Existing water quality data collected from domestic wells were summarized to develop the first national‐scale retrospective of self‐supplied drinking water sources. The contaminants evaluated represent a range of inorganic and organic compounds, and although the data set was not originally designed to be a statistical representation of national occurrence, it encompasses large parts of the United States including at least some wells sampled in every state and Puerto Rico. Inorganic contaminants were detected in many of the wells, and concentrations exceeded the U.S. EPA maximum contaminant levels (MCLs; federal drinking water standards used to regulate public drinking water quality) more often than organic contaminants. Of the inorganic constituents evaluated, arsenic concentrations exceeded the MCL (10 μg/L) in ∼11% of the 7580 wells evaluated, nitrate exceeded the MCL (10 mg/L) in ∼8% of the 3465 wells evaluated, uranium‐238 exceeded the MCL (30 μg/L) in ∼4% of the wells, and radon‐222 exceeded 300 and 4000 pCi/L (potential drinking water standards currently under review by the U.S. EPA) in ∼75% and 9% of the wells, respectively. The MCLs for total mercury and fluoride were each exceeded in <1% of the wells evaluated. The MCL was exceeded in <1% of all wells for all anthropogenically derived organic contaminants evaluated and was not exceeded for many contaminants. In addition, 10 contaminants evaluated do not currently have an MCL. Atrazine, however, was detected in 24% of the wells evaluated and was the most frequently detected organic contaminant of the 28 organic contaminants evaluated in this study. Simazine and metolachlor each were detected in ∼9% of all wells and tied for second in frequency of detection for organic contaminants. The third and fourth most frequently detected organic contaminants were methyl tert‐butyl ether (MTBE) (6%) and chloroform (5%), respectively. Because the water quality of domestic wells is not federally regulated or nationally monitored, this study provides a unique, previously nonexistent, perspective on the quality of the self‐supplied drinking water resources used by ∼45 million Americans in the United States.
An extensive 3 H −3He study was performed to determine detailed characteristics of a regional flow system and a sewage plume over a distance of 4 km in a sand and gravel aquifer at Otis Air Base in Falmouth, Massachusetts. 3 H −3He ages increase with depth in individual piezometer clusters and with distance along flowpaths. However, the age gradient with depth (Δt/Δ z) is smaller in the plume than that in the regional waters, due to the intense recharge in the infiltration beds. The 1960s bomb peak of tritium in precipitation is archived longitudinally along a flowline through the main axis of the plume and vertically in individual piezometer clusters. On the eastern side of the sampling area, where water from Ashumet Pond forces plume water deeper into the flow system, 3 H −3He ages are young at depth because the 3 H −3He “clock” is reset due to outgassing of helium in the pond. A reconstruction of the tritium input functions for the regional and plume samples shows that there is no offset in the peak [3H]+[3Hetrit] concentrations for the plume and regional water, indicating that the water from supply wells for use on the base is young. The 3 H −3He ages and detergent concentrations in individual wells are consistent with the beginning of use of detergents and the time period when their concentrations in sewage would have been greatest. Ages and hydraulic properties calculated using the 3 H −3He data compare well with those from previous investigations and from particle‐tracking simulations.
Abstract. The 3H-3He dating method is applied in a buried-valley aquifer near Dayton, Ohio. The study area is large, not all sampling locations lie along well-defined flow paths, and existing wells with variable screen lengths and diameters are used. Reliable use of the method at this site requires addressing several complications: (1) The flow system is disturbed because of high pumping rates and induced infiltration; (2) tritium contamination is present in several areas of the aquifer; and (3) radiogenic helium concentrations are elevated in a significant number of the wells. The 3H-3He ages are examined for self-consistency by comparing the reconstructed tritium evolution to the annual weighted tritium measured in precipitation; deviations result from dispersion, tritium contamination, and mixing. The 3H-3He ages are next examined for consistency with chlorofluorocarbon ages; the agreement is poor because of degradation of CFCs.Finally, the 3H-3He ages are examined for consistency with the current understanding of local hydrologic processes; the ages are generally supported by hydrogeologic data and the results of groundwater flow modeling coupled with particle-tracking analyses. IntroductionEstimates of aquifer properties that govern chemical transport have been made with increasing regularity using transient tracers, such as tritium (3H) and chlorofluorocarbons (CFCs), which have known time-dependent concentrations in precipitation and in the soil air, respectively, and thus in water recharged to aquifers. In groundwater flow systems where water residence times are less than 50 years, transient tracers can be used to estimate groundwater ages and velocities, aquifer porosity, and recharge rates, which in turn can be used in calibrating flow and transport models.Investigations of groundwater systems using tritium have commonly used the 1960s bomb peak of tritium as a time This contribution evaluates the usefulness of the 3H-3He method for dating groundwater in a complex buried-valley aquifer in southwest Ohio. Unlike many of the previous studies using this method, the study area is large, not all sampling locations lie along well-defined flow paths, and existing wells with variable diameters and screen lengths (up to 7 m) are used. In addition, successful application of the 3H-3He technique at this site is dependent on overcoming complications that in this combination, have not been important factors in previous 3H-3He studies. These complications include sampling flow systems altered by high rates of pumping, which may cause fractionation of the gases dissolved in the water; sampling groundwater affected by seepage of pollutants from sources associated with industrial, commercial, and military facilities, including a U.S. Department of Energy (DOE) research facility, near which tritium concentrations greatly exceed typical atmospheric precipitation levels; and sampling waters that have high concentrations of radiogenic helium. The radiogenic helium can cause analytical difficulties and further complicates t...
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