Organic Compounds Present at Minute Concentrations in the Hydrologic System

Introduction. Laboratory methods have continuously improved during the past few decades with much lower detection levels enabling some interesting results of human impact on the condition of the hydrologic system. Extremely low levels of certain organic contaminants are present in surface and groundwaters throughout the nation, even in remote wilderness areas. Methyl tert-butyl ether (MtBE) and some chlorinated solvents have been measured at low levels in public supply wells near rivers. Low levels of chloroform and other trihalomethanes produced as a result of the water disinfection process (also called disinfection byproducts) appear nearly ubiquitous in the urban environment.

The source of disinfection byproducts is from the use and application of treated water in the urban environment for landscape irrigation, transport of household wastewater through leaky sewer systems, and also from leaky water pipe systems emanating from water treatment plants. An additional constituent which has low risk-based level, N-nitrosodimethylamine (NDMA), has also been found associated with water treatment plant disinfection byproducts. Even caffeine has been detected in rural and wilderness areas at trace amounts as well as in streams and rivers in urban areas.

A well-known nationwide study was conducted by the U.S. Geological Survey (USGS) in 1999-2000. The USGS collected surface water samples from 139 streams in 30 states across the lower 48 states (Figure 1). The streams tended to be downstream of intense urban or livestock areas and consequently were suspected of contamination. The surface water samples were analyzed for organic wastewater contaminants (OWCs). The results of the surface water sample evaluation were published in Environmental Science & Technology and in a USGS Open-File Report. This article is an updated version of an earlier article from 2005.

Figure 1. Sampling locations of USGS study. Source: Courtesy of USGS

The USGS found that 80% of the streams sampled generally contained low concentrations of OWCs in the <1.0 pgL^-1 range, impacted primarily by residential, industrial, and agricultural products. Seven chemical groups (steroids, nonprescription drugs, insect repellent, detergent metabolites, disinfectants, plasticizers, and fire retardants) were found in more than 60% of the stream samples, and three groups (detergent metabolites, steroids, and plasticizers) contributed to almost 80% of the total measured concentration. More than one-third (33 of 95 target OWCs) of the chemicals detected are known or are suspected of exhibiting at least weak hormonal activity and have the potential to disrupt normal endocrine function (Figure 2).

Figure 2. Most frequently detected compounds. The analytical method number is provided (in parentheses) at the end of each compound

name

Sources of Chemicals. One source of the chemicals identified is products and materials typically used in farming and industry. In addition, a significant contribution was from residential use, drugs typically found in the medicine cabinet, and disposed of as is or as processed human waste in toilets and sinks. Some commonly detected constituents include caffeine in 70.6% of the samples at a maximum concentration of 5.7 µgL^-1, insect repellent N, N-diethyltoluamide in 74.1% of the samples at a maximum concentration of 1.1 µgL^-1, plant/animal steroid cholesterol in 84.3% of the samples at a maximum concentration of 60 µgL^-1, and fecal steroid coprostanol in 85.7% of the samples at a maximum concentration of 150 µgL^-1. The USGS study shows that many of the chemicals used in modern life survive human waste processing, current wastewater treatment plant processing, as well as biodegradation in the natural environment, and are detected as contaminants in potential water supplies.

Contaminant Studies. A long list of contaminants has been documented in surface waters throughout the country partly due to the low detection levels of analysis equipment at chemical laboratories. Considering the interconnection of surface water and groundwater, and the ability of these contaminants to move in the subsurface environment, our shallow groundwater resources are vulnerable to and likely impacted by contamination from these surface water sources. In California, the Groundwater Ambient Monitoring and Assessment Program (GAMA) implemented by the State Water Resources Control Board (SWRCB) is a program to assess the water quality and relative susceptibility of groundwater resources to contamination throughout the state.

This assessment of relative contamination vulnerability is made based on the results of two types of analyses that are not routinely carried out at public water supply wells: ultra low-level measurement (part per trillion) of volatile organic compounds (VOCs), and groundwater age dating (using the tritium-helium-3 method). In addition, stable isotopes of oxygen are measured to help determine recharge water source location. Interpreted together with existing water quality and hydrogeologic data, these parameters assist in assessing the flow field of a groundwater basin and indicate the degree of interconnection between near-surface sources (or potential sources) of contamination, and deeper groundwater pumped at high capacity production wells.

In GAMA studies that were conducted by the USGS and Lawrence Livermore National Laboratory under contract to SWRCB, the two most frequently detected VOCs were disinfection by-products (trihalomethanes) dominantly chloroform, and MtBE, which were found more frequently and at higher concentration in samples from rivers than in samples from nearby shallow public supply wells, suggesting the rivers may be the source of the groundwater occurrences. In addition, chloroform was found associated with wellhead disinfection systems and may be present due to back-flushing of treated water. In one study of the Los Angeles Basin area, it was determined that the overall distribution of low-level VOCs in groundwater was related to the hydrological and engineered recharge facilities. In a study of the Sacramento area, the frequent detection of one VOC (perchloroethylene PCE) was believed to be due to mobilization of numerous point sources from spills or leaks at dry cleaners or other businesses, or possibly, from leaky sewer lines that hold small amounts of PCE.

Trace Detections of Common Chemicals. Although these common chemicals are detected in very-low or trace amounts, the possible cumulative or synergistic effects of these chemicals on plants, animals, and humans is currently unknown. Compunds range from Caffeine, pyrene, fluoranthene and diazinon. There are no known potential adverse health effects of these chemicals at such low documented concentrations.

If treatment or remediation is deemed appropriate, advanced oxidation, enhanced bioremediation, or improved water polishing/filtering could be added to existing wastewater treatment plants to reduce contaminants of these types from entering the environment. More limited pretreatment for septic systems could also help to reduce these chemicals in the environment. For individual water users, small point-of-use treatment systems could be used to remove trace amounts of contaminants.