Tracer Technologies for Hydrological Systems (Proceedings of a Boulder Symposium, July 1995). IAHSPubl.no. 229, 1995. 57
Validation of a groundwater model by simulating the transport of natural tracers and organic pollutants
EMC ZECHNER, THOMAS NOACK & LUKAS HAUBER Institute of Geology, University of Basel, Bernoullistr. 32, CH-4056 Basel, Switzerland JÏJRG TRÔSCH Laboratory of Hydraulics, Hydrology and Glaciology, ETH Zurich, Switzerland RICHARD WÙLSER Chemical Laboratory, IWB Basel, Switzerland
Abstract The groundwater system in the porous aquifer at the artificial recharge and groundwater catchment site of Langen Erlen is modelled with a 2D-flow and -transport model. A transient flow model forms the basis for transport simulation (convective-dispersive) by which firstly the flow model is validated, especially the natural recharge influx, and secondly the propagation of natural tracers (chloride) and organic contaminant (tetrachlorethylene) is studied. The chloride simulation leads to a quantifying of the three main natural influxes, i.e. the highly mineralized recharge from a karstic area east of Langen Erlen (Dinkelberg), the low mineralized recharge from the northeastern upper valley (Black Forest) and the very low chloridated river infiltration. Finally the transport model of a tetrachlorethylene-spill in 1981 with an adsorption onto the aquifer material is in mainly good agreement to the analysed data.
INTRODUCTION
General situation
The groundwater catchment site of Langen Erlen is located northeast of Basel and extends along the River Wiese over more than 4 km to the German border at Lôrrach (Fig. 1). The discharge of 13 well groups of with about 60 000 m3 day"1 represents nearly 50% of the drinking water consumption of the city of Basel. To support this discharge the natural influxes are enriched with artificial recharge of filtered Rhine water. About 70 000 m3 day4 are recharged by flooding several forest fields. The water consumption of the city has been slowly decreasing over the last 20 years. In addition persistent organic pollutants appeared in the groundwater of the Langen Erlen with the beginning of the eighties.
Hydrogeological background
The free aquifer consists of mostly homogeneous Quaternary sandy gravels. The gravels 58 Eric Zechner et al.
i. Lprradh(D)
Geology: Quaternary valiey floor gravels _____ Quaternary low-terrace gravels ______Tertiary, Jurassic and triassic sediments
Signs: \ 1 Artificial recharge fields • catchment sites Rhine Fig. 1 Geological overview of the Langen Erlen area along the River Wiese northeast of Basel, km in Swiss coordinates. originate either from the Wiese valley (variscan gneiss and granites, Black Forest) or of the Rhine valley (calcite-rich rocks, Alps and Jura). The underlying Tertiary marls are considered impervious. The saturated thickness of the aquifer increases from the German border in the northeast towards Basel from 8 m to more than 20 m with a depth of the water table from 3 m to nearly 10 m below the surface. A dense mesh of observation wells helps to monitor the piezometric head and the groundwater quality. Several pumping tests were performed to determine the hydraulic conductivity and storage capacity. They showed a hydraulic conductivity increasing from 2 X 10"3 m s"1 in the southwest to nearly 1 X 10"2 m s"1 in the northeast of the investigated area and a storage capacity varying between 0.08 and 0.20. There are mainly three natural recharges in the Langen Erlen. Firstly the low mineralized groundwater from the Wiese valley in the northeast. Secondly the highly mineralized (especially chloride, sulphate, sodium, calcium and magnesium) water from the karstic Triassic limestones and evaporites of the Dinkelberg in the east. Thirdly, the infiltration from the Wiese River.
FLOW MODEL
To simulate the flow the numerical model GW2D with quadrilateral finite elements and quadratic interpolation developed at the ETH Zurich (Trôsch, 1975) was used. The Validation of a groundwater model 59 modelled area of about 8 km2 was discretized into over 250 elements and 800 nodes. To satisfy the changing hydraulic conditions due to the influences of the artificial recharge and catchment, the simulation had to be transient. The boundaries were defined as constant in-/outfluxes (Neumann-condition), whereas infiltration from the Wiese through the colmated riverbed was simulated with a leakage-condition. The calibration of the model showed that the parameters difficult to constrain were natural recharge, especially the infiltration from the Wiese River, and recharge from the karstic Dinkelberg in the east of the site. Therefore a transport model that is more sensitive on these parameters was calibrated.
DATA: ANALYSED TRACERS
(a) Chloride, sulphate: A set of samples taken in April and October 1994 were analysed for chloride and sulphate. The natural main influxes, i.e. the highly mineralized karstic Dinkelberg groundwater, the low mineralized Wiese groundwater, and the very low mineralized infiltration water of the River Wiese were analysed, as well as the mixed groundwater of the Langen Erlen. In order to minimize the influence of infiltrated Rhine water in the mixing zone, artificial recharge was stopped for one month. (b) Oxygen isotope (O18): In order to examine oxygen isotopes samples were taken under the same conditions as those for chloride and sulphate. As the results of test samples of April 94 show a significant gradient between the main natural recharges, a further set of samples taken in October 1994 will be analysed in January 95 and discussed later. (c) Tetrachlorethylene: A tetrachlorethylene (C2C14) spill of an illegal dry cleaner took place in June 1981 at the northeastern border of the catchment area. Under aerobic conditions this organic contaminant is very persistent and with a density of 1.6 g cm"3 it may also reach the bottom of the aquifer in phase (Schwille, 1982). The propagation of the dissolved contaminant in the saturated zone menaced the catchment wells in that area of the Langen Erlen. A removal of the contaminated soil in the unsaturated zone was necessary. Further two groups of decontamination wells were drilled. The first well started to pump out the polluted groundwater in August 1981 at the site of the contamination, the second well located 300 m further down started in December 1981 to inhibit the propagation of the plume. The propagation of tetrachlorethylene and the effect of the decontamination is documented in long-term analyses (1-2 datasets per month) of the groundwater. At the same time a diffuse background flux of mainly trichlorethylene, but also tetrachlorethylene, polluted the groundwater from the Wiese valley.
TRANSPORT MODEL
To simulate the transport of tracer the transport model of the finite element program GW2D (Trôsch, 1993) was used. To reduce the influence of numerical dispersion smaller elements of about 100 x 50 m were used in the northeastern part of the Langen Erlen where the main natural recharges occur and the contamination took place. This 60 Eric Zechner et al. mesh has now about 750 elements and 2300 nodes. The model takes into account the convective and dispersive part of the propagation.
Modelling chloride
For the nonreacting chloride retardation (adsorption) had not to be considered. Flow and transport were modelled for the month of April 1994 with time steps of 1 day. The computation was initiated with measured concentrations of chloride after stopping artificial recharge. The model boundaries were set as influx with fixed (known) concentration. This means that the mass in-/output is independent of the concentration in the border element. The boundary concentrations were interpolated from the analyses and well known on the critical northeastern boundary where the main mass-input occurs. The mass infiltration of the River Wiese was also simulated with the fixed (measured 4.0 mg l"1) concentration of the influx.
Modelling tetrachlorethylene
It was assumed that tetrachlorethylene (C2C14), similarly to most organic contaminants, would adsorb onto the aquifer material. Following an empirical relationship between the octanol- water distribution coefficient of tetrachlorethylene (kow, 760) and the adsorption coefficient (lcd) provided by Schwarzenbach & Westall (1981) and based on a low fractional organic carbon content (foc) of about 0.001 (Giger & Schwarzenbach, 1984), the retardation factor Rf should be in the range of 4.0.
012 kd = 3.09fockow (1)
Rf=l +kd- b{\ - ri)ln (2) where <5 is the density of aquifer material (= 2.5 g cm"3) and n is the total porosity (= 0.2). With the calibration runs the optimal retardation converged to a factor between 3.0 and 4.0. The model boundaries were set as influx with no mass input, except the diffuse flux from the northeastern Wiese valley, where the influx concentration was fixed corresponding to the interpolation of measured data. During the simulated period of 18 months (July 1981 to January 1983), the outflux-concentration at the western border was set between 0.5 and 1.5 /xg l"1 to prevent an artificial mass buildup. The contamination was simulated as a constant mass-input and compared to the analyses at the site of the spill (nearly 1000 [L% l"1 in June 1981). The simulation was initiated with the measured concentration of tetrachlorethylene at the end of June 1981.
RESULTS
Chloride
The simulation of chloride transport was compared to the analysed data after 25 days. Figure 3 shows the concentration-distribution in mg l"1 chloride of the final simulation Validation of a groundwater model 61
Waterbalance [1/s flow and transport/(flow m
Waterbalance midwater situation
Artificial recharge: 720 1/s
Catchment: 7671/s
Total water in-/output: 12371/s
Fig. 2 Modelled water balance with flux over boundaries, river infiltration and direct groundwater recharge through precipitation before and after transport modelling. in the relevant upper part of the Langen Erlen. The first computation runs showed a generally too high concentration in the modelled area. Therefore the higher mineralized influx of Dinkelberg was reduced in the flow model from over 2001 s"1 to about 1501 s4 (Fig. 2), also influx from the Wiese valley was lowered about 30% to 80 1 s"1. In addition the low mineralized river infiltration had to be nearly doubled (about 1801 s4). The new ratio of influxes required 2-3 times reduced permeabilities in some northeastern parts (Fig. 5). Although the final simulation generally represented a good fit to the measured data, at some places higher concentrations were measured than predicted, especially at the catchment wells. This most probably resulted from a remaining influence of the artificially infiltrated lower mineralized Rhine water. Due to the relatively high flow velocity in the Langen Erlen the transport simulation was not very sensitive on dispersivity. The phenomena of dispersion could better be constrained with time-dependent analyses of retarded tetrachlorethylene propagation.
Tetrachlorethylene The simulation of tetrachlorethylene propagation and its decontamination was compared with monthly analyses over a period of 18 months. Kg. 3 Isolines of simulated chloride concentrations in mg 1"' for the northeastern part of Langen Erlen end of April 1994 (black dots indicate analysed data, squares catchment wells).
Fig. 4 Isolines of simulated tetrachlorethylene concentrations in yug l"1 for the northeastern part of Langen Erlen in October 1981 four months after spill occurred (black dots indicate analysed data, squares catchment wells). Validation of a groundwater model 63
Figure 4 shows the simulation of October 1981, two months after the first decontamination well started to pump where the spill took place (computed concentration of 79 fig l*1, measured of 84.5 fig l"1). The general propagation of the plume could be predicted satisfactorily, only locally at the site of the plume did the simulation deviate from the measured data. This could be due to local vertical inbomogeneities of the aquifer which can only be considered with a 3D-simulation and data. Numerical problems with negative concentrations occurred in the upper part of the plume around the decontamination wells at the end of the simulated period, which resulted from a high mass gradient around the spill. The parameters of the flow model adjusted for chloride transport were confirmed with tetrachlorethylene, especially the lower flow velocity in the contaminated area (Fig. 5). The calibrated dispersivities especially the longitudinals increased with distance of propagation, an often observed phenomena (Pickens & Grisak, 1983) which is caused in 2D-modeîling by the influence of macrodispersion on large-scaled inhomogeneities. The longitudinal dispersivity varied from 5.0 to 10.0 m, the transversal dispersivity from 0.3 to 0,5 m.
/^
.sT.' . =•;-'.'.,'/•....'WW
m/s] 10. 0E H OF li Oil
Fig. 5 Distribution of hydraulic conductivities in the validated model for the northeastern part of Langen Erlen (black dots indicates spill location, squares catchment wells).
CONCLUSION
By modelling the transport of a natural tracer like chloride the three main natural recharge influxes in the modelled area could be modelled, i.e. the highly mineralized recharge from a karstic area east of Langen Erlen (Dinkelberg), the low mineralized recharge from the northeastern upper valley (Black Forest) and the very low chloridated 64 Eric Zechner et al. river infiltration. The interpretation of O -data and the transport simulation of sulphate will permit to further constrain the deviating simulation results on chloride around the catchment wells. Modelling the tetrachlorethylene-spill allowed a calibration of the dispersivities in the northeastern part of Langen Erlen and was proved to be a tool to predict the behaviour of eventual future pollution events.
REFERENCES
Giger, W. & Schwarzenbach, R. (1984) Grundwasserverschmutzung Lange Erlen. Unpublished report, EAWAG Diibendorf. Pickens, J. F. & Grisak, G. E. (1981) Modelling of scale-dependemdispersion in hydrogeologicsystems. "Wat. Resour. Res. 17(6), 1701-1711. Schwarzenbach, R. P. & Westall, J. (1981) Transport of nonpolar organic compounds from surface water to groundwater. Environ. Sci. & Technol. 15(11). Schwille, F. (1982) Die Ausbreitungvon Chlorkohlenwasserstoffenim Untergrund, erlâutert anhand von Modellversuchen. DVGW-Schrifienreihe Wasser 31, Eschborn, 203-234. Trôsch,J.(1975)NumerischeSimulationDupuit'scherGrundwasserstrômungen.V'erj«d!raniïa(