The state of the art We have shown that source-receptor

relationships must be viewed in terms of changes in deposition in response to changes in emission. The concept of source apportionment has little rele- vance to these relationships, which are determined by complex interactions among a large number of variables in the acid deposition system. Models are the only tools available to illuminate the effects of these interactions.

We have classified models as semi- empirical or comprehensive. Semi-em- pirical models, although relatively sim- ple, are well suited for directly probing the observations of the acid deposition system. Although they cannot provide mechanistic explanations for acid d e w sition processes, they can yield consid- erable insight. Comprehensive models, on the other hand, can provide explana- tions for the behavior of what is ob- served. However, their complexity, coupled with uncertainties in their for- mulation, can pose formidable difftcul- ties in their use and evaluation. We be- lieve that complementary use o f comprehensive and semi-empirical models will offer the greatest opportu- nity for understanding source-receptor relationships. Acknowledgment Some of the results described were ob- tained from simulations made with a long- range transport model developed under the sponsorship of the Ontario Ministry of the Environment, Environment Canada, the German Umweltbundesamt, and the Elec- tric Power Research InStiNte (Palo Alto, Calif.).

This article was reviewed for suitability as an ES&T feature by Robert G. Lamb, EPA, Research Triangle Park, N.C. 27711.

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(26) Lurmann. F. W.; Godden. D. A,; Collins. H. M. “User’s Guide to the PLMSTAR Air Quality Simulation Model.” ERT Document M-2206-100. Environmental Research and Technology: Newbury Park, Calif., 1985.

(27) Carmichael. G. R.; Peters. L. K.; Ki- tada. T Almos. Environ. 1986.20, 173-88.

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1175-110 (30) SCOII, B. c . J. ~ p p i . ~eteoroi . im, 17. . - . - .. , . (31) b a w r . R . C.eta l .”Over~icvof ihcOa~-

dation and Scavenging Characteristics of A ~ r l l Rains IOSCARl EaDcriment.” PNL- 4869; PacifiC Northwest hboratoiy: Rich- land. Wash.. 1984.

Ah& VrnknmM (1.) is a senior scientisr at Environmental Research and Technol- ogy, Newbury Park, Calif: He received a B.S. in mechanical engineering from the Indian lnsritute of Technology (Madras) and a Ph.D. from Purdue University (La- fayetre, lnd.). He is managing the develop- ment of an internarionally sponsored com- prehensive acid deposifion model.

F’rakash hhwnchandani ( r ) is an air quality scientist af Environmental Re- search and Technology. He has a 8. s. in chemical engineeringfrom the Indian Insti- tule of Technology (Bombay) and an M . S. and a Ph.D. from the Universiry of Ken- tucky. He is involved in developing a com- prehensive regional acid deposition model.

Letters, continuedfrom p . 1068

day, Nov. 13, 1985, p. 46883 [Table 31 and p. 46961 [Table 131; and personal communication from C. R. Cothern). Note that the value in Table 1 for Sr-90 should have read 8 X I@’. These esti- mates are acknowledged to provide up per bound estimates of ILR, with the true risk lying between these values and zero.

When ILR estimates exceed IW3, regulatory action is generally taken to reduce the risk. In the range of I t 6 - I@’, economic cost effectiveness esti- mates are the primary determinants of regulation in most agencies. Such esti- mates may incorporate population risk as a factor. For most substances in our Table 1, the population risk based on exposure at the MCL or RMCL is low. However, for substances with ILRs above 3 x 1U4, systematic use of pop ulation risk estimates based on pro- jected M C h , as well as on actual oc- currence data, may play a useful role in determining the adequacy of proposed standards.

Corrections In the introduction to the series on wa- ter treatment processes ( E M T Septem- ber 1986, p. 855) the synopsis of Wil- liam J . Jewell’s paper should have concluded, “He says that it has a major role in wastewater treatment and will become the preferred domestic sewage treatment process.”

An item in “Currents” (ES&T March 1986, p. 214) should have read as follows:

Travis Hughes and J. J. Dow do not recommend a specific monitoring-well spacing, but suggest that site-specific hydrologic conditions dictate proper spacing between wells. They add that the spacing of monitoring wells closer than I50 A is required as long as EPA continues to allow installation of haz- ardous-waste facilities in improper lo- cations. They also say that stainless steel well casing and screens are clearly inferior to polyvinyl chloride (PVC) materials when groundwater is acidic or saline and when analyses for transi- tion metals are to be performed on groundwater samples. Stainless steel casing and screens may, however, be slightly superior to PVC when wells are installed to sample for low levels of organic chemicals and may be appro- priate for some groundwater monitor- ing programs. The use of PVC should be permitted in most cases in which monitoring programs are required for detecting the release of hazardous con- stituents.

Environ. Sci. Technol.. Vol. XI. No. 11. 1986 1091