Simulating Errors in Statistical Tests of Leachate-lmpacted Ground-Water Quality

Data on 13 median leachate qualities at 56 landfills show a multivariate character, A simulation technique is used to include this multivariate character of leachate quality in assessing false negative error rates for statistical tests of ground-water quality. Although the technique makes gross simplifying assumptions regarding the conservative mixing of leachate and ground water, the technique is more appropriate than traditional statistical methods for simulating false negative error rates that neglect correlations among water quality parameters. The technique is applied to the simple case of a control chart, intrawell statistical test of ground-water contamination. Simulation results show that both the false positive and false negative errors of statistical tests can be reduced when tests include more parameters and a higher confidence level; thus, the technique can provide a “win-win” situation for regulators and the regulated.     

Relationships Between Ground-Water Silica, Total Dissolved Solids, and Specific Electrical Conductivity

Specific electrical conductivity (SEC), total dissolved solids (TDS), and silica (SiO₂) are ground-water quality parameters routinely measured in a laboratory. Electrical conductivity measurements are made quickly and are less costly than TDS measurements. Once the relationship between the parameters is determined by regression analysis, TDS can be estimated quickly from the SEC and SiO₂ measurements. Water quality data from 25 city wells in Fresno, California, and historical ground-water quality data from the adjacent San Joaquin River/Kings River alluvial interfan (central San Joaquin Valley, California), the Kaweah River alluvial fan, and the Kern River alluvial fan (southern San Joaquin Valley) were used in this investigation. For the specific hydrologic areas studied, the model's TDS predictive ability is improved when SiO₂ is included with SEC as the independent variables.     

Impact of Land Use on Ground-Water Quality in Southern Delawarea

A ground-water quality monitoring study was conducted in Kent and Sussex Counties, Delaware. In the coastal region of Sussex County, 210 wells were sampled 12 times each. In noncoastal Sussex and Kent Counties, 272 wells were sampled once each season over a period of one year. Over 99 percent of the wells were in the water-table aquifer. In coastal Sussex County, 32% of the wells had average nitrate concentrations above the EPA drinking-water standard of 10 mg/1 N. In noncoastal Sussex County and Kent County, 21% and 8% of the wells respectively, had nitrate concentrations above 10 mg/1. The highest nitrate concentrations occurred in areas with intensive broiler production or intensive crop production with excessively-drained soils. Nitrate concentrations in forest areas were <1.5 mg/1. Poultry manure, septic tanks and fertilizers all contributed to the high nitrate concentrations. Poultry manure was one of the major causes of nitrate contamination in four out of the five top prioritized ground-water problem areas. Bacteria contamination was low in all sampling areas. Lead, cadmium and chromium were far below drinking-water standards in all but a few wells.     

Effects of Agriculture on Ground-Water Quality in Five Regions of the United States

Water-quality conditions in surficial unconsolidated aquifers were assessed in five agricultural regions in the United States. The assessment covers the Delmarva Peninsula, and parts of Long Island, Connecticut, Kansas, and Nebraska, and is based on water-quality and ancillary data collected during the 1980s. Concentrations of nitrate in ground water in these areas have increased because of applications of commercial fertilizers and manure. Nitrate concentrations exceed the maximum contaminant level (MCL) for drinking water of 10 milligrams per liter as nitrogen established by the U.S. Environmental Protection Agency in 12 to 46 percent of the wells sampled in the agricultural regions. Concentrations of nitrate are elevated within the upper 100 to 200 feet of the surficial aquifers. Permeable and sandy deposits that generally underlie the agricultural areas provide favorable conditions for vertical leaching of nitrate to relatively deep parts of the aquifers. The persistence of nitrate at such depths is attributed to aerobic conditions along ground-water-flow paths. Concentrations of nitrate are greatest in areas that are heavily irrigated or areas that are underlain by well-drained sediments; more fertilizer is typically applied on land with well-drained sediments than on poorly drained sediments because well-drained sediments have a low organic-matter content and low moisture capacity. Concentrations of other inorganic constituents related to agriculture, such as potassium and chloride from potash fertilizers, and calcium and magnesium from liming, also are significantly elevated in ground water beneath the agricultural areas. These constituents together impart a distinctive agricultural-chemical trademark to the ground water, different from natural water.