# Recycled Materials Resource Center Progress Report for the U. S. Environmental Protection Agency

• Project Title: Evaluation of I W EM ForNon-Federal Users W ith Regard to Highway Applications

• USEP A Co ntact Mr. John Sager

• Name of PIs: Dr. Jeffrey S. Melton and Dr. Kevin H. Gardner

• Affiliation of PIs: University of New Ha mpshire

• Project Dates: October 1, 2004 to September 30, 2006

• Progress Report Number: Report #2

• Progress Report Dates : Septem ber 1, 2005 – February 28, 2006

• Next Report Due: May 31, 2006

• Funds Expe nded to Date: $20,966 of $35,000 Expended to Date.1.0 Introductio n : previous w o rk and objectiv es Com p rehensive m odeling has been perform e d wi th IW EM to determ ine how the m o del responds when sim u lating water and contam inant transport from heavy metal bearing secondary
m aterials into the subsurface. The objective of this work is to evaluate whether IW EM can be us ed as a pred ictive tool to accurately determ ine wh ether le achingf r om m aterials will re sult in significant changes in groundwater concentrations wh en the m aterials are reused as a base or sub
base in a roadway. Past m odeling included evaluating how I W EM tr eats the behavior of heavy m e tal transport as a function of tim e and recep tor well distance from the source. Additio nally, s i m u lations were perform e d with varying distri bution coefficient v a lues (Kd ) and waste m a nagem e nt unit (W MU) areas. Results and conclusions can be vi ewed in the prev i ous progress report. This phase of research involves com p aring IW EM’s results to those of other solute transport groundwater m odels usin g the sam e input. Doing so will allo w for the analysis of whether IW EM can accurately pred ict g r oundwater concentration s at a po int d o wn gradientfrom a secondary
m aterial source. IW EM sim u lates 1-D flow in th e unsaturated zone and 3-D flow in the saturated zone. A situation such as this requires the use of two m odels in order to effectively mim i c the secondary m aterial leaching s cenario modeled by IWEM. Aft e r careful consideration, HYDRUS
1D/2D and Visual MO DFLOW we re chosen as the com p arison tools to evaluate IW EM’s solute transport prediction capabilities. Both m odels have been widely used and proven to be effective in sim u lating solute transport in the situations for which they were developed (e.g. flow di m e nsion and level of m e dia saturation ) ther efore providing the highest level of confidence in their output. The rem a inder of the progress report di scusses the HYDR US -2D m odeling portion of research. MODFLOW r e sults will be pres ented in a subsequent progress report.2.0 Methods HYDRUS-2D was used to sim u late one dim e nsiona l (1D) ve rtical so lute transpo rt f r o mthe secondary m aterial source thr ough the unsaturated zone down to the water table. Not only can HYDRUS provide 1D flow to m i m i c that m odele d by IWEM, but it can do so through variably saturated m e dia representative of the unsaturated zone. Input includ ing initial m e tal concentrations and fluxes from the secondary m aterial source , soil type, recharge rates, and hydrogeologic param e ters (e.g. conductivity) has rem a ined unchanged from those used f o r IW EM simulations. Upon running HYDRUS, output concen trations and fluxes were read from the lower boundary of the m odeled dom a in which is rep r es e n tative of th e t op of the water table. These value s will th en b e used for input into MO DFLOW to com p lete th e m odeling process thro ugh the saturated zone. Two scenarios were run using HYDRUS: (1) witha cross-sectional leng th of 14 m e ters (m ) to m i m i c the arbitrarily chosen 200 m 2 W M U area ((14 m ) 2 ~ 200 m 2 ) used for m o st of the previous IWEM sim u lations; and (2) with a cross-section length of 1000 m to re present a real life application of secondary m aterials along a stretch of highwa y. The purpose of using these lengths is to com p are the output concentrations from both and dete rm ine if increasing the cross-sectional horizon has any dram atic effect on the concentration observe d at the water table along the plum e centerline. In previous IWEM work, it was observed that incr easing the WMU area had analm o st lin ear effect on the output groundwater concentrations. It was thought that this is an unr ealistic result that m ay be attribut ed to the fact that IW EM assum e s its WM Us to be squares wh ich is not representative of roadway geom etry (rectangular). Both scenarios were run using averag e tabulated Kdvalues compiled by an extensive USEPA literature s ear ch (Table 1 )
.Metal K d (L/Kg) Ag 398.1 Cd 501.2 Cr 6.3 Se 20 Table 1 – EPA tabulated K dvalues based on literature search (EPA, 1999). Additionally, the 14 m cross-secti onal scenario w a s run using a Kdvalue o f zero for each solute (Ag, Cd, Cr, and Se). The m a jority of th e IWEM sim u lations run to date have relied upon K d values drawn from the MINTEQA2 database incor porated into the software. Earlier work has dem onstrate d that these values ar e r e latively low (on the order of m a gnitude of one) and this allows for a good com p arison between the models. Moreove r, IW EM had been run as a function of tim e using the tab u lated EPA values a llow i ng once again for effective com p arisons. All HYDRUS sim u lations were run for 100 y ears using bottom ash data, which had the highest leachate concentrations. No runs were perform e d for the othe r m a ter i als in the resear ch (e.g. foundry slag) under the assum p ti on that these would yield identic al trends. The unsaturated zone was represented bya sim p le rectangular geo m etry. The boundary co nditions (B C) of the modeled dom ain were set up such thatthe top was defined by a daily constant flux B C representing the incom i ng water leaching from the secondary m ate rial as reported by S auer et. al; the bottom as a constant pressure head BC set equal to zero which is characteristic of the water table; and the vertical sides as no-flow BCs . Additionally, five observation nodes were set along the center of the dom a in extending from th e surface to the water table. Solute concentratio ns and fluxes, as well as water fluxes could be read from these nodes.3.0 Results and Discussion Solute concentrations from the five nodes for the 14 and 1000 m scenarios using the EPA K d s were compared for 100 years. Table 2 show s the concentrations at node 5 (bottom boundary) for select years. Table 2 – Com p arison of output concentrations (mg /l) at node 5 (groundwater table) for 14 and 1000 m cross-section scenarios. Cd Cr Se Ag Time (yrs) 14m 1000m 14m 1000m 14m 1000m 14m 1000m 1 0 0 8.35E 15 9.12E 15 7.38E 17 8.01E 17 0 0 10 4.70E 20 5.07E 20 1.04E 10 1.12E 10 8.48E 13 9.15E 13 8.27E 20 8.92E 20 40 4.78E 17 5.21E 17 9.58E 08 1.04E 07 8.38E 10 9.14E 10 8.40E 17 9.17E 17 100 4.64E 15 5.09E 15 7.60E 06 8.25E 06 7.69E 08 8.43E 08 8.15E 15 8.95E 15 Review of Table 2 clearly shows that increasi ng the cross-sectional le ngth of the dom a i n has
m i ni m a l effect on the solute output concentrations along the centerline. T h is is in contrast to the large increas es in concentration as a function of WMU area observe d with IW EM from earlie r work as shown in Figure 1. Figure 1 clearly dem onstrates in crea ses in concentration up to an order of
m a gnitude (see Section 4.0).1.0 E -0 3 1.0 E -0 2 1.0 E -0 1

20 40 60 8 0 1 0 0 1 2 0 1 40 16 0 1 8 0 20 0 2 2 0 Operat i ona l Life (y e a r s )Concentration (ppm) W i s D O T W M U Ar ea ( 7 90 m 2 ) Ar bi t r ary W M U Ar ea ( 2 00 m 2 ) O n e Mi le St r e t c h ( 8 3 67 m 2 ) 10 x O n e M i l e S t re tc h (8 36 70 m 2 )

E P A ' s M C L ( 0 .0 05 pp m)

Figure 1 – Plot of Cd concentration withtim e for differing IW EM W M U areas. Next, a comparison between HYDRUS-2D an d IWEM results was perform e d with both models using USEPA re ported K dvalues. Again, concentrations of interest from HYDRUS were taken from the bottom node. Results can be viewed in Table 3.

Cd Cr Se Ag Time (y rs ) IWEM H y d IWEM H y d IWEM H y d IWEM H y d 1 0 0 0 8.35E-1 5 5 .65E-0 6 7 .38E-1 7 0 0 10 0 4.70E-2 0 4.53E-0 6 1 .04E-1 0 5 .65E-0 5 8 .48E-1 3 0 8.27E-2 0 20 0 1.50E-1 8 7.24E-0 5 3 .21E-0 9 1 .00E-0 4 2 .68E-1 1 0 2.64E-1 8 40 0 4.78E-1 7 7.24E-0 5 9 .58E-0 8 3 .22E-0 4 8 .38E-1 0 0 8.40E-1 7 80 0 1.52E-1 5 7.24E-0 5 2 .67E-0 6 5 .06E-0 4 2 .58E-0 8 0 2.68E-1 5 100 0 4.64E-1 5 7.24E-0 5 7 .60E-0 6 6 .00E-0 4 7 .69E-0 8 0 8.15E-1 5 Table 3 – Co m p arison between HYDRUS-2D a nd IWEM results for botto m ash concentrations (m g/l) with tim e using USEPS tabulated K dvalues. Analysis of Table 3 shows that the models pr ed ict s i m ilar c oncentr ation s f o r Cd and Ag. Because HYDRUS is only sim u lating transport through the unsaturated zo ne, it is likely theconcentratio ns presented here w ould decrease due to dispersion and further re tard atio n in the saturated zone, bringing them closer to the zero values reported by IW EM. However, for Cr and Se, concentrations pr edicted by HYDRUS are consid erably lower than those of IWEM. This raises concern because , again, HYDRUS is only representing transport through the vadose zone and one would expect the c oncentrations to be larger because the solutes have not yet been subject to further transport, dilution and attenuation thr ough the saturated zone. Additionally, IWEM does not allow for user-defined K dvalues for Cr. Thus, it is assum e d that MINTEQA2 is being used to provi de this param e ter. As m e nt ioned before, previous w o rk with IWEM has observed that the m odel draws upon very low K dvalues when MINTEQA2 is used. This could very well be the cause for such high Cr concentrations observed at the receptor well (50
m e ters from source) b ecause retardat ion due to soil sorp tion is m i nim a l. Finally, a comparison was m a de withIWEM results using MINTEQA2 Kdvalues and HYDRUS r e sults using K dvalues equal to zero (Table 4).
Cd Cr Se Ag Time (y rs ) IWEM H y d IWEM H y d IWEM H y d IWEM H y d 1 0 1.24E-0 7 0 9.70E-0 8 1 .80E-0 3 2 .41E-0 7 1.67E-0 5 6.90E-0 8 10 7.00E-0 4 1.07E-0 3 4.53E -0 6 8 .40E-0 4 7 .30E-0 3 2
. 10E-0 3 7.00E-0 4 6.01E-0 4 20 1.40E-0 3 1.12E-0 2 1.84E -0 5 8 .17E-0 3 8 .30E-0 3 2
. 18E-0 2 1.30E-0 3 6.24E-0 3 40 2.30E-0 3 2.29E-0 2 3.54E -0 5 1 .61E-0 2 9 .40E-0 3 4
. 45E-0 2 1.90E-0 3 1.28E-0 2 80 3.10E-0 3 2.11E-0 2 8.68E -0 5 1 .52E-0 2 9 .50E-0 3 4
. 11E-0 2 2.30E-0 3 1.18E-0 2 100 3.40E-0 3 2.15E-0 2 2.00E -0 4 1 .50E-0 2 9 .80E-0 3 4
. 17E-0 2 2.50E-0 3 1.19E-0 2 Table 4 - Co m p arison between HYDRUS-2D a nd IWEM results for botto m ash concentrations (m g/l) with tim e . Results based on using K dvalues of zero and MINTEQA2 derived for HYDR US and IW EM r e spectively. Observation s from Table 4 show sim i lar resu lts in m a gnitude between the models for each solute and tim e . This appears to confirm th e notion that IW EM is drawing upon very low K dvalues (less than or equal to 1 L / Kg) from MINTE QA2 which contradicts the E P A tabulated data.4.0 Conclusions Based on the data presented in the previous s ection, several prelim inary conclusions can be
m a de. The m a jor conclusion is that, based on HYDR US 2-D data, it appears IW EM is very conservative in its m odeling and thus is overestimati ng output concentrations at the receptor well. This m ay be the result of several factors. K dvalues derived from MINTEQA2 appear to be extrem ely low (less than or equal to 1 L/Kg) when com p ared to the EPA tab u lated d ata ( T ab le 1) and those from other literature sources. The use of these sm all Kdvalues seem s to be accounting for th e lack of retardation being observed, thus allowing the contam inants to move freely through the aquifer with m i ni m a l sorption. This is likely resulting in the higher observed concentrat ions at the recepto r well when com p ared to HYDRUS-2D outputs. Furtherm ore, HYDRUS is only sim u lati ng flow/transport within the unsaturated zone while IWEM accounts for the entire aquifer. This raises further concern that IWEM m ay be too conservati ve it its predictions. Another concern is the lack of output files with respect to non-user defined input values. For instance, if MINTEQA2 is used, the m odeler can not view which Kdvalues were selected due to the absen ce of such a file. The sam e is tru e for Monte Carlo derived aquifer param e ters. While there is a file listing national averages, it is not cl ear whether these are th e va lues being used in the sim ulations. Such output files would allow for the m odeler to determ ine whether the chosen values are realistic, and if not, to m ake the necessary adjustm e nts. Finally, because IW EM assum e s WMUs to be square it does not allow the m odeler to accurately p o rtray the true geom etry of a roadway (rectangular). It seems likely that IWEM is contr i butingm o re solute to the c ente r lin e lin e of th e plum e than would re alis tic ally b e seen with a rectangular cross-section of the same area. The further you move away laterally from the plum ecenterline, the less likely it is that contam inants at those di stances would be introduced into the groundwater that is seen at the recep tor well down gradient(located alongthe plum e centerline). Factors such as advectio n, dispersion, dilution and sorption w oul d all act to count er any significant contributions from contam inants outside a ce rta i n late ral exte nt f r om the cente rlin e.
5.0 Future Work Upon com p letion of the HYDRUS-2D m odeling portion of th is res earch, MODFLOW will be used to simulate 3-D groundwater flow and solute transport in saturated m e dia at an observation well down gradientof th e secondary m aterial sour ce. Input will includ e th e m e tal concentrations and fluxes obtained at the lower boundary of the corresponding HYDRUS sim u lation. Several layers in th e MODFLOWdom a in will be c r eated so thatmultip le conc entrations can be obtain e d at various intervals along th e observatio n well screen. The average of these concentratio ns will be com p ared to that ob tained from I W EM inorder to evaluate its predictiv e accuracy. Additionally , arbitrarily selected hig hconcentr ations of solute (e.g. 10 x MCL) will b e input into MODFLOW to obs erve the m a gnitude of di lution encoun tered alongthe pathwayto the recep tor well. These de term inations will be co m p ared to IW EM to m ake a conclus i o non its transport accuracy.
6.0 References Sauer, J.J., Benson, C.H., and Edil, T.B. ( 2005). “Metals L eaching fro mHighway Test Sections Constructed with Industrial Byproduc ts.” Geo Engineering Report No. 05-21, Dept. of Civil and Environm ental Engin eering, University of W i sconsin-Madison. U.S. EPA (1999). “Partition Coefficients for Metals in Surface W ater, Soil, and W a ste, Washington, D.C.