Question

翻译段落(练习题参考答案）

Answer 1

( 1) As part of the hydrologic cycle，groundwater is always in motion from regions of natural and artificial replenishment to those of natural and artificial discharge. Bodies of stagnant，usually saline，w ater trapped in various porous geological formations do exist，but as long as they do not participate in the hydrologic cycle，they are of little interest to the groundw ater hydrologists.

( 2) The porosity n can be an important controlling influence on hydraulic conductivity K. In sampling programs carried out w ithin deposits of w ell-sorted sand or in fracture rock formations，samples w ith high n generally also have high K. How ever，the relationship does not hold on a regional basis to various possible rock and soil types. Clayey soils，for example， usually have higher porosities than sandy or gravelly soils but low er hydraulic conductivities.

( 3) In recent years the term aquitard has been used to describe the less-permeable beds in a stratigraphic sequence. These beds may be permeable enough to transmit w ater in quantities that are significant in the study of regional groundw ater flow ，but their permeability is not sufficient to allow exploitation of groundw ater w ithin them. Most geological strata are classified as either aquifers or aquitards.

( 4) Of all the words in the hydrologic vocabulary，there are probably none with more abundant meaning than the term aquifer. It means different things to different people，and perhaps different things to the same person at different times. It is used to refer to individual geologic layers，to complete geologic formations，and even to groups of geologic formations. The term must alw ays be view ed in terms of the place w here it is used.

( 5) One of the main characteristics of groundwater motion is that it occurs at very， sometimes extremely， low velocity. How ever， because of the large cross-sectional areas through w hich this motion takes place，large quantities of w ater are transported. This chapter deals mainly w ith the basic law s governing the motion of groundw ater in aquifers，and w ith the porous media and aquifer parameters. Only saturated flow is considered here. Unsaturated flow w ill be further discussed in Chapter 6. Darcy's law indicates that groundw ater velocity is directly proportional to the hydraulic gradient.

( 6) Salt ( as NaCl only) applied to a highway during the winter for prevention of icing problems has cause contamination of a shallow unconfined aquifer near the highw ay. It has been observed that Cl content of w ater from many domestic w ells，w hich w as formerly soft，has become hard as Cl has risen. The large increase in hardness can be contributed to the effect of road-salt contamination.

( 7 ) Many field workers have observed an anomalously large rise in water levels in observation w ells in shallow unconfined aquifers during heavy rainstorm. It is now recognized that this type of w ater-level fluctuation is the result of air entrapment in the unsaturated zone. Air pressure in this zone is much greater than atmospheric pressure. This type of w ater-level rise bears no relation to groundw ater recharge，but because it is associated w ith rainfall events，it can easily be mistaken for it. The most obvious feature is the magnitude of the ratio of w ater- level rise to rainfall. Changes in atmospheric pressure can produce large oscillations in w ater levels in w ells or piezometers penetrating confined aquifers. The relationship is an inverse one; increases in atmospheric pressure create declines in observed w ater levels.

( 8) Phreatic aquifers can be replenished from above by precipitation falling directly over the ground surface overlying the aquifer，provided the ground surface is sufficiently pervious. Some parts of a certain area may be completely impervious ( houses，streets，and highw ays or an impervious rock) and do not contribute to the natural replenishment of the aquifer beneath them. Confined aquifers are replenished by groundw ater inflow from an adjacent phreatic aquifer w hich，in turn，is replenished from precipitation. In principle，infiltration is unsaturated dow nw ard flow from the ground surface to the w ater table. The relationship betw een natural replenishment and total precipitation is governed，among others，by the follow ing factors: type of precipitation， climatic conditions， soil moisture prior to storm， storm characteristics， topography of ground surface，perviousness of ground surface and vegetation cover，etc. The natural replenishment of an aquifer can be determined，depending on the amount of precipitation and lithology of the unsaturated zone，etc.

( 9) Artificial recharge may be defined as man's planned operations of transferring water from the ground surface into aquifers. Artificial recharge can be implemented by several methods. The choice of method for each particular case depends on the source of w ater，the quality of the w ater，the type of aquifer，the topographical and geological conditions，type of soil，economic conditions，etc. Artificial recharge can be carried out through injection w ells and surface spreading pits. Artificial recharge plays an important part in augmenting groundw ater resources and in the improvement of groundw ater quality.

( 10) However，the depth to the fresh water-saline water interface is underestimated by the Ghyben-Herzberg relation. The exact position of the interface can be determined by draw ing flow -net for steady-state outflow of groundw ater to the sea. On the other hand，the fresh w ater- salt w ater interface is not a sharp boundary but a zone of diffusion mixed w ith salt w ater and fresh w ater. Both the field observation and numerical modeling confirm the necessity of considering dispersion effect in the analysis of seaw ater intrusion. ( 11) There exist many examples of the occurrence of land subsidence of large areal extent as a result of pumping groundw ater. The most spectacular one in an urban area is in Mexico City，w here almost the entire metropolitan area has subsided more than 3 m ( w ith up to 8 m at some locations) . The San Joaquin Valley in California has experienced subsidence at a rapid rate of 30 cm to 40 cm per year，reaching a total subsidence of 9 m at some locations. Other places in w hich significant land subsidence has occurred are Taipei Basin，Tokyo，the Texas Gulf Coast，London and Bangkok. In almost all these places，the aquifers contain compressible clay strata，or a sequence of confined aquifers are separated from each other by compressible clay strata.

( 12) Groundwater has an important impact on the generation of landslides. When the slope is w ater-bearing，pore w ater pressure along the potential sliding plane is needed to be determined. If observation w ells have been installed in the slope，the observed hydraulic head， h，can then be converted to pore pressure，p as follow s p = ρwg ( h － z)

w here z is the elevation of the potential sliding surface，ρwis the density of w ater，and g is the gravitational acceleration. Pore w ater pressure must be taken into account in the analysis of the potential of landslides.