Lakes and Groundwater of the Rieti Basin, Central Italy: Hydrochemistry, Paleolimnology, and Seismologic Influences
AuthorArcher, Carey Claire
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A combination of water and sediment core samples were recovered and analyzed to study the hydrochemistry and paleolimnology of two lakes in the Rieti Basin and adjacent groundwater springs. Chemical and stable isotopic tracers were used to characterize water samples and to examine downcore changes in sediment cores. The lakes, Lungo and Ripasottile (LUN and RIP) have been described as surface outcroppings of the groundwater table, yet the surface-groundwater interactions have not previously been investigated in detail. High discharge springs representing local and regional aquifers were sampled as a means of comparing and evaluating the chemical data from the lakes, including Vicenna Riara (VIC), an alluvial aquifer which has hydrochemistry similar to LUN, and Peschiera (PES), which flows from a major regional carbonate aquifer. Results from the modern study suggest that LUN is in connection with the alluvial aquifer of the basin, and that RIP receives substantial input from Santa Susanna Spring (SUS), which is sourced from a regional carbonate aquifer. SUS has a characteristic chemistry with high SO42-, HCO3-, Mg2+, and Ca2+concentrations. This work also shows that in the modern configuration, RIP has a shorter water retention time, an important consideration for the paleolimnological portion of this work. The seismic sequence of 2016-2017 provided an opportunity to study the hydrochemical response of groundwater springs located within a 40-60km region of the epicenters of the main shocks (>6.0 Mw). The springs considered included 3 which had been previously sampled during the modern study; PES, VIC, and SUS. A fourth spring < 5km from the epicenter of the October 26th and 30th 2016 main shocks, Nerea (NER), was added. Both SUS and PES exhibited transient increases in electrical conductivity, alkalinity, and trace metal concentrations immediately after the Aug. 24th mainshock. The Nerea spring had a similar response to the Oct. 30th mainshock. The mechanism proposed for the observed increases in elemental concentrations is increased co-seismic pore pressure that cleared faults, pore spaces, and/or long-residence time conduits containing high concentrations of dissolved constituents. Subsequent mainshocks elicited less chemical response after these fluids had already been cleared. The post-seismic enrichment in the stable isotopic composition of the dissolved inorganic carbon (δ13CDIC) may also indicate greater fluid-rock interaction, or alternatively may suggest a second mechanism a play; a potential input of deep-sourced CO2 upwelled along conduits provided by fault movement. All physiochemical and trace element parameters decreased to pre-earthquake values over the weeks-months following the mainshocks. These findings, along with reports of strong ground shaking during mainshocks and aftershocks in the Rieti Basin, led to a re-examination of sediment cores recovered from LUN and RIP within the context of paleoseismicity and the potential for these lakes to record past earthquakes. Event layers that occurred coevally in both lakes were identified according to a compilation of seismic signatures from past studies. After application of the age models and historically documented major (Mw>6.0) earthquakes within 40 km of the lakes, four seismites were proposed with distinct sedimentological and geochemical compositions. The common feature of seismites attributed to the 1298, 1349, and 1703 earthquakes was a homogenite formed either by a rapid influx of groundwater at the sediment-water interface or strong ground shaking causing sediment and pore water mixing and resuspension. All events, including one identified in 1639, contained a geochemical anomaly as well, namely the elements that were identified in Ch. 1 as indicative of an input of regional groundwater with high sulfate. Complications with earthquake attribution increased during the modern period as human landscape alteration and lake eutrophication may have overprinted earthquake signals. The variable lake level and extent of LUN and RIP through time serve as a backdrop for these and all paleoenvironmental interpretations, so another study was conducted to focus specifically on hydrological regime evolution over the past ~2000 years. The carbon of the inorganic and organic fractions from both sediment cores were studied, emphasizing major transitions at stratigraphic zones. Carbon cycling, evidenced by stable isotopes of carbon in organic matter and carbon and oxygen in carbonate, functioned differently during each historical period and in LUN versus RIP. Though historical records provide some information on lake extent and flooding, proved a useful proxy in discerning water depth, potential sources of inflows and outflows and extent of connection between the two lakes as well as surrounding marsh.