Analysis of Hydrogeochemical Characteristics of Tunnel Groundwater Based on Multivariate Statistical Technology

Following tunnel excavation, which is influenced by hydraulic fracturing and geological structure, a series of hydrochemical reactions occur in the karst aquifer, which has a significant impact on groundwater hydrology and the earth process. Based on five sets of 38 samples collected in the Tongzi Tunnel in 2020 and 2021, the main geochemical processes and water quality conditions of the karst aquifer system during tunnel construction were revealed by multivariate statistical analysis and graphical methods. The results showed that water-rock action is the main mechanism controlling groundwater chemistry in the study area; HCO3-, Ca2+, and Mg2+ are associated with the widely distributed carbonate rocks in the study area. SO42- is derived from gypsum and sulfate rocks and special strata, which are another important source of Ca2+. Sodium-containing silicates and reverse cation exchange as the causal mechanisms of Na+ whereas F- is derived from fluorite. According to the mineral saturation index calculations, the dissolution and precipitation of minerals such as alum, gypsum, calcite, dolomite, and salt rock have an important influence on the main chemical components in water. The 38 samples were subjected to cluster analysis, and the results could be classified into seven categories. The representative clusters 1, 3, and 5 were selected for principal component analysis. Clusters 1 and 5 of groundwater represent weathering, dissolution, and ion exchange of carbonate and sulfate rocks and are closely related to the lithologic limestone, limestone intercalated with carbonaceous mudstone, carbonaceous mudstone, and coal-measure strata in the aquifer. Cluster 3 is dominated by upper surface river water and characterizes the geochemistry in natural water bodies dominated by the dissolution of carbonate, sulfate, and salt rocks. Finally, groundwater quality is mostly found in Class IV, with NO3- and F- being the main contaminants in the water.

Simulation of solute transport behaviors in saturated karst aquifer system

The karst development makes aquifer have strong anisotropy and heterogeneity. In order to reveal the characteristics of solute transport in the karst fissure–conduit aquifer system, this study presents a physical model of fissure–conduit in laboratory experiments to carry out the solute transport simulation. In this paper, the tracer tests of fissure–conduit combination, fissure, and conduit solute transport process in saturated flow are designed. We found that different aquifer structures and tracer injection points have an influence on the shape of the breakthrough curve. Besides, the two-dimensional dispersion model of tracer injection of the instantaneous point was used to calculate the dispersion parameters of each group of experiments. Then, the dynamic responses of the linear distance (x) between the injection point and the receiving point, initial time (t0), peak time (tm), peak concentration (cm), average tracer transport velocity (V), and porosity (p) of aqueous media to the longitudinal dispersion coefficient are discussed. In addition, according to the measured data, Gaussian multi-peak fitting can be used to reflect the overall shape and change trend of the multi-peak BTC. These results demonstrate the solute transport behaviors in the saturated karst aquifer system, which have important reference significance for solving the engineering environmental problems in the karst area.

Karst springs in small islands: The Kamari spring (Mylopotamos) in Kythira Island, Greece

<p>Three main aquifer systems developed on Kythira Island (Greece) include (Pagounis, 1981; Pagounis & Gertsos, 1984, Danamos, 1991; Koumantakis et al., 2006; Filis et al., 2019):</p><ul><li>The porous aquifer system in Neogene and Quaternary formations.</li> <li>The karst aquifer system in the carbonate formations of the Pindos and Tripolis Units.</li> <li>The aquifer system (both shallow and deep) in the fractured hard rocks mainly of the Phyllites – Quartzites Unit.</li> </ul><p>The main discharge of the aquifer systems takes place in coastal and submarine brackish springs around the island, except for its northern part where the Phyllites – Quartzites Unit outcrops and its central part where springs of small capacity discharge the carbonate formations of the Pindos Unit.</p><p>Precipitation is the direct recharge of the three aforementioned aquifer systems while indirectly lateral discharge occurs in places between adjacent and tangential aquifer systems and from the streams runoff as well.</p><p>In the area of Mylopotamos village four springs discharge the karst aquifer of the Pindos Unit within the channel of Kako Laghadi stream forming downstream the known “Neraida or Fonissa waterfall”. Moreover, along the dell of Kako Laghadi stream 22 watermills were built, among the plane trees and the ivy.</p><p>The most significant of the aforementioned springs is the Kamari spring (+282.28 meters a.s.l.) which emerge at the thrust fault between the overlying permeable carbonates and the underlying impermeable flysch formation of the Pindos Unit. The discharge of the Kamari spring presents annual fluctuation which varies from app. 45-50 m<sup>3</sup>/h (during winter) to total recession (during summer), due to restriction of the precipitation and the prolonged drought and overpumping of its recharge area mainly with boreholes.</p><p>The inactive municipal borehole of Mylopotamos village (+299.15 meters a.s.l.) is located app. 310 meters SSE of the Kamari spring within its recharge area (karst aquifer of the Pindos Unit). This borehole of a total depth of 40 meters penetrates carbonates of the Pindos Unit which thickness exceeds 100 meters in that area. Monthly measurements of the Kamari spring discharge and the water table head in the inactive borehole demonstrate clear and direct hydraulic correlation between them. The Kamari spring presents outflow only in the case when the water level head of the borehole exceeds +282.28 meters. This means that the water level head in the borehole should not exceed 16.87 meters from the earth surface. Taking into account all the aforementioned, the Kamari spring is designated as an overflow spring.</p><p>Finally, microbiological analysis from the Kamari spring showed qualitative degradation, due to human activities in the wider area (Pagounis, 1981; Filis et al., 2019).</p>