Eiichi Shimojima

Water vapor transfer beneath bare soil where evaporation is influenced by a turbulent surface wind

Abstract A laboratory experiment on evaporation in various porous material layers was undertaken by applying a turbulent air flow to the exposed surface, to investigate the mechanism of the water vapor transport beneath a bare ground surface influenced by a turbulent surface wind. Variation of air pressure within these layers was also measured. Based on a physical model of water vapor transfer, an observed function decribing bulk resistance for water vapor transfer with evaporating front depth was investigated, using an observed power spectrum of the pore-air pressure. The model is based on the so-called turbulent mixing theory. The following conclusions were reached. When porous material layers are of low permeability such as fine sand, water vapor transfer under the exposed surface occurs by molecular diffusion as well as by turbulent diffusion caused by variation of the pore-air pressure due to the turbulent surface wind (P-type). The surface turbulence significantly affects the transport just beneath the surface. Turbulent diffusivity near the surface is determined by horizontal and vertical fluctuating components of the pore-air pressure, but with increasing depth is driven only by the vertical component. When the porous material layer is more permeable, water vapor transfer in this layer is analogous to that within a vegetation canopy (F-type), so that turbulent diffusivity decays exponentially with depth. The turbulent diffusivity is a function of the mixing length. The mixing length for F-type was of the order of the particle size. For P-type the mixing length ranged from the particle size to ten times larger.

The influence of pore water velocity on transport of sorptive and non-sorptive chemicals through an unsaturated sand

A set of laboratory column experiments were conducted to study the transport of two relatively non-sorptive (Br, NO3) and three sorptive (PO4, simazine, linuron) chemicals at three water flux conditions (q =6.9, 3.1, 0.62 cmh−1) through an unsaturated sand. Special attention was paid to the evaluation of the effects of pore-water velocity on transport of the chemicals as well as on their adsorption-desorption characteristics. The breakthrough curves (BTC) for a non-sorptive chemical measured at three water fluxes were more similar when the pore water velocity (q/θ) was computed using mobile water content (θm) than total water content (θ). The BTCs for Br and NO3 were almost identical in shape at any given water flux. There was evidence of some attentuation of NO3, and its extent was larger at lower fluxes. The BTCs for the adsorption process, observed at different depths under three different flux conditions could be scaled approximately using only two similarities, i.e. pore volume for ‘mobile’ water phase and Brenner number, VX/D∗, where V is the average pore-water velocity, X is the depth and D∗ is the hydrodynamic dispersion coefficient. Establishment of the similarity requires the condition that the retardation factor (R) be represented approximately as a function only of concentration. This suggests that in the assumed equilibrium isotherm, S = G(θ)F(C). The assumption that G(θ) is a linear function of θ is satisfactory for practical purposes. Here, S is the specific sorbed chemical (weight per unit dry soil weight), θ the volumetric water content and C the concentration. A comparison between the modified BTCs of phosphate and simazine for adsorption and desorption processes leads to the conclusion that both processes are more or less reversible. Based on the similarity, this requires a severe condition that R must be constant, i.e. dF/dC is constant. A similar analysis could not be applied to linuron data, as they exhibited a two-step concentration increment in the BTCs, and it is suggested that this may be caused by the presence of organic matter in the sand. The retardation factors for the three fluxes were found to be only approximately constant, with an average of 1.5 for phosphate and 2 for simazine. Strictly, R was dependent on V in that R increased with deceasing flux. Furthermore, this dependence was larger for chemicals with a higher sorption characteristic (i.e. the dependence of simazine was greater than that of PO4). Calculated BTCs for PO4-P and simazine using these average values of R showed a good agreement with observation and the dispersion coefficients were directly proportional to pore water velocity. It is concluded that transport of some sorptive solutes can be described adequately on the assumption that R is constant.

Seepage into a mountain tunnel and rain infiltration

Abstract To clarify the processes governing recharge of rainwater into an unconfined aquifer in a mountain basin, the seepage rate of water into a mountain tunnel was monitored continuously between May 1988 and December 1991 in Yura, Wakayama, Western Japan. The electrical conductivity and ionic composition of the seepage water was also measured regularly from June 1989. The mountain is composed of fractured sedimentary rocks such as sandstones and cherts. Seepage appeared only at specific locations on the roof of the tunnel. Seepage was measured through overlying sandstones of 10 m depth and cherts of 5 m depth. Time variations in the discharge suggested that seepage is formed both by rapid flow and basic flow components which correspond to the so-called fissure flow and matrix flow. This was confirmed by analysis of time variations in the concentration of chemical species in the seepage water. Fissure flow contributes the initial increment of seepage discharge, immediately after the occurrence of rainfall, and its velocity for sandstone can be represented by a function only of water content (θ), αθ n . α is constant and larger in winter than in summer, but n remains constant, at approximately two. Infiltration time in fissures through the base of the cherts is negligible, compared with that through the overlying soil layer. Whereas matrix flow through sandstones persists through the year, for cherts it disappears within a week after rainfall events and its decay is dependent on the seasonal magnitude of evapotranspiration. The behavior of the matrix flow for sandstones can be analyzed through a kinematic wave model, as can the fissure flow.

Using short- and long-term transients in seepage discharge and chemistry in a mountain tunnel to quantify fracture and matrix water fluxes.

Abstract Infiltration of rain-water into a fractured sedimentary rock mountain is explored through continuous observations of discharge rate, Q , and electrical conductivity, EC, of seepage water into a mountain tunnel. Also concentrations, C CO 2 , of carbon dioxide gas near the tunnel ceiling, and the chemistry of the seeping water are examined. Earthquake events occurred in the period of the seepage observation and influenced characteristics of the time trends in Q and EC. This provided a mechanism for the identification of rapid flow (fissure flow) and slow flow (matrix flow) in the infiltration components in the fractured rock base. Also, a cycling of discharge water from the matrix via the fissures and back into the matrix was expected to occur. C CO 2 increased due to rainfall events, and its response was with a phase-shift to increased Q . For a heavy rainfall event, the increase in Q was mainly caused by the occurrence of fissure flow, and as soon as Q began to decrease moderately after a rapid decrease from a peak value, C CO 2 showed a peak value. The C CO 2 peak seemed to coincide with increased matrix flow. Wetting in the rock matrix was assumed to behave as a shock wave. For a light rainfall event, where only matrix flow is likely to occur in the fractured rock base, Q increases were delayed in comparison to C CO 2 increases. The variations in C CO 2 due to rainfall events appeared to relate to the movement of the matrix wetting front, when high moisture contents were apparent. The wetting front was inferred to be pushing void-airs with high concentrations of CO 2 gas towards the tunnel. High CO 2 concentrations were assumed to be formed near the ground surface via dissolution of organic matter and respiration of plant roots. The chemistry of seepage water observed at two close locations is seen to differ distinctly. Time-variations in EC for one location (A1) are consistent with those for C CO 2 , while for the other location (A3) this was not the case. The variations are due to dominant anions in the seepage water; HCO 3 − for A1 and SO 4 2− for A3. These occur via dissolution of CaCO 3 and CaSO 4 into infiltrating water, and CO 2 gas plays an important role in the former process. The time trends and integrated interpretation of the seepage volumes, chemistry of seepage water, and the concentration of CO 2 gas are shown to be useful indicators for understanding rainwater-infiltration process in the fractured rock mountain, and for separation of the seepage into fissure flow and matrix flow components.

Precipitation, Groundwater and Ground Deformation

Strainmeters and tiltmeters installed on or at a shallow part under the ground surface record ground deformations caused by rainfall. Such deformations are detected to a depth of several ten meters with these instruments, and accordingly they are main noises to observations of crustal movements and/or earth tides with these instruments.