ENHANCEMENTS OF NONPOINT SOURCE MONITORING OF VOLATILE ORGANIC COMPOUNDS IN GROUND WATER¹

Abstract

The U.S. Geological Survey (USGS) has compiled a national retrospective data set of analyses of volatile organic compounds (VOCs) in ground water of the United States. The data are from Federal, State, and local nonpoint‐source monitoring programs, collected between 1985–95. This data set is being used to augment data collected by the USGS National Water‐Quality Assessment (NAWQA) Program to ascertain the occurrence of VOCs in ground water nationwide. Eleven attributes of the retrospective data set were evaluated to determine the suitability of the data to augment NAWQA data in answering occurrence questions of varying complexity. These 11 attributes are the VOC analyte list and the associated reporting levels for each VOC, well type, well‐casing material, type of openings in the interval (screened interval or open hole), well depth, depth to the top and bottom of the open interval(s), depth to water level in the well, aquifer type (confined or unconfined), and aquifer lithology. VOCs frequently analyzed included solvents, industrial reagents, and refrigerants, but other VOCs of current interest were not frequently analyzed. About 70 percent of the sampled wells have the type of well documented in the data set, and about 74 percent have well depth documented. However, the data set generally lacks documentation of other characteristics, such as well‐casing material, information about the screened or open interval(s), depth to water level in the well, and aquifer type and lithology. For example, only about 20 percent of the wells include information on depth to water level in the well and only about 14 percent of the wells include information about aquifer type. The three most important enhancements to VOC data collected in nonpoint‐source monitoring programs for use in a national assessment of VOC occurrence in ground water would be an expanded VOC analyte list, recording the reporting level for each analyte for every analysis, and recording key ancillary information about each well. These enhancements would greatly increase the usefulness of VOC data in addressing complex occurrence questions, such as those that seek to explain the reasons for VOC occurrence and nonoccurrence in ground water of the United States.

Volatile organic compounds in the nation's ground water and drinking-water supply wells

“…An assessment level is necessary because the detection frequency computed for a specific VOC depends on the laboratory reporting level for that compound. (21) Laboratory reporting levels for VOCs may vary from compound to compound and from one laboratory to another due to differences in laboratory equipment, equipment sensitivity, experience and skill of equipment operators, or laboratory conditions. In addition, data sets collected for different monitoring objectives or analyzed by different laboratory methods also can have different reporting levels.…”

VOLATILE ORGANIC COMPOUNDS IN GROUND WATER FROM RURAL PRIVATE WELLS, 1986 TO 19991

“…Since the laboratory reporting levels for individual VOCs in both NAWQA and retrospective data were variable, an assessment level of 0.2 µg/l was chosen for all analyses. An assessment level was applied uniformly to all VOCs because detection frequency is strongly dependent on the laboratory reporting level (Lapham et al, 2000). As a result, comparisons of detection frequencies between individual VOCs, or between groups of VOCs, with variable laboratory reporting levels may not reflect true differences in water quality unless a uniform assessment level is applied.…”

Occurrence of Volatile Organic Compounds in Aquifers of the United States1

“…For both NAWQA and retrospective data, the minimum reporting levels varied among individual VOCs. The occurrence frequency computed for a VOC is dependent in part on the minimum reporting level for the compound (Lapham et al. , 2000).…”