Luca Alberti

Borehole Heat Exchangers: heat transfer simulation in the presence of a groundwater flow

The correct design of the Borehole Heat Exchanger is crucial for the operation and the energy performance of a Ground Source Heat Pump. Most design methods and tools are based on the assumption that the ground is a solid medium where conduction is the only heat transfer mechanism. In turn in regions rich in groundwater the groundwater flow influence has to be assessed, by including the convection effects. In this paper a numerical model of a 100 m U-pipe in a saturated porous medium is presented. The model is created adopting MT3DMS coupled to MODFLOW. A Darcy flow is imposed across the medium. The typical operation of a Borehole Heat Exchanger operating both in winter and in summer is simulated for two years, under different groundwater velocities. The energy injected to and extracted from the ground is derived as a function of the Darcy velocity and compared with the purely conductive case. Temperature fields in the ground at key moments are shown and discussed. From both the energy and the aquifer temperature field points of view, the velocity ranges for respectively negligible and relevant influence of the groundwater flow are identified.

Numerical modeling of regional groundwater flow in the Adda-Ticino Basin: advances and new results

The Adda Ticino basin is a densely-populated area, dependent massively on groundwater resources not only for industrial uses but also for agricultural and domestic ones. A large scale hydrogeological assessment study over 8000 Km2 area northwest of Lombardy Region was done in reason to implement a groundwater flow model. The main objective was to give a quantitative estimate of groundwater budget into the Adda-Ticino basin and to deliver a tool to support Public Authorities in regional groundwater management required by the European Directive 2000/60/CE. The approach consists of improving the hydrogeological conceptual model and better define hydraulic properties of the aquifer units and all components in the basin influencing the groundwater flow (rivers, wells withdrawal). Then the collected GIS information were added into the numerical model Modflow. Regional steady-state flow was calibrated under current stress conditions with an inverse calibration code (Doherty, 2005) mainly by changing hydraulic conductivity and vertical recharge inflow. The water mass balance computed shows a total flow of 3279 Mm3/y. Vertical recharge provides 65% of the total groundwater inflows, whereas the 30% is coming from pre-Alps up-gradient boundary. Discharge from basin aquifers occurs through flow to rivers (60%), groundwater supply wells (30%) and underground outflow at the S-SE border (10%).

Fingerprinting and Groundwater Model Application to Evaluate Hydraulic Barrier Efficiency (Italy)

Remediation actions at contaminated sites are based on multiple numerical model scenarios considering different parameter distributions, source positions and contaminant transport paths. In some cases the excess of scenarios is due to uncertainties in the conceptual model as a result of the spread of contamination through heterogeneities in the physical system. Reduction of project hypotheses and conceptual model uncertainty is therefore needed. This result can be achieved by coupling hydrogeological investigations with environmental forensic techniques, better localization of the source and understanding of contamination history. In this respect, in the present study, compositional fingerprinting and groundwater flow modeling were applied to a former oil storage facility where, even though a hydraulic barrier had been built to stop the hydrocarbon plume, the presence of some hydrocarbons was still found in downgradient monitoring wells. The final aim was to evaluate the efficacy of the hydraulic barrier and identify of the source of pollution. Fingerprinting results indicated pollution with a gasoline-diesel mixture much altered by water washing and/or biodegradation. Comparison of seven groundwater samples collected in wells and monitoring wells was performed by analyzing the volatile fraction (BTEX) and the total ion chromatogram (TIC), focusing attention on: n-alkanes (m/z 85), alkylcyclohexanes (m/z 83), isoprenoids (m/z 113), C4-alkylbenzenes (m/z 134), C3-C6 alkylbenzenes and polycyclic aromatic hydrocarbons (PAHs). The most probable scenario was then identified by combining the results of fingerprinting with different contaminant paths obtained using the numerical model.

Assessment of sources of PCE contamination in groundwater using a Monte Carlo method in the Functional Urban Area of Milano

Water Framework Directive and Groundwater Directive (2006/118/CE) represent the main European regulations on water resources quality, requiring the identification and management ofcontamination sources threatening the achievement of acceptable groundwater quality status. The sources of groundwater contamination can be classified into two different categories: 1) point sources (PS), which are identified areas releasing plumes of high/very high concentrations (i.e. hot-spots) and 2) multiple-point sources (MPS) constituted by a series of unidentifiable small sources clustered in a large area, generating a diffuse contamination. The latter category predominates in European Functional Urban Areas and cannot be managed with the usual remediation techniques such as those conceived for large/medium contaminated sites, mainly because of the difficulty to identify the many different source areas releasing small contaminant mass. Consequently, the usual remediation procedures are not economically sustainable and often fail to provide results in an acceptable time frame. A powerful tool in this context is transport modeling, as it can be used in combination with statistical methods to assess MPS groundwater contamination even in a highly undetermined setting. Due to the uncertainty related to the exact position and intensity of MPS, a numerical model (MODFLOW/MT3DMS) was implemented in a pilot area in the North-Eastern sector of the Milano FUA. Using the inverse calibration code PEST, a model was calibrated representing the diffuse PCE source field (Italian Law limit is within 1.1 mg/l). This was then used as input to a statistical process based on the Null-Space Monte Carlo (NSMC) method, which allows to generate unlimited sets of sources, all respecting the measured concentrations.99 different realisations were thus obtained, each attributing a contaminant inflow (with varying concentrations) to every cell of the model top layer. The model minimizing the objective function (composed by the diffuse concentration targets in monitoring wells), gives useful information to assess the overall contaminant sources distribution and to identify the probability of each domain sector to contribute to the contaminant mass inflow.

Assessment of aquifers groundwater storage for the mitigation of climate change effects

In the coming decades, the effects of expected climate change will affect key aspects of our society including quantity and quality of available water resources. It is believed that water resources will play a growing important role in Europe and in particular in the countries south of the Alps, where climate change could hit harder (EEA, 2014). Without appropriate interventions, water will turn from a good hearty undervalued to a scarce and precious resource, whose distribution will be less and less equitable, resulting in growing conflicts for its use. As already recognized by the European directives (2000/60/EC and 2006/118/EC), the theme of quantitative management of water resources is therefore a strategic issue that can allow the reduction of the risks and costs associated with climate change. Integrated management of both surface and groundwater for sustainable use nowadays represents more and more a key concern in environmental policies and water management. The goal of this work is to assess the effects on groundwater storage of a new management of irrigation waters.The highly variable availability of water resources during the hydrological season could be particularly troublesome in the management of irrigation systems, since they need water in seasons (spring-summer) when usually its availability has a depletion. Storage of groundwater in aquifers in colder seasons could be used to soften irrigation systems water deficit in drier ones. Specifically, the practice of winter irrigation, i.e. the use of the main irrigation channels to recharge aquifers during the colder seasons, could increase water resource availability for irrigation during the drier growing seasons.A pilot groundwater flow transient model (MODFLOW2000, Harbaugh et al., 2000) with a domain of about 255 km2 was implemented in the north zone of Lodi Province, including a large part of the Consorzio Muzza irrigation district. The model was then run to estimate change in groundwater resource availability in two main scenarios: in the first scenario the storage in the shallow aquifer was computed in the colder period (October–March), when irrigation is not occurring; for this case, it was used measured monthly rainfall, average temperature for the period 2004-2013 (representing actual climate conditions) and an estimations of water volume used for irrigation. In the second scenario, an integrated management approach of irrigation and groundwater was tested, adopting winter irrigation. Both scenarios want to assess if water storage in aquifers in colder periods could represent an additional water volume useful to mitigate possible climate changes effects on crops.

APPLICATION OF COMPOSITIONAL FINGERPRINTINGTO AN ITALIAN SITE CONTAMINATED BY HYDROCARBONS

(*) Tethys srl Viale lombardia 11 20131 milan, italy (**) Politecnico di milano, Dipartimento ingegneria Civile e ambientale (D.i.C.a.), ed. 9, Sezione geoscienze 20133 milan, italy e-mail: patrizia.trefiletti@tethys-geco.it ilaria.pietrini@polimi.it dario.rigamonti@tethys-geco.it luca.alberti@polimi.it ApplicAtion of compositionAl fingerprinting to An itAliAn site contAminAted by hydrocArbons

37Cl-compound specific isotope analysis and assessment of functional genes for monitoring monochlorobenzene (MCB) biodegradation under aerobic conditions

A laboratory approach was adopted in this study to explore the potential of 37Cl-CSIA in combination with 13C-CSIA and Biological Molecular Tools (BMTs) to estimate the occurrence of monochloroenzene (MCB) aerobic biodegradation. A new analytical method for 37Cl-CSIA of MCB was developed in this study. This methodology using a GC-IRMS allowed to determine δ37Cl values within an internal error of ±0.3‰. Samples from a heavily MCB contaminated site were collected and MCB aerobic biodegradation microcosms with indigenous cultures in natural and enhanced conditions were set up. The microcosms data show a negligible fractionation for 13C associated to MCB mass decrease of >95% over the incubation time. Conversely, an enrichment factor of -0.6±0.1‰ was estimated for 37Cl, which is a reflection of a secondary isotope effect. Moreover, the dual isotope approach showed a pattern for aerobic degradation which differ from the theoretical trend for reductive dehalogenation. Quantitative Polymerase Chain Reaction (qPCR) results showed a significant increase in todC gene copy number with respect to its initial levels for both natural attenuation and biostimulated microcosms, suggesting its involvement in the MCB aerobic degradation, whereas phe gene copy number increased only in the biostimulated ones. Indeed, 37Cl fractionation in combination with the dual carbon‑chlorine isotope approach and the todC gene copy number represent valuable indicators for a qualitative assessment of MCB aerobic biodegradation in the field.

Multivariate data analysis and numerical modeling: a combined approach to assess the Groundwater contamination in Milan Functional Urban Area

Abstract from 88th Congress of the Italian Geological Society, 2016-09-07 - 2016-09-09, NaplesAbstract from 88th Congress of the Italian Geological Society, 2016-09-07, 2016-09-09, Naplesbook Edited by D. Calcaterra, S. Mazzoli, F.M. Petti, B. Carmina & A. Zuccari doi: 10.3301/ROL.2016.79

Borehole Heat Exchangers in aquifers: simulation of the grout material impact

In Ground Source Heat Pump installations the U-shaped pipes are located in boreholes filled with grouts that are enhanced from the thermal conductivity point of view and usually hydraulically impermeable. Here two cases are considered: (1) a numerical finite-difference model of a single U-pipe in a sandy aquifer, implemented using the MT3DMS code, where the grout material surrounding the Borehole Heat Exchanger is not simulated and (2) the same numerical model where, in turn, the presence of the grout is accounted for in a small volume around the Borehole Heat Exchanger, as in real in situ conditions. Simulations were carried out first keeping the heat rate constant, as when simulating a Thermal Response Test, and then maintaining the inlet temperature constant in order to reproduce the yearly operation of the Ground Source Heat Pump system. Then, fitting the numerical ground response with analytical solutions and comparing the two models, the effect of the grout material was assessed in terms of exchanged energy and temperature distribution in the subsoil. Results show that the presence of the grout around the Borehole Heat Exchanger is negligible from the exchanged energy and temperature distribution point of view. Whereas, looking at the Darcy velocities it was noticed that the ones returned by the analytical solutions (that necessarily neglects the presence of the grout) are at least 30% smaller than the velocities implemented in the grouted model. So when comparing the MT3D simulations, or using MT3D for TRT interpretation, the presence of the grout around the BHE can at a first approximation be disregarded in terms of both exchanged energies and temperature distribution in the subsoil. In turn when Darcy velocity, obtained by Thermal Response Test interpretation through analytical solutions (MLS), is used in a numerical model it must be corrected specially for advection-dominated cases.