Envlron. Scl. Technol. 1984, 18, 631-632
EPA Contract 68-01-5975,pp 1-160. (26) Hogan, M. D.; Hoel, D. G. In “Principles and Methods of Toxicology”;Hayes, A. W., Ed.; Raven Press: New York, 1982; pp 711-731. (27) Tinsley, I. J. “Chemical Concepts in Pollutant Behavior”; Wiley: New York, 1979. (28) Crouch, E. A. C.; Feller, J.; Fiering, M. B.; Hakanogly, E.; Wilson, R.; Zeise, L. “Non-Regulatory and Cost-Effectiveness Control of Carcinogenic Hazard”; Energy and Environmental Policy Center, Harvard University: Cambridge, MA, 1982. (29) Hesselberg, R. J.; Seelye, J. G. “Identification of Organic Compounds in Great Lakes Fishes by Gas Chromatography/Mass Spectrometry: 1977”;Great Lakes Fishery Lab: Ann Arbor, MI, 1982; p 3. (30) Crouch, E. A. C.; Wilson, R.; Zeise, L. J. Water Res. 19 (6), 1359-1375. (31) Chem. Week 1983,132 (16), 26-27. (32) Doll, R.; Hill, A. B. Br. Med. J . 1964, 1 , 1399. (33) Todhunter, J. A. Science (Washington, D.C.)1983,219,794. (34) Miller, S. Environ. Sci. Technol. 1983, 17, llA-14A.
(15) Nassau County Department of Health “Chemical Quality
of Untreated Water from Community Supply Wells in Nassau County”; Nassau County: Mineola, NY, 1981. (16) National Research Council “Drinking Water and Health”; National Academy of Sciences: Washington, DC, 1977;p 11.
(17) “Seafood Consumption Study, 1973-1974”; National Marine Fisheries Service: Washington, DC, 1976; p 146. (18) Food and Drug Administration Fed. Regist. 1979, 44, 38330-38340. (19) OccupationalSafety and Health Administration Fed. Regist. 1983,48, 17284-17319. (20) OccupationalSafety and Health Administration Fed. Regist. 1983,48,45956-46003. (21) Consumer Product Safety Commission Fed. Regist. 1982, 47, 14366-14419. (22) Staffa, J. A.; Mehlman, M. A., eds. J. Environ. Pathol. Toxicol. 1980, 3 (3), 1-250. (23) Consultative Panel on Health Hazards of Chemicals Pesticides ”Pest Control: An Assessment of Present and Alternative Technologies”; National Academy of Sciences: Washington, DC, 1975. (24) Crouch, E. A. C.; Wilson, R. J. Toxicol. Environ. Health 1978,5, 1095-1118. (25) Crump, K. S.; Howe, R. B. “Approaches to Carcinogenic, Mutagenic, and Teratogenic Risk Assessment”; U.S. Environmental Protection Agency: Washington, DC, 1980;
Received for review June 20,1983. Revised manuscript received October 31, 1983. Accepted March 7, 1984. This work was partially supported by Grant CR-807809to the Interdisciplinary Programs in Health from the EPA. The contents of this paper do not necessarily reflect the views and policies of the EPA.
CORRESPONDENCE Comment on Comment on “Acid Precipitation in Historical Perspective” and “Effects of Acid Precipitation” SIR: Apparently, soil scientists have difficulties in accepting that lakes and stream waters have become acid due to acid rain, because the natural production of acidity in ecosystems is large compared to the contribution from acid rain. Richter (1) concludes that “many of the reported changes, where real, may well result from natural processes with relatively minor contributions from acid precipitation”. This conclusion is not based on scientific evidence documented in his letter. Richter does not even try to explain why regional acidification is only reported from areas receiving acid precipitation. If he believed his hypothesis, he should look for areas in the world not receiving acid precipitation where acid clear water lakes have been reported. If acid precipitation was only a small contributor, regional acidification should occur in all areas undergoing natural ecological changes. I assume Richter does not believe that natural acidification processes only take place in areas receiving acid precipitation? Natural acidification processes are well-known also to water chemists and lead to an excess of hydrogen ions in the soil where the base saturation is reduced. If the hydrogen ion concentration in the runoff water also increases because of natural acidification, this increase can be compensated either by an increased concentration of anions (organic anions) or by an equivalent decrease in cations. In the first case increased concentrations of organic carbon (more colored waters) should be found. In the latter case the H+ increase must be compensated by a decreased leaching of basic cations from the soil, because its base 0013-936X/84/0918-0631$01.50/0
saturation is decreased. Thus, the formation of bicarbonate from COz is correspondingly reduced, and the net effect on the runoff water will be lower concentrations of Ca, Mg, and HCO,, and thus a decrease in pH (which in oligotrophic clear water lakes is a function of the HCO, concentration). In areas not receiving external acid inputs, the pH of lakes will most likely not be lower than about 5.5 because some HCO, will still be present. This process does not lead to any relative change in concentrations of Ca, Mg, and HCO,, and this change will be difficult to detect. Thus, increasing acidity of soil does not necessarily lead to increasingly acid lakes, merely to lakes with lower alkalinity. We believe that the latter process dominates for natural acidification processes, because in sensitive areas both receiving and not receiving acid precipitation, clear water lakes are dominant. Krug and Frink (3) recently suggested that SO4 from acid rain is exchanged with organic anions originally present in the water, leaving pH essentially unchanged. This implies that all clear water lakes that are strong acid dominated today (and they are found in areas receiving acid precipitation) originally were acid dystrophic lakes with organic anion concentrations approximately equal to their present sulfate concentrations. To maintain the pH, the pK of the organic acid must have been the pK of H2S04! Their hypothesis is thus not acceptable, also because waters with organic anion as major anion are rather atypical. Harriman and Morrison (2)have compared the chemistry of a moorland stream with an adjacent forest stream in an area receiving acid precipitation (pH 4.3-4.4). They assumed that any differences in stream water chemistry would be due to afforestation of the catchment of the forest stream. Table I gives their results together with values
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Environ. Sei. Technol. 1984, 18, 632-634
Table I. Comparison of the Chemistry of a Moorland Stream with an Adjacent Forest Stream in the Loch Ard Forest Area of Central Scotland (after Harriman and Morrison (2)) stream
pH
forest moorland
4.34 5.40
Ca‘,b M p b Sodb Albb pHo Acb 45 75
osea salt corrected values. liter.
31 26
135 121
37 63
6.15 6.35
83 67
units of microequivalents per
I
,Time
Figure 1. To illustrate pH changes of a lake undergoing “natural” acidification and superlrnposed on this acidification due to acid precipitation.
for “original alkalinity” (Alk,,), “preacidification”pH (pH,), and “acidification” (Ac) estimated according to procedures suggested by Henriksen (4),Wright & Henriksen (5),and Henriksen (6). If we assume that the streams had similar water chemistry before afforestation, the change in land use (in the absence of acid rain) would have reduced the pH from 6.35 to 6.15. The present low pH values of both streams are thus due to acid rain. The forested stream has a lower pH than the moorland stream today because afforestation has made the stream more sensitive (lower Ca Mg concentrations) to acidification. Also the acidification (Ac) is higher in the forested stream, probably because, as suggested by Harriman and Morrison ( 2 ) ,trees are more effective collectors of gaseous pollutants than moorland vegetation. These results illustrate that natural acidification may lead to runoff waters with lower alkalinity, thus rendering them more sensitive to acidification by external acid inputs, as illustrated in Figure 1. Thus, reported changes in acidities of lakes and streams in areas in the world that receive acid precipitation may well result from acid precipitation with relatively minor contribution from natural processes.
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Literature Cited (1) Richter, D. D. Enuiron. Sci. Technol. 1983, 17, 568-570. (2) Harriman, R.; Morrison, B. R. S. Scott. For. 1981,35,89-95. (3) Krug, E. C.; Frink, C. R. Science (Washington,D.C.)1983, 221, 520-525. (4) Henriksen, A. “Ecological Impact of Acid Precipitation”. Oslo, 1980, SNSF Project. (5) Wright, R. F.; Henriksen, A. Nature (London) 1983,305, 422-424. (6) Henriksen,A. “Changesin Base Cation Concentrations due to Freshwater Acidification”;XXII S.I.L. Congress, Lyon, Aug 21-28, 1983, in press.
Arne Henriksen Norwegian Institute for Water Research Blindern, Oslo 3, Norway 632 Environ. Sci. Technol., Vol. 18, No. 8, 1984
SIR: Henriksen (17) appears to have misunderstood the intent of my original correspondence (26),which was not to document evidence but rather to assert two generalities: (i) Adverse effects of acid deposition on ecosystems are commonly overstated. (ii) The biogeochemistry of ecosystems is easily oversimplified, and natural sources of acidity are often ignored. Henriksen (17)asks why “regional acidification” of “clear waters” has only been reported in areas receiving acid precipitation. The most obvious answer is that there has been little intensive study of most of the world’s surface waters. Moreover, regional acidification does not mean that most lakes in a region have low pH. About 17% of all small lakes in southern Norway (south of 63’N) have pH less than 5.0 (34),and similarly, about 19% of lakes and ponds in the Adirondacks have pH less than 5.0 (24, 25). On an area basis, about 3.7% of the Adirondack lake area has pH less than 5.0. Some data, however, are presented in Table I for acidic Tasmanian lakes which range in color from 20 to 150 Pt (platinum unit). Although clear water is not rigorously defined by Henriksen, Scandinavian lakes with color up to a t least 56 Pt can have water characterized as “clear” (Lake Horsikan (9)). Moreover, if clear water is determined by transparency, then southeastern Norwegian lakes (16) that average 4.1-m Secchi transparency and pH 5.03 (34) are comparable to 11 lakes on Frazier Island, Australia, that average 3.8-m Secchi transparency and pH 4.59 ( I ) . Without a perusal of the aquatic chemistry literature and possibly more field study, the question of why acidic clear water lakes exist only in areas with acid rain has relatively little meaning. Henriksen asserts that clear water lakes and streams have pH greater than 5.5, unless they receive acid precipitation, and thus he concludes that natural processes only contribute relatively minor amounts of acidity to natural waters (ref 17, Figure 1). This assumption, however, is as misleading as was the similar assumption that unpolluted precipitation could only have pH greater than 5.5 (5,11,20).For example, theory and experimental data indicate that the pH of runoff from acid soils is controlled more by precipitation electrolyte content than by precipitation pH (27-29, 31). This is well illustrated by Norwegian data (34)in Table I which show that lake water pH was decreased markedly by runoff of precipitation greatly enriched in sea salts. Low pH in these lakes cannot be caused by acid rain as these data show small amounts of and little change in SO4 derived from man-made sources (SO4* in Table I). This salt effect involves adsorption of base cations (e.g., Na+ of NaC1) to exchange sites of acid soils and a release of soil acidity into solution (Na+ + soil-H H+ soil-Na). Only a small portion of the Na+ needs to exchange for H+ to lower pH of water considerably. Under certain watershed conditions, it seems possible for the salt effect to lower pH of runoff or lake water far below that observed in the two Norwegian lakes (e.g., precipitation high in salts, acid soils with very low base saturation, and hydrologic conditions which limit contact of runoff and weatherable minerals). Although deposition of sea salts decreases markedly with increasing distance inland, a number of major areas considered affected by acid rain are also affected by sea salt deposition. Skartveit (29) documented in some detail seasonally variable salt effects in Norway, and Wiklander (31) indicated that salt effects were important in southwestern Sweden. Sea salt effects should also be important in Nova Scotia (30),the British Isles, and other coastal environments. By suggesting that surface waters are not likely to have pH less than 5.5 except in areas receiving acid rain, Henriksen ignores a
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