VOC's in Streams

VOC data has been collected from only a limited number of streams by the NAWQA program and are not representative of National conditions. Therefore VOC data were not used in calculating the occurrence and significance of contaminants in streams.

VOC's in ground water 

Data on concentrations of  85 aromatic hydrocarbon, reformulated fuel components, and halogenated hydrocarbon compounds. were used from shallow wells screened near the top of the water table where land use near the well was primarily agricultural or urban. These wells were sampled in NAWQA study units begun in 1994. The reporting of volatile organic compounds (VOCs)  only included data from the 1994 Study Units because a change in the analytical method and reporting strategy occurred in 1993 at the end of the 1991 Study Unit high intensity period.  The analytical method was modified to include more compounds (60 for 1991 Study Units to 85 for 1994 Study Units).  The reporting strategy for the analytical method was also changed from a common reporting level of 0.2 ug/L to a lower reporting level based on a long-term method detection limit for each compound and reporting of detections less than the minimum daily standard  The long-term method detection limit is based on multiple instruments and analysts, and was developed over a long time-period (6-months to 1-year).  The minimum daily standard is the lowest daily standard run with a set of samples.  The VOC data from the 1991 Study Units was not included in the analysis because of difficulties in comparing it to the later data of the 1994 Study Units, which had lower reporting levels, and more compounds analyzed Contaminant exceedances were defined using only human-health standards.