Freshwater acidification from atmospheric deposition of sulfuric acid: A conceptual model A new way of addressing the complex temporal relationships between changes in sulfur deposition and changes in surface water chemistry
James N. Galloway University of Virginia Charlottesville, Va. 22903 Stephen A. Norton University of Maine at Orono Orono, Maine 94469
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4~ in surface waters result in decreased alkalinities, and that the concentrations of the BCs (Ca 2 + + Mg2+ + Na+ + K+) are constant. Implicit in this assumption is the assumption also that alkalinities will increase and BC concentrations will remain constant if concentrations of SO4"" in surface waters are reduced by the control of S 0 2 emissions. These models are oversimplified in two respects. First, a change in SO4 - (and H + ) concentrations will not result in a change in alkalinity (e.g., Equations 1 and 4) if increased H + concentrations cause an increase in BC concentrations (Equations 1.-3). Although some reports assume that BC concentrations do not change in surface waters during acidification (2), others assume that they do (5-8). A number of researchers have documented increased leaching of soils and sediments during acidification (9, 10). The magnitude of the changes in BC concentrations depends on the characteristics of the terrestrial system and on the residence time of the SOl" in the system. A second problem is the assumption that a reduction of SO4 - concentrations in surface waters cause an immediate increase in alkalinity. This response is not immediate. The characteristics of the terrestrial system determine 542A
whether the lag in time is weeks, decades, or centuries. This article represents a new way of addressing the complex temporal relationships between changes in sulfur deposition and changes in surface water chemistry by creating a conceptual model that includes • the stages of acidification of aquatic ecosystems, • the stages of recovery, • the time lags in acidification and recovery, and • the processes that control the time lags. The first two points of the model provide a conceptual framework for the last two, which must be addressed before we can state the time scale over which aquatic systems respond to changes in sulfur deposition. Our model is applicable to those lakes where terrestrial processes mediate the lake water acidification process. It does not apply to those unique hydrologie situations where acidification results from the dilution of more alkaline water by less alkaline water (a virtual titration and dilution) or where more acidic water, unaltered by terrestrial processes, replaces less acidic water, volume by volume (for example, during a snow melt). We assume that the net biomass in the terrestrial system is constant. Because S04~ is more important than NOJ in promoting long-term acidification of aquatic systems, we include only SO4 - in the model (8). Controlling processes The acidification of freshwaters is the result of a series of complex, interrelated processes. The series begins with increased emissions of sulfur to the atmosphere. This is followed by "instantly" increased deposition to terrestrial ecosystems and increased concentrations of SO4 - in terrestrial and aquatic ecosystems. We assume that, on the geographical scale of eastern North America, changes in the emission rates of sulfur to the atmosphere result in proportional changes in the deposition rates of sulfur (11). The difference between the residence time of sulfur in the atmosphere and that in watershed-lake systems is reflected in the time lag between the change in atmospheric sulfur deposition and the attainment of new steady-state conditions in watershedlake systems. This time lag can be substantial largely because terrestrial systems are able to act, through sulfate adsorption, as a sink for anthropogenic sulfur (12, 18).
Environ. Sci. Technol., Vol. 17, No. 11, 1983
Increased concentrations of sulfur (as SO4 - ) in the aquatic system also must result in decreased alkalinity, increased BC concentrations, or a combination of the two. Both, however, may not change at the same rate. An initial increase in SO^ - concentrations may result in proportionally large increases in BC concentrations relative to decreases in alkalinity in the aquatic system. This continues for some time until the easily weathered or exchangeable reservoirs of BCs in the soils associated with the hydrologie pathway through the terrestrial system are depleted (that is, the percent base saturation [%BS] approaches zero). Then, the concentration of H + (and possibly aluminum species) increases more rapidly with a concurrent decrease in alkalinity (or an increase in strong acidity). Consequently, a time lag also will exist between the decreased sulfur deposition resulting from emission controls and significantly increased lake alkalinity. The conceptual model The model has seven stages. The effect of each stage on the water composition of an oligotrophic lake is shown in Figure 1. Stage 1. Preacidification stage (t\-t0). This is the steady state prior to significant emissions of anthropogenic sulfur. The concentrations of SO4 - , alkalinity, and BCs are relatively constant over time, and the solution chemistry is dominated by BCs and HCOJ formed by soil solution neutralization during the weathering of primary minerals. Stage 2. Undersaturated sulfate adsorption capacity (SAC) (f 2 -ii). Increased sulfur emissions result in immediately increased sulfur deposition. All of the deposited sulfur, however, does not enter the aquatic systems until the SAC of the soil is saturated. As the increased sulfur deposition partially accumulates in the terrestrial system, the BCs gradually increase in the aquatic system and alkalinity gradually decreases (Figure 1). Stage 3. Saturated SAC in soil (ti-ti)· The soil system is now saturated with respect to the new level of SO4 - deposition. This allows an amount of SÔ4 - equivalent to the sulfur deposition from the atmosphere to be discharged to the aquatic system. Because the concentration of SO4 - in the aquatic system has increased, the anion (HCO3") concentration must decrease or the cation (BC, H + , or Al n+ ) concentration must increase. The exact proportions depend on the
FIGURE 1
The temporal variation of the concentrations (/u,eq/L) of S0 4 ~,BCs and alkalinity (alk) during the seven stages of acidification
Stage 1 to
Stage 2 tl
Stage 3
Stage 4 t3
rate of primary weathering, the %BS and the cation exchange capacity of the soil along the hydrologie pathway of the precipitation, the cation content of the precipitation, and adsorption characteristics of the ion exchange surfaces. Because the BCs are held much less tightly to ion exchange surfaces than are H + ions, initially a greater proportion of BCs accompanies the S0 4 ~ into the lake. Therefore, as Figure 1 shows, the BC concentration increases substantially and H + concentration increases only slightly as the SAC of the soil becomes saturated (t2) in Stage 2. The ratio of [BCs] to [H+] and the %BS of the soil along the hydrologie pathway decrease slowly unless BCs are resupplied from primary weathering or if primary weathering is increased by acid deposition. As the %BS approaches a lower equilibrium value, the [BCs]:[H + ] ratio then decreases relatively quickly until it reaches the new steady-state value (that is, at t3 on Figure 1). At this time, the BCs are supplied mostly from primary weathering of minerals in the hydrologie pathway and only secondarily from depletion of BCs on the ion exchange surfaces. Note that as [BC] decreases
Stage 5
u
Stage 6
Stage 7
ts
during t3-Î2, [H+] also increases. This prevents the formation of HCO^ from primary weathering and results in a negative alkalinity (strong acidity). Stage 4. Steady-state period of lake acidification (Î4-/3). During this stage both the terrestrial and aquatic systems are in a new steady state as a result of a higher level of steady-state deposition of S0 4 ~. Aquatic alkalinities remain low and %BS values in terrestrial systems are near zero. Stage 5. Supersaturated SAC (Γ5-?4). This stage begins with de creases in the deposition of S0 4 ~ (and H + ) resulting from decreases in sulfur emissions. Concurrently, the higher pH shifts the adsorption reactions, and both the terrestrial and aquatic sys tems (soil and sediment) scavenge cations, causing a decrease in BCs. The S0 4 ~ concentration is lowered in sur face waters, although the soils release S0 4 ~ for some time if the S0 4 ~ in the soil is reversibly adsorbed. If the S0 4 ~ is irreversibly adsorbed, the decrease of [S0 4 ~] in the lake or stream ap proximately parallels the S0 4 ~ de crease in atmospheric deposition. Stage 5 ends at ts, when the terres trial system reaches a new steady state as a result of lower rates of SO|~ and
H + deposition. Stage 6. Recovery of%BS (t6-ts)This stage is one in which the sulfur budget of the terrestrial and aquatic systems is in a steady state with respect to the constant rates of S0 4 ~ deposi tion. The changes occurring during this stage are an increase in the %BS of the terrestrial system and a recovery of BCs in surface waters because the BCs supplied by primary weathering now exceed adsorption demands. If the hydrologie pathway through the terrestrial system is such that it bypasses the zone of primary weath ering, the %BS of the soils along the hydrologie pathway recovers only if the supply of BCs from atmospheric deposition and litterfall decomposition exceeds that lost by the decreased SOj" flux. Stage 7. Stable period of lake re covery (?7-f6). The systems are now in steady state with the lower levels of S O ! - deposition. Use of conceptual model The model can be used by defining the pertinent characteristics of each stage and identifying geographical regions that have those characteristics. Because sulfur deposition has not yet begun to decrease significantly, if at all, the pertinent characteristics of Stages 1 -4 only are listed below. • Stage 1 : Systems are not receiving acid deposition. • Stage 2: Systems are receiving acid deposition, soil is unsaturated with S0 4 ~, and the BC concentrations are increasing. • Stage 3: Systems are receiving acid deposition, soil is saturated with S0 4 ~, alkalinity is decreasing, and the BC concentrations are decreasing. • Stage 4: Systems are receiving acid deposition, soil is saturated with sulfur, alkalinity is constant but lower than initially, and the BC concentra tions are constant. Examples of low-alkalinity aquatic ecosystems in Stage 1 are those in the Rocky Mountains (13) and Sierra Nevada Mountains (14). These are not receiving the significantly elevated sulfur deposition that accompanies acid deposition. (Figure 2, Area I). For most of the high-altitude lakes in these areas, alkalinities are < 100 μeq/L and SO4" concentrations are
