Analysis of travel time, sources of water and well protection zones with groundwater models

Transcription

1 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 Analysis of tavel time, souces of wate and well potection zones with goundwate models ZHOU Yang-xiao 1*, Pavez Sawe Hossain, Nico van de Moot 3 1 UNESCO-IHE Institute fo Wate Education, Delft, The Nethelands. The Institution of Enginees, Bangladesh. 3 Wate Supply Company, Denthe, The Nethelands. Abstact: This study compaes numeical models with analytical solutions in computing tavel times and adius of potection zones fo a pumping well located in an unconfined aquife with unifom echage and in a semi-confined aquife. Numeical models wee capable of delineating potection zones using paticle tacking method in both cases. Howeve, potection zones defined by tavel time citeion can only potect small pecent of souce wate to the well; lage pecent of souce wate is not potected which may pose a isk of pollution of souce wate to the well. The case study of Leggeloo well field in the Nethelands indicates that although a well field potection aea was enfoced in 198s, elevated nitate concentation has been monitoed in the abstacted wate since 199s. The analysis of potection aeas shows that the cuent potection aea only potects 37.4% of echage wate to the well field. A lage potection aea must be adopted in ode to safeguad the sustainable wate supply fo the local community. Keywods: Tavel time; Souces of wate; Potection zone; Goundwate models Intoduction Goundwate povides a vey impotant souce fo dinking wate supply. As goundwate is geneally of good quality and equies less teatment, goundwate is inceasingly being used in pefeence to suface wate fo dinking wate poduction. In Austia, Denmak, Potugal, Iceland and Switzeland, ove 75% of the wate fo public wate supply is abstacted fom goundwate. Goundwate accounts fo 5% to 75% of dinking wate supply in Belgium, Finland, Fance, Gemany, Ieland, Luxemboug and the Nethelands (EEA, 1999). Goundwate is also an impotant souce fo wate supply in China. Aound two-thids of cities depend on goundwate * Coesponding autho. y.zhou@unesco-ihe.og. fo uban wate supply in China (China s Agenda 1, 1994). Especially in Noth China, goundwate accounts fo moe than half of the total wate supply and 7% of uban wate supply in the Noth China Plain (HAN, 1998). The withdawal of wate by a well causes loweing of goundwate levels in the suounding aea of the well. The aea that is coveed by echaging o contibuting wate to the well o well field is called the captue zone (Keely and Tsang, 1983). Theats of contamination of goundwate souces fo domestic wate supply wells necessitated the definition of goundwate potection zones. Thee ae thee common theats of contamination of goundwate souce: Shot cicuits of accidental contaminants to the wellhead; tanspot of micobial contaminants (pathogens) such as bacteia and viuses; and tanspot of 363

2 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 chemical contaminants. In ode to pevent these theats, potection aeas aound wells o well fields ae delineated to limit land suface activities. These potection zones include in geneal (Chave et al. 6): -A zone immediately adjacent to the site of the well to pevent apid ingess of contaminants o damage to the wellhead (often efeed to as the wellhead potection zone); -A zone based on the time expected to be needed fo a eduction in pathogen pesence to an acceptable level (often efeed to as the inne potection zone); -A zone based on the time expected to be needed fo dilution and effective attenuation of slowly degading substances to an acceptable level (often efeed to as the oute potection zone). A futhe consideation in the delineation of this zone is sometimes also the time needed to identify and implement emedial intevention fo pesistent contaminants; -A much lage zone sometimes coves the whole catchment aea of the well o well field. Many counties have developed citeia to delineate the above zones. These citeia include (US EPA, 1987, 1994): -Distance citeion defines the potection aea aound a pumping well with a adius; -Dawdown citeion defines the potection aea as the zone of influence (cone of depession of a pumping well); -Tavel time citeion bases the bounday of the potection aea on the time equied fo contaminants to each the pumping well; -Flow bounday citeion includes goundwate divide and inflow boundaies; -The assimilative capacity citeion is based on the capacity of the geological fomations to dilute o attenuate contaminant concentations to the acceptable level befoe they each the pumping well. Table 1 Definition of the dinking wate potection aea in the Nethelands (Van Waegeningh, 1981) Potection zones Wellhead potection Inne potection zone Oute potection zone Tavel time 6 days 1 yeas to 5 yeas Radius 3 m Aound 1 to 15 m Aound 8 to1 m, espectively Potection against Limits of activities Owned by the wate supply company. No any activity is allowed. Pathogenic bacteia and viuses Only activities in elation to wate supply ae admissible Hadly-degadable chemicals Following activities ae not admissible: -stoage of dangeous goods -industies -waste-sites -building -militay activities -intensive agicultue and cattle beeding -gound-, sand- o limestone pits -waste wate Remaining captue aea Application of act on soil and goundwate potection The methods used to delineate the potection zones depend on the choice of citeia and include the following geneal methods (US EPA, 1987, 1994; Chave et al. 6): 364 -Abitay fixed adius. Daws a cicle of fixed adius aound a pumping well. -Calculated fixed adius. Daws a cicle of specified time of tavel using a simple equation

3 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 based on volume of wate dawn to the well in a specified time. -Simplified vaiable shapes. Deived fom hydogeological and pumping figues simila to those at the wellhead, and oientates the shape accoding to goundwate flow pattens. -Analytical methods. Uses equations to define goundwate flow and contaminant tanspot. -Hydogeological mapping. Use of hydogeological infomation to map flow boundaies o use tace methods to identify flow paths. -Numeical models of goundwate flow and/o contaminant tanspots to delineate the captue zone of the wells o well fields and plot isochones map and/o contaminant concentation map to define the potection aea. In pactice, most counties adopted the tavel time citeion to define the potection aea because it is undestandable by wate manages and police-makes. In the Nethelands, fou potection zones wee defined in the poous media aquife to potect the dinking wate souces as listed in Table 1 (Van Waegeningh, 1981). Howeve, the ecent eseach (ZHOU, 11) shows that a potection aea defined with tavel time citeion only potects a small pecent of souce wate to the well. In this pape, the numeical models wee developed fo an unconfined aquife with unifom echage and a semi-confined aquife. Dawdown and tavel times computed with numeical models wee compaed with analytical solutions. Wellhead potection aeas wee delineated with tavel time citeion and checked with the pecentage of souce wate to the well. A case study of Leggeloo well field in The Nethelands was conducted to demonstate the limitation of tavel time citeion and necessity of using souce wate as additional citeion to delineate the wellhead potection aea in ode to achieve sustainable dinking wate supply. Q w h H H Q R R w Fig. 1 Coss-section of a pumping well located in the cente of an unconfined aquife with unifom echage 1.1 Conceptual model 1 Models fo the unconfined aquife with unifom echage The unconfined aquife is homogeneous and isotopic (hydaulic conductivity K=constant) 365

4 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 bounded with an impemeable base at bottom (Fig. 1). In the cente of the aea, a pumping well fully penetates the aquife with a constant pumping ate (Q ). The pumping ate is maintained by the unifom echage (w) fom the suface. At the adius of the influence (R), goundwate level is kept constant (H ). Dupuit's assumption is valid so that goundwate low is hoizontal adial flow. 1. Analytical solution 366 The analytical solution of the goundwate level fo the steady adial flow to the well in an unconfined aquife with unifom echage was developed by Bouwe (1993) as: h = H Q R w ln( ) + π K K ( R ) (1) The total catchment aea of the pumping well is A=πR ; total echage within the catchment is πr w which is equal to the pumping ate Q unde the steady state flow condition. The tavel time of the echage wate at adial distance fom the well can be calculated as: t = w dρ = v( ρ) w π neρh( ρ) dρ Q π ρ w Whee n e is the effective poosity of the aquife. () An appoximate solution fo tavel time was found by Zhou and Haitjema (1) as: ~ Q π w w = 1 π w w / Q t ( ) = τ w ln τ ln (3) w Q π w 1 π w / Q Whee: πr H ne H ne τ w = = (4) π R w w is the mean esidence time (stoage divided by total echage) of an aquife with unifom echage and constant thickness (H ). The tavel time Equation (3) can be futhe simplified (ZHOU and Haitjema, 1) as: ~ 1 t ( ) = τ ln (5) w 1 η π w η = (6) Q Whee η epesents the atio of the echage wate within adius to the total well dischage (Q ). When tavel time t p is used as a citeion to delineate the well potection aea, the adius of the potection aea ( p ) can be calculated fom Equation (5) as: p = The combined use of Equation (7) and (8) leads to the design of the well potection aea that includes both a time of tavel citeion (eaction time to contamination) and a pecentage of souce wate to the well that is potected by the well potection aea. 1.3 Numeical model A numeical goundwate model was designed to simulate goundwate flow and calculate tavel time in compaison with analytical solutions. The values of paametes used ae: Q = 5 m 3 /d, w=.1 m/d, H =5 m, K = 5 m/d, n e =.3. These values esults in a adius of the influence R = m. Theefoe, the numeical model consisted of a cicula unconfined aquife with a adius of 1 65 m. The model aea is discetized by 53 ows and 53 columns with a unifom cell size of 1 m by 1 m. The cente cell whee the pumping well is located was futhe efined to m t w ( e p τ 1 ) Q (7) πw ZHOU (11) agues that the pecentage of echage wate to the well dischage should be used as an additional citeion to define the well potection aea in esponding to sustainable dinking wate supply. The adius of the potection aea ( p ) can be calculated fom Equation (6) as: Q = η (8) πw p

5 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 by m cells. MODFLOW- (Habaugh et al. ) was used to simulate goundwate flow, and PMPATH (Chiang and Kinzelbach, 1994) was used to calculate tavel times. Dawdown (m) MODFLOW Analytical solution Radial distance fom the well (m) Fig. Compaison of dawdown calculated by the analytical solution and MODFLOW fo the unconfined aquife with unifom echage Fig. shows the compaison between analytical solution and numeical solution with MODFLOW in calculating dawdown. The numeical model is able to calculate goundwate dawdown with vey high accuacy except at the location of the well. With a adius of the well equal to.1 m, the analytical solution (Equation 1) computed a dawdown of.93 m. MODFLOW calculated a dawdown at the location of the well as.53 m. MODFLOW calculated a smalle dawdown at the location of the well which epesents an aveage value of the dawdown fo the cell with a size of m by m. Tavel times (yea) PMPATH Analytical solution Radial distance fom the well (m) Fig. 3 Compaison of tavel times calculated by the analytical solution and PMPATH fo the unconfined aquife with unifom echage PMPATH eads cell-by-cell velocities fom MODFLOW and uses paticle tacking method to compute tavel times of wate paticles. One paticle was placed at the cente of evey cell at the middle ow (ow numbe 17); fowad paticle tacking was used to move all paticles to the well step by step (3 days); and ecoded the tavel times of each paticle to the well. Fig. 3 plots tavel times computed with PMPATH and compaed with the appoximate tavel time Equation (3). It can be seen that PMPATH can estimate tavel times with vey high accuacy, only slightly undeestimates the tavel times at fa distance away fom the well. The backwad paticle tacking was used to delineate the potection zone with PMPATH. Thity paticles wee placed aound the well and wee tacked backwads until the time of tavel citeia was eached. The isochone line of the tavel time gives the potection aea. Table lists the citeia and the calculated adius of the potection zone by PMPATH and the analytical solution (Equation 7). Fo taditional well potection zones of 1 yeas and 5 yeas, PMPATH computed adii of potection aeas almost the same as the analytical solution. Howeve, only 1.6% and 45.6% of echage wate to the well wee potected with the 1-yea and 5-yea potection aeas, espectively. Fo long-tem wate supply secuity, highe pecent of echage wate to the well should be potected. Table shows the calculated adii of potection aeas fo 85%, 9%, and 95% of echage wate to the well using PMPATH and analytical solution. A much lage echage aea must be potected

6 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 Table Compaison of analytical and numeical models fo calculating adius of potection aea Tavel time (yeas) Citeia Pecentage of echage wate (%) Analytical solution Radius of potection aea (m) Numeical solution Relative eo (%) Q Pheaticaquife c=d/k v Aquitad q L φ T=KH φ φ Semi-confined aquife w Fig. 4 Coss-section of a pumping well located in the cente of a semi-confined aquife Models fo the semi-confined aquife.1 Conceptual model Fig. 4 shows the coss-section of a cicula semi-confined aquife. The semi-confined aquife is homogeneous and isotopic with unifom thickness (H) and extends to infinite. The semi-pemeable laye (aquitad) is homogeneous with constant vetical hydaulic conductivity (K') and unifom thickness (d). A pumping well fully 368 penetates the semi-confined aquife with a constant pumping ate (Q ). Befoe the pumping, goundwate head in the semi-confined aquife is the same as the wate table in the unconfined (pheatic) aquife. Afte pumping, goundwate in the unconfined aquife leaks into semi-confined aquife though the aquitad. Flow is assumed to be vetical in aquitad and is adial towads pumping well in the semi-confined aquife.. Analytical solution The analytical solution of the dawdown fo

7 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 steady adial flow to the well in the semi-confined aquife was deived by Hantush (1964) as: Q K (/ λ ) s = π T ( / λ) K ( w / λ ) w 1 (9) Whee λ is the leakage facto (λ =Tc), c=k'/d is the esistance of the aquitad, K (x) is called the second kind of modified Bessel function of ode zeo, K 1 (x) is the second kind of modified Bessel function of ode one, and T is the tansmissivity of the semi-confined aquife. The souce of the wate to the well comes fom leakage. The cumulative leakage though the aquitad fom the well adius w to any adial distance can be calculated as: Q QL= πqld = K( ) d (1) λ λ w An analytical solution of Equation (1) was found by Zhan and Bian (Equation 16 in ZHAN and BIAN, 6), and was eaanged as (Zhou, 11): Q 1 ( L = K1 ) Q λ λ (11) Equation (11) shows that the pecentage of leakage wate to the well is only a function of the adial distance scaled by the leakage facto. When the adial distance appoaches infinity, the total leakage (Q L ) equals the pumping ate (Q ). When the adial distance equals 4 times the leakage facto (=4 λ), Equation (11) yields a value of.95. It means that 95% of the leakage wate to a well will always come fom within a adial distance of 4 times the leakage facto egadless of the actual pumping ate. Only 5% of the leakage wate is deived fom distances beyond 4 times the leakage facto. Theefoe, the adial distance of 4 times the leakage facto can be defined as the pactical adius of influence of a well in a semi-confined aquife. w The tavel time of the leakage wate at adial distance fom the well can be calculated as: w d H n K ( / ) t = π w e 1 w λ = d v Q K 1(/ λ ) (1) w Howeve, an explicit fomula of the tavel time Equation (1) is not available. Appoximate tavel time was calculated by the numeical solution of the integation (ZHOU, 11)..3 Numeical model Also, a numeical goundwate model was designed to simulate goundwate flow and calculate tavel time in compaison with the analytical solutions. The values of paametes fo the semi-confined aquife ae: Q =5 m 3 /d, H=5 m, K=5 m/d, n e =.3. The paametes fo the aquitad ae: K'=.1 m/d, d=1 m. These values esults in a leakage facto, λ=5 m. Theefoe, a adius of 1 times λ was used to detemine the numeical model aea which would be sufficiently lage to epesent the infinite semi-confined aquife. The model aea consisted of a cicula semi-confined aquife with a adius of 5 5 m. The model aea is discetized by 5 ows and 5 columns with a unifom cell size of 5 m by 5 m. The cente cell whee the pumping well is located was efined to 1 m by 1 m cells. Since the analytical solution teats the leakage though the aquitad fom the pheatic aquife with a vetical leakance, aquitad and pheatic aquife wee not explicitly simulated. The numeical model consisted of a confined aquife, leakage was simulated as an extenal souce using Geneal Head Bounday package of MODFLOW whee the conductance was calculated fom vetical leakance. Fig. 5 shows the compaison of dawdown calculated by the analytical solution and the 369

8 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 MODFLOW model. The numeical model is able to calculate goundwate dawdown with vey high accuacy except nea the location of the well. With a adius of the well equal to. m, the analytical solution (Equation 9) computed a dawdown of.53 m. MODFLOW calculated a dawdown at the location of the well as 1.85 m. MODFLOW calculated a smalle dawdown at the location of the well which epesents an aveage value of the dawdown fo the cell with a size of 1 m by 1 m. Dawdown (m) MODFLOW Analytical solution potection aeas fo 85%, 9%, and 95% of leakage wate to the well using PMPATH and analytical solution. Tavel times (yea) Analytical solution PMPATH Radial distance fom the well (m) Fig. 6 Compaison of tavel times calculated by the analytical solution and PMPATH fo the semi-confined aquife Numeical model fo Leggeloo well field Radial distance fom the well (m) Fig. 5 Compaison of dawdown calculated by the analytical solution and MODFLOW fo the semi-confined aquife 37 Fig. 6 plots tavel times computed with PMPATH and compaed with the appoximate tavel time Equation (1). It can be seen that PMPATH estimates tavel times vey accuately compaing to the analytical solution. Similaly, PMPATH was used to delineate the potection zone with backwad paticle tacking. Table 3 lists the citeia and the calculated adius of the potection zone by PMPATH and the analytical solution. PMPATH calculated simila adius of the potection aea compaing to the analytical solution. The elative eo is less than.5%. Again, the taditional well potection zones of 1 yeas and 5 yeas can cove only 4.3% and 58.4% of leakage wate to the well, espectively. Fo long-tem wate supply secuity, highe pecent of leakage wate to the well should be potected. Table 3 shows the calculated adii of 3.1 Conceptual model The Wate Supply Company of Denthe is esponsible fo supplying dinking wate to half million population in the povince of Denthe, the Nethelands. One well field was installed in Leggeloo in 197s. The well field consists of two pumping wells with a capacity of abstaction of m 3 /yea. A well field potection aea was demacated by the povincial authoity in 198s to potect wate souce against pollution fom agicultual activities (Fig. 7). Despite this potection, highe nitate concentations wee found in pumped wate since 199s (Fig. 8). The aquife system consists of 3 layes. The top unconfined laye is composed of clay and fine sand; the middle poduction aquife is composed of coase sand; and the bottom laye is composed of fine sand. The land suface is vey flat with elevation vaying between 7.5 m to 1 m above the

9 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 mean sea level. The land use of the aea includes aable land, gass land, conifeous foest, open wate, and built-up aea. Goundwate echage comes mainly fom pecipitation infiltation. Long-tem aveage annual ainfall is about 78 mm/yea. Goundwate dischage mainly takes place with agicultual dains. Table 3 Compaison of analytical and numeical models fo calculating adius of potection aea Citeia Radius of potection aea (m) Tavel time (yeas) Pecentage of echage wate (%) Analytical solution Numeical solution Relative eo (%) Numeical model The calibated model is good enough to analyze A steady goundwate flow model was the potection aeas. constucted to analyze the well potection aea. 3.3 Compaison of well field potection The model aea of 5 km by 6 km was discetized aeas by 1 columns and 1 ows with a unifom cell size of 5 m by 5 m. Thee model layes epesent PMPATH was used to delineate potection coespondingly 3 aquifes. Since thee ae no zones fo 5, 5, 15, and 5 yeas. The physical hydogeological boundaies in the aea, demacated potection aeas ae plotted in Fig. 9 specified head boundaies wee used fo all side togethe with the cuent potection aea enfoced boundaies. Goundwate heads on the boundaies by the povincial authoity. The wate budget fo wee intepolated fom measuements of the each potection aea was calculated and listed in national goundwate monitoing netwok. Table 4. The aea of 5 yeas coves the total Rechage package of MODFLOW was used to catchment of the well field; theefoe, total simulate ainfall echage. Dain package of echage in the catchment is slightly moe than the MODFLOW was used to simulate dainages. Two abstaction. The cuent potection aea does not pumping wells wee placed in the second model fall completely coincide with the catchment and laye to simulate abstaction. potects only 37.4% of the echage wate. A lage The model was calibated by adjusting potection aea is needed as indicated by 15-yea hydaulic conductivities in 3 model layes and potection aea. The 15-yea potection aea can dain conductance. The diffeences between potect almost 95% of the echage wate to the computed heads and measued heads ange fom well field; theefoe, a sustainable opeation of the -1 m to +1 m; ae less than.5 m in most aeas. well field can be guaanteed

10 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 Fig. 7 Location of Leggeloo well field (two wells in ed cicles) in the Nethelands and existing well field potection aea (demacated by ed polygon line) LEPP1 LEPP 1 Nitate (mg/l) //8 14/11/84 11/8/87 7/5/9 31/1/93 8/1/95 4/7/98 19/4/1 14/1/4 1/1/6 6/7/9 1/4/1 Date Fig. 8 Nitate concentations in two pumping wells in Leggeloo well field

11 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec Table 4 Compaison of wate budget (m /d) and pecentage of echage to the well field fo diffeent potection aeas Potection aea Net inflow Rechage Dainage Abstaction % of echage to the wells 5 yeas yeas yeas yeas cuent Fig. 9 Well field potection zones fo diffeent tavel times: ed colo epesents the existing potection zone; blue and yellow colos daw potection zones fo 5 and 5 yeas, espectively; puple and violet colos designate potection zones of 15 and 5 yeas, espectively. used to delineate well field potection aeas. 4 Conclusions Numeical model has advantages in application to eal wold cases with complex hydogeological When designed popely, numeical models ae stuctue and bounday conditions, anisotopy and capable of simulating dawdown and tavel times heteogeneity with good accuacy in compaing to analytical spatial-tempoal vaiation of hydological stess, solutions. Numeical models have been widely and multiple well fields. of hydogeological paametes,

12 Jounal of Goundwate Science and Engineeing Vol.3 No.4 Dec. 15 Although well field potection aeas wee usually delineated with the time of tavel citeion in past, these potection aeas may only potect small pecent of souce wate to the well. Fo sustainable dinking wate supply, it is impotant to potect lage pecent of souce wate to the well. The pecentage of souce wate to the well should be used as additional citeion to define the well field potection aea. Fo unconfined aquife, souce wate usually comes fom ainfall echage, echage aea which poduce 9% to 95% of souce wate to the well should be potected. Fo semi-confined aquife, souce wate comes fom leakage of top unconfined aquife. Rechage aea of the unconfined aquife must be potected. The potection aea should be delineated in consideation of pecentage of amount of leakage wate to the well, fo example, 9% to 95% of the leakage wate to the well. Leggeloo well field case study indicated that a well field potection aea defined by the time of tavel citeion is not sufficient to potect the wate souce. A lage potection aea must be adopted in ode to safeguad long-tem sustainable dinking wate supply fo local community. Refeences Bouwe H Goundwate hydology. McGaw-Hill seies in wate esouces and envionmental engineeing. U.S.A.: Mcgaw- Hill College. Chave P, Howad G, Schijven J, et al. 6. Goundwate potection zones. In: Schmoll O, Howad G, J Chilton, Chous I (eds) Potecting goundwate fo health: managing the quality of dinking-wate souces. London: IWA. Chiang W H, Kinzelbach W PMPATH. An advective tanspot model fo Pocessing Modflow and Modflow. Hambug: Geological Suvey of Hambug, Gemany. China s Agenda White pape on China s population, envionment, and development in the 1 st centuy. Beijing: China Envionmental Science Pess. Euopean Envionment Agency (EEA) Envionmental assessment epot No 1: Sustainable wate use in Euope, Pat 1: Sectoal use of wate. Luxemboug: Office fo Official Publications of the Euopean Communities. HAN Zai-sheng Goundwate fo uban wate supplies in nothen China-An oveview. Hydogeology Jounal, 6(3): Hantush M S Hydaulic of wells. New Yok: Advances in Hydoscience, Academic Pess. Habaugh A W, Banta E R, Hill M C, McDonald M G.. MODFLOW-, the U.S. Geological Suvey modula gound wate model-use guide to modulaization concepts and the gound wate flow pocess. Reston: US Geological Suvey. Keely J F, Tsang C F Velocity plots and captue zones of pumping centes fo goundwate investigation. Gound Wate, 1(6): US Enviomental Potection Agency Guidelines fo delineation of wellhead potection aeas. EPA/44/ Washington D.C.: US EPA. US Enviomental Potection Agency Handbook, goundwate and wellhead potection. EPA/65/R-94/1. Washington DC: US EPA. Van Waegeningh H G A poposal fo the dimensions of potection aeas. Science of the Total Envionment, 1: ZHAN Hong-bing, BIAN Ai-guo. 6. A method of calculating pumping induced leakage. Jounal of Hydology, 38(3): ZHOU Yang-xiao. 11. Souces of wate, tavel times and potection aeas fo wells in semi-confined aquifes. Hydogeology Jounal, 19(7): ZHOU Yang-xiao, Haitjema Henk. 1. Appoximate solutions fo adial tavel time and captue zone in unconfined aquifes. Goundwate, 5(5):