Geochemical processes and solute transport at the seawater/freshwater interface of a sandy aquifer
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The authors are to be complimented on an interesting case history presenting an original method of solving the commonly occurring problem of development of excess pore pressures, when driving piles in soft clays. In Fig. 5, the authors present the measured pore pressures and number of piles driven on specific days. This figure indicates that the developed pore pressures were considerably smaller after the installation of the Geodrain on the piles, as compared with the measurements before the drain application. However, as the magnitude of the measured pore pressures is less dependent on the number of piles driven per day and more on the distance from the piles to the piezometers, a different plotting of the results gives a better basis for the conclusions drawn by the authors. In Fig. 1, the writer has plotted £or each driven pile the distance against time to two piezometers, numbers P-1 and P-3. A black dot shows the distance (and date) of a pile relative to piezometer number P-1 and an open dot that of the same pile relative to piezometer number P-3. The lower diagram in Fig. 1 shows the piezometric elevations measured in piezometer numbers P-1 and P-3. A study of the measurements taken before
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Abstract Data from a large‐scale canal‐drawdown test were used to estimate the specific yield (s) of the Biscayne Aquifer, an uncon‐fined limestone aquifer in southeast Florida. The drawdown test involved dropping the water level in a canal by about 30 cm and monitoring the response of hydraulic head in the surrounding aquifer. Specific yield was determined by analyzing data from the unsteady portion of the drawdown test using an analytical stream‐aquifer interaction model (Zlotnik and Huang 1999). Specific yield values computed from drawdown at individual piezometers ranged from 0.050 to 0.57, most likely indicating heterogeneity of specific yield within the aquifer (small‐scale variation in hydraulic conductivity may also have contributed to the differences in s y among piezometers). A value of 0.15 (our best estimate) was computed based on all drawdown data from all piezometers. We incorporated our best estimate of specific yield into a large‐scale two‐dimensional numerical MODFLOW‐based ground water flow model and made predictions of head during a 183–day period at four wells located 337 to 2546 m from the canal. We found good agreement between observed and predicted heads, indicating our estimate of specific yield is representative of the large portion of the Biscayne Aquifer studied here. This work represents a practical and novel approach to the determination of a key hydrogeological parameter (the storage parameter needed for simulation and calculation of transient unconfined ground water flow), at a large spatial scale (a common scale for water resource modeling), for a highly transmissive limestone aquifer (in which execution of a traditional pump test would be impractical and would likely yield ambiguous results). Accurate estimates of specific yield and other hydrogeological parameters are critical for management of water supply, Everglades environmental restoration, flood control, and other issues related to the ground water hydrology of the Biscayne Aquifer.
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The Vipava Valley is a unique region in south-western Slovenia. In addition to surface water, groundwater is also important, although it is hidden from the eye. The paper presents an analysis of groundwater levels in piezometers in the Vipava Valley. The analysis was performed on 10 piezometers, which are still operating today, and includes a display of levels and basic statistics, correlations, the impact of distance from the Vipava riverbed, trends, and seasonality. According to the groundwater level, piezometers in the Vipava Valley can be divided into three groups. The first group with the highest levels includes piezometers Gradišče, Vipavski Križ, and Ajdovščina, the second group piezometers Prvačina, Šempeter, Volčja Draga, Renče, and Vrtojba, and the third group with the lowest groundwater levels includes the piezometers Miren and Orehovlje. The results of the analyses showed good or bad connections between groundwater levels in piezometers, as well as between groundwater levels and the Vipava River water level at various gauging stations. The fluctuation of the groundwater level is conditioned by the distance from the Vipava riverbed and the area’s geological or tectonic structure. An unambiguous trend of groundwater levels cannot be determined. The seasonality of groundwater level fluctuations is not pronounced, but the highest values of groundwater levels occur in autumn and winter, and the lowest in summer.
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The natural pore pressure response of a shallow aquifer/aquitard system can be used to assess aquitard continuity. This is the first paper to examine aquitard continuity at the field scale based on the barometric head response function (BHRF) obtained from multiple-regression analysis. The pore pressure measurements made during winter provided more representative BHRFs for the bounding aquifers and the aquitard. Distinct BHRFs were found for the shallow unconfined aquifer, clay aquitard and confined aquifer. This implies how the clay aquitard isolates the underlying confined aquifer from the shallow unconfined aquifer. It is found that the BHRFs for intact clay formation are delayed and differ from those for the fractured zone and confined aquifer. For the fractured zone, the observed BHRF quickly reached equilibrium. The BHRFs for intact clay and fractured zone equilibrated at high BHRF values. For the confined aquifer, the BHRF was low and constant. Numerical simulations provided a hydraulic conductivity below 1 × 10 −8 m/s for the observed BHRF. Fast Fourier transform showed that the pore pressure components with a period smaller than one cycle per day were fully explained by barometric pressure fluctuations. Furthermore, the response of a deep piezometer to the moisture loadings provided an opportunity to estimate that the annual snowfall, for the study area, lost 25–35% of its weight.
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The compressibility of calcite to 40 kbar has been remeasured by using a piston-cylinder apparatus. Calcite 1 is found to transform to calcite 2 at 14.5 kbar with a volume change of 0.00483 cm3/g, and calcite 2 is found to change to calcite 3 at 17.4 kbar with a volume change of 0.01291 cm3/g. The volume compression data for the three phases are described by the following quadratic relations: Calcite 1 Calcite 2 Calcite 3 where P is pressure in kilobars. The compression data for calcite 1 and calcite 3 are in good agreement with those available in the literature. The data exhibiting an abnormally high compression of calcite 2 have been reported for the first time. The compression data for calcite 2 have been used to explain quantitatively the abnormal drop near 15 kbar observed in the ultrasonic sound velocity in calcite.
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