USGS NAWQA TANC Background

About Us

The study of public-supply well vulnerability to contamination from compounds commonly found in the environment is one of five national priority topics being addressed by the National Water-Quality Assessment (NAWQA) Program. The Transport of Anthropogenic and Natural Contaminants to Supply Wells (TANC) study began in 2001 with the following general objectives...

Identify the dominant contaminants and sources of those contaminants in public-supply wells in representative water-supply aquifers across the Nation
Assess the effects of natural processes (such as degradation) and human activities (such as irrigation) on the occurrence of contaminants in public-supply wells in representative aquifers
Identify the factors that are most important to incorporate into public-supply well vulnerability assessments in different settings and at different spatial scales
Develop simple methods and models for screening public-supply wells for vulnerability to contamination in unstudied areas and from newly emerging contaminants
Increase understanding of the potential effects of water-resource development and management decisions on the quality of water from public-supply wells

Unique Characteristics of the TANC Study

Sampling at different depths -- The screened or open intervals of public-supply wells are commonly from tens to hundreds of feet in length; therefore, water from these wells is generally a mixture of waters of different ages that enter the well at different depths and are associated with different potential sources of contamination. Using a USGS-developed sampler (Izbicki and others, 1999 and Izbicki, 2004), the TANC study is collecting samples at multiple depths in pumping public-supply wells to ascertain where and how contaminants from different sources enter the wells.

Evaluating multiple settings and scales -- Consistent methods are being used to collect and analyze data, and investigations are being conducted at both regional (tens to thousands of square miles) and local scales (less than 10 square miles). Regional-scale investigations are allowing for comparison of important contaminant-transport mechanisms in water-supply aquifers across the Nation. Local-scale investigations build upon the regional-scale efforts and are focusing on understanding common transport and transformation processes within the contributing areas of public-supply wells.

Exploring the consequences of uncertainty -- To make informed decisions about activities at a particular location, decision-makers need to know whether the location is contributing recharge to public-supply wells. They also need information about traveltimes between potential contaminant sources and public-supply wells. Because this information cannot be measured directly, decision-makers must rely on estimates that are inherently uncertain (due to limitations in the methods). The TANC study is exploring the consequences of this uncertainty, and helping decision-makers understand these consequences, by comparing estimates from traditional and probabilistic modeling approaches with actual water-quality data from public-supply wells.