The computer program, RORA, allows automated analysis of streamflow hydrographs to estimate ground‐water recharge. Output from the program, which is based on the recession‐curve‐displacement method (often referred to as the Rorabaugh method, for whom the program is named), was compared to estimates of recharge obtained from a manual analysis of 156 years of streamflow record from 15 streamflow‐gaging stations in the eastern United States. Statistical tests showed that there was no significant difference between paired estimates of annual recharge by the two methods. Tests of the slopes of best‐fit lines through paired estimates showed that the slopes were not significantly different from unity. Tests of results produced by the four workers who performed the manual method showed that results can differ significantly between workers. Twenty‐two percent of the variation between manual and automated estimates could be attributed to having different workers perform the manual method. The program RORA will produce estimates of recharge equivalent to estimates produced manually, greatly increase the speed of analysis, and reduce the subjectivity inherent in manual analysis.
Two new methods are described for compensating for discharge when evaluating trends in water quality data. One method, discharge normalization, adjusts daily discharges using a central value calculated for the period of record and recalculates daily specific conductance from the adjusted discharges and discharge versus specific conductance regressions. Normalized concentrations for many constituents can then be calculated from linear relationships between specific conductance and constituent concentrations. The second method, discharge‐frequency weighting, weights each observed concentration by a fraction of the total area underneath the discharge‐frequency distribution of the period of record. This fraction is determined using the stream discharge at the time of sampling and the discharge‐frequency distribution for the period of record. The weighted concentrations are summed for each year. Both normalized values and weighted values can be plotted against time to produce trends essentially independent from discharge effects. Results from the methods are statistically similar to each other and to results from other trend detection techniques.
Hydrogeologic and groundwater quality data were collected near the wastewater-treatment plant and associated polishing lagoons at the Marine Corps Air Station, Cherry Point, North Carolina, in 1988. Between March and May 1988, two observation wells were installed upgradient and six wells were installed downgradient of the polishing lagoons and sampled for organic and inorganic U.S. Environmental Protection Agency priority pollutants. Placement of the well screens allowed sampling from both the upper and lower parts of the surficial aquifer. Natural gamma-ray geophysical logs were run in the four deepest wells. Lithologic logs were prepared from split-spoon samples collected during the drilling operations. Laboratory hydraulic conductivity tests were conducted on samples of fine-grained material recovered from the two confining units that separate the surficial aquifer and the drinking-water supply aquifer; values ranged from 0.011 to 0.014 foot per day (4xlO~6 to 5xlO~6 centimeters per second). Static water levels were recorded on April 25, 1988. Relatively low concentrations of purgeable organic compounds (up to 2.2 micrograms per liter for dichlorodifluoromethane), acid and base/neutral extractable compounds (up to 58 micrograms per liter for bis(2-ethylhexyl) phthalate), or pesticides (up to 0.03 micrograms per liter for diazinon and methyl parathion) were detected in water samples collected from all of the wells. Trace metals were detected in concentrations above minimum detectable limits in all of the wells and were found to be higher in water samples collected from the downgradient wells (up to 320 micrograms per liter for zinc) than in water samples from the upgradient wells.
Multiply By To obtain Unit depth per year Volume Description of Study Area Orange County covers approximately 401 square miles (mi 2) in the eastern part of the Piedmont physiographic province in North Carolina (fig. 1). The major population areas in Orange County are Carrboro, Chapel Hill, and Hillsborough. The estimated County population in 1998 was 109,288 people; an increase of 16.4 percent since 1990 (North Carolina Office of State Budget and Planning Management, 1999). Of the total population, about 65,000 people obtain water from public water systems that are dependent upon surface water as the raw water source. The remaining residents, about 40 percent of the total population, obtain water
Prolonged drought, allocation of surface-water flow, and increased demands on groundwater supplies resulting from population growth are focuses for the need to evaluate groundwater resources in the Blue Ridge and Piedmont Provinces of North Carolina. type areas considered representative of the larger terranes. Detailed geologic mapping, core drilling, well installation, and surface and borehole geophysical surveys are in progress at four of the sites.
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