Peter McMahon

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Reduction/oxidation (redox) conditions in 15 principal aquifer (PA) systems of the United States, and their impact on several water quality issues, were assessed from a large data base collected by the National Water-Quality Assessment Program of the USGS. The logic of these assessments was based on the observed ecological succession of electron acceptors(More)
[1] In 2000–2002, three rangeland and six irrigated sites were instrumented to assess the storage and transit time of chemicals in thick (15 to 50 m) unsaturated zones (UZ) in the High Plains. These processes are likely to influence relations between land use and groundwater quality, yet they have not been documented systematically in the High Plains. Land(More)
ubsurface hydrologic response to natural climate variability is of particular interest in semiarid and arid regions, such as the western USA, where groundwater resource availability and sustainability are important (Hanson et al., 2004). The response of soil ecosystems in these regions to climate and precipitation variability, including carbon and nutrient(More)
[1] In 2003–2005, systematic studies in four contrasting hydrogeologic settings were undertaken to improve understanding of source and transport controls on nitrate movement to public supply wells (PSW) in principal aquifers of the United States. Chemical, isotopic, and age tracer data show that agricultural fertilizers and urban septic leachate were the(More)
A new generation of radio telescopes is achieving unprecedented levels of sensitivity and resolution, as well as increased agility and field-of-view, by employing high-performance digital signal processing hardware to phase and correlate large numbers of antennas. The computational demands of these imaging systems scale in proportion to BM N 2 , where B is(More)
Ground water systems dominated by iron- or sulfate-reducing conditions may be distinguished by observing concentrations of dissolved iron (Fe(2+)) and sulfide (sum of H(2)S, HS(-), and S(=) species and denoted here as "H(2)S"). This approach is based on the observation that concentrations of Fe(2+) and H(2)S in ground water systems tend to be inversely(More)
1172 P in the vadose zone is nonuniform fl ow that can cause water and solute movement to bypass much of the soil matrix, often resulting in greater water fl uxes and shorter solute transit times than expected (Flury et al., 1994). Understanding preferential fl ow is crucial for quantifying aquifer susceptibility, defi ned as the inherent hydrogeologic(More)
[1] Understanding how changes in land use affect water quality of public supply wells (PSW) is important because of the strong influence of land use on water quality, the rapid pace at which changes in land use are occurring in some parts of the world, and the large contribution of groundwater to the global water supply. In this study, groundwater flow(More)
—In this report we discuss the application of machine learning techniques to the problem of reading text in CAPTCHAs. We first present techniques for reading a CAPTCHA that requires only single character classification and trivial segmentation, and show that we are able to successfully read such CAPTCHAs. We then show how we attempted to read a more(More)