B and Li isotopes as intrinsic tracers for injection tests in aquifer storage and recovery systems

Boron and Li isotopes have been tested as environmental tracers of treated sewage injected into a sandy aquifer (Shafdan reclamation project, Israel). During a 38 days injection test in a newly dug injection well, a conservative artificial tracer (Br⁻) was monitored together with δ¹¹B and δ⁷Li in the injectate, in the unsaturated soil zone (porous cup) and an observation well in the aquifer. In spite of B and Li concentrations in the injectate close to background values, significant shifts of the isotope signatures could be observed over the duration of the injection test. Boron isotope ratios show a breakthrough curve delayed with respect to Br⁻ breakthrough due to some reversible sorption on the aquifer material. No isotope fractionation was observed in the unsaturated or the saturated zone so that B isotopes can be considered as conservative in the investigated part of the aquifer system. Lithium isotopes are strongly fractionated, probably due to sorption processes. Lithium concentrations point to a Li sink in the system, δ⁷Li values vary strongly with a tendency of ⁷Li depletion in the liquid phase over the duration of the experiment. This is opposite to the expected preferential sorption of ⁶Li onto clay minerals. Boron isotopes reveals a valuable tracer of artificial recharge of freshwaters derived from treated sewage, both for short term tracer tests and for long-term monitoring of artificial recharge, even if in aquifers with higher clay contents, sorption-linked isotope fractionation cannot be excluded. More data are needed on Li isotope fractionation in natural groundwater systems to assess the potential of this tracer as monitoring tool.     

Validation of a kinematic and parametric approach to calculating intensity scenarios

We employ our semi-empirical kinematic model for shear body waves (KF) [Sirovich L. A simple algorithm for tracing out synthetic isoseismals. Bull Seism Soc Am 1996;86(4):1019–27; Sirovich L. Synthetic isoseismals of three earthquakes in California-Nevada. Soil Dyn Earthquake Eng 1997;16:353–62] to back-predict and then validate the regional intensity scenario of a destructive earthquake (Loma Prieta, California, M_s 7.1, October 17, 1989). Only the pre-1988 geological and seismotectonic knowledge was used to set the 11 source parameters of KF (in this sense, our procedure was deterministic). Then, the ranges of the pre-1988 uncertainties were explored by grid search and the parametric combination produced 59,049 sources. The quality of our prediction was measured using the field intensities of 1989 by the US Geological Survey (in the MMI scale). The squared scenario residuals are: 73 for the mean KF scenario and 123 for the best available empirical attenuation of MMI intensity. We stress that, before using KF in the forward mode, its automatic inverse application has already been validated by refinding a series of earthquake sources [Gentile F, Pettenati F, Sirovich L. Validation of the automatic nonlinear source inversion of the US geological survey intensities of the Whittier Narrows, 1987 Earthquake. Bull Seism Soc Am 2004;94(5):1737–47; Pettenati F, Sirovich L. Intensity-based source inversion of three destructive California earthquakes. Bull Seism Soc Am 2007;97(5):1587–606; Sirovich L, Pettenati F. Source inversion of intensity patterns of earthquakes: a destructive shock in 1936 in northeast Italy. J Geophys Res 2004;109:B10309, doi:10.1029/2003JB002919:1–16]. If our technique had been available at the time, the 1989 pattern of damage south of San Francisco would have been conservatively foreseen even from 1983 on.     

Test of Source-Parameter Inversion of Intensity Data

We demonstrate that the approximate source kinematics of the San Fernando, 1971 earthquake can be back-predicted by analysing its macroseismic intensity data set (felt reports) objectively and quantitatively. This is done by inverting either the data set of the intensity values observed in all sites, or the intensities tessellated with the Voronoi polygons technique. It is shown that the kinematic characteristics found following our method (epicentral coordinates, source depth, seismic moment, rupture length, Mach number, fault plane solution) match those determined by other authors, via instrumental measurements, rather well. The prerequisite for obtaining these results is that local amplification must not affect groups of neighboring sites. It was possible to invert the U.S.G.S. ``felt reports' for the source because this data set is sufficiently uncontaminated by local site responses, and retains relevant regional traces of source effects. Isoseismal maps cannot be safely used for this task, because qualitative drawing criteria give subjective results. Isoseismals, based on incomplete space frequency samplings, give rise to spurious effects, whereas the Voronoi polygons produce easy-to-grasp, quantitative and objective, representations of macroseismic intensity data. The tests performed, up to now on a series of earthquakes, suggest that the combined use of tessellation and of our KF model is promising mostly for inverting intensities of preinstrumental earthquakes.     

Capillary geochemistry in non-saturated zone of soils. Water content and geochemical signatures

The unsaturated zone (UZ) retains aqueous solutions against gravity by capillary forces. This suction state corresponds to a decreasing internal pressure of the water, which modifies its thermodynamic properties. Accordingly, the speciation of solutes and the solubility of solids and gases in such capillary solutions change. The volumetric capillary water content of the soil at high suction can be calculated extrapolating the water retention curves (WRC) with the Rossi–Nimmo model. Interestingly, several tens of liters per cubic meter of soil can be thus suctioned, a sufficiently large volume to support that: (1) capillary water is not restricted to nanosized pores, which means it disobeys the Young–Laplace law and is metastable with respect to vapor (superheating); and (2) the geochemistry of capillary solutions might significantly influence the subsurface mass transfer. Two field situations are here interpreted using the capillary thermodynamic properties: (1) the trapping of sand grains during the growth of desert roses (gypsum), and (2) the development of abnormal paragenetic sequences in some saprolites.The capillary approach is extended to the soil solids, so that the micro-mineralogy can be explicitly (though sketchily) integrated in the calculations. The key conclusion is that capillarity changes the saturation indexes (and so the reaction rates) at given solution composition, in a way consistent with the field observations. This perspective amounts to geochemically distinguishing the capillary and percolating solutions, which is interestingly analogous to the immobile and mobile water distinction already often integrated in UZ flow models.     

Investigation of recharge dynamics and flow paths in a fractured crystalline aquifer in semi-arid India using borehole logs: implications for managed aquifer recharge

The recharge flow paths in a typical weathered hard-rock aquifer in a semi-arid area of southern India were investigated in relation to structures associated with a managed aquifer recharge (MAR) scheme. Despite the large number of MAR structures, the mechanisms of recharge in their vicinity are still unclear. The study uses a percolation tank as a tool to identify the input signal of the recharge and uses multiple measurements (piezometric time series, electrical conductivity profiles in boreholes) compared against heat-pulse flowmeter measurements and geochemical data (major ions and stable isotopes) to examine recharge flow paths. The recharge process is a combination of diffuse piston flow and preferential flow paths. Direct vertical percolation appears to be very limited, in contradiction to the conceptual model generally admitted where vertical flow through saprolite is considered as the main recharge process. The horizontal component of the flow leads to a strong geochemical stratification of the water column. The complex recharge pattern, presented in a conceptual model, leads to varied impacts on groundwater quality and availability in both time and space, inducing strong implications for water management, water quality evolution, MAR monitoring and longer-term socio-economic costs.     

Seismotectonic outline of South-Eastern Sicily: an evaluation of available options for the earthquake fault rupture scenario

Seismotectonic information and interpretations available for SE Sicily suggest three groups of possible sources for the M=7.1-7.5 mainshock of 1693 and its strong foreshock: (1) normal faults belonging to the Ibleo Maltese Escarpment (also: Malta Escarpment); (2) normal faults associated with the two adjacent Simeto and Scordia-Lentini structures; (3) a transfer structure between the Sicily Straits rift system and the two grabens to the north. We use a new kinematic model to invert the data sets of macroseismic intensities of the two earthquakes to retrieve information on their sources. For this, we invert point observations, or intensities tessellated with the Voronoi polygons technique, and treat residuals of inversion in the matrix of points, or in the tessellated plane. Our inversions of the regional intensity patterns using this technique show that family N°3 is a good candidate for the foreshock of 9 January, 1693. For the mainshock of 11 January, 1693, an almost perfect synthesis of its intensity IX area was obtained with our model and a source belonging to family N°3. However, all information considered (tsunami included), this earthquake could have been produced either by (3) or by a fault located along the Ibleo-Maltese Escarpment, and tangential to the Augusta and Siracusa promontories.