853 Environmental Toxicology and Chemistry, Vol. 20, No. 4, pp. 853–865, 2001 Printed in the USA 0730-7268/01 $9.00 + .00 FREQUENTLY CO-OCCURRING PESTICIDES AND VOLATILE ORGANIC COMPOUNDS IN PUBLIC SUPPLY AND MONITORING WELLS, SOUTHERN NEW JERSEY, USA PAUL E. STACKELBERG,* LEON J. KAUFFMAN,MARK A. AYERS, and ARTHUR L. BAEHR U.S. Geological Survey, 810 Bear Tavern Road, West Trenton, New Jersey 08628 ( Received 24 February 2000; Accepted 14 August 2000) Abstract—One or more pesticides were detected with one or more volatile organic compounds (VOCs) in more than 95% of samples collected from 30 public supply and 95 monitoring wells screened in the unconsolidated surficial aquifer system of southern New Jersey, USA. Overall, more than 140,000 and more than 3,000 unique combinations of pesticides with VOCs were detected in two or more samples from the supply and monitoring wells, respectively. More than 400 of these combinations were detected in 20% or more of the samples from the supply wells, whereas only 17 were detected in 20% or more of the samples from the monitoring wells. Although many constituent combinations detected in water from the supply and monitoring wells are similar, differences in constituent combinations also were found and can be attributed, in part, to differences in the characteristics of these two well types. The monitoring wells sampled during this study yield water that typically was recharged beneath a single land-use setting during a recent, discrete time interval and that flowed along relatively short paths to the wells. Public supply wells, in contrast, yield large volumes of water and typically have contributing areas that are orders of magnitude larger than those of the monitoring wells. These large contributing areas generally encompass multiple land uses; moreover, because flow paths that originate in these areas vary in length, these wells typically yield water that was recharged over a large temporal interval. Water withdrawn from public supply wells, therefore, contains a mixture of waters of different ages that were recharged beneath various land-use settings. Because public supply wells intercept water flowing along longer paths with longer residence times and integrate waters from a larger source area than those associated with monitoring wells, they are more likely to yield water that contains constituents that were used in greater quantities in the past, that were introduced from point sources, and/or that are derived from the degradation of parent compounds along extended flow paths. Keywords—Co-occurrence Pesticides Volatile organic compounds Public supply wells Monitoring wells INTRODUCTION Recent investigations indicate that synthetic organic com- pounds in surface and groundwaters of the United States occur most frequently in combinations of two or more compounds, generally at small concentrations [1–3]. These findings are significant for several reasons. Recent investigations indicate that the co-occurrence of multiple organic compounds may modify anticipated properties of individual compounds. For example, multiple pesticides can compete for soil sorption sites, thereby reducing the overall sorption rate for certain individual pesticides [4]. Furthermore, microbial degradation of an individual pesticide can be inhibited by the presence of additional pesticide compounds as a result of the enhanced toxicity of the pesticide mixture to micro-organisms [5]. These effects also may interact in an antagonistic way whereby com- petitive sorption maintains the availability of certain pesticide residues for microbial attack, thereby increasing microbial degradation of those compounds [5]. The identification of fre- quently occurring combinations of organic compounds and the determination of the effect of co-occurrence on properties that govern environmental fate and persistence are, therefore, of interest. Drinking-water criteria currently are based on the toxicity of individual compounds and not to combinations of com- pounds. The possibility that exposure to multiple organic com- pounds, even at low concentrations, may have a synergistic adverse human health consequence is an area of recent re- search. Findings from this research indicate that combinations * To whom correspondence may be addressed (pestack@usgs.gov). of organic compounds have toxic effects on aquatic organisms that cannot be attributed to the presence of any individual compound within the mixture [6]. In addition, the U.S. De- partment of Agriculture reports that exposure to combinations of two pesticides (e.g., atrazine and metolachlor or atrazine and dimethenamid) proved lethal to certain soil invertebrates at concentrations four to five times lower than exposure to the individual compounds [7]. Several researchers [8,9] have dem- onstrated that low-level concentrations of certain herbicides, similar to those commonly observed in environmental samples, can cause chromosomal damage during cell development. It is, therefore, of interest to determine which organic compounds frequently co-occur in water used for public supply. The 1996 Food Quality Protection Act mandates that the U.S. Environmental Protection Agency consider information on aggregate exposure and common modes of action when establishing acceptable pesticide-residue levels. Previously, acceptable levels of pesticide residues were established solely on the basis of dietary intake of individual active ingredients. Under the 1996 Food Quality Protection Act, exposure from all nonoccupational routes, including food, drinking water, and home- and lawn-pest control, must be taken into account. Thus, low-level concentrations in a given exposure route, such as drinking water, become a factor to be considered. Analysis of water samples collected from a network of supply and monitoring wells in the Glassboro area of southern New Jersey (Fig. 1) as part of the U.S. Geological Survey’s (USGS) National Water Quality Assessment Program shows that pesticides and volatile organic compounds (VOCs) occur frequently, generally at low-level concentrations [10,11].