3D density-dependent groundwater modeling in assessing ...

3D density-dependent groundwater modeling in assessing optimisation of a pumping strategy within a chemical plant

D. Campoli, E. Crestaz, M. Fehervari, L. Patata & P. Scipioni Snamprogetti SpA – Divisione Aquater (ENI Group), San Lorenzo in Campo (PU), Italy

Abstract

This paper focuses on the key-role of density-dependent groundwater modeling in designing an emergency pumping strategy for a small area within an Italian chemical site, located close to the seashore. The finite-element numerical code Feflow has been used, due to its numerical features and its full-3D integrated processing environment. Following a schematic geological, hydrological, hydrogeological and contamination picture of the area, the initial conceptual model has been investigated by numerical modeling, which has highlighted the influence of leakage from human-made technological networks in controlling groundwater recharge. As most of the leaking waters are pumped from the sea, a general salinisation of the aquifer system occurred, resulting in a few meters of “relatively fresh” water and a sharp to gradual transition to sea water; numerical modeling helped in assessing the transient hydrogeological history from the unknown pre-industrial site configuration. Focusing on risks of contaminated water flowing to the sea, different pumping schemes have been simulated, including shallow vs. deep pumping wells, optimisation of well location and number. In the face of uncertainty issues in groundwater recharge and flow estimation, conceptual guidelines have been provided, recommending proper monitoring strategies to support the decision making process. Keywords: groundwater modeling, salt-water intrusion, Feflow.

1 Introduction

A preliminary 3D density-dependent numerical flow model has been developed for a small flat area located in southern Italy (fig. 1). The area, falling within a

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Water Resources Management III 131

chemical site and bounded by the sea along both its northern and southern limits, has been traditionally affected by contamination phenomena, causing heavy quality impacts on groundwater environment. A groundwater remediation strategy is currently under analysis and modeling should support pumping systems design, including optimisation of wells location and discharge rates, face to possibly complex interactions with intruding seawater. This application is of general conceptual interest as most of Italian oil and chemical sites are located close to the sea and groundwater remediation strategies are strongly influenced by fresh-salt water interface dynamics.

2 Geological and hydrogeological framework

A relatively uniform stratigraphic sequence can be outlined from about 20 piezometers located within the area (fig. 2). From upper limit of the stratigraphic sequence, main geological and hydrogeological features can be summarized as follows: Continental deposits: interlayered silts, fine silts and clays, with relatively

low hydraulic conductivity; Marine deposits: markedly more sandy than previous one and characterized

by 1-2 m thick fractured sandstone horizons; medium to high (fractured sandstone) hydraulic conductivity, inferred from tracer tests, available in neighbouring areas;

Transition deposits to underlying continuous clay basement; silt to silty-clay and clay of low to very low hydraulic conductivity.

Figure 1: Study area conceptual scheme.


132 Water Resources Management III

Figure 2: Electrical conductivity and stratigraphy for 2 wells.

Figure 3: Monthly rainfall over the period 2000-2002.

From the hydrological point of view, the area is characterized by a relatively dry clima, with mean yearly rainfall in the order of 800 mm and a typical dual-



peaks distribution with maximum values in autumn and spring. Precipitation on a monthly basis over the period 2000-2002 is reported in fig. 3. High evapo-transpiration rates are assumed, due to high temperatures and to low vertical permeability of upper layers in the stratigraphic sequence. Piezometric data, collected over an extended time period, show a general trend of groundwater flow towards northern and southern coast, with a major distinctive piezometric high, confined at the northeastern limit. This last feature is basically related to leakage from existing infrastructures and particularly from buried water pipelines, according to field observations. General piezometric trend is well captured by January 2004 water table, interpreted after fully screened wells (up to sandstone horizon) and reported in fig. 4. Yearly piezometric head oscillations in the order of 0.30 m (0.4 –0.7 m a.s.l.) have been observed at the western boundary of the study area.

Figure 4: January 2004 piezometric surface.

A clearly distinctive feature in the area is the extremely high electrical conductivity of groundwater (fig. 2); empirical rules (i.e. Ghyben Herzberg – see Bear and Verruijt, 1987; pp. 202-204 [1]), relating observed piezometric heads to fresh-salt water interface geometry, make clear that aquifer salinisation in the area can not be properly explained with salt water intrusion phenomena. As a network of buried pipelines with seawater under pressure conditions exists in the area, the hypothesis of leakages occuring from these pipelines to the groundwater sounds as a reasonable starting point. Additionally, this hypothesis seems also to be confirmed by the generalised salinization phenomena observed within the site, on the west of the study area, face to relatively high piezometric heads, which should prevent major sea water intrusion phenomena.



3 Main features of numerical model

Due to the specific hydrogeological framework, the occuring salisation phenomena and to the nearness to the coastline, the 3D finite-element density-dependent numerical code Feflow FM3 (Diersch, 2002 [2]) has been adopted. A discretisation mesh (fig. 5), properly refined along the N-E limit of the study area, has been defined, including a total of 12 vertical layers (from the piezometric surface down to –15 m a.s.l., each layer being 1 m thick, except for a fractured sandstone layer from –10 to –12 m a.s.l. and a transition silty-clay layer from –12 to –15 m a.s.l.). This mesh configuration, leading to 18204 elements, resulted in acceptable computation times under a Pentium IV 2.4 GHz HW platform.

Figure 5: Discretisation mesh: plan view and a 3D perspective from E.



Boundary conditions have been set to no flow along the western limit of the modeled area, where observed groundwater flow directions have been confirmed to be parallel to the boundary and virtually unaffected by piezometric head variations. Same considerations can be applied to the 120 m wide boundary at the eastern limit of the study area. N and S limits, bounded by the sea water, have been set, according to code specification (Diersch, [2]), to an equivalent fresh water head profile, assuming sea water salinity concentration to be 37000 mg/l.

4 Groundwater model calibration

Face to previous considerations, the role of leakages from human-made infrastructures and water pipelines should have been properly investigated by comparing current piezometric and hydrochemical (mainly water salinity) framework against pre-site construction conditions. Unfortunately, no relevant information dating back to early 70s was made available; considering the generalised salinization of groundwater and in-situ observations of occuring leakage phenomena, recharge from human-made infrastructures has been assumed, particularly at the N-E limit of the study area in connection with the local piezometric high. A long-time transient simulation (100000 years) was run, looking to natural site conditions. Starting with fresh water conditions everywhere within the modeling domain, the initial vertical salt-water concentration along N and S boundaries was left free to evolve to a quasi-steady state condition. The resulting piezometric configuration, still not reporting the piezometric head high at the N-E limit, was used as the initial condition for next calibration steps. As a further step, steady state calibration against January 2004 piezometry was performed (Fig. 6), by assuming a local relevant leakage (estimated in the order of 150 m3/day) from pipelines resulting in the piezometric high observed at the N-E limit of the study area. Piezometric trend is well reproduced, particularly in the central and eastern part of the study area. Very high saline concentration, as shown in a cross section from E, reveals the impact of the local leakage on groundwater; this situation is confirmed by the salinity profile of a well drilled along the piezometric high and reporting salt water starting from the water table downward. Relevant aquifer salinisation phenomena are still observed in case of low-leaking pipelines; this situation has been conceptually captured in the calibration process by assuming a limited leakage (10 m3/day) occuring along a central-southern saltwater pipeline, trending approximately E-W (the elongated high concentration line in Fig. 6). The lack of well distributed spatial data did not permit any further analysis of the issue and the location of leaking pipelines is intended as purely demonstrative, except for the one observed at the N-E limit of the study area. Considering that the contamination sources are distributed over a relatively large sector, at the N-E limit of the study area, and a higher contaminated sector, also referred to as “Hot spot”, can be also identified (Fig. 1), a simulation of the 3D groundwater flow pathways has been made, after a range of random shallow



depth sources. Fig. 7 highlights how contaminated groundwater flow would affect both N and S sea boundaries, depending upon specific starting location. Flowlines tend to migrate towards the higher conductivity fractured sandstone layer, while they markedly move upward as the front of salt-water interface is approached.

Figure 6: Model calibration: steady state computed vs. January 2004 observed piezometric head and resulting salinity concentration distribution on a vertical cross section after a view from E.



Figure 7: Model calibration: plan view and 3D perspective from E of the groundwater flow pathways from hot spot at the N-E limit of the study area.

Figure 8: Pumping test curve interpreted with Neuman method.

Table 1: Hydrogeological properties of calibrated numerical model.

Formation Horizontal K (m/s) Vertical K (m/s) Continental 2*10-5 5*10-6 Marine (mainly sands) 5*10-5 1*10-5 Marine (fractured sandstones) 5*10-4 5*10-5 Transitional to underlying clays 1*10-6 1*10-7

The higher hydraulic conductivity of the fractured sandstone layer is confirmed by the interpretation of a 24 hour pumping test (Fig. 8) made within the study area, close to the southern coast. Pumping test interpretation highlights semi-confined conditions with delayed recharge from upper layers, and this is basically confirmed by other pumping tests on wells still located within the chemical site and with a similar stratigraphic sequence. Hydraulic properties used along the calibration process are reported in tab. 1.

5 Simulation

Following model calibration, groundwater management strategies have been investigated by assuming pumping at two existing wells. While other



remediation measures will be investigated in the next future, the emergency nature of the problem suggested that pumping should be regarded as the reference measure to keep under control groundwater contaminated flow towards the sea. Results of a first simulation, focused on 1 l/s/well pumping rate, clearly highlights a wide capture zone; fig. 9 shows pumping impact on groundwater pathlines in both 2D and 3D views. Despite the same pumping rate is assumed for both wells, capture zone of eastern well looks like much narrower, basically due to the significative local leakage from water pipelines. The eastern well keeps in charge for the hydraulic containment of the local additional recharge.

Figure 9: Simulation with 2 pumping at a rate of 1 l/s/well: flowlines plan view and 3D perspective from E.

Provided that further field data should be collected to properly assess pumping impact on groundwater flow and to validate modeling calibration, a major relevant effect on groundwater flow would be played by cut-off of leakages from existing pipelines. Fig. 10.A captures the groundwater system behaviour with a total discharge of 2 l/s, assuming that no more leakages occur; if compared with outcomes from previous simulation (ref. Fig. 9), it comes clear the widely increased zone affected by pumping, particularly around the eastern well. Based on the results of previous simulation, assessment of pumping rates reduction effects has been taken into account. The study case with 0.25 l/s/well (fig. 10.B) clearly highlights a critical framework, resulting in an unacceptable containment effect on groundwater.

6 Conclusions

The groundwater model presented in this paper supported the data analysis and interpretation process, leading to a conceptual framework with the following relevant features: • isolated piezometric high, confined at the N-E limit of the study area,

reasonably related to leakage from existing buried pipelines;



A.

B.

Figure 10: Forward flowlines plan view of the groundwater flow, following cut-off of leakage from NE pipeline. A. Simulation with 2 wells with a pumping rate of 1 l/s/well- B. Simulation with 2 wells with a pumping rate of 0.25 l/s/well.

• salinisation of the aquifer system mainly related to leakage from salt-water pipelines, running within the chemical plant area. Salt-water intrusion, related to seawater, would be really quite limited, along the coastline.

Resulting hydrologic balance and next modeling simulation steps highlighted that even low pumping rates would result in a satisfactory containment of contaminated groundwater flow to the sea. Despite uncertainty affecting estimation of hydrogeological properties and the lack of an exhaustive number of reliable pumping tests, a preliminary analysis focused on a total 2 l/s pumping



rate at 2 wells located not far away from the northern limit of the study area as a reasonable target to produce satisfactory impact on groundwater flow related to potentially contaminated area. Provided that more information would be required (pumping tests and monitoring of piezometric levels, once pumping will be started), a relevant outcome of modeling activities has been the focus on the urgency of restoration measures on existing pipelines. This would significatively reduce the leakages to groundwater, improving at the same time the efficiency of pumping. Moreover, a relevant problem in the area continues to be a proper balance between pumping rates and effects on fresh-salt water interface dynamics. Current conceptual hydrogeological framework suggests the idea of 2 l/ pumping rate coupled with repair measures on existing pipelines is a viable option, not causing significative salt water intrusion. Further information will support a refinement of the measures suggested above. We would like to thank Alfredo Battistelli for reviewing the paper. Thanks go also to Mr. R. Fratini for his support in the final editing phase.

References

[1] Bear, J. & Verruijt, A., Modeling Groundwater Flow and Pollution, D. Reidel Publishing Co., Dordrecht, Holland, 1987.

[2] Diersch, H.J.G., Feflow: Finite Element Subsurface Flow & Transport Simulation System: Reference Manual. Wasy, Institute for Water Resources Planning and Systems Research Ltd., Berlin, Germany, 2002.



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