Whatever Happened to Acid Rain?

Sept. 1, 2001

In the 1970s, acid rain was a top-level environmental problem that garnered great public attention. Automobiles and industries burning fossil fuels expelled large amounts of sulfur dioxide and nitrogen oxides into the atmosphere. Reacting with water and oxygen in the atmosphere, the substances formed acidic compounds that fell back to earth, killing trees, polluting surface waters, and damaging building facades.

Over the past 30 years, improvements in auto emissions and the use of scrubber technologies have decreased pollution levels, and public attention has waned. But airborne pollutants are still a concern. “The problem has not disappeared,” says Robert Hirsch, Ph.D., associate director for water with the United States Geological Survey (USGS). “There have been real improvements because of scrubbers. We have cleaned up sulfur emissions. But autos especially exhaust lots of nitrates, and regulations have not concentrated on that.”

Many water-quality experts are now concerned about atmospheric deposition. As the name suggests, certain substances are deposited on the earth through the atmosphere. Wet atmospheric deposition can be synonymous with acid rain. Atmospheric deposition also occurs during dry periods, however; trace metals and other pollutants fall to earth, perhaps at a site close to the pollution source or, if carried by winds, to a site miles away. Washed into a water body during storm events–or deposited directly onto its surface–the particles can change the pH of surface water. Deposition of mercury and other substances contributes to surface water pollution. For those developing total maximum daily loads (TMDLs), determining how much of a water body’s impairment is caused by atmospheric deposition of pollutants–as well as the sources of those pollutants and the atmospheric transfer mechanisms that bring them to a particular location–is a growing concern.

Acid Rain and Water Quality

One of Tetra Tech MPS’ elevated sampling stations

After reacting with atmospheric carbon dioxide, rain and snow become slightly acidic (about 5.5 pH); however, this is a natural occurrence. It’s only after precipitation bonds with other airborne chemicals (the main culprits, sulfur dioxide and nitrogen oxides) that it becomes “acid rain.” Precipitation rating 5 pH or less is considered acid rain. According to the EPA’s most recent (2000) figures, precipitation in some parts of the US has a pH as low as 4.3. (As a point of comparison, most healthy soils have a pH between 6.5 and 7.5; tomatoes rate between 4.1 and 4.4 pH.)

Although pollution controls have decreased sulfur dioxide (SO2) levels, the USGS predicts nitrogen oxide (NOx) pollution will increase in the coming years. A recent study prepared for the Hubbard Brook Research Foundation and published in the March 2001 issue of BioScience, “Acidic Deposition in the Northeastern United States: Sources and Inputs, Ecosystem Effects, and Management Strategies,” noted NOx pollution increases as well. The study also concluded that SO2 emissions are still too high, and that it would take an additional 80% cut to neutralize the pH levels of some water bodies.

As for water quality, the report stated that 15% of New England lakes have become acidic. That figure rises to 41% in the Adirondacks, where around 24% of the region’s lakes have become too acidic for fish life.

Assessing the Deposition Problem

Several networks are in place to monitor wet and dry atmospheric deposition. The National Atmospheric Deposition Program (NADP), established in 1978 to monitor long-term trends in precipitation, is a collaboration among more than 100 organizations (including eight federal agencies, state governments, universities, tribal nations, and private companies) that share resources to provide a database of atmospheric deposition throughout the US. The data are generated from networks of precipitation-chemistry monitoring sites, the largest of which is the 220-site National Trends Network (NTN). Every NTN site (there’s at least one in each of the 48 contiguous states, Alaska, the US Virgin Islands, and Puerto Rico) uses identical equipment and operating protocols. Data are gathered weekly, and samples are analyzed for sulfate, nitrate, ammonium, phosphate, pH, chloride, calcium, magnesium, sodium, potassium, and specific conductance. The USGS, the lead federal agency in the NADP, supports 72 of the NTN stations and provides an extensive external quality-assurance program for the entire NTN.

Different regions of the US experience slightly different precipitation trends. NADP data from 1981 to 1998 show the heaviest nitrate concentrations in the Northeast and in the manufacturing areas around the Great Lakes; ammonium in precipitation for the same period is heaviest in the agricultural Midwest. NADP produces maps showing pollutant concentrations year by year.

Estimated ammonium ion deposition, 1999

As electric utility deregulation accelerates, some are concerned (especially after the “power crises” of the past few quarters) that the use of coal and its subsequent emissions will increase. NADP/NTN will continue to monitor for evidence of such activities and any changes in pollution levels.

With growth in confined animal agriculture facilities in some regions, ammonium deposition is increasing, which can result in changes in nutrient runoff, precipitation acidity, and watershed nutrient status. Monitoring nitrogen loading will become increasingly important throughout the network.

In the early 1990s, the Mercury Deposition Network, another component of NADP, was set up to measure nationwide deposition of total mercury and methyl mercury, to define spatial and temporal deposition trends, and to help measure the effectiveness of controls that are put in place.

Another network under the NADP, the Atmospheric Integrated Research Monitoring Network (AIRMoN), was established in 1992 to provide shorter-term tracking of data and to monitor the effects of emission controls. Sponsored by the National Oceanic and Atmospheric Administration’s Air Resources Laboratory, AIRMoN consists of three subnetworks: AIRMoN-wet to monitor wet deposition; AIRMoN-dry to monitor dry deposition; and a planned third network to monitor only air concentrations of various substances. AIRMoN-wet currently collects daily samples at a limited number of sites, tracking the same constituents that are measured at the NADP/NTN sites.

What the TMDL’s Going On?

Atmospheric deposition can be a significant source of nonpoint-source pollutants in a watershed. In his keynote presentation at the TMDL Science Issues Conference in St. Louis last March, Hirsch of the USGS identified the three nonpoint or areal contributors to surface water pollution: surface runoff; groundwater, from which contaminants can migrate to surface waters; and atmospheric deposition. Of the top 15 categories of impairment identified on the 1998 303(d) lists, several (metals, including mercury; substances affecting pH; ammonia) can enter water sources through atmospheric deposition. “Researchers studying the Chesapeake Bay and the Mississippi River noted that atmospheric deposition led to their problems,” notes Hirsch. “Atmospheric deposition is a small but significant contributor.”

Top 15 Categories of Impairment

  1. Sediments
  2. Pathogens
  3. Nutrients
  4. Metals
  5. Dissolved oxygen
  6. Other habitat alterations
  7. Temperature
  8. pH
  9. Impaired biologic community
  10. Pesticides
  11. Flow alterations
  12. Mercury
  13. Organics
  14. Noxious aquatic plants
  15. Ammonia

In the scramble to assess TMDL listings for their waterways, state and local entities are trying to determine what the pollutants are and where they’re coming from. In the case of atmospheric deposition, the task’s not easy. Is the pollution from a local source? Is it from a neighboring city or state? Is it possible to determine how much of the pollutant loading is from atmospheric sources?

Equipment installed on the sampling platform: a dry deposition sampler, ambient air samplers, an ambient monitoring unit for Hg, and a rainwater sampler

In its comments on EPA’s proposed TMDL program, the Pennsylvania Chamber of Business and Industry strongly argued that in cases in which atmospheric deposition of pollutants is the cause of water-body impairment, the water body should be excluded from a state’s 303(d) list. The chamber maintained that it is technically infeasible to quantify the impacts of atmospheric deposition: “While the presence of atmospheric deposition should be considered in developing TMDLs (so as to realistically determine the improvement achievable through reductions from various sources), current science cannot reliably measure, model, calculate, or otherwise assess the impact of atmospheric deposition on water quality or determine its ultimate source.” A further argument was that, since airborne pollutants may cross jurisdictional lines, EPA would impose an “overwhelming” burden on states by requiring them to address this issue.

Nevertheless, several states—Arizona, Florida, Louisiana, North Carolina, South Carolina, and Wisconsin—are developing TMDLs for water bodies impaired by atmospheric deposition of mercury. Other states have identified atmospheric deposition of nitrogen as a significant contributor to surface water impairment.

Given the lack of clear data on the relationship between atmospheric deposition of pollutants and water quality and states’ lack of authority to regulate sources of pollutants that lie across state, tribal, or even national boundaries, suggestions for dealing with the issue have taken on an “airshed” approach rather than a watershed approach. When pollutant sources are clearly outside a state’s control (or when atmospheric pollutant loadings cannot be attributed to a single source), EPA might assist the state in developing a TMDL. If several water bodies are affected by pollutants from the same source or geographical region, EPA, working through the Clean Air Act, might develop TMDLs and a single implementation plan covering all the impaired water bodies.

The typical layout of an air deposition and runoff monitoring site

USEPA has, over the last six years, come at the atmospheric deposition problem from a couple of other directions. In 1998, EPA issued the NOx SIP Call rule, which requires more than 20 eastern states to prepare state implementation plans (SIPs) for reducing NOx emissions; the intent is to block an estimated 40 million lb. of nitrogen per year from being deposited in coastal areas, reducing nitrogen loadings. This is significant because excessive algal growth and outbreaks of the toxic organism Pfiesteria have been linked to excessive nutrient loadings.

In 1995, EPA’s Office of Water launched the Air Deposition Initiative to help characterize atmospheric deposition. In cooperation with EPA’s Office of Air and Radiation, this effort has examined the relationship between water quality and emissions of specific pollutants, especially mercury and nitrogen.

Case Study: Wayne County

The rainwater collector shown closed during dry weather

In the Rouge River watershed in Michigan’s Wayne County (Detroit area), fairly extensive data have been collected on atmospheric concentration and deposition of trace metals. Specially designed sampling and monitoring equipment allowed monitoring of concentrations in stormwater runoff, precipitation, and ambient air.

“Detroit has elaborate sewer systems; some overflow with stormwater,” says consultant Dale Bryson, who at the time of the study was consulting for Wayne County. “Detroit has been trying to keep the CSOs out of the Rouge River. As a rule, Michigan had only been looking for solids and bacteria in the water—cities could use basins and chlorine to remove them. But then someone asked, ‘Hey, what about mercury?’ Detroit tried to assess how much mercury existed in the air, and found it was an uncontrollable source; they [Detroit] were not responsible for the mercury pollution.”

The rainwater collector open
The rainwater collector for metals

While preparing its National Pollutant Discharge Elimination System permit requirement in 1995, the county worked with Tetra Tech MPS in Detroit to measure atmospheric concentration and deposition of polychlorinated biphenyls (PCBs), cadmium (Cd), and mercury (Hg). Tetra Tech placed elaborate monitoring equipment at four sites to identify pollution concentrations in the atmosphere and determine how much was deposited during dry conditions and during rain events.

Carol Hufnagel, a Tetra Tech MPS vice president with expertise in water resources, was heavily involved in the research. “How much of the pollutant mass was originating in rainfall, and what impact would that make in the river? Our basis of comparison was Michigan’s water-quality standards,” she says. “Michigan has ‘designated use’ goals; it wants to make all water fishable and swimmable, and that’s the status we’d like the Rouge River to have.”

Monitoring stations were set up in four areas with different land uses. One was located close to major industry (Livernois Center); another was sited in a densely populated Detroit residential neighborhood characterized by small lots (Rouge River Park); a third was situated in an area of light industry (St. Maron’s Church). The fourth station, built 50 mi. west of Detroit in Dexter, MI, sought to uncover background level pollution from sources outside and upwind from the Detroit metro area.

The study revealed that both wet and dry deposition of all three pollutants (PCBs, Hg, Cd) occurred. The Livernois (industrial) site showed the highest levels of PCB and Hg; the St. Maron’s Church (light industrial) site showed slightly less of each, and Rouge River Park (residential site) had the lowest levels.

As points of comparison, Wayne County’s PCB levels (the highest, 0.9–1.5 ng/m3) were lower than those from a residential site in Bloomington, IN (2.0 ng/m3), and much lower than readings taken at a Chicago urban site (13.6 ng/m3). Mercury readings ranged from 1.8 to 3.2 ng/m3 (range of all the Wayne County sites combined), and were compared to Providence, RI’s 2.4 ng/m3. The rural Dexter site showed mercury levels at 1.7 ng/m3, very close to the lowest Wayne County reading (residential site).

A follow-on study by Tetra Tech as part of the Rouge River national Wet Weather Demonstration Project quantified the concentrations of 12 trace metals (aluminum, antimony, arsenic, cadmium, chromium, copper, lead, manganese, mercury, nickel, vanadium, and zinc) in surface runoff, precipitation, and ambient air at three of the sites: Livernois Center, Rouge River Park, and Dexter. Again, Dexter was included to provide background-level atmospheric deposition. Spatial differences in pollutant concentrations were found, with concentrations generally heavier at the industrial site for most pollutants as expected. The study concluded that atmospheric deposition can be a significant nonpoint pollution source for trace metals.

The Quicksilver Quandary

The underside, or interior view, of the rainwater collector for metals
This part of the rainwater sampler was used to monitor PCB readings.

“There are some pollutants that are uniformly distributed in the atmosphere, either from natural sources or historic pollutants,” says Hufnagel. “Mercury exists in the atmosphere, in some quantity, just about anywhere, not necessarily from a local source. If it’s from a stack emissions source, it could be a fairly distant source.” Mercury was found at the Dexter sampling station sited 50 mi. upwind from Detroit, which indicated that some of the pollution was entering the area from points west (perhaps Illinois, Indiana, or Wisconsin).

“Mercury is pretty much everywhere, at some level,” says Hufnagel. “Even in the Great Lakes, which is high-quality water, there are advisories for consuming certain levels of fish, especially for pregnant women and children, because of mercury pollution.”

Mark Morris of the Great Lakes Initiative points out the many mercury sources: “It’s found in some soils, coal, and fossil fuels. It gets into the atmosphere during the combustion of coal or from medical waste incinerators. Mercury is sometimes in sewage sludge, likely from dental waste, or improper consumer disposal of thermometers.”

“The standard techniques used to measure [mercury] concentrations in water were a couple orders of magnitude higher than what most standards say are the allowable levels in mercury,” Hufnagel notes. “When TMDLs are performed, many that are impaired are from mercury. In our case, the amount of mercury falling from the sky was higher than the standards we were working against.”

“Mercury’s very bad in very small concentrations; it’s caused problems for many years,” Bryson says. “What can make it so deadly is the ‘bioaccumulation’ factors–it goes up the food chain–so humans can get lots of mercury in their systems. You couldn’t measure how much was coming out, so [EPA] made a guideline. You could measure in effluent 1 part per billion, but the standard might be 1/100th part per billion, extrapolated back from humans to the first link in the food chain.

“Now, just in the past months, you can measure it more accurately,” Bryson adds. “The EPA lab in Cincinnati established a new method for measuring mercury at minute levels.” Method 1631 allows detection at levels of 0.5 ng/l, approximately 400 times lower than the level detectable by previous methods.

Mercury remains a large concern, not only for the citizens of Wayne County but also for those downstream of the Rouge River. Just as Wayne County’s pollution levels are affected by industry in states west of Lake Michigan, the Rouge River feeds into the Detroit River and Lake Erie, which could cause increased pollution levels for Ohio, Pennsylvania, and Ontario.

About the Author

Janis Keating

Janis Keating is a frequent contributor to Forester Media, Inc. publications.

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