The 1990 U.S. Clean Air Act Amendments (CAAA) set target reductions for both sulfur and nitrogen emissions to reduce acidic deposition and improve the biologically-relevant chemistry of low ANC surface waters in the United States. The Maine High Elevation Lake Monitoring (HELM) project was designed to complement other acid rain status and trend assessments in the northeast that were known to have underestimated the number of acidic lakes. HELM lakes are more susceptible to the effects of acid deposition than lowland lakes typically included in other surveys because they receive higher amounts of precipitation, and the watersheds are less able to neutralize acidic inputs because of steep slopes, shallow soils, and resistant bedrock. Furthermore, development impacts that affect water quality and cloud our interpretation of recovery from deposition in many lowland lakes are absent in the HELM lakes.
Since 1986, HELM surface water SO4-2 concentration has decreased at a rate of 1.6�eq/L/yr.. HELM lake ANC has increased at a rate of 0.58 �eq/L/yr. and hydrogen ion has decreased at a rate of 0.05 �eq/L/yr. since 1986, highlighting the positive effect the CAAA is having on HELM acidity. Over the same time period, HELM DOC has increased at rate of 0.03 mg/L/yr., raising the median DOC in HELM lakes by 21%. Furthermore, we calculate that organic anions (OA-) now contribute 10% to 15% more to total anionic charge while at the same time, the lakes have become 23% more dilute. The increase in DOC has led to a shift in the source of acidity from anthropogenic inorganic (acid rain), to natural organic DOC sources. While this shift appears to complicate the interpretation of acid-base data coming from acid-sensitive lakes, in reality it highlights recovery to a more natural state for these surface waters.
A comparison of HELM recovery data to recent data from the New Hampshire Remote Pond (NHRP) project serves to put the NHRP in regional perspective as well as enabling us to make inferences about the recovery status of NH's most sensitive aquatic ecosystems. We estimate that the number of acidic NHRP lakes has been cut in half since the CAAA took effect, from 26% to 13%.