SOME PHYSICAL PROPERTIES OF WATER TRANSPORT IN WASTE ROCK MATERIAL

Transcription

1 MWA Proceedngs 1999 nternatonal Mne Water Assocaton MNE, WATER & ENVRONMENT MWA Congress. Sevlla, Span SOME PHYSCAL PROPERTES OF WATER TRANSPORT N WASTE ROCK MATERAL Mchael M. Noel and A. an M. Rtche Managng Mne Wastes Project, Australan Nuclear Scence and Technology Organzaton Prvate Mal Bag 1, Mena NSW 2234, Australa Phone , Fax e-mal: Mchael.noel@anto.gov.au ABSTRACT A seres of water transport smulatons of typcal waste rock materal was carred out to assess flux and mosture content changes over tme. Typcal seasonal cycles of precptaton were used for the smulatons. The results ndcated that the mosture content vares lttle over the seasonal cycles. Also, the tme for the flux at the base of a 20m hgh dump to react (response tme) to a flux change at the top s short (jew weeks) whle the water resdence tme s much longer (jew years). These observatons are crtcal for the proper selecton of methods and devces to montor water and contamnant transport n unsaturated waste rock materal. NTRODUCTON Water s the domnant transport mechansm n the transport of pollutants n sulfdc waste rock materal. Understandng water transport s also an mportant aspect n the management of the envronmental mpact caused by these dumps. The modellng of water transport n waste rock materal can provde essental nformaton for selectng the approprate methods and devces to montor water and contamnant transport n waste rock dumps. Waste rock by ts nature s hghly heterogeneous wth partcle sze varyng from boulders of several metres to clay sze partcles. The bulk and ntrnsc physcal propertes of waste rock are relatvely well documented n the lterature. However, the lterature contans lttle nformaton on sol-water relatonshps and the unsaturated hydraulc conductvty for waste rock. Also, these parameters are often consdered to have large uncertantes. Ths can be explaned n large part by the dffcultes and the costs assocated wth measurng these parameters n waste rock. The tme scale of water and contamnant transport n waste rock materal s another area where there appears to be uncertantes and some confuson. Mne operators have some understandng and quantfcaton of these tme scales when they develop ther plans for managng pollutant dranage comng from waste rock dumps. Such polluted dranage s often called acd rock dranage. For example, mne operators may want to estmate the tme t wll take for the water qualty n dranage to mprove after a sol cover s placed on top of a waste rock dump. n an effort to clarfy ths ssue of tme scales, we nvestgate two basc parameters: the response and resdence tmes. For the purposes of ths paper, the response tme s the tme requred for the water flux at a gven depth n the dump to start to change followng a change n the nfltraton flux condton at the surface. f the change at the surface s a step change then the response tme at depth corresponds to the tme halfway between when the flux change was frst detected and when the flux reached ts maxmum value. The resdence tme corresponds to the tme t takes for a conservatve pollutant dssolved n water to travel from the surface to a gven pont along a vertcal profle n the dump. ts quantfcaton s smlar to that for the response tme. The lterature contans numerous studes on unsaturated water transport. These studes are often appled to "conventonal" sols (clay to sand sze partcles) and the water table s often relatvely near the surface. Waste rock dumps are generally unsaturated wth the bulk of the materal stuated far above the water table. Also, the water nfltraton rate n waste rock dumps s usually much lower than the rate requred to saturate the materal. Such condtons usually lead to mosture contents n the dump at or near the mnmum water content achevable for the type of waste rock present n the dump. To the casual observer, such waste rock appears to be qute dry. Water flow under low nfltraton condtons tends to be the opposte to that observed n saturated meda. Under saturated or unsaturated condtons close to saturaton, water flows mostly n the coarse porton of the medum where the larger 449

2 MWA Proceedngs 1999 nternatonal Mne Water Assocaton vods are present. However, under "dry" unsaturated condtons, the only water left n the medum s n the fne porton where the small pore sze can hold the water by surface tenson. Newman and al. ( 1997) presented a case where the water "preferred" to travel n the fnes when the porous materal was dry and n the coarse fracton where t was near saturaton. Brkholzer and Tsang (1997) carred out a detaled modellng exercse and arrved at a smlar concluson. El Boush (1969) studed the nfltraton of water n coarse rock partcles and suggested that the fne porton present n coarse partcles wll lkely act as a wck to attract the water nfltratng coarse materal. Consderng that waste rock s hghly heterogeneous, s often under dry condtons and that the sol-water relatonshps are dffcult to measure, we have carred out a seres of water transport smulatons to tentatvely address the followng questons: What s the varaton n water content n waste rock materal for a range of nfltraton rates typcal of those expected from ranfall? How quckly wll the water flux nsde a waste rock dump react to a change of nfltraton flux at the surface (response tme)? How long wll t take for a conservatve pollutant dssolved n water to travel through from the top of the waste rock dump to varous depths wthn the dump (resdence tme)? What measurng devces are approprate or napproprate to montor the movement of water n waste rock dumps? How senstve are the response and resdence tmes to the hydraulc propertes used to descrbe water transport n the dump under typcal nfltraton rates? MODEL DESCRPTON The smulatons were carred out usng SWM v.2.1 (Verburg et al., 1996). SWM s a one-dmensonal model based on a numercal soluton of Rchards' equaton. SWM has the capablty of modellng runoff, nfltraton, redstrbuton of water and solute, solute transport, plant uptake and transpraton, sol evaporaton, deep dranage and leachng. SWM accepts tme dependent boundary condtons and uses the fnte dfference method. SWM was developed by the Commonwealth Scentfc and ndustral Research Organsaton (CSRO) n Australa. Water retenton curves and hydraulc conductvty functons are normally measured n laboratory and are then ftted by a mathematcal expresson. Several expressons are avalable n the lterature. SWM has the capablty of usng 7 dfferent expressons for the water retenton relatonshp and 3 dfferent hydraulc conductvty functons. nstead of usng e, these relatonshps use a normalsed parameter defned as the effectve degree of saturaton se. e{lfl) - e, S 8 (\jf)= (1) es- e, where e = volumetrc water content \jf =matrc potental (m) er = resdual volumetrc water content es =saturated volumetrc water content The resdual volumetrc water content corresponds to the mnmum volumetrc water content value obtaned by free dranage. The saturated volumetrc water content s the maxmum value that can be reached for a gven sol. The saturated water content should not be equated to the porosty of the sol (usually 5 to 1 0% smaller) because of entrapped or dssolved ar (van Genuchten 1991 ). However, the saturated volumetrc water content s often consdered equal to the porosty of the sol for modellng purposes. The smulatons presented heren used the van Genuchten formulaton (van Genuchten 1980, 1991) for the water retenton relatonshp. The expresson s: se (\jf) = [1 + (a \jf l nj m (2) where: a= emprcal constant (m 1) n, m = emprcal constants Two hydraulc conductvty functons were used for the smulatons dependng on the sol type: van Genuchten (van Genuchten, 1980, 1991) and Brooks-Corey (Brooks and Corey, 1964, 1966). The van Genuchten functon s defned as: K(lfl) = KsatSt [1 - (1- s; 1 mynj2 wthm=1-1/n,n>0 (3) where: K =hydraulc conductvty (m s 1) Ksat =saturated hydraulc conductvty (m s 1) p = pore nteracton ndex The Brooks-Corey hydraulc conductvty functon s as follows: K(lfl) = K 581 St +M (4) where: b = emprcal constant The van Genuchten formulaton has the dsadvantage of havng an nfnte slope at saturaton ( lfl = 0) whch can sometmes cause numercal convergence problems. The Brooks-Corey expresson does not have ths property. THE SMULATONS Three dfferent dump heghts, 10 m, 20 m and 40 m, were used n the smulatons. The node spacng was 0.1 m for the 10 m and 20 m profles whle a node spacng of 0.16 m was used for the 40 m profle. The node spacng was ncreased for the 40 m profle because of the maxmum number of nodes allowed by SWM. The bottom boundary was set to a matrc potental of 0 m. t corresponds to a stuaton where there s a groundwater table rght at the base of the dump from whch water can dran freely. Ths s smlar to the stuaton n a sol column experment n laboratory where the bottom of the column s open to ar and s free to dran. The boundary condton at the surface was specfed by a tme dependent nfltraton flux. No nfltraton flux was appled 450

3 MWA Proceedngs 1999 nternatonal Mne Water Assocaton SOME PHYSCAL PROPERTES OF WATER TRANSPORT N WASTE ROCK MATERAL for the ntal 50 years of smulaton. The 50 years dranage perod wth no nfltraton was selected long enough to allow each sol type to reach draned steady state condton before the cyclc nfltraton rate was appled. A cyclc nfltraton flux was then appled from 50 years to 72 years. The 22 year perod of cyclc nfltraton was selected to assure that a pseudo-steadystate condton wth the flux would be reached wthn that tme and that t was long enough to measure the resdence tme through the profle of the dump. The ntal 20 years of nfltraton had an output nterval of 146 hours and the nterval was reduced to 4 hours for the last 2 years (70 to 72 years). The shorter nterval was used to measure the relatvely short response tme. Although the smulaton tme may seem long, SWM solved these smulatons wthn a few mnutes. The ntal condton was set to a matrc potental equal to -0.5 m for the entre profle except for the bottom 0.5 m where the t decreased lnearly from -5 to 0 m (bottom boundary). The low ntal matrc potental was ntended to have a mosture content hgh enough to allow for some water to be draned durng the ntal 50 years. The cyclc nfltraton rates used n the smulatons were based on rates typcal of those expected from ranfall n many places around the world. The cycles were used to smulate the seasonal varaton that s generally assocated wth ranfall. For the purpose of smulaton, we used 1500, 750 and 375 mm of nfltraton per year appled over a perod of 5 months. These precptaton quanttes correspond to fluxes of 1.14 x 1o- 7 m s 1, 5.70 x 1 o-a m s 1, 1.14 x 10 8 m s 1 mantaned over the 5 month perod. The remanng 7 months of the year had no ran. For smplcty, we assumed that there s no evaporaton and that all the water nfltrated the waste rock materal. The flux at the surface was too low to have runoff because t s well below the saturated hydraulc conductvty of the materal (Ksat = 1o- 3 m s 1). Solute transport was used to measure the resdence tme n the profle. A solute wth the propertes of water was appled at the surface boundary at 55 years. t was appled contnuously at a constant concentraton for the remanng 17 years of the smulaton. The solute was appled after the flux condton had already reached pseudo-state condtons n the profle. Eght materal types were used for the smulatons, rangng from coarse waste rock to clay sols. Propertes from four dfferent waste rock materals were selected from the lterature: Mne Doyon (Lefebvre, 1994), Kenncott Mnes (Gardner, 1992), Gas Hlls (Troncoso and Shackelford, 1999), Kdston Gold Mne (Sews et al., 1997). Four basc sol types were selected from Rawls et al. (1982) to complement the waste rock data. To compare the parameters defnng the unsaturated propertes, we normalsed the sols to the Kenncott waste rock for the saturated hydraulc conductvty, the resdual and saturated volumetrc water content. The saturated hydraulc conductvty (Ksatl was set to 10 3 m s 1, the resdual volumetrc water content (8,} to 0.18 and the saturated volumetrc water content ( esat) to Table 1 presents the parameters assocated wth the eght dfferent sol types. 451 Methodto calculate Kunsat (m-1) ==n a n p ' ' Clay be l Clay loam be l Gas Hlls vg Kenncott be ! Kdston be Mne Doyon vg Sand be Sandy loam be ooks-corey method; vg = van Genuchtten Table 1. Parameters defnng the unsaturated propertes. The extremely hgh value of a reported for the Mne Doyon materal should be noted. RESULTS The modfed materal propertes were plotted as a functon of the volumetrc water content and the hydraulc conductvty (Fgure 1 ). t shows the wde range of propertes used for the smulatons. Ths fgure also demonstrates that typcal water nfltraton rates are several orders of magntude smaller than the rate requred to saturate the materals. Fgure 1 also shows that the Kenncott materal exhbts the lowest volumetrc water content for a gven nfltraton rate whle the clay and the Gas Hlls materals have the hghest values. The response tme s llustrated n Fgure 2 where the water flux and the volumetrc water are plotted as a functon of tme over 2 years for the Kenncott materal (20 m profle). Ths typcal case shows that t takes about 45 days before the water flux at the bottom to change after the nfltraton had started at the surface. The response tme at the end of the nfltraton Kennlcottlab values 0 1 ; 0 -oo 10 w' w' 10 10' 1o 10, 10 ' Hydraulc conductvty (m s ') Fgure 1. Volumetrc water content versus hydraulc conductvty. J

4 MWA Proceedngs 1999 nternatonal Mne Water Assocaton r;:: ".. cycle ("dry" front) was shorter. Ths behavour was observed for all the smulatons. The volumetrc water content for the 20 m depth s not shown n Fgure 2 because t corresponds to the boundary condton where we mposed saturated condtons. The response tme vares dependng on nfltraton rate and materal type. Table 2 lsts the response tmes for a 20 m hgh dump, for all the materal types and for the three nfltraton rates. The response tme s as short as three weeks and s as long as 6 months but wth the excepton Mne Doyon, t vares only by about a factor of two for a gven nfltraton rate over the range of materal types. s=-'ls:t S 2E-08 t- s: :; " 0-70 '!, Depth (m) 1\ 1.\ 1\!..., 1\ :! \! \ 1\! 1\ 1\! 1\ '! \'. '.! \ \\! \'- \.: '... '... _..., J 71.l! \: ' _ L..._._..._._....._..._._..._...-.._..._..._.._..._...._.-J_ > Tme (years) Fgure 2. Water flux (top) and volumetrc water content (bottom) versus tme, Kenncott materal, 20m profle, nfltraton flux of 5.70 x 10 8 m s 1 rr = ;;;; =:: H l " ll n ResRonse tme (days) [! nfltraton rate (m s 1): 1.14 x x x ! Clay l Clay loam !! Gas Hlls :1 Kenncott =yon ;; Sand ll=s=an=d=yl=oa=m==========4=6======7=6======1=1=8== Table 2. Response tme at the bottom of the 20m profle. Fgure 3 shows the water flux and the volumetrc water plotted as a functon of tme for the Kenncott materal (20 m profle) over the ntal 20 years of cyclc nfltraton. The water flux curves ndcate that the pseudo-steady state condton was acheved wthn the frst two years. A pseudo-steady state condton was obtaned n less than 3 years for all the cases. The bottom graph of Fgure 3 shows the resdence tme obtaned for the Kenncott materal for the 20 m profle under an nfltraton rate of 5.70 x 1 o-s m s 1. Ths typcal case shows that t wll take about 5 years for a conservatve pollutant dssolved SE-08., _ 4E-08 :;- 5E-08 3E-08 r;:: S 2E-08.. S: 1E c 6 0 r ' "". Depth (m)., ;; 10.0 " c " 2 s..2 0! E:''J C/l Tme (years) Fgure 3. Water flux (top) and solute concentraton (bottom) versus tme, Kenncott materal, 20 m profle, nfltraton flux of 5.70 x 10 3 m s 1. n water to travel from the surface down to the base of a 20 m dump. Snce the pseudo-state condton was acheved wthn the ntal 2 years of nfltraton, the solute could have been appled at 52 years to obtan the same resdence tme. Table 3 below summarses the resdence tmes for all the materal types obtaned wth the 20 m profle. As for the response tme, the resdence tme vared consderably between the smulatons. Fgure 4 shows the varablty of the response tme and the resdence tme as a functon of the water flux for the 20 m profle wth the Kenncott materal. t ndcates that the response and resdence tmes both ncrease when the nfltraton flux s reduced. Ths was also the case wth the other materal types. ::: -= :::: Response tme (years) nfltraton rate (m s 1): 1.14x x x Clay Clay loam l Gas Hlls Kenncott Kdston Mne Doyon Sand jndyloam Table 3: Resdence tme at the bottom of the 20m profle 70 JJ The volumetrc water content was plotted as a functon of depth for the 20 m profle usng the Kenncott and Gas Hlls materals. These two materals reflect the range n the capacty of the materal used n the smulatons to hold water (se Fgure 1 ). The "dry" condton s the lowest water content obtaned durng the annual cycle and the "wet" condton s the hghest water content obtaned for the maxmum flux used n the smulatons (1.14 x 1 o 7 m s 1). Ths fgure shows that the maxmum varablty n volumetrc water content wll be n the range of 2% to 3%. 452

5 MWA Proceedngs 1999 nternatonal Mne Water Assocaton SOME PHYSCAL PROPERTES OF WATER TRANSPORT N WASTE ROCK MATERAL 60 u; ;' 50 :. " 40 " g 30 c. c:: 20 ',,_ '.,, Response tme.\.. aflersurface - flux stopped..._, ,;;REJsldencetme 10 O ot_-'---"---'--5..j.e-0_8_... 1E.L '---'---'--:-1_5=E-7 Water flux (m s 1) Fgure 4. Response and resdence tme versus water flux, Kenncott materal, 20 m profle. \ "Ory"Gas 1 Hlls j 4 'Wet" Kennlcott 1 8 l.'[ "Dry" Kenncott \ = 10 c \ 'Wef' Gas Ht'lls \,\_ 18._ 20-'--==== Volumetrc water content Fgure 5. Volumetrc water content versus depth, Kenncott and Gas Hlls materals, 20 m profle. DSCUSSON The approxmaton used to defne the materal-water relatonshp for the Kenncott materal (Fgure 1) may appear to be nadequate near saturaton but s well defned n the range of fluxes used n the smulatons. As ndcated n Fgure 5, the regon wth near saturaton condtons s neglgble and occurs only at the bottom boundary. Ths emphasses the mportance of havng adequate unsaturated propertes that correspond to the condtons for the range of nfltraton fluxes occurrng n the system. The normalsaton of the materal propertes resulted n some materal types that are probably unrealstc, such as the clay and the Gas Hlls materal where ther water retenton curves would normally correspond to a much smaller hydraulc conductvty value. Agan, our nterest s to assess the mportance of the parameters used to descrbe the unsaturated propertes of waste rock materal. The smulatons ndcated that the response and resdence tmes depend on the water flux appled to the system (see Table 3). The longest resdence tme was obtaned wth the clay and Gas Hlls materals and they both have the hghest water content values. The shortest resdence tme was obtaned wth the Kenncott materal whch mantans the lowest water content. The dfferent tme scales obtaned for the response tme and the resdence tme have mportant mplcatons for understandng water and contamnant transport n waste rock dumps. An example s the response of the dranage from a waste rock dump followng the placement of a sol cover on ts top. The sol cover wll lkely decrease the nfltraton flux by about an order of magntude or more. From Fgure 4, a reducton n the nfltraton flux wll ncrease consderably the response and resdence tmes. Although the placement of a cover over waste rock wll reduce the dscharge quanttes wthn weeks, the benefts of the cover on the pollutants could take tens of years before t can be measured. The varaton n water content as shown n Fgure 5 ndcates that the volumetrc water content hardly vares over tme when typcal nfltraton rates of those expected from ranfall are appled to waste rock dumps. The varaton n water content ranged from only 2% to 3% between the "dry" and "wet" season. The low range was obtaned for all materal types. Ths s consstent wth the measurements reported by Danel et al. (1979), and Goodman et al. (1981). They measured water content profles durng the wet and dry season at Rum Jungle and dd not see any sgnfcant dfference between the dry and wet season. Goodman et al. (1981) also observed that the water content dd not vary much wth depth over the top 5 m. The low range n varaton of the water content wth changng nfltraton rate has another mportant consequence. The detecton lmts of the devces (e. TORs) for measurng water content s about 1 to 2%. t follows that water content montorng s not a practcal way to measure nfltraton rates n waste rock dumps. CONCLUSON Provded that Rchards' equaton descrbes water transport n waste rock and provded t s reasonable to normalse the hydraulc propertes of dfferent materal types such that the saturated hydraulc conductvty s the same, the smulatons ndcated that: o The range of varaton of the water content n waste rock s only about 2 to 3% over the range of nfltraton rates typcal of those expected from ranfall. o The response tme s short, n the order of a few weeks for a 20 m hgh dump. o The resdence tme s long, n the order of a few years for a 20 m hgh dump. o The range n varaton of the water content wth changng nfltraton rate s of smlar magntude to the detecton lmts (about 1 to 2%) of current devces for measurng water content. t follows that water content montorng s not a practcal way to measure nfltraton rates n waste rock dumps. 453

6 MWA Proceedngs 1999 nternatonal Mne Water Assocaton For a gven nfltraton rate, the response and resdence tmes vary only by about a factor of 2 over the range of hydraulc propertes used n the smulatons. ACKNOWLEDGEMENTS Ths research was carred out as part of the Mne Management Wastes Project (MMWP) presently on-gong at the Australan Nuclear Scence and Technology Organsaton (ANSTO). The authors would lke to thank Dr Eugena Kuo and Mr Bll Plotnkoff for ther advce and collaboraton n preparng ths paper. REFERENCES Bartlett, R.W., Soluton mnng: leachng and flud recovery of materals. Gordon and Breach Scence Publshers, The Netherlands. Sews, B.E., M.A. O'Kane, G.W. Wlson, D. Wllams, N. Currey, The desgn of a low flux cover system, ncludng lysmeters, for acd generatng waste rock n sem-ard envronments. n proc. 4th nternatonal Conference on Acd Rock Dranage, Vancouver, Canada. Vol. 2: Brkholzer, J. and C.-F. Tsang, Solute channelng n unsaturated heterogeneous porous meda. Water Resour. Res. 33(1 0): Brooks, R.H. and A.T. Corey, Hydraulc propertes of porous meda. Hydrol. Pap. No. 3. Cv. Eng. Dep., Colorado State Unv., Fort Collns, USA. Brooks, R.H. and A.T. Corey, Propertes of porous meda affectng flud flow. J. lrrg. Dran. Dv. Am. Soc. Cvl Eng. 92(1R2), Danel, J.A., J.R. Harres, A..M. Rtche, Water movement caused by monsoonal ranfall n an overburden dump underrgong pyrtc oxdaton. n proc. of the 4th nt. Symp. On Env. Bogeochemstry and Cont. On Bo- geochemstry n Relaton to the Mnng ndustry and Env. Polluton, Canberra, Autrala, El Boush,.M., Amount of water needed to ntate flow n rubbly rock partcles. Journal of Hydrology. 27: El Boush,.M. and S.N. Davs, Water-retenton characterstcs of coarse rock partcles. Journal of Hydrology. 8: Goodman, A.E., A.M. Khald, and B.J. Ralph, Mcrobal ecology of Rum Jungle Part. Envronmental study of sulphdc overburden dumps, expermental heap-leach ples and talngs dam area. Report AAEC/E531, Australan Atomc Energy Commsson. 170 p. Lefebvre, R., Caractersaton et modelsaton numerque du dranage mner acde dans les haldes de sterles. Ph.D. Thess, Unverste Laval, Canada. 375 p. Newman, L.L., G.M. Herasymuk, S.L. Barbour, D.G. Fredlund, and Smth, T., The hydrogeology of waste rock dumps and a mechansm for unsaturated preferental flow. n proc. 4th nternatonal Conference on Acd Rock Dranage, Vancouver, Canada. Vol. 2: Rawls, W.J., D.L. Brakensek, and Saxton, K.E., Estmatng Sol Water Propertes. Transactons, ASAE, 25(5): and Troncoso, J.C. and C.D. Shackelford, Solute Transport n Unsaturated Sol Beneath Two Abandoned Evaporaton Ponds, Gas Hlls, Wyomng: 1. Characterzaton of Sols. n Proc.: Talngs and Mne Waste'99, Colorado, USA. Balkema, Rotterdam van Genuchten, M. Th., A Closed-Form Equaton for Predctng the Hydraulc Conductvty of Unsaturated Sols. Sols Sc. Soc. Am. J., 44: van Genuchten, M. Th., The RETC Code for Quantfyng the Hydraulc Functons of Unsaturated Sols. EPA/600/2-91 / Verburg, K., P.J. Ross, K.L. Brstow, SWM v.2.1 User Manual. Dvsonal Report No. 130, CSRO, Dvson of Sols, Australa. 77 p. 454