Fifty sediment cores from 13 soft-water lakes in north- and south-central Florida were collected to examine the role of surficial sediments in buffering lake water against changes in pH induced by additions of sulfuric acid (H2SO4). Intact cores were collected in duplicate from nearshore and pelagic locations to stimulate in situ lake sediment-water conditions. In addition, batch titrations of sediments treated with chloroform were performed in continuously shaken sediment-water slurries to evaluate ion exchange mechanisms and total mineralogical potential for acid neutralization.
Results from batch bottle experiments showed higher pH values in sediment-water slurries than corresponding lake water pH, suggesting that chemical processes in the sediments tend to counter inputs of acidity to the water column. The acid-neutralizing capacity (ANC) was moderately related to organic content; consequently, ANC was more pronounced in pelagic sediments, which typically accrete organic matter, than littoral sediments, which are more subject to wind-induced scour and resuspension and characteristically are impoverished in organic matter. Calculated areal buffering capacities from the batch studies ranged from 5.5 to 149 meq/m2/cm, indicating that sufficient buffering is available in 1 to 2 cm of sediment to neutralize annual proton loadings via ion exchange alone.
Significant neutralization of acid inputs also was observed in intact cores, and analysis of changes in cation chemistry in the overlying water of the cores suggests that microbially mediated sulfate reduction exceeded ion exchange as the principal alkalinity-producing mechanism. High rates of sulfate removal from the experimental cores support this conclusion; sulfate losses ranged from 32 to 94% in pelagic cores and 39 to 91% in littoral cores. Cores that received no acid showed losses of sulfate ranging from 55 to nearly 100% with concomitant increases in pH in the overyling water. Neutralization of added acid directly attributable to the sediments ranged from 31.8 to 101 meq/m2 over the 4-month duration of the study. Neutralization was independent of particle size distribution or sediment organic content.