHF with asize of about 20 m ð 5 m 30 years ago. This is consistentwith the description in GCO (1979a).

Geology and hydrogeology of the Po Hing Fong area

The geology and hydrogeology of the Mid-Levels areahas been described elsewhere (GCO, 1982). In brief,

Copyright 2007 John Wiley & Sons, Ltd. Hydrol. Process. 22, 1080–1092 (2008)DOI: 10.1002/hyp

1082 C.-M. LEUNG AND J. J. JIAO

the geology is dominated by two rock types: acidicvolcanic rocks and a granitic intrusion. The volcanicrocks have been subject to low-grade regional metamor-phism and deformation and affected by contact metamor-phism where close to the granite. Both lithologies weresubsequently intruded by basaltic dykes. The irregularcontact between the granite and volcanic rocks crossesthe area and is disrupted by normal faults at severallocations (Figure 1). Colluvium overlies several metresof decomposed rock above the bedrock. Granite under-lies most of the developed area, and is composed ofquartz (23–42%), potassium feldspar (31–42%), plagio-clases (16–35%) and biotite (¾5%) according to Allenand Stephens (1971). Volcanic rock underlies the upperundeveloped slopes.

Although it is likely that the subsurface lithologywould be extremely heterogeneous and anisotropic, GCO(1982) have grouped them into three aquifer units corre-sponding to (a) colluvium, (b) decomposed volcanic andgranite rocks, and (c) volcanic and granite bedrocks. Thebedrock aquifers are the least permeable and, togetherwith the decomposed rock aquifers, contain the mainwater tables. The colluvium aquifers contain transientand permanent perched water tables. These are connectedto the main water table by both saturated and unsatu-rated flows. The upper slopes are generally covered byyoung colluvium, which is more open and more per-meable than the older colluvium, which mainly coversthe lower parts of the developed spaces (GCO, 1981).High rates of natural recharge from rainfall could occurin the upper slopes, as most of the ground surface is stillexposed.

As shown in Figure 1, the PHF area lies wholly withinthe granite. Since there is a lack of deep borehole logsaround the spring, the exact depth of granitic bedrockremains unknown. A normal fault extending from Victo-ria Peak passes through the PHF area. The retaining wallsare located near the lower boundary of the colluvial fan(which is largely decomposed volcanic material) probablyeroded from the slopes rising southwards to Victoria Peak(GCO, 1979a,b). The colluvium is commonly 10 to 15 mthick, but can be up to 36 m thick, and is found to covermost of the Mid-Levels area between levels of about 300and 50 m above the principal datum (Randall and Tay-lor, 1982). Natural soil pipes, which are continuous orpartly continuous subsurface voids formed by internalerosion of naturally occurring voids in the soil matrix,can be found in the upper colluvium slopes. Some ofthe soil pipes can be sizable, 200 mm diameter or more,and may carry water rapidly to the lower slopes throughthe partially continuous networks running downslope, asspeculated by GCO (1981). The interface between thecolluvium and the decomposed granite beneath is a sourceof concern, since shear zones can often be found. Drill-hole log reports indicated the presence of a weak andclayey layer at the base of the colluvium (Randall andTaylor, 1982). This clay bed is at a flat angle in a down-hill direction, but it curves upwards again in the PHFarea. Two previous disastrous slope failures that involved

old masonry walls near the lower boundary of the col-luvial fan were recorded in 1925 and 1976. Accordingto the water level records stored at the library of CivilEngineering Department of the Hong Kong Government,artesian flows have been observed in the piezometers(with depths from 5 m to more than 20 m) installedalong PHF.

Origins of the Po Hing Fong spring

The PHF area is within a topographic depressioncompared with the surrounding areas. According to GCO(1979b), the PHF area was originally the bottom of avalley before urban development. The present landscapealso supports this view.

A photolineation study was conducted in the Mid-Levels area using high-flight aerial photographs takenin 1945, 1956 and 1963. It was found that there isa photolineament (possible fault) passing through thePHF area. PWD (1979) and GCO (1982) found a majorlocalized thick zone of decomposed granite of about90 m, possibly fault related, at the area which is about200 m south of PHF. The thick zone decomposed granitebecomes much thinner in the downslope direction (GCO,1982).

It is suggested by Nash and Dale (1984) that mostof the favourable factors for the formation of natu-ral soil pipes and erosional tunnels can be found inHong Kong and in the Mid-Levels area. The steepslopes provide sufficient hydraulic gradient to initiateand develop pipes from naturally occurring voids andpreferred flow paths. The pipes form partially continu-ous networks running downslope, carrying water rapidlydownslope and possibly to depth. A number of voids (andsome pipes) have been found from test pits and drill-holes in the study area, and at the surface, particularlyin the upper slopes (GCO, 1981; Randall and Taylor,1982; Nash and Dale, 1984; Au and Pang, 1993; Au,2001).

It is speculated that the presence of the PHF spring ispossibly due to a combination of the topographic depres-sion, faulting and natural soil piping. The presence ofwater-conducting faults and naturally derived soil pipescould preferentially bring waters to the PHF area fromupper natural slopes.

Long-term changes in the discharge of Po Hing Fongsprings

Since no systematic measurements on the flow ratesof PHF have been undertaken, it is hardly possibleto have quantitative conclusions about the long-termchanges in the flow rates of PHF springs. However,some indirect information could provide insights to inferthe changes of flow rates over the last few decades. Asmentioned, seepages and pools of waters were observedat nos. 20–30 PHF in 1979. However, the waters havedisappeared nowadays. In addition, some man-madestructures for seepage collection were found on somecurrently ‘dry’ weepholes in retaining walls. As shown



in Figure 1, the number and the coverage of seepagesin the PHF area have decreased in the past 20 years.It is possible that some original flow paths may havebeen either redirected or obstructed by the extensiveconstruction of subsurface engineering structures in theupper slopes, resulting in the overall reduction in theamount of recharging water from the upper slopes to thePHF area.

METHODOLOGY

Figure 2 shows the weephole distribution of the retain-ing wall where seepages can be found. A U-shapedchannel was constructed at the toe of the wall to col-lect the seepages and divert the waters to the subsur-face drainage nearby. As MW and BW are the twomajor springs contributing most of the water in the U-shaped channel, they were monitored in greater detail.The pH, dissolved oxygen (DO), electrical conductivity(EC) and temperature of waters from MW, BW and theU-shaped channel were measured weekly at the samplingsites from February 2002 to February 2003 by varioushandheld meters. The flow rate of the U-shaped chan-nel was also monitored weekly for the same period.The flow rate of BW could not be measured directlybecause of its physical limitations. A traditional methodwas employed for flow estimation. The velocity wasestimated by dropping a small piece of paper on theflowing water. The flow rate was calculated by multi-plying the velocity and the cross-sectional area. The flowrates of MW, R4 and R11, determined by measuring thetime required to fill up a 500 ml beaker, were recordedfrom May 2002 to February 2003. Water samples fromMW were collected weekly and analysed for chemicalcompositions following standard sampling and analyticalguidelines (Clescerl et al., 1998). The sampling and ana-lytical methods are described in detail in Leung (2004).In brief, water samples for chemical analysis were filteredthrough a hand-held Hanna filter system using 0Ð45 µmcellulose filter papers and collected in a 500 ml cleanhigh-density polyethylene bottle. Three 125 ml aliquotswere collected each time; two were unacidified and theother was acidified to pH <2 using ultra-pure nitric acid.Two aliquots, one acidified and the other unacidified,were then refrigerated at 4 °C before chemical analy-sis. The acidified aliquot was analysed for major cations

and trace metals by inductively coupled plasma massspectrometry, except for Si and B, which were anal-ysed by inductively coupled plasma atomic emissionspectrometry. The second 125 ml aliquot, unacidified,was measured for major anions by ion chromatogra-phy. Sample batches were regularly interspersed withstandards, including NIST SRM 1640, and blanks, andall data were corrected for instrument drift. A three-point calibration curve was constructed for each element.The precision of measurement of three replicate analy-ses for major and trace elements was generally betterthan 5%. HCO�

3 was measured by titration with stan-dard hydrochloric acid solution. The third 125 ml aliquot,unacidified and not refrigerated, was collected for activecarbon dioxide (CO2) determination. All the chemicalanalyses were completed within 3 days after sampling.Satisfactory charge-balance errors of within š7% wereachieved in most spring samples. The detailed phys-iochemical and analytical results are listed in Leung(2004).

WEATHER CONDITIONS DURING MONITORINGPERIOD (FEBRUARY 2002–FEBRUARY 2003)

The weather conditions during the monitoring period,which are recorded by the Hong Kong Observatory(HKO), are briefly described here. Generally, the year2002 was the second warmest year on record. Themean temperature of 23Ð9 °C was 0Ð9 °C above average.Only three tropical cyclones affected Hong Kong in thatyear, which was about half of the normal number. Only4Ð6 mm of rainfall was recorded in February, amountingto less than 10% of the normal rainfall of 48Ð0 mm. Themonthly total rainfall of 238Ð7 mm for March 2002 wasmore than three times the normal amount for March.About 134 mm rainfall was recorded for 23–24 March asa result of a cold front. The monthly rainfall of 723Ð0 mmin September was about 2Ð4 times the normal amount andranked the fourth highest for that month. More than half(about 440 mm) of the rain in September fell from the15th to 17th of the month because of the approach of atropical storm, leading to serious flooding in the lowerpart of the Mid-Levels area.

Figure 2. Distribution of weepholes on retaining wall where PHF springs (MW, BW, R4 and R11) could be found



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Figure 3. Temporal variations of flow rates at U-shaped channel, MW, R4 and R11

RESULTS

Variations of flow discharges of U-shaped channel, MW,R4 and R11 with rainstorm events

Seepage discharge characteristics could shed importantlight on the recharge pattern and hydrogeology of thestudy area. As shown in Figure 2, there are three rowsof weepholes found on the retaining wall. The holesin the top row were dry throughout the whole yearof 2002, even though it was one of the wettest yearson record. This suggests that the fluctuation of thewater level near the spring outlets is less than 1 m(the vertical spacing between holes). Some of the holesin the bottom row, although strong spring flows wereobserved in the adjacent holes, remained dry throughoutthe monitoring period. Some weepholes may have agood supply of water from hydrogeological features,such as natural soil pipes, erosion tunnels or fractures,whereas others may not or may be blocked by siltsand clay. As shown in Figure 3, the flow rates ofthe U-shaped channel and weepholes MW, R11 andR4 increased sharply shortly after rainstorms and thendecreased quickly, with the rate of increase being fasterthan the rate of decrease. During the rainstorm eventthat occurred in mid September, the amount of flowincreased from 110 to 260 ml s�1 at MW in 1 day.Compared with its average flow rate, the dischargemeasured in the U-shaped channel doubled in 1 day.Such an immediate response may demonstrate that watertable changes induced by infiltrating rain are transmittedquickly from the area behind the retaining wall to thespring vents.

MW and R11 showed general declines in flow ratesfrom the wet season (May–October) to the dry seasonthroughout the monitoring period, as shown in Figure 3.However, this decreasing trend was not observed eitherat R4 or at the U-shaped channel.

Variations of dissolved oxygen contents of MW and BWwith air temperature

Figure 4 shows the variations of DO in waters at BWand MW with air temperature. The hand-held oxygenmeter has a precision of š0Ð5% of the measured values.

It is found that the variations of DO at BW and MWwere strongly consistent with each other and inverselycorrelated with air temperature (Figure 4). The DO atBW and MW started to decrease gradually from April2002 (spring) to August 2002 (midsummer). A higher airtemperature could lead to lower DO because biochemicalreactions proceed faster and the solubility of oxygen inwater decreases at higher temperature (Metcalf and EddyInc., 1979). DO began to increase from September 2002and peaked in early January 2003, following a significantdrop in air temperature in late December 2002. Theincrease in DO in mid September 2002 could possiblyrelate to the intense rainstorm events.

Temporal variations of spring water temperature at MWand BW

Figure 5 shows the variation of water temperatures atMW and BW with air temperature. The water temperatureat MW ranged from 21Ð8 to 26Ð3 °C, with an average of24Ð5 °C. At BW, it ranged from 22Ð0 to 26Ð3 °C, withan average of 24Ð4 °C. During the monitoring period, the



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Figure 5. Variations of water temperatures at MW and BW with air temperature

mean water temperatures at both weepholes were close tothe average air temperature, which was 23Ð5 °C. There isa systematic lag time of about 40–50 days between theair and spring temperature peaks. On a yearly basis, thespring temperature varied approximately 4Ð4 °C, whereasthe air temperature varied about 23Ð5 °C. The results alsoshow that water from MW appears to respond faster tothe changes in air temperature than does that from BW.

Variations of hydrochemistry of MW with rainstormevents

Variations of aggressive CO2 at MW with rainstormevents. Figure 6 shows the variations of aggressive CO2

concentration at MW with rainfall. Free aggressive CO2

is likely to exist when CO2 levels get high enough that thecarbonic acid level becomes high and could cause morerapid dissolution of calcium carbonate. The aggressiveCO2 contents, ranging from 4Ð93 to 11Ð15 mg l�1, usu-ally increased shortly after rainfall and then decreased.Higher aggressive CO2 concentrations could be observedfrom late June to August. It may be possible that the

aggressive CO2 contents could be controlled by micro-bial activity, such as organic carbon oxidation. However,no corresponding decrease in DO contents was observedduring the monitoring period, suggesting that the impactsof microbial activity were insignificant.

Variations of major ions with rainstorm events. It isfound that not all major ions behaved the same wayin response to rainstorm events. The results of majorion variations with rainstorm events are illustrated inFigure 7. The chloride concentrations, ranging from 202to 309 mg l�1, usually exceeded the level of 250 mg l�1,which is the drinking water limit based on the guidance ofWHO (1993), during the monitoring period. Like sodiumand magnesium ions, chloride and sulphate behaved sim-ilarly in response to rainstorm events. From Figure 7, noclear patterns can be observed that can be related to rain-storm events, suggesting that the variations of major ionconcentrations may be controlled by many factors. Unlikeother major elements, the concentration of bicarbonateincreased almost every time after a rainstorm event. The



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Figure 6. Variations of aggressive CO2 in water at MW with rainfall

increase was most obvious after the exceptionally heavyrainstorm in mid September.

Seawater has long been used for flushing in theMid-Levels area since the 1960s. Leung et al. (2005)demonstrated that urban groundwater samples in theMid-Levels area are extremely sensitive to small fractionsof leakage from flushing water pipes. However, like mostof the urban groundwater samples in the area, the springsamples collected at MW were dominated by Na–Cland Na–Ca–Cl water types and had Na/Cl ratios closeto the seawater value throughout the monitoring period,Leung and Jiao (2006b) successfully demonstrated that itis largely composed of natural groundwater. However, theoriginal water chemistry was effectively masked by salinewater leaking from flushing water pipes, even at a smallamount. Detailed discussions on the impacts of leakagefrom flushing water pipes on the hydrogeochemistry ofthe Mid-Levels area are described in Leung et al. (2005)and Leung and Jiao (2006a)

Variations of total dissolved solids at MW with rain-storm events. Figure 8 shows the variations of total dis-solved solids (TDS) at MW with rainfall. The TDS variedfrom 604 to 740 mg l�1 and appears to be correlatedwith rainfall. Three major rainstorm events are chosen fordetailed investigation because rains of more than 100 mmfell in each day, and sufficient chemical data were col-lected before and after the rainstorm events.

Rain event 1 occurred on 23 March 2002. Beforethis heavy rainstorm, the study area experienced a dryFebruary with total rainfall of only 4Ð6 mm. The TDS ofMW increased from 671 mg l�1 on 22 March to 740 mgl�1 on 29 March. It took about 1 week for the TDS toreach its maximum level after the rainfall. After that, theTDS decreased gradually back to the original level.

Rain event 2 occurred on 11 June 2002. Rain hadfallen nearly every day for the previous 2 weeks. TDSwas 676 mg l�1 on 7 June and decreased to 637 mg l�1

on 13 June. After that, it increased gradually and peakedat 713 mg l�1 on 4 July. It took about 3 weeks for theTDS to reach a maximum in rain event 2.

A total of about 440 mm rain fell on 15–17 September2002 (rain event 3), resulting in serious flooding inthe lower part of the Mid-Levels area. TDS decreasedfrom 665 mg l�1 on 5 September to 622 mg l�1 on 18September. No flow and TDS were measured until 17October 2002, when the TDS was 694 mg l�1.

Compared with flow rates, which showed instanta-neous response to rainfall, it took about 1 weekand3 weeks for TDS to reach maximum in rain events 1and 2 respectively. The response of TDS in rain event 1seems to be different from that in rain events 2 and 3.No TDS decrease was observed shortly after rain event 1,whereas TDS first decreased and then increased graduallyto a peak in both rain events 2 and 3.

Variations of trace elements with rainstorm events. Theanalytical results of trace elements are summarized inTable I. No significant elevations in the concentrationsfor most of the trace elements were observed at MWcompared with that of the natural slopes, except for Sr,Se and B. Leung and Jiao (2006a) examined the sourcesof Sr, Se and B in groundwater in the Mid-Levels area.As seen from Figure 9, most trace elements did not showclear relationships to rainstorm events. Trace elementsexhibited even more varying responses to rainfall thanthe major elements did.

DISCUSSION

Size estimation of recharge area of the Po Hing Fongsprings

The size of recharge area of the springs is estimatedbased on the amount of rainfall, infiltration rate and flowrate:

Rainfall ð Infiltration rate ð Recharge area D Flow

�1�

The flow rate of springs was 1300 ml s�1. Table II liststhe calculated recharge areas under different infiltration



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Figure 7. Variations of aqueous major ion concentrations at MW with rainfall

rates. Given that the average rainfall of the area is2200 mm year�1 and the infiltration rate is 20%, thenthe calculated recharge area is approximately 0Ð1 km2.This value is clearly too small in view of the sizeof the Mid-Levels area, which is about 1Ð5 km2. Asthe PHF springs are permanent springs with relativelystable recharging sources, lower infiltration rates, e.g.5%, would give a more reasonable size of the rechargearea, e.g. 0Ð4 km2. The low infiltration rate is likely tobe caused by extensive impermeabilization of the landsurface of the Mid-Levels area.

Comparison of physiochemical responses of springwaters from different weepholes

The springs at MW and R11 are only 1 m above thoseat R4 and BW. However, the two groups of springsexhibited significant differences in response to rainfalland temperature changes. As shown in Figure 3, flowrates at R4 fluctuated around a value of 47 ml s�1 in themonitoring period, whereas the flow rates at MW andR11 showed obvious declines with time. The relativelystable flow rates measured at R4 suggest that the rechargesources may be more regional and deeper with a large



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Figure 8. Variations of TDS at MW with rainfall

Table I. Comparison of trace element contents at MW andbackground values (Leung, 2004)

Element Concentration (ppb)

Background Weephole ‘MW’

Min. Mean Max. Min. Mean Max.

Al 0Ð78 8Ð94 43Ð72 0Ð00 0Ð17 0Ð43Fe 2Ð23 8Ð35 63Ð25 2Ð23 4Ð90 6Ð93Mn 0Ð36 2Ð33 7Ð34 0Ð37 1Ð13 5Ð53Cu 0Ð00 0Ð55 4Ð53 0Ð00 0Ð81 2Ð59Zn 1Ð50 26Ð35 73Ð75 7Ð64 20Ð58 57Ð20Sr 5Ð82 24Ð92 47Ð47 280Ð00 347Ð46 403Ð80Se 0Ð00 0Ð19 0Ð70 6Ð19 8Ð62 11Ð54Li 0Ð20 1Ð94 6Ð79 0Ð09 0Ð47 0Ð76Be 0Ð03 0Ð41 1Ð12 0Ð00 0Ð02 0Ð03V 0Ð00 0Ð41 1Ð06 1Ð70 3Ð29 5Ð63Cr 0Ð00 0Ð61 5Ð12 0Ð28 0Ð78 1Ð64Co 0Ð00 0Ð01 0Ð07 0Ð04 0Ð12 0Ð22As 0Ð00 0Ð35 1Ð78 0Ð84 1Ð24 2Ð39Rb 2Ð39 5Ð85 11Ð38 23Ð66 26Ð61 33Ð45Mo 0Ð00 0Ð19 0Ð88 0Ð62 0Ð84 1Ð71Ag 0Ð00 1Ð89 7Ð10 0Ð08 3Ð96 11Ð13Cd 0Ð01 0Ð06 0Ð18 0Ð10 0Ð35 2Ð23Sb 0Ð00 0Ð04 0Ð20 0Ð00 0Ð16 0Ð24Ba 8Ð07 29Ð96 55Ð79 50Ð59 59Ð54 73Ð06Pb 0Ð01 1Ð06 12Ð10 0Ð21 0Ð31 0Ð59B 12Ð54 18Ð84 31Ð22 110Ð57 148Ð71 171Ð89

drainage area. On the other hand, the recharges of MWand R11 may be controlled more by local factors, suchas rainfall near the outlets.

Moreover, the pH at BW, ranging from 6Ð19 to6Ð73, was generally lower than that at MW, rangingfrom 6Ð34 to 6Ð83, in the monitoring period and thetemporal variations of pH were consistent with each other(Figure 10). As shown in Figure 5, water temperatureat BW appears to be changed at a slower rate thanthat at MW in response to variations in air temperature.This implies that the spring at BW was ‘heated up’more slowly than that at MW when the air temperature

Table II. Estimated recharge area with different infiltration rates

Infiltration rate (%) Recharge area (km2)

20 0Ð0915 0Ð1210 0Ð195 0Ð37

increased. And the spring at MW ‘cooled down’ quickerthan that from BW when air temperature dropped.

From Figure 11, the EC of BW was generally higherthan that of MW from February 2002 to August 2002,whereas from September 2002 to February 2003 the ECof MW became higher than that of BW. Throughoutthe monitoring period, the trends in EC variation forboth waters were consistent with each other. The EC ofboth waters dropped significantly and became very closeto each other shortly after the rainstorm event in midSeptember 2002. From Figure 4, the temporal variationsof DO are very similar for the two weepholes. MW wasgenerally higher than that of BW from April 2002 to midDecember 2002, whereas the DO of BW became higherfrom mid December 2002 to February 2003.

The above physiochemical monitoring results furtherdemonstrate that BW represents a deeper groundwatersource and MW represents a shallower groundwatercomponent and is more sensitive to local environmentchanges. Further studies are required to explain thedifference better over such a small vertical distancebetween the two groups of waters.

Hydraulic characteristics and solute transportmechanism of water at MW

The variation of TDS in response to rainstorm eventscould shed important lights on the recharge pattern ofsprings. As stated, TDS showed a delayed response torainstorm events compared with flow responses. This isbecause subsurface hydraulic responses occur much morerapidly than the transport phenomena that would governTDS fluctuations.



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Figure 9. Variations of trace element concentrations at MW with rainfall

Before the heavy rainstorm event on 23 March 2002, asmall amount of rainfall, only 5Ð7 mm, was recorded fromFebruary 2002. Dry soils may inhibit water infiltration,ultimately forcing water to flow through preferential pathsthrough the vadose zone (Ritsema et al., 1998). Largeamounts of solutes, which accumulated in the vadosezone during dry seasons, may be washed out. Moreover,pore water, which was already present in the soil profileas ‘old water’, was pushed in front of the newly infiltrated

water. Because of this, no obvious decrease in TDS wasrecorded shortly after rain event 1 in late March 2002.

On the other hand, a significant reduction followedby a gradual increase in TDS could be observed inthe subsequent rainstorm events. A possible explanationis that some of the solutes accumulated in the majorpreferential flow paths were washed out in the rainstormevent of March, resulting in the overall dilution ofgroundwater flowing through the preferential paths after



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Figure 11. Variations of EC in waters at BW and MW with rainfall

subsequent rainstorm events. As soil moisture increasesgradually from dry season to wet season, the proportionof water flowing through the preferential paths maydecrease. The infiltrating water could then dissolve thesolutes in soils where they are passing through. Asthe velocity of water in soils is lower than that in thepreferential paths, it may take longer for this portion ofgroundwater recharging to the spring outlets, resulting inthe delayed response of TDS observed.

Limitations of study

The results presented are not without their limitations.First of all, the groundwater regimes in the PHF areahave possibly been modified by a number of anthro-pogenic factors. According to GCO (1979a), the retainingwall at nos. 20–30 PHF (11SW-A/R36) was backfilledafter 1880 to provide a platform on which residentialbuildings were constructed. As shown in Figure 1, anunderground tunnel and a groundwater drainage adit wereconstructed under the primary school and Hong KongGardens respectively in order to intercept groundwater

flow beneath (GCO, 1979b, 1989). In the early 1980s, thisunderground tunnel was backfilled with concrete whichresulted in an acute increase in the groundwater levelsbehind (GCO, 1988). The above modifications could pos-sibly modify the local groundwater flow systems, whichmay in turn complicate the flow responses of springs.

Second, the hydrochemistry of springs could be alteredby anthropogenic pollution sources, such as leakage fromwater mains, as the springs emerge in highly urbanizedareas. Leung and Jiao (2005) and Leung et al. (2005)demonstrated that groundwater in the Mid-Levels areahas been affected by leakage from sewage pipes tovarious extents. The nitrate level (expressed as NO�

3 ) atMW was significantly higher than the background levelof 1Ð7 mg l�1 during the monitoring period, suggestingthe presence of sewage. Leakage from water mainscould certainly add complexities and uncertainties on theinterpretation of variation of chemicals in response torainstorm events. More studies are required in order togive a better explanation on the observed hydrochemicalvariations of springs.



Third, being limited by resources, only weekly orsometimes biweekly monitoring of physical and chemicalparameters of the springs was conducted. This precludesthe observation of any short-term responses of flowand chemicals to most rainstorm events. Further studiesaiming at examining such instantaneous responses may bevery useful to constrain the hydrogeology of the springsfurther.

CONCLUSIONS

This paper presents the results of a 1-year comprehen-sive monitoring programme of the PHF springs in ahighly urbanized coastal area in Hong Kong. The his-tory, geology and origin of the springs were examined.The variations of physical and hydrochemical propertiesof the spring in response to rainstorm events provideinformation useful to an improved understanding of thehydrogeology of areas around the springs.

The flow rates of PHF springs responded immediatelyand sharply to rainstorm events. Part of the water couldbe recharged through the preferential flow paths nearthe springs, which may complicate the hydrogeologyof the area. The flow rates could be approximatelydouble shortly after certain rainstorm events, possiblyresulting in the sharp increase in pore pressure in thesoils. As supported by the reduction in the numberof seeps and the flow rates, some of the rechargewaters may possibly be either redirected or blockedby the widespread underground engineering structures.The abundance of deep building foundations and othersubsurface engineering structures around the springs maychange the groundwater flow pattern in the area.

Variations in the physiochemical parameters of thesprings in response to rainfall and air temperature changeswere also studied. The DO of waters was found tobe inversely correlated with air temperature. Althoughno clear trend of pH variation with rainfall could beobserved, the pH of water from BW was always lowerthan that from MW throughout the monitoring period.In addition, the average temperatures of the waterswere close to the average air temperature during themonitoring period. There was a systematic lag time ofabout 40–50 days between the peak air temperature andthe peak spring water temperature. The waters fromBW and MW showed significantly different responsesto the changes in rainfall and temperature, although theirvertical distance is just about 1 m. It is speculated that thewaters from BW and MW are of different origins: BWpossibly represents a deeper groundwater source and MWa shallower groundwater component.

The TDS of the spring at MW showed a delayedresponse after rainstorm events. The peak of TDS usuallyappeared at least 1 week after rainstorm events. Thechemical responses of springs may be controlled byfactors such as the amount and frequency of rainstormevents. For individual ions, it appears that each ionexhibited a unique response to rainstorm events that

cannot be fully explained based on the limited dataobtained in this study. Their responses could also becomplicated by anthropogenic factors, such as leakagefrom service pipes.

ACKNOWLEDGEMENTS

This study is supported by the Hong Kong ResearchGrants Council (RGC) (HKU 7013/03) of the HongKong Special Administration Region, China, and theDevelopment Budget for Area of Excellence in WaterEnvironment Engineering, the University of Hong Kong.

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Documents

Temporal variations of physical and hydrochemical ...presents the results of a 1-year comprehensive monitoring programme of the springs in the Mid-Levels area. The variations of physical