RECOVERY OF LAKES IN SOUTH CENTRAL MAINE FROM ACIDIC DEPOSITION

Since the passage of the Clean Air Act Amendment in 1990 atmospheric deposition of sulfur has been substantially reduced. This study examines the chemistry of a suite of 15 randomly selected lakes and ponds in south central Maine in order to determine how changes in atmospheric deposition have impacted surface waters under a variety of surficial geologic conditions.

Samples collected during the summer of 2017 were filtered and analyzed for major cations and silica (ICP-OES); anions (IC); and Total Organic Carbon (TOC combustion analyzer). Alkalinity and pH were measured by Gran Titration

Sulfate concentrations in the lakes fall into three distinct categories. The lowest concentration (7μeq/L in Pickerel Pond) occurs in a small perched seepage lake where sulfate reduction is likely important. Most of the other lakes (10) have concentrations that average around 30μeq/L. Sulfate in these lakes is mainly from atmospheric deposition and reflects the decrease in sulfate deposition since 1990 when the average sulfate concentration was 77μeq/L. Four lakes had higher concentrations and likely reflect local anthropogenic sources.

None of the sampled lakes were acidic, but the lowest pH lakes (5.9 and 6.0) had alkalinities less than 25μeq/L. Most of the other lakes had alkalinities in excess of 100μeq/L and some as high as 500μeq/L. The close correlation of Ca and alkalinity suggest that the weathering of Ca bearing minerals (calcite) are likely important in controlling alkalinity in these systems. Larger watersheds and watersheds with large watershed to lake ratios had higher alkalinities. About half the lakes were impacted by road salt with concentrations in affected watersheds ranging from 10 to over 125mg/L.

The decrease in atmospheric deposition of sulfate has not impacted all lakes equally. Those that currently have the lowest alkalinities should have benefitted the most. It is possible that some of the perched seepage lakes may have recovered from a previous acidic condition.