The relation between sinkhole density and water quality was investigated in seven selected carbonate aquifers in the eastern United States. Sinkhole density for these aquifers was grouped into high ([25 sinkholes/ 100 km 2 ), medium (1-25 sinkholes/100 km 2 ), or low (\1 sinkhole/100 km 2 ) categories using a geographical information system that included four independent databases covering parts of Alabama, Florida, Missouri, Pennsylvania, and Tennessee. Field measurements and concentrations of major ions, nitrate, and selected pesticides in samples from 451 wells and 70 springs were included in the waterquality database. Data were collected as a part of the US Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program. Areas with high and medium sinkhole density had the greatest well depths and depths to water, the lowest concentrations of total dissolved solids and bicarbonate, the highest concentrations of dissolved oxygen, and the lowest partial pressure of CO 2 compared to areas with low sinkhole density. These chemical indicators are consistent conceptually with a conduit-flow-dominated system in areas with a high density of sinkholes and a diffuse-flow-dominated system in areas with a low density of sinkholes. Higher cave density and spring discharge in Pennsylvania also support the concept that the high sinkhole density areas are dominated by conduit-flow systems. Concentrations of nitrate-N were significantly higher (p \ 0.05) in areas with high and medium sinkhole density than in low sinkhole-density areas; when accounting for the variations in land use near the sampling sites, the high sinkhole-density area still had higher concentrations of nitrate-N than the low sinkholedensity area. Detection frequencies of atrazine, simazine, metolachlor, prometon, and the atrazine degradate deethylatrazine indicated a pattern similar to nitrate; highest pesticide detections were associated with high sinkhole-density areas. These patterns generally persisted when analyzing the detection frequency by land-use groups, particularly for agricultural land-use areas where pesticide use would be expected to be higher and more uniform areally compared to urban and forested areas. Although areas with agricultural land use and a high sinkhole density were most vulnerable (median nitrate-N concentration was 3.7 mg/L, 11% of samples exceeded 10 mg/L, and had the highest frequencies of pesticide detection), areas with agricultural land use and low sinkhole density still were vulnerable to contamination (median nitrate-N concentration was 1.5 mg/L, 8% of samples exceeded 10 mg/L, and had some of the highest frequencies of detections of pesticides). This may be due in part to incomplete or missing data regarding karst features (such as buried sinkholes, low-permeability material in bottom of sinkholes) that do not show up at the scales used for regional mapping and to inconsistent methods among states in karst feature delineation.

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Cover. Depiction of the 11 mixtures resulting from 4 detected analytes (diagrams of electron shells by Greg Robson, http://commons.wikimedia.org/wiki/User:GregRobson; used with permission). Software for Analysis of Chemical AbstractThe composition, occurrence, distribution, and possible toxicity of chemical mixtures in the environment are research concerns of the U.S. Geological Survey and others.

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One of the great challenges faced by the Nation's water-resource scientists is providing reliable water-quality information to guide the management and protection of our water resources. That challenge is being addressed by Federal, Tribal, State, interstate, and local water-resources agencies, by academic institutions, and by private industry. Many of these organizations are collecting water-quality data for a host of purposes, including compliance with permits and water-supply standards, development of remediation plans for specific contamination problems, decision of operational procedures for industrial, wastewater, or water-supply facilities, and refinement of research to advance our understanding of water-quality processes. In fact, during the past two decades, tens of billions of dollars have been spent on water-quality data collection programs. Unfortunately, the utility of these data for present and future regional and national assessments is limited by such factors as the areal extent of the sampling network, the frequency of sample collection, the variety of collection and analytical procedures, and the types of water-quality characteristics determined. The Lower Columbia River Bi-State Water Quality Program, with involvement from private industry, sports and commercial fishing, public ports, environmental groups, municipal, State, Northwest Power Planning Council, and Federal interests, has provided guidance to: 1. Provide a regionally consistent description of water-quality conditions; 2. Define seasonal and long-term trends (or lack of trends) in water quality; and 3. Identify, describe, and explain, as possible, the major factors that affect observed water-quality conditions and trends.