Determination of the Fresh Water-Salt Water Interface

翻译+解答【问题也要翻译噢】~

文章的意思：
现在的世界不仅很“饿”而且也很“渴”。地球上大约有97%的水是海水或是碱水。只占另外3%的来自于河流、湖泊和地下的淡水才是人类能直接饮用或使用的。另一方面，由于世界人口不断增长，人类对淡水的需求也越来越大。因此水资源短缺成为一个很大的问题。
但是我们要怎样来解决这个问题呢？科学家们已经尝试了用不同方法把海水转化为淡水。其中一个很重要的方法就是把海水加热，使其达到沸点让里面的淡水变成蒸汽脱离盐分而蒸发出来。通过这种方法可以快速得到大量的淡水。
但是要得到淡水不是只有加热海水这一种方法。其它的方法也被尝试过，例如：挖深井、储存雨水、防止河流污染。
到底哪种方法才是最好的呢？能用最少的代价换来最多的淡水的方法才是最好的。

问题：
1、水资源缺乏成为了一个很严重的问题，不是吗？
2、科学家们都发现了哪些方法？
3、通过挖深井、储存雨水和保护河流的方法（来获得淡水）困难吗？

第一题填yes,it is 就是说：不，它是个很严重的问题
第二题：boil sea water / digging deep wells / keeping rain water / preventing rivers from being polluted
第三题：yes ,they are

Measurements of piezometric heads at w ells near the coastline indicate that piezometric heads at points in the salt w ater zone change w ith time and may have an important bearing on the location of the fresh w ater-salt w ater interface. They should be considered in estimating the location of the interface.

Figure 20. 11 show s a fresh w ater-salt w ater interface which generally occurs in a homogeneous unconfined aquifer in a coastal zone. Groundwater flow is in a steady state and discharges into the sea. It is assumed that a piezometer taps any given point A in the salt w ater zone below the interface; h_sstands for the elevation of piezometric head of the salt w ater measured at point A above the mean sea level; h_fis the elevation of w ater table at point D at the same location of the piezometer above sea level.

Figure 20. 11 Schematic diagram show ing a sharp interface betw een fresh w ater-salt w ater in a coastal unconfined aquifer

At point A，the follow ing relation can be established w hen the subsurface flow system is in an equilibrium state:

w here ρ_sand ρ_fare densities of salt w ater and fresh w ater，respectively，Msand M are the depths of point A and the interface at point B below sea level， g is the gravitational acceleration. Rearrangement of Equation ( 20. 18) gives

and the follow ing relation is obtained

or

w here

For any given datum ( Figure 20. 11) ，the follow ing relation can also be established:

w here h'_sand h'_fare the piezometric heads of salt w ater at point A and w ater table at point D above the datum ，respectively ; M '_sand M ' are elevations of point A and the interface at point B above the datum ，respectively. From Equation ( 20. 23 ) the follow ing relation is obtained

or

Equation ( 20. 25) can be converted into Equation ( 20. 24) by letting h'_s= h_s+ H₀，h'_f= h_f+H₀and M' = H₀－ M，w here H₀is the elevation of the sea level above the datum.Equation ( 20. 21) gives the depth of the interface，w hich is determined by ρ_s，ρ_f，h_sand h_f. According to Equation ( 20. 25 ) the elevation of the interface above the datum can be calculated，w hich is determined by ρ_s，ρ_f，h'_sand h'_f. In particular，if hsequals to zero， Equations ( 20. 20) or ( 20. 21 ) becomes the Ghyben-Herzberg relation. Thus，the Ghyben- Herzberg relation is a special case of Equation( 20. 21) . It is also noted that M in Equations ( 20. 20) or ( 20. 21) is not related to Msand that M' in Equations ( 20. 24) or ( 20. 25) is not related to M's，indicating that the depth or elevation of the interface is independent of the depth or elevation of the measured point in the salt zone below the interface.

For ρ_f= 1. 0 and ρ_s= 1. 025，Equation ( 20. 21) becomes

水文地质专业英语

and Equation ( 20. 25) becomes

水文地质专业英语

Figure 20. 12 show s different depth_s of the interface in an ideal model for different hf( ranging from 0 m to 3 m ) w hen h_s= － 1 m，－ 0. 5 m，0 m，0. 5 m and 1 m according to Equation ( 20. 26) . It is seen that the depth of the interface increases with increasing hffor a given h_s， and also increases w ith increasing h_sfor a given h_f. For example，the interface is 121 m and 39 m deep below the sea level for h_f= 2 m w hen h_s= － 1 m and 1 m，respectively.

Figure 20. 12 Depth of interface for different hsand hfin a theoretical model

Equation ( 20. 26) is used to estimate the depth of the interface in the northw estern coastal zone of Beihai，w here a sharp interface is assumed to exist betw een the fresh w ater and salt w ater. Piezometric heads at w ell cluster H5 are used as the upper Q₁z clay terminates near this w ell cluster. Piezometric head at H5-1 stands for the w ater table of the fresh w ater at the w ell and piezometric head at H5-3 represents the hydraulic head of the salt w ater at a point in the salt w ater zone below the interface. Data of piezometric heads at H5-1 and H5-3 are first smoothed using the Haning formula of running average to eliminate the minor fluctuations in the measured piezometric heads ( Figure 20. 13a) . The estimated depths of the interface at w ell H5 during the measurement period are show n in Figure 20. 13b，w hich range from 40 m to 80 m below the sea level. The interface w as deeper in dry season than in rainy season due to the increasing piezometric heads of the salt w ater. Also show n in Figure 20. 13b are the estimated depths of the interface w ith the Ghyben-Herzberg relation. The results estimated w ith the Ghyben- Herzberg relation appear opposite of those calculated w ith Equation ( 20. 26) and are considered to be not as reasonable as the latter，because the Ghyben-Herzberg relation does not take the piezometric head of the salt w ater into account.

Figure 20. 13 Changes in ( a) average piezometric heads at H5-1 and H5-3 and ( b) calculated depth of the fresh w ater-salt interface below sea level from January，1996 to M arch，1997 A—estimated w ith the Ghyben-Herzberg relation; B—estimated w ith Equation ( 20. 26)

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