Water quality is a fundamental metric of environmental health, and its chemical properties directly influence ecosystems and human infrastructure. A primary indicator of water quality is its acidity, which determines how corrosive the water is and what biological life it can support. Understanding the geographic distribution of water acidity in the United States requires examining both natural geology and human activity. This exploration reveals specific regions where water pH levels have dropped significantly below a healthy range, creating substantial environmental challenges.
Defining Water Acidity and the pH Scale
The acidity or alkalinity of water is quantified using the pH scale, which ranges from 0 to 14. This scale measures the concentration of hydrogen ions (\(\text{H}^+\)) in an aqueous solution, operating logarithmically so that each whole number change represents a tenfold shift in concentration. A pH value of 7.0 is considered neutral; numbers decreasing toward 0 indicate increasing acidity, and numbers increasing toward 14 indicate greater alkalinity.
Natural, unpolluted water is slightly acidic, registering closer to pH 5.6. This occurs because atmospheric carbon dioxide (\(\text{CO}_2\)) dissolves into rainwater, forming a weak carbonic acid. Water below this natural range is considered acidic, but a pH drop below 4.5 or 5.0 indicates a severely altered condition. The United States Environmental Protection Agency (EPA) recommends that drinking water fall within a pH range of 6.5 to 8.5.
Primary Drivers of Acidic Water in the US
Water acidity is primarily driven down by two major mechanisms: atmospheric deposition and industrial discharges. Atmospheric deposition, commonly known as acid rain, results when sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)), largely from the combustion of fossil fuels, react with water and oxygen in the atmosphere. This process generates strong acids, specifically sulfuric acid (\(\text{H}_2\text{SO}_4\)) and nitric acid (\(\text{HNO}_3\)), which can be carried over long distances before falling as rain, snow, or dry particles.
The second major source is Abandoned Mine Drainage (AMD) in historical coal and metal mining regions. When mining exposes sulfide minerals, such as pyrite (\(\text{FeS}_2\)), to air and water, a chemical reaction occurs that generates a high concentration of sulfuric acid. The resulting effluent can be extremely acidic, sometimes with a pH falling below 3.0.
The susceptibility of a water body to these acid inputs depends heavily on the surrounding geological composition, known as buffering capacity. Areas rich in alkaline minerals like limestone, which contains calcium carbonate, can naturally neutralize incoming acid. Conversely, regions characterized by granite bedrock or thin, poorly buffered soils, such as the high-elevation areas of the Northeast, are unable to counteract the acid influx, leading to a rapid and sustained drop in water pH.
Geographic Regions with the Lowest Water pH
The most pronounced areas of water acidification in the country are concentrated in regions affected by historical industrial activity coupled with low geological buffering capacity. The Appalachian Coal Belt, spanning states like Pennsylvania, West Virginia, and Kentucky, contains some of the lowest pH water due to abandoned mine drainage (AMD). Here, thousands of miles of streams are continually polluted by AMD, with some discharges in Pennsylvania having been measured at a pH less than 3.0. The extreme acidity in streams like Aaron Run in Maryland, which once fell to a pH of 3.5, is characteristic of this widespread regional issue.
The Northeast and Upper Midwest, particularly the Adirondack and Catskill Mountains of New York and areas of New England, represent another major acidification hotspot. These regions feature hard, crystalline bedrock like granite with minimal capacity to neutralize acid precipitation. The lakes and ponds in the Adirondack Park have been severely impacted by acid rain, with a quarter of the lakes studied registering a pH of 5.0 or below, a level considered too acidic to support most fish species.
Acid mine drainage also creates pockets of severe acidity in the mountainous Western United States, particularly in areas of historic hard-rock mining in Colorado and Montana. In Colorado, abandoned mines contribute to waterways with a pH of 4.0 or lower. For example, Cement Creek, a tributary of the Animas River, is known for its low-pH, metal-rich water that severely impacts the downstream environment.
Environmental and Infrastructure Effects
The consequences of highly acidic water extend across both natural ecosystems and human-built systems. In aquatic environments, a drop in pH below 5.0 is toxic to most fish species, amphibians, and invertebrates. Low pH water mobilizes aluminum from surrounding soils and sediments. This aluminum is harmful to fish, interfering with gill function and causing suffocation. Furthermore, at a pH of 5.0, the eggs of most fish cannot hatch, leading to the collapse of populations and a loss of aquatic biodiversity.
On land, acidic water percolating through soil leaches essential nutrients, such as calcium and magnesium, vital for forest health. The loss of these nutrients weakens trees, making them more susceptible to disease and environmental stress. Mobilized aluminum is also toxic to vegetation, damaging tree roots and inhibiting the plant’s ability to absorb water and nutrients, contributing to forest decline.
For human infrastructure, water with a pH consistently below 6.5 becomes corrosive, posing a risk to drinking water systems. Acidic water aggressively dissolves metals in plumbing, leading to the leaching of heavy metals like lead and copper into the water supply. This corrosion compromises the integrity of the delivery system and introduces contaminants toxic to human health.