Ion Budgets and Sediment -Water Interactions during the Experimental Acidification and Recovery of Little Rock Lake, Wisconsin CAROLYN J. SAMPSON † AND PATRICK L. BREZONIK* Department of Civil Engineering, University of Minnesota, Minneapolis, Minnesota 55455 Ion budgets for the two basins of experimentally acidified Little Rock Lake (Vilas County, WI, U.S.A.) indicate that Ca 2+ ,Mg 2+ ,andK + were released from the bottom sediments to the water column during 1984-1994, and NH 4 + , NO 3 - , and SO 4 2- were removed for a net internal alkalinity generation (IAG). Sulfate removal contributed ∼50% of the IAG in the reference basin, and cation production generated ∼40%. In-lake processes in the reference basin removed ∼38% of the sulfate input; 58% was lost to outflow, and 4% remained in the water column. As a result of acid additions that stimulated sulfate reduction and lower pH that enhanced ion exchange, sulfate removal and Ca 2+ production were more important for IAGin the treatment basin. During 1984- 1994, sulfate removal contributed about 61% of the IAG, and Ca 2+ production contributed about half of the IAG from cation production. In the treatment basin, in-lake processes removed about 46% of the total input of sulfate (including acid additions);36% was lost to outflow and 18% remained in the water column (representing ∼25% of the added acid). In both basins of LRL, NH 4 + consumption roughly balanced NO 3 - consumption, and net N transformations provided only 3-12% of the IAG.Overall,Na + and Cl - were conservative in both basins during 1984-1994. Most ion budget components, including calculated internal reaction terms, showed fairly large interannual variations; e.g., ion inputs (dominated by atmospheric deposition) varied by a factor of about two. Over the 10-year period, ANC terms calculated from the budgets as the difference between base cation and acid anion terms agreed well with measured ANC terms for the budget components, indicating that the budgets accounted for all important IAG constituents. Introduction Prior to the early 1980s, microbial reduction of sulfate was not considered to be an important process in freshwater lakes. Atmospheric inputs of sulfuric acid thus were not expected to be neutralized within lakes (reduction ofsulfate to sulfide consumes H + , thus neutralizing acidity). Early models of acid impacts on lakes assumed that inputs of alkalinity to lakes were derived from terrestrial weathering (1).Studiesduringthe experimentalacidification ofELALake 223 in Ontario showed that adding sulfuric acid stimulated microbial sulfate reduction (2) and that this process con- tributed significantly to the lake’s alkalinity. This finding led to further investigations of the in-lake processes that contribute to alkalinity(i.e.,internalalkalinitygeneration or IAG). Studies in the 1980s confirmed the importance of IAG in lake responses to acidification and elucidated the processes contributing to IAG. For example, IAG neutralized 66-81% ofthe acid added to Lake 223 (3),and IAGwas4.5timesmore important than terrestrial processes in providing alkalinity to ELALake 239 (4). IAG is not important in lakes with large watersheds and short hydraulic retention times; terrestrial processes are more important for these lakes. However, IAG isusuallyimportant in regulatingpH and alkalinityin seepage lakes, which have long water residence times and receive most of their water as direct precipitation (5-7). Various methods,from laboratoryexperiments to whole- ecosystem manipulations, have been used to study IAG and define its contributing processes (4, 8-10). These methods provide insights into IAG on a range of spatial and temporal scalesdependingon the mannerofinvestigation.Thispaper evaluatesIAGin Little RockLake,WI,based on annualwhole- basin ion budgets for 1984 to 1994. During that time, the lake’s north basin was acidified experimentally from pH 6.1 to 4.7 in three 2-year steps (11), and recovery from acidifica- tion was followed for 4 years (12). Site description, experi- mentaldesign,and methodsofsample collection and analysis were discussed elsewhere (13). The budgets we developed used data from severalsources,but except where we modified data or quality was an issue, methods used by others are not described here. Little Rock Lake Hydrology and Water Budgets Overview. Little Rock Lake (LRL), Vilas County, WI, is a softwater, oligotrophic lake in an uninhabited, forested watershed with highly permeable sandy soil. The lake has two basins: north (treatment, symbolized by TB): 9.8 ha; south (reference, symbolized by RB) 8.1 ha, but the north is deeper: z max,TB ) 10.3 m ; z m ax,RB ) 6.5 m. LRL has no surface inlets or outlets and is a groundwater recharge lake (Figure 1). It receives almost all its water from direct precipitation. Groundwater inflowoccursintermittentlybut is <2%oftotal annual inflow (14). Installation of a PVC barrier in 1984 to separate the basins (15, 16) caused slight differences in their hydrology. The regional water table slopes down from SE to NW, and groundwater flows only into the SE corner of the reference basin. The resulting head difference caused water to leak around the barrier as transfer outflow (TO) from the reference basin and inflow (TI) to the treatment basin. Precipitation, Evaporation, and Lake Level. Precipitation (P), evaporation (E), and lake stage were measured by the USGS(14).MonthlyPand Edata showseasonalcyclestypical of north temperate regions and a drought from 1986 to 1990 (Figure 2). For annual budgets, P and E were summed over a treatment year (April-March). From 1986 to 1989, annual P decreased from 91% of the average for 1951-1980 to 69%, and Eincreased so that Eexceeded P during1989(14). Above normal rainfall and cooler temperatures ended the drought in 1990. The level of LRL varied during the study period, causing significant changes in basin volumes and area. The highest value (April 7-9, 1986) was 0.93 m higher than the lowest value (August 8, 1990). Because LRL has virtually no watershed, its level responds quickly to rainfall events. For example, the second highest monthly rainfall in the record *Correspondingauthor phone: (612)625-0866;fax: (612)624-1263; e-mail: brezo001@umn.edu. † Current address: General Mills, Inc., Minneapolis, MN. Environ. Sci. Technol. 2003, 37, 5625-5635 10.1021/es034684x CCC: $25.00  2003 American Chemical Society VOL. 37, NO. 24, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5625 Published on Web 11/04/2003