WATER RESOURCES RESEARCH, VOL. 28, NO. 1, PAGES 231-236, JANUARY 1992 Stable Sulfur Isotopes of Sulfate in Precipitation and Stream Solutions in a Northern Hardwood Watershed A. C. STAM A• M. J. MITCXqELL College of Environmental Science and Forestry, StateUniversity of New York, Syracuse H. R. KilOUSE University of Calgary,Alberta, Canada J. S. KAHL University of Maine, Orono Stable S isotopes of SO•- in precipitation, throughfall, and stream water solutions in a northern hardwood watershed (Bear Brook Watershed, Maine) were examined todetermine sources of stream SO•- and to identify watershed processes that may affect atmospherically deposited SO¬- prior to reaching the streams. Similarity among temporal patterns in b34S of prec•_itation, throughfall, stream SO42- (range: +3.4 to+9.4%o), and [SO42•narine]/[SO42•'otal] of stream SOg- (5-10%) indicate that the 4 '>- /53 S values of stream SOg are controlled principally by atmospheric deposition. Lack of correlation between •34S of stream SO•- and stream [SO4-total], e!evational position, and stream discharge supports the interpretation that fractionation ofstable S isotopes within the watershed appears to have, atmost, a minor influence on $34S of stream SOj- INTRODUCTION Sulfur cycling in forestecosystems has a potentially important influence on acidification of streams by acidic precipitation. It has been hypothesized that movement of SO•- derived from acidic deposition to surface waters can acidify streams if it is accompanied by acidifying cations (H + or A13+) [e.g., Reuss and Johnson, 1986]. Unless another mobile anion ispresent, retention ofSO•- within the watershed through adsorption or biologic retention pro- cesses would prevent the enhanced movement ofacidifying cations to the streams and thus minimize stream acidification [Mitchell et al., 1991]. Therefore anunderstanding of water- shed Sdynamics isimportant inevaluating the potential for anthropogenic SO42- to beretained within a watershed. Stable S isotopes can provide information regarding wa- tershed processes which cannot be discerned solely through the study ofthe distribution and fluxes ofS species inforest ecosystems. Hesslein et aI. [1988] used stable S isotopes to infer isotopic fractionation processes within watersheds at theExperimental Lakes Area(ELA), Ontario, Canada. Through the use of such isotopes, Fuller et al. [1986] were able to detect an elevational gradient in watershed proc- ess(es) affecting stream SO•- at Hubbard Brook Experi- mental Forest (HBEF). In neither case could these patterns have been detected strictly by quantifying concentrations and fluxes of SO42-. In the present study, which ispart ofthe ongoing U.S. Environmental Protection Agency Watershed Manipulation Project, stable Sisotopes ofSO42- inprecipitation, through- fall, and stream waters were examined toprovide insight into Copyright 1992 bythe American Geophysical Union. Paper number 91 WR02481. 0043-1397/92/91 WR-02481 $05.00 Scycling processes inanorthern hardwood watershed (Bear Brook Watershed, Lead Mountain, Maine).The specific objective was tocompare the isotopic signatures ofprecip- itation, throughfall, and stream SO•- to determine sources ofstream SO•- and to identify watershed processes which may affect atmospherically deposited SO42- as it moves through the terrestrial ecosystem before reaching surface waters. METHODS Bear Brook Watershed (BBWM) is on the south-facing slope of Lead Mountain in eastern Maine (44ø51'75"N, 68o6'25 ") approximately 40 km from the Atlantic coast. Forest vegetation inthe watershed isclassified as northern hardwood and consists primarily of American beech (Fagus grandifoIia Ehrh.) with subordinate yellow birch (Betula aIleghaniensis Gritton), red maple (Acer rubrum L.) red spruce (Picea rubens Sarg.), striped maple (Acer pennsyl- vanicum L.), and sugar maple (Acer saccharum Marsh.) [David et al., 1990]. Soils inthe watershed are predominantly Spodosols (frigid Typic Haplorthods) and have formed from till [David et al., 1990]. Bedrock in the watershed consists of metamorphic silicate rocks and granite and iscovered by0-4.6 m of soil. The climate is classified (modified from Koppen) asDbf humid, continental, cool summer [Trewartha, 1957]. From 1987 to 1989, mean annual precipitation was 115 cm yr -•, with approximately 25% as snow. During this period, mean annual temperature was 5øC, with the range from -30 øto +34øC. The twostreams in the watershed [East Bear Brook (EB) and West Bear Brook (WB)] are first-order intermittent, with observed ranges indischarge from 0 to0.26 m 3 s -•. The elevational gradient of thewatershed from itstopto the 231