Arsenic, a naturally occurring chemical element, poses a significant concern when present in well water. This metalloid is odorless, colorless, and tasteless, making it impossible to detect without specialized testing. Its presence in drinking water is a widespread issue, affecting millions globally.
Natural Presence in Geology
Arsenic is a common element widely distributed within the Earth’s crust, found in various rock types and mineral deposits. It frequently occurs in sulfide minerals like arsenopyrite (FeAsS). As groundwater flows through these geological formations, natural processes like weathering and erosion can dissolve arsenic from rocks and soils, allowing it to enter the groundwater.
The concentration of naturally occurring arsenic in groundwater varies significantly by region, influenced by the specific geology of an area. Some geological formations are inherently richer in arsenic than others, leading to elevated levels in the local water supply. This geogenic arsenic is a primary source of contamination in many parts of the world.
Human Activities and Their Role
Beyond natural geological sources, human activities can also introduce arsenic into the environment, potentially reaching well water. Industrial processes, such as mining and smelting, release arsenic into the air and water. Coal burning and the manufacturing of products like pesticides and wood preservatives also contribute to contamination.
Historically, agricultural practices, particularly the use of arsenic-based pesticides and herbicides, have played a role. Runoff from fields where these chemicals were applied can carry arsenic into soil and water systems. Improper disposal of arsenic-containing industrial waste also allows the element to leach into groundwater.
Groundwater Chemistry and Release
The release of arsenic from solid materials into groundwater is influenced by specific environmental and chemical conditions within the aquifer. Groundwater pH levels play a significant role, with arsenic mobility high in the typical pH range of 6.5 to 8.5. The oxidation-reduction (redox) potential is also important. Under anaerobic (reducing) conditions, arsenic becomes more soluble and mobile.
Iron oxy-hydroxide minerals, which bind arsenic, can dissolve under reducing conditions, releasing trapped arsenic into the water. Organic matter promotes these conditions by stimulating microbial activity, leading to the dissolution of arsenic-bearing minerals or desorption from surfaces. Arsenic exists primarily as arsenite (As(III)) and arsenate (As(V)) in groundwater, with arsenite being more mobile and prevalent in reducing environments.
Vulnerability of Well Water Systems
Private well water systems are more susceptible to arsenic contamination than municipal supplies. Private wells draw water directly from local geological formations and shallow aquifers, which are influenced by natural arsenic sources. Unlike public water systems, private wells are unregulated by federal and state drinking water standards, placing responsibility for water safety solely on the well owner.
The absence of centralized treatment and routine monitoring means that arsenic, undetectable by taste or smell, can go unnoticed in private wells for extended periods. Well construction, including depth and casing integrity, influences vulnerability by determining which geological layers are accessed. This direct interaction and lack of oversight place private well users at risk of exposure.