Methods. the Residence Times of the Elements Determined by Goldberg
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methods.The residencetimes of the elementsdetermined causeone can predict stable statesthat are not at thermo- by Goldberg (1963a) are given in table 7. These range dynamic equilibrium. from 2.6~10~ years for sodium to only 100 years for aluminum. Similar results were reported by MacKenzie EVALUATION OF WATER COMPOSITION and Garrels (1966). The composition of natural water must be deter- In most respects, the concept of residence time in mined by physical and chemical means,usually by collec- the ocean is more satisfying to the chemist interested in tion and examination of samples.The standard practice aqueousbehavior of the elementsthan is the Goldschmidt of collection of samplesand later analysisin the laboratory model with its dependenceon averageigneous and sedi- is changing somewhat in responseto the growing trend mentary rock composition. As Barth (1961) has pointed to use automatic sampling and continuous-sensing de- out, the igneous rocks that lie near the surface of the vices. It is with the study and interpretation of water continents, which are the ones available for collection of composition, however the water is obtained, that we are rock samples for analysis, represent material that has in principally concerned. all probability been reworked many times and may, therefore, have a composition very different from its Collection of Water Samples original composition. The present compositions of rocks and the oceans represent the result of a long-continued Sampling is a vital part of studies of natural-water process of fractionation, and the residence time of ele- composition and is perhaps the major source of error in ments in the ocean is, therefore, useful as an index of the whole processof obtaining water-quality information. their geochemicalbehavior. The elements whose chemis- This fact is not well enough recognized,and some empha- try definitely favors retention in aqueous species have sis on it seemsdesirable. long residencetimes, and those preferentially bound into In any type of study in which only small samples of solids have short residencetimes. the whole substanceunder comideration may be exam- Processesof sea-floor spreading and plate tectonics ined, there is inherent uncertainty because of possible can be viewed as the way in which the cycle of weathering sampling error. The extent to which a small sample may is closed and the oceanic sediments are returned to the be considered to be reliably representative of a large continental crust. However, the quantitative evaluation volume of material depends on several factors. These of such processesdoes not appear feasible at this time. include, first, the homogeneity of the material being Cycles of some elements are interrelated. Lasaga sampledand, second,the number of samples,the manner (1980) evaluated some aspectsof coupling between the of collection, and the size of the individual samples. cycles of carbon and oxygen, using concepts of thermo- The sampling of a completely homogeneous body dynamics of irreversible processes.This approach has is a simple matter, and the sample may be very small. interesting possibilities for future theoretical studies, be- Becausemost materials are not homogeneous,obtaining Table 7. Average residence time of elements in the ocean [After Goldberg (1963a)] Residence Residence Residence Element time Element time Element time (Yr) (Yr) (Yd Na ______________________2.6~10’ Rb . .._______________2.7~10~ SC __________....._........5.6x103 Mg ______________________4.5~10~ Zn . ..__________________1.8x10’ Pb _______.......__________2.0~10~ Li ..__..____________2.0x107 Ba ________________........8.4~10~ Ga .._.______________1.4x103 Sr ..____________________1.9x107 cu . ..____ 5.0x104 Mn ___________________...1.4~10~ K _____________..._.......1.1~10~ Hg ..__________________....4.2~10~ w . .._____ 1.0x103 Ca .._._.________________8.0~10~ cs . .._____ 4.0x104 Th ____________________..3.!ix102 Ag ..____________________2.1~10~ Co ______.........._.____ 1.8~10~ Cr ____________________....3.!ix102 Au . .._..______________5.6~10~ Ni ____________._.._.......1.8~10~ Nb _...__________________3.0~10’ Cd ______._._.._____.____5.0~10~ La __________..__..........1.1x104 Ti .__.____________________l.6x102 MO __________________._..50x10’ v 1.0x104 Be ____._.........._.__.... l.!ixlO’ Sn ________________________50x10’ Si _...____________________8.0~10~ Fe _____...........________1.4~10’ u . .._.__________5.0Xld Y . ..________7.5x103 Al ________________._......1.0~10~ Bi _.______________________4.5~10~ Ge ..__________________..7.0x103 Sb . .._.__________________3.5~10~ Ce _._.........._________6.1~10~ 42 Study and Interpretation of the Chemical Characteristics of Natural Water truly representative samples depends to a great degree on may persist for many kilometers downstream. The effect the sampling technique. A sample integrated by taking is more pronounced if the water of the tributary differs small portions of the material at systematically distributed markedly from the water of the main stream in concen- points over the whole body represents the material better tration of dissolved or suspended solids or in temperature. than a sample collected from a single point. The more Where a river enters the ocean there is, of course, also a portions taken, the more nearly the sample represents the possiblity of seawater mixing incompletely with the flow. whole. The sample error would reach zero when the size These effects may be of special interest in some studies, of the sample became equal to the original volume of but if the average composition of the whole flow of a material being sampled, but for obvious reasons this stream or its changes in composition over a period of method of decreasing sampling error has practical limits. time are the factors of principal significance, sampling One of the primary goals of a water-quality investi- locations where mixing is incomplete should be avoided. gation may be to provide information that can be used to An outstanding example of incomplete mixing across determine the composition of the whole volume of water the stream is afforded by the Susquehanna River at within or available to a region. The object of study may Harrisburg, Pa. The stream at the highway bridge where be a slowly circulating mass in a lake or reservoir, the samples were collected is about half a mile wide and is water in an aquifer, or the water carried by a river during split into two channels by an island. The composition of some finite time period. Also, information may be re- the water is indicated by six samples spaced across the quired on the variations in composition at a point, or stream and is given in figure 2. More than 20 years of over the whole water body, with passage of time. For observations by the U.S. Geological Survey (Anderson, other types of studies, a synoptic evaluation of water 1963) show that this pattern is always present in some composition in a river drainage system may be desired, degree, except at very high stages. The anthracite-mining with the goal ofemphasizing spatial rather than temporal region northeast of Harrisburg produces large volumes variations. The design of sampling programs that will of drainage containing high sulfate concentrations and accomplish all these objectives encounters different kinds having a low pH. Tributaries entering the river from the of problems in surface- and ground-water systems, and west above Harrisburg, especially the Juniata River, are rather careful attention to sample collection is required. less influenced by mine drainage and usually carry alkaline The purpose underlying a water-quality study largely water having much lower sulfate contents. Obviously, it governs the sampling procedures that should be followed. is difficult to characterize the whole flow of the stream at Commonly, the investigator wishes to know the composi- Harrisburg, although samples at one point would indicate tion of a cross section of a river at a specific time. For what an intake located there would obtain. some purposes, however, only the composition that would A composite sample that will represent accurately occur at a fixed water-intake point is of interest, and in the water in a vertical cross section of a stream can be this case the procedure would be somewhat simpler to obtained by combining appropriate volumes of samples design. taken at a series of points along the cross section. At each point, samples should be obtained at enough different Sampling of River Water depths to compensate for vertical inhomogeneity. Obvi- ously, it is physically impossible to obtain all these samples To determine adequately the instantaneous compo- at one instant. The water in the stream is in motion at sition of a flowing stream, the sample, or set of samples different rates in different parts of the cross section, and taken simultaneously, must be representative of the entire this further complicates the problem. flow at the sampling point at that instant. Furthermore, In practice, the collection of river-water samples is the sampling process must be repeated if the results of somewhat simplified by use of portable integrating sam- analysis are to be extrapolated in time, and the sampling pling devices which allow water to enter the sample interval chosen must represent adequately any changes container at a rate proportional to water flow rate at the that might occur. Changes occurring along the length of intake nozzle. The sampling device is raised or lowered the stream can be evaluated by adding more sampling from a selected position on a bridge or cableway to points. obtain a sample that will represent all the river flow at The homogeneity of a stream at a cross section is the particular point along the cross section. This process determinedby suchphysical factors asproximityof inflows is repeated at other points along the cross section (com- and turbulence in the channel. Locally, poor lateral or monly five or more) and the individual depth-integrated vertical mixing can be observed in most stream systems.