Synthetic organic compounds have been detected in ground and surface water, sediment, and biota of the aquifers, rivers, and lakes throughout the United States. These organic compounds enter the hydrologic system in point-source discharges, nonpoint-source runoff, atmospheric deposition, and ground-water discharges.
Pesticides are used to control many different types of weeds, insects, and other pests in a wide variety of agricultural and urban settings. Concerns have grown steadily about the potential adverse effects of pesticides on the environment and human health through contamination of the hydrologic system. Water is one of the primary means by which pesticides are transported from their application areas to other parts of the environment. Through physical processes such as erosion, surface runoff, and ground-water recharge, trace amounts of pesticides used on lawns, gardens, road rights-of-way, and crops can eventually end up in the ground-water system and in streams. Although many modern pesticides are designed to degrade rapidly, the short distance between the land surface and shallow water table makes shallow wells more susceptible to contamination than deeper wells.
Organochlorine pesticides are synthetic hydrophobic organic chemicals that pose a threat to the environment because of their persistence (their use has been banned in the U.S. since the early 1970's) and toxicity to most organisms. They tend to adsorb to organic carbon in suspended or bed sediments rather than dissolve in the water column. Because of this characteristic, these compounds can be present in sediments in concentrations that are much larger than those in the water column. Sediments can provide a mechanism by which organochlorine pesticides remain in a surface-water system many years after their initial introduction and are available for downstream transport and bioaccumulation in aquatic organisms.
During 1993-95, 65 wells that tap shallow ground water were sampled in two agricultural land-use studies (table 2). Eighteen wells had detectable pesticides, with 6 different pesticides detected a total of 23 times. Of the 24 wells that represent urban land use overlying shallow ground water, 8 wells had detectable pesticides, with 5 different pesticides detected a total of nine times. The aquifer subunit survey, which sampled water from existing wells that are completed in the deeper aquifer, yielded only 1 pesticide detection for 30 wells. The pesticides detected most often were prometon (12) and metolachlor (10) (table 2). No concentrations of pesticides exceeded any EPA MCL or HA.
1Pesticide identified but concentration was not determined at a 99-percent confidence level.
The small number of different pesticides detected and the small concentrations of those pesticides indicate little leaching of pesticides from land surface to shallow ground water in these land-use areas. The detection of only one pesticide in the samples from the aquifer subunit survey suggests that shallow ground water containing pesticides has not moved into deeper parts of the aquifer.
(78,527 bytes)
Ground-water sampling at one of the monitoring wells in the San Luis Valley (photograph by Sherman R. Ellis, U.S. Geological Survey).
During 1994-95, 156 water samples were collected at 40 stream and drain sites to study the occurrence and seasonal variability of pesticides. The samples were collected as part of four separate study components (table 3). Collectively, 322 detections of 23 pesticides occurred in 125 of the 156 samples. The most commonly detected pesticides were DCPA (65 samples), metolachlor (53), prometon (37), and simazine (36). The pesticides detected at the largest number of sites were DCPA (25 sites), metolachlor (23), prometon (14), and carbofuran (14). The maximum pesticide concentration detected was an estimated 0.75 microgram per liter carbofuran. The presence of pesticides in surface water is erratic and probably highly dependent on the amount applied and the timing, location, and method of application.
On the basis of data collected, pesticide concentrations are usually small and do not presently appear to be a major concern in the surface waters of the Rio Grande Valley. Table 4 lists the eight pesticides with the largest concentrations; all but one site is located in the Rincon or Mesilla Valley. Of these pesticides, only carbofuran has an established EPA MCL and it was not exceeded. No pesticide concentration exceeded EPA MCLs, HAs, or any applicable Federal or State ambient criteria or exposure guidelines. The relatively frequent detection of pesticides at low levels, however, indicates ongoing exposure that merits careful monitoring. Water-quality standards have not been set for many pesticides and existing standards do not consider cumulative effects of several pesticides in the water at the same time.
The pesticide synoptic study conducted in the Mesilla Valley enabled pesticide concentrations during the irrigation and the nonirrigation seasons to be compared. There were more detections of different pesticides during the nonirrigation season than during the irrigation season. The synoptic study also allowed comparison between urban contributions (samples from wastewater-treatment plant effluent) and agricultural contributions (drains). About one- fourth of the pesticide detections in the Mesilla Valley may be from urban sources.
During the temporal study of drains in the Rincon Valley, pesticides were detected more frequently in drains in April than in October or January. Also, some pesticides were detected in drains but not in shallow ground water. This may mean that pesticides are entering drains as a result of surface runoff from fields or that the timing of ground-water sampling in the area (April-May) was not representative of ground-water quality throughout the year.
1Pesticide identified, but concentration was not determined at a 99-percent confidence level.
As part of the pesticide sampling in the Mesilla and Rincon Valleys, bed sediment also was sampled near the mouths of nine drains. Samples of bed sediment from all nine drains contained detectable concentrations of DDT, DDD, and DDE.
Bed sediment was sampled at 18 sites between September 1992 and March 1993 to characterize the geographic distribution of organochlorine pesticides and polychlorinated biphenyls (PCBs). Six of the bed-sediment samples had detectable concentrations of at least one DDT-related compound. No other organochlorine pesticides were reported in bed sediment.
As part of the whole-body fish-tissue contaminant study, fish were collected at 11 of the 18 bed-sediment sites and analyzed for organochlorine pesticides and PCBs. Concentrations of DDE were detected in composited samples of fish collected at 10 of the 11 sites; these concentrations, however, were below the median reported by the FWS National Contaminant Biomonitoring Program. Cis-chlordane, trans-chlordane, and trans-nonachlor were other compounds detected in whole-body samples of fish from at least one site; these concentrations also were below the median reported by the FWS National Contaminant Biomonitoring Program.
Comparison of pesticide data for bed sediment and whole-body fish tissue indicates (1) organochlorine pesticides were reported more frequently in fish samples and (2) more types of pesticides were detected in fish samples. The presence of DDT and its metabolites, DDD and DDE, in bed sediment and whole-body fish confirms the persistence of this pesticide in the environment.
In July 1995, a site in Elephant Butte Reservoir was cored and analyzed for DDT metabolites (Van Metre and others, 1997). The core was age-dated by correlating sample depth with the radioactive isotope cesium-137. Total DDT concentrations in the core reached a maximum of about 10.5 µg/kg in sediments deposited in the late 1960s, then decreased exponentially, indicating the gradual removal of residual total DDT from the watershed. The largest concentration detected in the core does not exceed sediment-quality guidelines published for aquatic life (Environment Canada, 1995, and Environmental Protection Agency, 1996b).
A site in Elephant Butte Reservoir, one of four major reservoirs on the main stem of the Rio Grande, was cored for analysis of selected pesticides (photograph by Sherman R. Ellis, U.S. Geological Survey).
Volatile organic compounds (VOCs) are carbon-containing chemicals that readily evaporate at normal air temperature and pressure. They are contained in many commercial products such as gasoline, paints, adhesives, solvents, wood preservatives, dry-cleaning agents, pesticides, fertilizers, cosmetics, and refrigerants. Some VOCs are suspected carcinogens and are toxic to humans or wildlife.
Improper disposal of VOCs can result in the leaching or infiltration of these compounds to the shallow ground-water system. VOCs can also be removed from ground water by adsorption onto clays or organic materials, or they can be broken down by bacteria and other microbes in soils. The detection of VOCs in ground water indicates compounds generally associated with human activities leaching into ground water.
Samples were collected from 79 shallow wells and 30 deeper wells for analysis of 60 VOCs. No concentrations exceeded an EPA MCL or HA (table 5).
Water from 9 of the 79 wells sampled had detectable concentrations of one or more VOCs. In the Albuquerque urban land-use study, five different VOCs, two of which were found in more than one sample, were detected in water from 5 of the 24 shallow wells (table 5). Four of the VOCs detected are solvents or metal degreasers. The other is methyl tert-butyl ether (MTBE), a gasoline additive that was detected at a concentration of 7.9 µg/L, which was the largest VOC concentration measured in the Study Unit. In the Rincon Valley agricultural land-use study, only one VOC, xylene, was detected in 3 of 20 samples. In the San Luis Valley agricultural land-use study, MTBE was detected in one well.
Although few VOCs were detected and the concentrations were relatively small, their presence in shallow ground water means that shallow ground-water quality has been adversely affected by human activities. VOCs were detected more frequently in the Albuquerque urban land-use study than in the agricultural land-use studies, perhaps because urban areas have more sources of VOCs.
The wells sampled in the aquifer subunit survey had no VOC detections. This may mean that VOCs have not moved into deeper parts of the aquifer.
Semivolatile organic compounds are a large group of environmentally important organic compounds. Three groups of compounds, polycyclic aromatic hydrocarbons (PAHs), phenols, and phthalate esters, were included in the analysis of bed sediment collected at 17 sites in the Study Unit. These compounds are abundant in the environment, are toxic and often carcinogenic to organisms, and could represent a long-term source of contamination.
The analysis of the PAH data show one or more PAH compounds were detected at 14 sites. Four of these sites had about 60 percent of the detections; three sites had no detections. Two of the four sites are downstream from urban land-use areas, one is downstream from a mining area, and one is in a forested area.
Phenol compounds were detected at 13 sites with 50 percent of the detections at 5 sites. Two sites had no detections. No relation to land use was apparent for the phenol compound detections.
Four phthalate ester compounds were detected at 10 sites. Only one site, downstream from an urban land-use area, had detections of more than one phthalate ester.
