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The U.S. Geological Survey, in cooperation with the U.S. Centers for Disease Control and Prevention and the Maine Center for Disease Control and Prevention, assessed the chemical characteristics and the occurrence, distribution, and oxidation state of inorganic arsenic in drinking water from selected domestic well-water supplies in Maine in 2001–2 and 2006–7. The data collected provide support for evaluating arsenicremoval efficiencies of household water-purification systems and provide information to State and local officials that can be used in determining a water-treatment approach for the removal of arsenic from drinking water. Introduction Arsenic is toxic to humans and widespread in surface water and groundwater throughout the United States and world (National Research Council, 1999; Welch and others, 1999; Welch and others, 2000; Focazio and others, 2000; Ryker, 2001; Smedley and Kinniburgh, 2002; DeSimone and others, 2009). Arsenic in surface waters and groundwater aquifers has both anthropogenic and natural sources. Anthropogenic sources include current and historical industrial uses, as well as broad-spectrum uses in the lumber, agriculture, and livestock industries (National Research Council, 2001). Arsenic occurs naturally in the Earth’s crust, ranking as the 20th most abundant element, with an average crustal abundance of approximately 2 milligrams per kilogram (0.0002 percent). It is common in weathered volcanic and marine sedimentary rocks containing iron and manganese oxides, in fossil fuels, and in a number of minerals, including arsenopyrite (FeAsS), orpiment (As2S3), scorodite (FeAsO4), and realgar (AsS) (Cullen and Reimer, 1989; Korte and Fernando, 1991; Smedley and Kinniburgh, 2002). 1U.S. Geological Survey. 2U.S. Centers for Disease Control and Prevention. 3Maine Department of Health and Human Services. Arsenic in groundwater is primarily inorganic, either in the oxidized state as arsenate [As(V)], the form typical in oxic waters, or the reduced state as arsenite [As(III)], the form typical in anoxic waters (Cullen and Reimer, 1989; Smedley and Kinniburgh, 2002). Roughly half of the population of Maine derives its drinking water from domestic wells, of which about 75 percent are drilled bedrock wells. Arsenic concentrations above the current U.S. Environmental Protection Agency (EPA) drinking-water standard of 10 micrograms per liter (μg/L or parts per billion) (U.S. Environmental Protection Agency, 2001) have been detected in water from drilled bedrock wells throughout Maine (Marvinney and others, 1994, 1995; Peters and others, 1999; Ayotte and others, 1999, 2003, 2006; Lipfert and others, 2006; Yang and others, 2009; Nielsen and others, 2010; Zheng and Ayotte, 2015; Flanagan and others, 2012). Effective removal of arsenic by household water-purification systems is constrained by the chemical form (for example, oxidation state) in which it occurs, as well as the prevailing chemical characteristics of the well water, including pH, and the presence of competing ions, such as phosphate (U.S. Environmental Protection Agency, 2001). The U.S. Geological Survey (USGS), in cooperation with the U.S. Centers for Disease Control and Prevention and the Maine Center for Disease Control and Prevention, assessed the chemical characteristics and the occurrence, distribution, and oxidation state of inorganic arsenic in drinking water from selected domestic well-water supplies in Maine in 2001–2 and 2006–7. The data collected provide support for evaluating arsenicremoval efficiencies of household water-purification systems and provide information to State and local officials that can be used in determining a water-treatment approach for the removal of arsenic from drinking water. Purpose and Scope This report describes methods used to collect arsenic and other water-chemistry data from domestic well water in support of studies evaluating arsenic removal efficiencies of household water purification systems. Data consist of water-chemistry results from samples collected over two time periods: 2001–2 and 2006–7 (also two samples collected 2 Methods of Collection and Quality Assessment of Arsenic Data in Well-Water Supplies in Maine in 2008), well characteristics (depth and type), and chemical characteristics of well water from approximately 120 selected domestic wells in known high-arsenic clusters throughout the State of Maine. All data are available in the USGS National Water Information System (NWIS) database, are searchable by USGS site number (U.S. Geological Survey, 2019), and are published in Culbertson and others (2020). Description of Study Area The study area includes most of the populated areas in the State of Maine apart from Aroostook County in northeastern Maine. The samples collected were from singlefamily dwellings relying on private well water as the primary drinking-water supply; additionally, these households had water-purification systems designed for the removal of arsenic installed either at the well-water point of entry (POE) or point of use (POU). The initial pilot study included 31 households sampled for arsenic and arsenic species in 2001–2, whereas the full-scale study included 120 households sampled in 2006–7. Households were recruited based on their locations in areas of Maine known to have high concentrations of arsenic (>10 μg/L) in groundwater (fig. 1). Methods of Data Collection Households with domestic wells included in this study were identified and prescreened prior to both sampling periods by project personnel associated with concurrent studies by the U.S. Centers for Disease Control and Prevention and the Maine Center for Disease Control and Prevention. To be considered for the study, households needed to have well water containing elevated (>10 μg/L) concentrations of arsenic and a previously installed water-purification system designed to remove arsenic; systems used reverse-osmosis, anionexchange, or adsorption technologies. Most homes tested had point-of-use reverse-osmosis systems installed. Well-water samples (pre-water-treatment system) were collected at the house POE, prior to the pressure tank and household watertreatment system(s), by using the existing domestic well-water pump. Samples collected after water treatment were collected at the kitchen (POU) faucet. A list of constituent names, USGS NWIS parameter codes, and minimum reporting limits for all samples collected for laboratory and field analyses are shown in tables 1 and 2. A summary of field measurements and analytical determinations, along with the entity or entities that analyzed or measured each, is presented in table 3. Measurement of Field Water-Quality Constituents Prior to collection of POE well water for arsenic and other chemical analyses, field measurements of temperature, specific conductance, pH, and dissolved oxygen were measured onsite using a multiparameter water-quality monitor immersed in a flow-through chamber under a steady water flow of approximately 1–1.5 liters per minute. Field measurements and water-chemistry results are published in Culbertson and others (2020). During the 2006–7 sample collection period, low dissolved oxygen readings (<1 mg/L) were confirmed by an alternate, colorimetric method (CHEMets colorimetric dissolved oxygen test kit, 0–1 part per million; CHEMetrics, Inc., Calverton, Va.). Samples for water chemistry determinations and arsenic analyses were collected after water-quality-monitor readings had stabilized (generally 0.25–0.75 hour). Except for specific conductance, field measurements of the other water-quality constituents in POU water samples were not made because of physical and chemical alterations imparted by the water-treatment systems. Specific conductance, a proxy for total dissolved solids, was measured in both prefiltration and postfiltration water, specifically to assess the performance efficiencies of reverse-osmosis water-treatment systems in removing arsenic (Fravel and Lindsey, 2014). Sample Collection for Well-Water Chemical Analyses Performed in the Field Upon obtaining stabilized readings of temperature, specific conductance, and pH, well-water samples for field chemical analyses were collected. Field measurements (or analyses) of POE well water included total iron, ferrous iron (FeII), total manganese, reactive phosphorus, sulfate, sulfide, and nitrate. These measurements were made immediately upon sample collection using standard colorimetric methods (American Public Health Association, 1998; Hach Company, 2000). Equipment blanks and sample replicates for quality-control (QC) purposes were collected and analyzed with the field samples. Additionally, during the 2006–7 sample collection period, replicate samples for some constituents were collected for chemical analysis by an alternate laboratory and method (Underwriters Laboratories Inc. [UL], South Bend, Indiana). Results of field chemical and quality-control analyses are published in Culbertson and others (2020). Point-of-Entry and Point-of-Use Sample Collection for Laboratory Chemical Analysis of Well Water According to study protocols for the 2006–7 study, two laboratories (Underwriters Laboratories Inc. [UL], South Bend, Indiana, and the Health and Environmental Testing Laboratory [HETL] of the Maine Department of Human Services4 in Augusta, Maine) were used to measure arsenic concentrations. The laboratory results from these two facilities are published here as a means of comparison of the analytical 4Since July 2004, the Maine Department of Health and Human Services. Methods of Data Collection 3 SOMERSET PISCATAQUIS

Methods of collection and quality assessment of arsenic data in well-water supplies in Maine, 2001–2 and 2006–7