Two models to analyze freshwater lenses in coral atolls are presented first; a freshwatersaltwater interface model and a dispersion model. In the freshwatersaltwater interface model, especially when the horizontal hydraulic conductivity varies exponentially with the horizontal coordinate, the analytical solution can be obtained. In the dispersion model, the horizontal and vertical dispersion coefficients are treated as a function of the hydraulic conductivity respectively, determined with experimental and field data. The estimated values of natural replenishment in the dispersion model were approximately 63∼78% of those in the freshwater-saltwater interface model at the New Zealand Airfield district of Christmas Island in the Republic of Kiribati. Secondly groundwater environment in Gogo Isla n d in Japan is investigated through simulations of seawater intrusion into the confined aquifer at the Yura district and the phreatic aquifer at the Kitaura district. The ratios of the horizontal dispersion coefficient to hydraulic conductivity obtained at both districts and from other experimental and field data are proportional to the thickness of the aquifer, and the proportional constant is as large as 10-3. A hydrologic cycle was simulated at the Kitaura district, and the outflow amount of groundwater into the sea was approximately 9% of annual precipitation in Matsuyama City in 1985. Key words: small island, coral atoll, freshwater lens, freshwater-saltwater interface model, dispersion model
The drainage basins in the Akiyoshi-dai Plateau limestone area (Yamaguchi Prefecture, Western Japan) are divided into six areas, on the basis of the topographic divide and the existence of two groundwater systems: Hirotani (where Akiyoshi-do Cave is located), Kanoide, Kuroiwa, Ono, Narutaki and Sowa. In this limestone area, groundwater flows out from 4 springs and 5 caves at the foot of the plateau. The origin of the dissolved components in the groundwater was estimated by using the selfconsistent least squares method proposed by Tsurumi (1982). The contents of the dissolved components for all spring waters could be explained only by the mixing of three source waters: one characterized by a high contribution of limestone components, another by that of non-carbonate rock components, and the other by calcium sulfate probably originating from fallout and sulfur-oxidation in the soil in the plateau. The drainage basin of Akiyoshi-do Cave, located in Hirotani Polje, occupies about a half of Akiyoshi-dai Plateau, where the annual mean precipitation is 1,974mm, and the average run-off from Akiyoshi-do Cave is. estimated to be 955mm. The calcium concentration of the baseflow showed seasonal fluctuations, which were followed by changes in CO2 partial pressure in the soil. Measured soil CO2varied from a minimum of 0.15 % at a soil temperature of 7.2°C, and a maximum of 1.7% at 19.0°C in the humus soil, whereas in the meadows which cover most of the area, it ranged from 0.08% at 3.8°C to 1.2% at 20.8°C. The calcium concentration in groundwater issuing from Akiyoshi-da i Plateau is controlled by water-limestone dissolution equilibrium, under open system conditions depending on the meadow's soil CO2 contents.
In this study, to clarify purifying effect of water quality in paddy fields, small lysimeters packed with the paddy soil were irrigated 30 mm every other day. NH4-N, Total-P, and K concentrations of the irrigation water were 0∼10,0.26 and 5.97 mg/l, respectively. The purifying capacity of NH4-N in the paddy soil was clarified to investigate the relationship between the load of NH4-N and the Total-N concentration in the percolating water. 1) The paddy soil can purify the irrigation water containing the average concentration of 0.93mg/ l Total-N by the suitable water management of the irrigation water. 2) The NH4-N concentration of the irrigation water was 10 m g/ l, and the annual Total-N load was 44.5 g/m2. Under these conditions, the Total-N removal rate was about 39.4 g/m2 per year during the percolation through the paddy soil. 3) Suppling the irrigation water o f 5.97 mg/l, the annual load of K was 24.8 g/m2. Under these conditions, the removal rate of K was 10.6∼12.6 g/m2 per year. 4) As the inflow and outflow loading of total organic carbo n and inorganic carbon were calculated, it became clear that these materials were added to the percolating water from the paddy soil. 5) In order to acquire the percolating water which will satisfy the Environmental Qualit y Standards for Lakes and Ponds (Class V; Total-N concentration below 1 mg/l ), it is necessary to supply the irrigation water whose NH4-N concentration is lower than 6 mg/l , and control annual Total-N loading below 28.3 g/m2.
An examination is made on the proposed permeability test to be conducted directly on a thin-wall tube specimen and a theoretical investigation is made on the seepage flow across the section of a soil specimen in the thin-wall tube. Since the boundary conditions are not too complicated, the flow can be mathematically analyzed by a quasi-1dimensional analysis. It is demonstrated that the shape factor a is proportional to the natural logarithm of n-1within a small range of n. The value of n is the ratio of the width of the inlet or outlet to the diameter of the thin-wall tube which affects the seepage flow. In addition, several different seepage flow models having different values of n were examined experimentally by performing laboratory permeability tests; each case was analysed and its characteristics were clearly defined. From the result of verification, it is shown that the seepage condition within the testing apparatus is approximately the same as the theoretical flow based upon the steady-state analytical solution. It is also shown thus that the small holes on a close spacing provide satisfactory inlets and outlets for the two-dimensional seepage flow. Key words: horizontal permeability test, thin-wall tube sample, quasi-1dimensional analysis, seepage modelling, shape factor
For an accurate prediction of contaminant migration in a porous medium, each of the various processes governing solute transport needs to be represented correctly in mathematical models. The transports of many contaminants of environmental concern are influenced by adsorption reaction in the subsurface. In this study, two models commonly used to represent adsorptive solute transport in homogeneous media, namely equilibrium model and two-site kinetic model, were examined for describing strontium transport in a homogeneous sand column. Carefully designed one-dimensional laboratory column experiments were conducted at three different fluid velocities using 85Sr as an adsorptive tracer. Observed breakthrough curves (BTCs) showed asymmetric shape with large dispersion and slight velocity dependence in dispersion and skewness. The equilibrium model which was based on batch K,d and a dispersivity of non-reactive tracer showed considerably smaller dispersion and apparently, failed to predict strontium transport in the sand. On the other hand, with a few optimizing parameters, the two-site kinetic model as well as the equilibrium model were able to describe observed BTCs successfully. However, the parameter sets required to describe BTCs for different fluid velocities are inconsistent with theory. In spite of good agreement observed between fitted and observed curves, inconsistent parameter sets indicate that processes occurring in the system are not accurately represented by these models. Since these models are virtually the only ones available to describe inorganic solute transport in homogeneous media, the presence of an additional unknown mechanism which causes considerably large dispersion in reactive BTCs is strongly suggested.