Samples of untreated groundwater from 1255 domestic drinking-water wells and 242 public supply wells were analyzed as part of the National Water-Quality Assessment Program of the U.S. Geological Survey between 1992 and 1999. Wells were sampled to define the regional quality of the groundwater resource and, thus, were distributed geographically across large aquifers, primarily in rural areas. For each sample, as many as 60 volatile organic compounds (VOCs), 83 pesticides, and nitrate were analyzed. On the basis of previous studies, nitrate concentrations as nitrogen g3 mg/L were considered to have an anthropogenic origin. VOCs were detected more frequently (44%) than pesticides (38%) or anthropogenic nitrate (28%). Seventy percent of the samples contained at least one VOC, pesticide, or anthropogenic nitrate; 47% contained at least two compounds; and 33% contained at least three compounds. The combined concentrations of VOCs and pesticides ranged from about 0.001 to 100 µg/L, with a median of 0.02 µg/L. Water from about 12% of the wells contained one or more compounds that exceeded U.S. Environmental Protection Agency drinking-water standards or human health criteria, primarily because of nitrate concentrations exceeding the maximum contaminant level in domestic wells. A mixture is defined as a unique combination of two or more particular compounds, regardless of the presence of other compounds that may occur in the same sample. There were 100 mixtures (significantly associated with agricultural land use) that had a detection frequency between 2% and 19%. There were 302 mixtures (significantly associated with urban land use) that had a detection frequency between 1% and <2%. Only 14 compounds (seven VOCs, six pesticides, and nitrate) contributed over 95% of the detections in these 402 mixtures; however, most samples with these mixtures also contain a variety of other compounds.
Attempts by previous investigators to measure the dielectric constant and electrical conductivity of moist rock and soil samples in the laboratory have produced unexpectedly high values of dielectric constant at low frequencies. These high values have been attributed to measurement error by some critics, making their validity subject to doubt. The present investigation was made to trace the source of the suspected errors and eliminate them if possible. The study resulted in the development of electrochemically reversible electrodes that appear to minimize measurement errors of previous investigators. Measurements were made with the new electrodes over the frequency range 10² to 106 cps using a group of rock and soil samples containing various amounts of moisture. Values of dielectric constant were quite high at low frequencies but were lower than the values obtained by previous methods by as much as an order of magnitude. Statistical correlations were made and empirical equations were derived by computer that gave the following relationships: σ = ƒ(ƒ,w), ϵ/ϵ0 = ƒ(ƒ,w), and ϵ/ϵ0 = ƒ(ƒ, σ100), where σ is conductivity, ϵ/ϵ0 is dielectric constant, ƒ is frequency, w is water content, and σ100 is conductivity measured at 100 cps.
A computer program has been developed by the U.S. Bureau of Mines for designing a two‐dimensional, layered earth model as an aid in interpreting seismic refraction field measurements. The program requires input data consisting of shot‐point and geophone locations, refraction traveltimes, and identification of the refraction layer associated with each traveltime. In the program, a first approximation model is generated by a computer adaptation of the delay‐time method, followed by a series of improved approximations that are made by use of a ray‐tracing procedure. The final result of the program is a model designed to minimize the discrepancy between field‐measured traveltimes and computed traveltimes of rays traced through the model. Test applications indicate that the accuracy of interpretation is improved, and that time and effort of the professional interpreter are greatly reduced by use of the computer technique.
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