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From 2001 until 2011, the U.S. Geological Survey (USGS) conducted a study of the Transport of Anthropogenic and Natural Contaminants to Supply Wells through its National Water Quality Assessment (NAWQA) Program to assess the vulnerability of public drinking water wells. Earlier NAWQA studies found 90 percent of monitoring wells showed contaminant mixtures at low concentrations near the water table in urban areas nationwide. In approximately 22 percent of public water well samples, contaminant concentrations were greater than drinking water standards or other human health benchmarks, implying that water from nearly one in five U.S. drinking water wells may need additional treatment before reaching the public.
The new report, Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality, (aka USGS Circular 1385) presents new ways to understand how these contaminants get into wells, their mobility and persistence and what factors influence contaminant concentrations. The study revealed three measures relating to different vulnerability aspects that are key to assessing well water quality:
1) A measure of contaminant input – those recharge sources and associated contaminants contributing water to a well,
2) A measure of contaminant mobility and persistence – the geochemical conditions influencing water drawn into a well, and
3) A measure of intrinsic susceptibility – the groundwater-age mixture of waters that blend in a well.
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Each of these measures alone defines individual processes and conditions impacting specific aspects of contamination potential. As a group, they provide a comprehensive and systematic way to evaluate both current water quality and long-term potential for contamination.
To begin, groundwater recharge, the process by which water infiltrates the ground and eventually reaches the saturated zone, should be considered for its potential to contribute contaminants to the water supply. The study emphasizes the importance of understanding that contamination from recharge is directly related to the aboveground activities through which water flows before it reaches underground supplies. For example, agriculture land will introduce contaminants like high concentrations of nitrate, while recharge from urban activities is more likely to contain volatile organic compounds (VOCs). At the same time, both land uses have potential to contribute chemicals such as pesticides, although even these may differ according to respective uses. Also important is the relative contribution of each recharge source, such as urban or agricultural land, infiltrating surface water, forests, or wetlands.
The geochemical conditions of groundwater are those factors that influence the chemical reactivity of substances dissolved in water. Specifically, geochemical conditions can define whether contaminants will do one of three things:
- Travel with the groundwater,
- React with the aquifer material, or
- Degrade before reaching a public water supply well.
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According to the study, different conditions (i.e., redox, pH and alkalinity) and how they are distributed within an aquifer have different affects on both manmade and natural contaminants. Understanding these complex chemical processes can provide the information necessary to both anticipate which chemical constitutes may be expected to occur and to define the most appropriate treatments methods for the impacted water.
Intrinsic susceptibility measured by groundwater age-mixture has long been utilized, especially regarding the impact of “young” groundwater often associated with manmade contaminants. The problems arise when a single value is used to define the age of groundwater, not revealing the entire recharge history of the well and resulting in misdiagnosis of vulnerability because contamination influx changes over time.
Another important consideration regarding groundwater age-mixture analysis is the use of tritium. Because it is short lived, radioactive tritium has been used for years to determine recently recharge waters. However, because available atmospheric tritium has declined since aboveground nuclear testing ended, it is becoming less useful as a tool to define recently recharged water and the study offers several alternatives that may be used.