Evaluation of the susceptibility of 

public water supplies to contamination 

is of nationwide interest. To this end, 

the U.S. Environmental Protection 

Agency, through the Safe Drinking 

Water Act Amendments of 1996, 

requires the Texas Natural Resource 

Conservation Commission (TNRCC) 

to complete a Source Water Assessment. The assessment will include the 

identification of potential contamination sources and will generate information regarding the susceptibility of 

public water-supply source waters to 

contamination. This information will, 

in turn, help identify source waters that 

might qualify for less monitoring or 

that might require additional monitoring. The U.S. Geological Survey, in 

cooperation with the TNRCC, is collecting ground- and surface-water 

quality data and using the data to compile a database for use in the Source 

Water Assessment. 

A Need for Sound Data 

One of the requirements for the 

assessment of susceptibility to contamination of public water-supply source 

waters is a comprehensive waterquality database. Currently (1999), few 

data on the quality of Texas drinkingwater sources are available for a number of constituents. Particularly scarce 

are data on the low-level occurrence 

of dissolved pesticides and volatile 

organic compounds (VOCs), including methyl tert-butyl ether (MTBE), 

which are among the most widespread 

contaminants in water supplies nationwide (table 1). These data can be used 

to target those source waters most susceptible to contamination for protection efforts long before contaminants 

reach levels that pose a risk to public 

health.

Building a Water-Quality 

Database

 A comprehensive survey of public 

water-supply sources in Texas is 

underway. The reservoirs and wells 

selected for sampling were chosen to 

cover a large range in the environmental characteristics that could influence 

susceptibility to contamination—for 

example, land use, soil type, and, in 

the case of wells, aquifer type and 

depth to water. Some of the sampled 

water sources might show a limited 

susceptibility to contamination, and 

others might show a higher risk. By 

choosing reservoirs and wells in this 

way, the importance of each of these 

factors in causing contamination can 

be determined. 

Forty-eight of the approximately 

200 water-supply reservoirs have 

already been sampled; 44 reservoirs 

were sampled once, and 4 were 

sampled twice. One hundred and 

eighty of the approximately 16,000 

public water-supply wells across Texas 

will be sampled during December 

1999–June 2000 (fig. 1). One sample 

will be collected from each selected 

public water-supply well before the 

water has been filtered, chlorinated, or 

treated in any other way. Samples will 

be analyzed for 83 dissolved pesticides 

and 6 VOCs (table 1). In addition, the 

ground-water samples will be analyzed 

for tritium to determine the “age” of 

the water. 

Putting the Data to Work

Water-quality data from this survey 

will be used to relate the environmental characteristics of the source watersheds or aquifers to the occurrence of 

contaminants. These relations will be 

used to predict the likelihood that 

other, unsampled source waters are at 

risk. By determining which public 

water-supply sources are most likely to 

be contaminated, monitoring and 

water-quality-protection dollars can be 

used more efficiently.

Determining the Occurrence of Pesticides and 

Volatile Organic Compounds in Public WaterSupply Source Waters in Texas

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 010–00

January 2000

Texans trust in the quality of 

their drinking water. Millions 

of dollars are spent each year 

on water-quality monitoring, 

source-water protection, and 

water treatment to ensure that 

this trust is well-founded. 

These programs are important 

but expensive. 

Just how susceptible are Texas 

drinking-water supplies to 

contamination?

Assessment of susceptibility 

to contamination would enable 

more efficient use of limited 

dollars for monitoring and 

protecting the water supply.

In cooperation with the Texas Natural Resource Conservation Commission
Amarillo

Lubbock

El Paso

Dallas

Austin

San Antonio

Houston

0 50 100 150 MILES

–B.J. Mahler

Any use of trade, product, or firm names 

is for descriptive purposes only and 

does not imply endorsement by the 

U.S. Government.

For more information, please contact:

District Chief

U.S. Geological Survey

8027 Exchange Dr. 

Austin, TX 78754–4733

E-mail: dc_tx@usgs.gov

Phone: (512) 927–3500

FAX: (512) 927–3590

World Wide Web: http://tx.usgs.gov/

Figure 1. Location of public water-supply wells proposed for sampling.

Table 1. Dissolved pesticides and volatile organic compounds included in each sample analysis 

Pesticides

Acetochlor Carbofuran Dichlobenil 3-Hydroxycarbofuran Oryzalin Propham

Acifluorfen Chloramben Dieldrin Lindane Oxamyl Propoxur

Alachlor Chlorothalonil 2,6-Diethylaniline Linuron Parathion Propyzamide

Aldicarb Chlorpyrifos 4,6-Dinitro-2-methylphenol Malathion Parathion-methyl Simazine

Aldicarb sulfone Clopyralid Dinoseb MCPA Pebulate 2,4,5-T

Aldicarb sulfoxide Cyanazine Disulfoton MCPB Pendimethalin Tebuthiuron

Atrazine Dacthal Diuron Methiocarb cis-Permethrin Terbacil

Azinphos-methyl Dacthal monoacid EPTC Methomyl Phorate Terbufos

Benfluralin 2,4-D Ethalfluralin Metolachlor Picloram Thiobencarb

Bentazon 2,4-DB Ethoprophos Metribuzin p,p’-DDE 2,4,5-TP

Bromacil Deethylatrazine Fenuron Molinate Prometon Triallate

Bromoxynil Diazinon Fonofos Napropamide Propachlor Triclopyr

Butylate Dicamba Fluometuron Neburon Propanil Trifluralin

Carbaryl Dichloroprop alpha-HCH Norflurazon Propargite

VOCs

Benzene Ethylbenzene MTBE Toluene m- and p-Xylene o-Xylene

Personnel collecting a sample from a well. 

The sampling outlet is protected with a sheet 

of plastic to avoid contamination during 

sample collection.