Groundwater flow system under a rapidly urbanizing coastal city as determined by hydrogeochemistry

In the Jakarta area (Indonesia), excessive groundwater pumping due to the rapidly increasing population has caused groundwater-related problems such as brackish water contamination in coastal areas and land subsidence. In this study, we adopted multiple hydrogeochemical techniques to demonstrate the groundwater flow system in the Jakarta area. Although almost all groundwater existing in the Jakarta basin is recharged at similar elevations, the water quality and residence time demonstrates a clear difference between the shallow and deep aquifers. Due to the rapid decrease in the groundwater potential in urban areas, we found that the seawater intrusion and the shallow and deep groundwaters are mixing, a conclusion confirmed by major ions, Br^?:Cl^? ratios, and chlorofluorocarbon (CFC)-12 analysis. Spring water and groundwater samples collected from the southern mountainside area show younger age characteristics with high concentrations of ¹⁴C and Ca–HCO₃ type water chemistry. We estimated the residence times of these groundwaters within 45 years under piston flow conditions by tritium analysis. Also, these groundwater ages can be limited to 20–30 years with piston flow evaluated by CFCs. Moreover, due to the magnitude of the CFC-12 concentration, we can use a pseudo age indicator in this field study, because we found a positive correlation between the major type of water chemistry and the CFC-12 concentration.     

Prediction of ground motion in the Osaka sedimentary basin associated with the hypothetical Nankai earthquake

We studied the long-period ground motions in the Osaka sedimentary basin, Japan, which contains a 1- to 3-km thickness of sediments and is the site of many buildings or construction structures with long-natural period. We simulated the broadband ground motions likely to be produced by the hypothetical Nankai earthquake: the earthquake expected to give rise to the most severe long-period ground motion within the basin. For the simulation, we constructed multiscale heterogeneous source models based on the Central Disaster Management Council of Japan (CDMC) source model and adopted a hybrid computation method in which long-period motion and short-period motion are computed using a 3-D finite difference method and the stochastic Green’s function method, respectively. In computing long-period motions, we used a 3-D structure model of the crust and the Osaka sedimentary basin. The ground motions are estimated to have peak velocities of 50–90 cm/s, prolonged durations exceeding 300 s, and long predominant periods of 5–10 s in the area with great thickness of sediments. The predominant periods are in agreement with an approximate evaluation by 4 H/V _s where H and V _s are the thickness of the sediment and the average S wave velocity, respectively.     

Earthquake nests

Summary. Making use of extensive observations of micro-earthquakes available in the Tokyo region, Usami & Watanabe constructed a simple but effective method of exhibiting those regions where small earthquakes have clustered during a 4.5 yr period. Such clusters are defined by contour surfaces, and for convenience are termed earthquake nests .
If we accept the assumption that the magnitude of an earthquake is directly related to the volume previously undergoing intensified strain, and if we regard the nests as measures of such volumes, we can make rough estimates of the magnitudes of potential earthquakes in different parts of the Tokyo region. This method of assigning seismicity can be one way of calculating local risk. Repeated investigations can also detect changes in the tress field.     

Influence of Salinity on pH and Aluminum Concentration on the Interaction of Acidic Red Soil with Seawater

Contamination of acidic red soil in the coastal areas of Okinawa Islands is a serious environmental problem. This study was conducted to examine the effects of the salinity on pH and aluminum concentration when the acidic red soil interacts with seawater. Acidic red soil from Gushikawa recreation center was fractionated into bulk soil, coarse sand and silt + clay. Different weights of each fraction were equilibrated with seawater solutions. The pH and concentrations of Al³⁺, Na⁺, K⁺, Ca²⁺ and Mg²⁺ were then analyzed in the extracts. The results showed a decreasing trend of pH with increasing soil to solution ratio while the extracted Al³⁺ revealed an increasing trend. The lowest pH values were 3.85, 4.06, 4.41, 4.66 and their corresponding highest Al³⁺ concentrations were 2.50, 1.01, 0.062 and 0.036 mmolL⁻¹ in the seawater extracts, one-tenth seawater extracts, one-hundredth seawater extracts and one-thousandth seawater solution extracts, respectively. Mostly, the concentrations of Na⁺, Ca²⁺, Mg²⁺ and especially K⁺ decreased with increasing soil weight in the high salinities but showed the opposite trend in the low salinity samples. Potassium concentration decreased by 39%, 53% and 40% in the seawater extracts, one-tenth and one-hundredth seawater extracts but increased by 200% in one-thousandth seawater extracts. The coincidence of the increase in Al³⁺ and H⁺ concentrations, and the decrease of Na⁺, K⁺, Ca²⁺ and Mg²⁺ concentrations in the solutions suggests ion exchange/adsorption, while the increased patterns, particularly at low salinity could be attributed to the dissolution of the species from the soils.     

Distribution of larvae ofPinnixa rathbuni Sakai (Decapoda: pinnotheridae) in Ise Bay and its neighbouring coastal waters,central Japan part 3. Aggregation of the benthic adult population with reference to the spatial distribution of the planktonic larvae

According to studies made to date, the planktonic larvae ofPinnixa rathbuni, especially the early zoeal stages, have been found mainly in the eastern part of Ise Bay (Pacific coast of central Japan), while the benthic adult population is restricted to the southwestern part of the bay. The present study was made to obtain a better understanding of the origin of the dense population of early zoea found along the coast of the Chita Peninsula in the eastern part of the bay. Field surveys made with a Smith-McIntyre grab from May to November 1981 (every two months) ruled out the possibility that the benthic adult crabs in the southwestern part of the bay migrated to the eastern part of the bay to release their larvae, or that a substantial population of the crabs occurred in the eastern part of the bay. Thus, the larvae may be derived wholly from the benthic adult populations localized in the southwestern part of the bay. The spatial distribution of the early larvae can be explained by assuming that the larvae, released by the benthic adult population in the southwestern part of the bay, are rapidly transported toward the Chita Peninsula and form a dense larval population along the coast in the eastern part of the bay.     

Temporal Variability of High Vertical Wavenumber Shear over the Izu-Ogasawara Ridge

Recent numerical studies (Hibiya et al., 1996, 1998, 2002) showed that the energy cascade across the internal wave spectrum down to small dissipation scales was under strong control of parametric subharmonic instabilities (PSI) which transfer energy from low vertical mode double-inertial frequency internal waves to high vertical mode near-inertial internal waves. To see whether or not the numerically-predicted energy cascade process is actually dominant in the real deep ocean, we examine the temporal variability of vertical profiles of horizontal velocity observed by deploying a number of expendable current profilers (XCPs) at one location near the Izu-Ogasawara Ridge. By calculating EOFs, we find the observed velocity profiles are dominated by low mode semidiurnal (∼double-inertial frequency) internal tides and high mode near-inertial internal waves. Furthermore, we find that the WKB-stretched vertical scales of the near-inertial current shear are about 250 sm and 100 sm. The observed features are reasonably explained if the energy cascade down to small dissipation scales is dominated by PSI.     

A laboratory study of formative conditions for characteristic ripple patterns associated with a change in wave conditions

A wave‐?ume experiment was conducted to examine the formative condition for three types of distinctive bedforms that emerged through deformation of existing ripples due to waning wave power. They were ripple marks with: (1) a single secondary crest, (2) double secondary crests, and (3) a rounded crest. Data were analysed using two parameters, kh and d₀/λ_*, where k is the wave number, h is the water depth, d₀ is the near‐bottom orbital diameter, and λ_* is the spacing of existing ripples. The former quantity, kh, was employed as a surrogate for the degree of ?ow asymmetry. The result showed that ripples with secondary crests developed under a rather symmetrical ?ow ?eld with kh ? 0·7, if d₀/λ_* ? 1·2, whereas rounded‐crest ripples emerged under asymmetrical ?ow ?eld with kh ? 0·7, if d₀/λ_* ? ?2·9 kh + 3·2. The number of secondary crests, which initially occurred in each trough, was single if d₀/λ_* ? 0·8, or double if d₀/λ_* ? 0·8. Copyright © 2004 John Wiley & Sons, Ltd.