Water-table and discharge changes associated with the 2016–2017 seismic sequence in central Italy: hydrogeological data and a conceptual model for fractured carbonate aquifers
Marco PetittaLucia MastrorilloElisabetta PreziosiFrancesca BanzatoMarino Domenico BarberioAndrea BilliCostanza CambiGaetano De LucaGiuseppe Di CarloDiego Di CurzioCristina Di SalvoTorquato NanniStefano PalpacelliSergio RusiMichele SaroliMarco TalliniAlberto TazioliDaniela ValigiPaola VivaldaCarlo Doglioni
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Abstract:
A seismic sequence in central Italy from August 2016 to January 2017 affected groundwater dynamics in fractured carbonate aquifers. Changes in spring discharge, water-table position, and streamflow were recorded for several months following nine Mw 5.0–6.5 seismic events. Data from 22 measurement sites, located within 100 km of the epicentral zones, were analyzed. The intensity of the induced changes were correlated with seismic magnitude and distance to epicenters. The additional post-seismic discharge from rivers and springs was found to be higher than 9 m3/s, totaling more than 0.1 km3 of groundwater release over 6 months. This huge and unexpected contribution increased streamflow in narrow mountainous valleys to previously unmeasured peak values. Analogously to the L'Aquila 2009 post-earthquake phenomenon, these hydrogeological changes might reflect an increase of bulk hydraulic conductivity at the aquifer scale, which would increase hydraulic heads in the discharge zones and lower them in some recharge areas. The observed changes may also be partly due to other mechanisms, such as shaking and/or squeezing effects related to intense subsidence in the core of the affected area, where effects had maximum extent, or breaching of hydraulic barriers.Keywords:
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Abstract The vertical transport of contaminants from source areas is employed in many risk assessment models and screening tools in order to estimate the contaminant mass discharge (CMD) into underlying aquifers. The key parameters for estimating CMD are the contaminant source area and concentration, and the vertical water flux, the latter of which depends on the vertical hydraulic conductivity and the vertical hydraulic gradient in the subsurface. This study focuses on advancing the use of the combined membrane interface probe hydraulic profiling tool (MiHPT) to investigate the vertical hydraulic gradient across a clay till overlying a sandy aquifer at a contaminated site in Denmark. Only the HPT is necessary for the estimate of vertical hydraulic gradient. The hydraulic head, clay till thickness, and resulting vertical hydraulic gradients found using the MiHPT compared well with observations from nearby nested wells. The parameter with the largest discrepancy was the thickness of the clay till. The advantage of the MiHPT is its relatively quick depth discrete access to information regarding subsurface permeability, vertical hydraulic gradients and contaminant distribution across a site. In this case study, performance of additional dissipationtests during the HPT log to acquire determination of the vertical hydraulic gradient increased the cost by 3% compared to standard HPT logs.
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A practical new field method is proposed to evaluate hydraulic conductivity in partially saturated media near a water impoundment. The new method is an inverse procedure which utilizes a flow net constructed from the steady state total hydraulic head distribution around the water source. In the vicinity of constant head sources and downstream along any stream tube, wetness and conductivity generally decrease. Knowing the stream tube geometry and hydraulic gradient from the flow net, Darcy's law is used to determine unsaturated hydraulic conductivities within the stream tube relative to some segment of the stream tube where conductivity is known. This approach was used successfully to predict the unsaturated hydraulic conductivity which was input to two variably saturated numerical models, utilizing total hydraulic head fields generated by the models. This procedure is also applied to pressure head and water content data collected in the field surrounding a constant head borehole infiltration test originally designed to determine only saturated hydraulic conductivity above the water table. For practical purposes the new procedure compares very favorably with (1) results of a field experiment to obtain unsaturated hydraulic conductivity in situ using the instantaneous profile method and (2) values of unsaturated conductivity calculated from field and laboratory measurements of water content and pressure head.
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A novel direct-push (DP) probe was developed for in situ hydraulic conductivity measurements in unconfined sandy aquifers. It is a small-diameter tool with an intake screen and a built-in pressure transducer, and is directly pushed into the ground. Hydraulic conductivity k is estimated by pumping from an aquifer through the intake screen and, while doing so, measuring flow rate and water pressure. This paper describes first the DP system and then the procedures for estimating k values. The finite-element method was employed to obtain intake factors that correlate the measurements with the k values, and these intake factors were compared with those proposed for conventional borehole packer tests. Laboratory and field experiments were also performed to assess the usability of the DP equipment. The results showed that the k estimates obtained from the new probe agreed with those obtained by other methods. The results of this assessment indicate that the proposed DP technique is a promising tool for simple and rapid in situ measurements of hydraulic conductivity. Limitations to the technology are discussed, and further work is suggested.
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