2025 Acid Rain Monitoring (ARM) Report and Results

Welcome to the 2025 Acid Rain Monitoring (ARM) Report page. The ARM Project continues to provide long-term, science-based insights into the effects of acid deposition on Massachusetts' surface waters. This year marks the 24th year of Phase IV of the project, which began in 1983 and remains one of the most enduring citizen-science monitoring efforts in the nation.

Executive Summary

Supported by the Massachusetts Department of Environmental Protection and led by the Water Resources Research Center (WRRC) at the University of Massachusetts Amherst, the Acid Rain Monitoring Project continues to assess and track chemical changes in the Commonwealth’s streams and ponds.

In 2025, 57 primary volunteers from 40 towns sampled 147 water bodies across Massachusetts, analyzed by the UMass Environmental Analysis Lab and nine volunteer-staffed laboratories. All 26 long-term sites—with data dating back to 1985—were successfully analyzed for color and 11 ions (Cl, NO₃, SO₄, Mg, Mn, Fe, Cu, Al, Ca, Na, and K).

Key statewide findings (1983–2025):

• 31% of water bodies showed an increase in pH, while 5% showed a rise in alkalinity.
• 9% exhibited a decrease in pH, and 10% showed a decrease in alkalinity.
• 60% of sites showed no change in pH; 85% were stable in alkalinity.

From 2024 to 2025, acidic sites increased from 45.6% to 59.7%, while neutral sites dropped from 43.0% to 33.6%. Similarly, low-alkalinity sites rose from 39.6% to 45.0%, and moderate-alkalinity sites decreased from 43.6% to 38.3%. These shifts are likely linked to seasonal conditions such as high spring precipitation and runoff, which can temporarily reduce buffering capacity even amid overall long-term chemical recovery.

Ion and color trends (1983–2025):

• Sulfate (SO₄) continued to decline at nearly all sites, reflecting lasting improvements from air quality regulations.
• Sodium (Na) and Chloride (Cl) increased at 20 and 17 sites respectively, linked to road salt applications.
• Potassium (K) rose at 23 sites.
• Color increased at 21 of 26 long-term sites, continuing the "browning" trend associated with rising dissolved organic matter and hydrologic changes.
• Most other ions, including Calcium, Magnesium, Aluminum, and Nitrate, remained stable.

Correlation analysis revealed:

• Strong positive relationships between pH and alkalinity, Na and Cl, and Ca and Mg.
• Strong negative correlations between Color and SO₄, and between pH and Al.

These findings underscore ongoing improvements in water quality due to emission controls, while identifying road salt impacts and localized acidification as continuing concerns.

Methods

Analyses were performed at:

• Volunteer Laboratories and UMass Environmental Analysis Lab (EAL):
  pH and Alkalinity (ANC) analyses were conducted both at the volunteer laboratories and at EAL, ensuring statewide data consistency and cross-validation.
• UMass Environmental Analysis Lab (EAL):
  Color analysis was performed for all long-term sites, in addition to overall QA/QC coordination.
• UMass Extension Soils Laboratory:
  Cation analysis.
• University of New Hampshire Water Quality Analysis Laboratory:
  Anion analysis.

Statistical and visual analyses were conducted using JMP Statistical Software and Matplotlib, allowing detailed trend, correlation, and regression evaluations across the 1983–2025 dataset.

Key Results

pH and Alkalinity Trends

• 31% of all sites showed increasing pH; 5% showed increasing alkalinity.
• 9% of sites showed decreasing pH; 10% showed decreasing alkalinity.
• Most ponds showed higher pH and alkalinity than streams, indicating stronger buffering capacity.
• Streams in central and southeastern Massachusetts remained more acidic and less buffered.

2024–2025 Year-to-Year Changes

• Slight decreases in both median pH and alkalinity were observed in 2025, reflecting natural variability.
• Acidic sites rose to nearly 60%, while neutral sites declined to one-third of the total.
• Streams showed the most pronounced increase in acidity, while ponds remained largely stable.

Long-Term Trends (1983–2025)

• pH recovery continues at many sites, especially in western Massachusetts.
• Alkalinity increased or remained stable at most long-term locations.
• Color continued rising at 21 sites, reinforcing the long-term “browning” trend.
• Sulfate continued its long-term decline, demonstrating the positive legacy of the Clean Air Act Amendments of 1990.

Ion and Color Relationships

• Na–Cl correlation: Strong and consistent, indicating common salt sources.
• Ca–Mg correlation: Reflects similar geologic origins.
• Color–SO₄ inverse correlation: Suggests increasing organic content as sulfate declines.
• Al–pH negative correlation: Higher aluminum solubility in more acidic waters.

Volunteer Network

This year’s success was made possible by 57 volunteers from 40 towns, assisted by family members and student participants. Nine laboratories supported analyses, including:

• Cape Cod National Seashore
• Analytical Balance Corp
• UMass Environmental Analysis Lab
• Upper Blackstone Clean Water
• MIT Sea Grant
• Bristol Community College
• Springfield Water and Sewer Commission
• Westfield State University
• Greenfield Community College

Conclusion

The 2025 ARM Report confirms that Massachusetts’ surface waters continue to recover from historic acid deposition. However, localized acidification, increasing color, and road salt impacts highlight emerging challenges requiring attention. Stable conditions across most sites reaffirm the program’s importance for long-term environmental assessment.

The full dataset has been uploaded to EPA’s Water Quality Exchange (WQX), and the comprehensive 2025 Annual Report has been submitted to MassDEP’s Bureau of Waste Prevention. You can also download the full 2025 ARM Annual Report here and view photos from this year’s volunteer sampling events here.