represents a major milestone in an ongoing cooperative effort 

between the Texas Natural Resource Conservation Commission 

(TNRCC) and the U.S. Geological Survey (USGS) to develop 

and implement a scientifically-defensible methodology for 

assessing susceptibility of Texas’ public water supplies (PWS) to 

contamination.

Background

 The 1996 Amendments to the Safe Drinking Water Act 

require, for the first time, that each state prepare a source water 

assessment for all PWS. Previously, Federal regulations focused 

on sampling and enforcement with emphasis on the quality of 

delivered water. These Amendments emphasize the importance of 

protecting the source water.

States are required to determine the drinking-water source, the 

origin of contaminants monitored or the potential contaminants to 

be monitored, and the intrinsic susceptibility of the source water. 

Under the amendments to the Act, States must create SWAP Programs. The programs must include an individual source water 

assessment for each public water system regulated by the State. 

These assessments will determine whether an individual drinking 

water source is susceptible to contamination. 

During 1997–99, TNRCC and USGS staff met as subjectmatter working groups to develop an approach to conducting 

Source Water Susceptibility Assessments (SWSA) and a draft 

workplan. The draft workplan was then presented to and reviewed 

by various stakeholder and technical advisory groups. Comments 

and suggestions from these groups were considered, and a final 

workplan was produced and presented to the EPA. After EPA 

approval, work formally began on the Texas SWAP Project. The 

project has an expected completion date of September 2002. At 

that time, initial SWSA of all Texas public water supplies should 

be complete.

Ground-water supplies can be considered susceptible if a possible source of contamination (PSOC) exists in the contributing area 

for the public-supply well field or spring, the contaminant travel 

time to the well field or spring is short, and the soil zone, vadose 

zone, and aquifer-matrix materials are unlikely to adequately 

attenuate the contaminants associated with the PSOC. In addition, 

particular types of land use/cover within the contributing area may 

bility of the publicsupply well even though there ma

tifiable PSOC or land use activity.

Surface-water supplies are by nature susceptible to 

tion from both point and non-point sources. The degr

tibility of a PWS to contamination can vary and is a f

the environmental setting, water and wastewater man

practices, and land use/cover within a water supply's 

watershed area. For example, a PWS intake downstre

extensive urban development may be more susceptib

point source contamination than a PWS intake downs

a forested, relatively undeveloped watershed. Surface

supplies are also susceptible to contamination from p

which may include permitted discharges, as well as a

spills or other introduction of contaminants.

The development of a scientifically-defensible me

for assessing the susceptibility of Texas PWS to cont

based on the most accurate, readily available hydrolo

geologic, point source, non-point source, and other n

resource and environmental data, will better enable th

SWAP staff to:

• Focus its source water protection efforts on PWS th

susceptible to contamination.

• Potentially reduce monitoring costs associated with 

safe drinking water.

• Assist the public in developing an improved unders

the source of their water.

• Support the implementation of best management p

needed to protect source waters.

Approach

For the Texas SWAP Project, susceptibility of a PW

as the potential for the PWS to withdraw water conta

contaminant(s), at a concentration that would pose co

through any of the following pathways: (1) direct inje

discharge; (2) soils; (3) geologic strata including faul

or other types of secondary porosity; (4) overland flo

gradient water or streamflow; and (6) cracks in a wel

intake pipe. 

U.S. Department of the Interior

U.S. Geological Survey

USGS Fa
2

Ground-Water Assessments and Surface-Water Assessments subject areas are further defined as sets of components, where each 

component deals with a specific problem domain of the SWSA. 

Assessment Software and Database Structures

The objectives of this subject area are to design and develop 

database structures, assessment software, and technical documentation specifically designed to support staff in performance of 

SWSA on TNRCC computers.

SWSA are technically complex activities dependent on relational database programming and spatial analysis techniques. 

Spatial analysis techniques are mostly available in commercial 

Geographic Information Systems (GIS) software as lower-level 

computer functions. These functions are available as macros or 

“system commands,” but require specialized expertise to combine 

them into usable software components capable of performing the 

higher-level analyses required for SWSA. 

Decision rules governing the assessment of PWS susceptibility 

must be encoded and made available so that they may be 

applied to data derived from spatial analysis, local SWAP-specific 

databases, as well as to data retrieved from TNRCC’s agencywide databases. In some cases, these rules are simple yes/no tests; 

in other cases, a series of logic tests involving several relational 

database files must be applied. The software system developed 

will be easy to use by staff charged with assessing PWS and will 

be compatible with TNRCC's existing databases. Specialized 

training in GIS technology will not be required. Because of the 

volume and variety of required data and the level of technical 

detail of SWSA, the staff requires access to software documentation and help files, metadata, bibliographic, and other supplementary information. The system being developed will make these 

data files and references available to the analyst at all times.

To complete the large number (more than 17,000) of assessments required, the software must be capable of supporting unattended (batch) processing of SWSA. As larger-scale datasets are 

produced for Texas, SWSA will be repeated, hence the ongoing 

requirement for unattended processing. In cases where a single 

assessment (a new PWS) or a small number of assessments are to 

be completed, an interactive version is desired. It is anticipated 

that as SWSA become more technically complex, and larger-scale 

user interface to provide user access to databases, ass

ware, on-line help, and documentation and support fo

or batch processing.

Ground-Water Assessments

The Ground-Water Susceptibility Assessments sub

consists of seven components, each addressing a spec

domain. The primary focus of this subject area is the 

development of databases and software to enable SW

with ground water as the primary source of the water

Identification Component

It is necessary to identify which aquifer a well deri

from, as all subsequent determinations in SWSA are 

aquifer type and hydrologic characteristics of the aqu

In Texas, 9 major and 20 minor aquifers have been 

These 29 aquifers have been subdivided and assigned 

aquifer codes, each having its own geologic, hydrolo

water-quality characteristics. These aquifer codes hav

developed for several uses, including regulation of pu

water, however the 29 major and minor aquifers do n

sufficient detail for the purposes of the SWAP. Altern

requirements for 450 aquifer codes are beyond the sc

component. Thus, agreement was reached between th

stakeholders, including representatives of TNRCC, T

Development Board (TWDB), and USGS, regarding a 

of about 45 aquifer codes that provide adequate detai

Texas aquifers, for the purposes of SWAP, are des

one of five major aquifer categories. Four of the categ

unconfined isotropic aquifers, confined isotropic aqui

aquifers along major rivers, and anisotropic karst aqu

tionally, there are some public ground-water supplies 

do not obtain water from the mapped major and mino

tems, or that obtain water where an aquifer determina

be made. Thus, a fifth aquifer category of “unknown,

for the susceptibility-assessment purposes. Separate a

have been developed for the five aquifer categories, b

their hydrogeologic characteristics.
3

chosen because of the regional scale of the problem, as well as 

knowledge of, and assumptions made about the hydrogeologic 

properties of Texas aquifers. Using specially-developed GIS software, the portion of the flow net that defines the contributing area 

for the water-supply well or spring will be identified, and a determination of time-of-travel to the well for all aquifer categories 

will be made, with the exception of the Edwards aquifer, where 

data from the USGS flowpath investigations for the Edwards aquifer will be used.

Using this approach, the characterization of the aquifer is such 

that the vertical movement of water to the water table is not 

approximated; only the horizontal movement. The assumption is 

that the contributing area to a well in an unconfined system is the 

area directly above the flowpaths for a specified end time (2, 5, 10, 

20, and 100 years). In a confined system, the contributing area is 

that area within specified end times or terminating in the outcrop 

of the aquifer for similarly specified end times.

Tasks associated with this component are focused on developing datasets and software for delineation of contributing areas to 

PWS wells or springs that derive their water from the five categories of aquifers. GIS coverages produced under this component 

include (1) time-of-travel and contributing area for wells or 

springs in confined or unconfined isotropic aquifers and alluvial 

aquifers, (2) contributing area for wells or springs in the Edwards 

aquifer; and (3) contributing area for wells or springs in unknown 

aquifers.

Non-Point Source Component

This component will involve a statewide investigation to 

develop statistical relations between known occurrences of nonpoint source contaminants in ground water and the natural and 

anthropomorphic factors or activities (referred to as environmental 

variables) within the capture zone contributing the water. To supplement existing TNRCC and USGS contaminant occurrence 

databases, 160 PWS wells are being sampled during 1999–2000. 

The PWS wells selected for sampling are located primarily in 

shallow, unconfined aquifers (those most susceptible to non-point 

source contamination) and have characteristics representative of a 

range of environmental variables that may influence source water 

susceptibility. Samples are being collected using specialized, lowlevel detection sampling procedures developed by the USGS and 

are analyzed for selected soluble pesticides, volatile organic com

supply to contamination is locating PSOCs that are w

contributing area of a supply. Selected categories of P

may contribute contaminants to the PWS well or spri

ground storage tanks; operative and closed solid and 

waste-management units, including landfills, surface 

ments, and waste piles; uncontrolled hazardous waste 

spill sites, including Superfund sites; waste injection 

including the family of Class V wells; and septic syst

Texas state databases hold records for an estimate

known PSOCs. While locational information for the 

these sites is available from the databases, the inform

accessible using the spatial analysis software within t

SWAP components. Approximately 10,000 PSOCs ha

locational information (latitude/longitude) as is requi

information for these sites may be available from a p

review of paper files maintained by TNRCC's variou

grams. In some cases, PSOCs may have been located 

topographic maps whereas in other cases only paper 

reports, site drawings, or field sketches may exist. In 

cases, only street address information is available in t

A large amount of work is ongoing to obtain accu

tional data for PSOCs. Interviews with pertinent TNR

who manage PSOC programs were conducted to dete

type, attributes, locations, quality, availability, and do

A comprehensive flowchart and list of interview ques

facilitate this process were developed and followed. F

PSOC that locational data are required, the paper file 

pulled, reviewed, and pertinent information extracted

PSOC, if possible, to be located on a USGS topograp

equivalent. Supplemental maps or commercial databa

address or location information will be required to lo

PSOCs. 

This information is then input into a GIS database 

vide a variety of information on PSOCs, including th

program that collects and manages the PSOC data, th

material for the data, descriptions of data quality, and 

accuracy standards (or needs) for PSOC locations. Th

will be linked to the list of regulated contaminants (a

nant groups) and to Standard Industrial Codes. A rela

base providing technical data on the environmental b

fate contaminants will be developed to assist evaluati
4

also may uncover sources of contamination caused by breaches of 

the confining unit for a confined aquifer. Several existing databases contain ground-water quality data useful for this analysis. 

Using spatial analysis techniques, water-quality sampling sites 

will be identified within a 1-mile search radius around each PWS 

well and spring. If detections of contaminants within this area are 

found, then the PWS would be assessed as being susceptible to 

either anthropogenic or naturally occurring contamination. These 

data will be used to identify sites with contaminant occurrences 

exceeding designated thresholds for specific constituents within a 

1-mile search radius of the PWS well or spring. 

Attenuation of Contaminants

Contaminants released from a point source or from the land 

surface that enter aquifers as solutes in ground water undergo 

physical, chemical, and biochemical processes that lower their 

concentrations in the ground water. The concentration of a contaminant in ground water and time-of-arrival at the point of 

exposure are also determined by the physical, chemical, and biochemical processes that may attenuate (lower) the concentration 

of the contaminant in ground water. Conservative behavior could 

mean that a contaminant might exceed the EPA maximum contaminant level (MCL) within a 20-year time-of-travel period of 

consideration at a PWS. Non-conservative behavior could mean 

that a contaminant might be attenuated in the soil, vadose zone, or 

aquifer matrix, depending on its specific properties, perhaps never 

arriving at the PWS, or arriving at concentrations below levels of 

concern. Thus, it is important to include considerations of fate and 

transport based on behavioral data of each contaminant, and physical properties of soil, unsaturated (vadose) zones, and aquifer 

matrices. 

Although the time-of-travel is the most critical element in the 

evaluation of a PWS susceptibility, an assessment of the attenuation property of the soils, vadose zone, and aquifer matrix in the 

contributing area of the well or spring will be considered in the 

assessment. Some of the most important properties of the soil 

zone affecting contaminant fate and transport are permeability, 

thickness, and total organic material content. Additionally, the 

greater the depth to water, the longer the travel time will be to the 

aquifer through the vadose zone. The rock type of some aquifers 

may also inhibit the transport of some contaminants. A decision 

matrix will be developed for these properties to assess the general

a pre-defined susceptibility determination. The softw

pare the codes generated for each water system again

sion rules and apply a summary determination of sus

This complex and extensive information will be simp

form easily comprehended; a detailed report prepared 

water purveyor; and a summary report produced for t

similar reporting method has been used for the last se

by the TNRCC Vulnerability Assessment Program an

vide a simple, objective, rapid, and automated evalua

Surface-Water Assessments

The Surface-Water Susceptibility Assessments sub

consists of seven components, each addressing differ

domains. The primary focus of this subject area is the 

development of databases and software to enable SW

with surface-water as the primary source of water.

Delineation Component

The contributing watershed area must be determin

face-water intakes or outlets of PWS reservoirs so tha

within the contributing watershed may be identified an

Land use types within the contributing watershed mu

mined to assess their potential non-point source impa

water supply. Characteristics such as rainfall, runoff, 

storage must be obtained for the contributing watersh

the intrinsic susceptibility of each surface-water supp

of watersheds are used in SWSA, as follows:

• Contributing watershed to the intake (delineated at 

reservoir outlet or at the mapped location of the 

stream).

• Contributing watershed to a stream, reservoir, mun

stormwater, or other water-quality monitoring si

• Contributing watershed for all non-PWS reservoirs 

storage capacity greater than 1,000 acre-feet and 

within the contributing area of the PWS intake.

• Truncated watershed (as required) for the area with

contributing watershed to the intake but excludin

contributing watersheds of non-PWS reservoirs 

storage capacity greater than 1,000 acre-feet.
5

(multiple intakes in various reservoirs). Contributing areas may 

also be required for an estimated 90 additional reservoirs located 

within the contributing areas of PWS reservoirs. Finally, areas-ofprimary-influence for all surface PWS must be delineated. 

Using specially developed software, watershed delineations are 

generated and then manually adjusted as necessary. Statewide 

coverages used in the watershed delineation process and created 

specifically or modified for use in this project include: 

• Digital Elevation Models (a new statewide database, developed 

at a 60-meter resolution by the USGS for SWAP).

• Flow direction and flow accumulation datasets.

• Hydrography (streams and reservoir boundaries).

Intrinsic Characteristics Component

Surface-water supplies are all susceptible to contamination to 

some degree because contaminants released at the land surface 

can potentially reach supplies in relatively short time frames. Factors that can affect the relative magnitude of susceptibility are 

geology, soil characteristics, vegetative cover, amount of runoff, 

and attenuation of contaminants in watersheds. Eroded soil may 

carry, adsorbed on the surface of the sediment particles, organic 

chemicals, pesticides, nutrients, and heavy metals. The dilution 

capacity and contaminant degradation capability of a stream or 

reservoir affect the fate, transport, and degradation of contaminants. Finally, the slope of the land is a major control on the timeof-travel of contaminants in runoff. Assessment of each of these 

factors would require very detailed, site-specific data that are not 

readily available in many cases; if the data were available, adding 

each of these components would result in the susceptibility assessment tool being too complex for source water assessment purposes. Instead, the following four broad measures will be used to 

assess the intrinsic susceptibility of a PWS:

• Intrinsic susceptibility associated with mean-annual and meanseasonal surface runoff. 

• Intrinsic susceptibility associated with soil erodibility for 

contributing watersheds of water-supply intakes.

ity soils is being developed that will be used to determ

ceptibility of the PWS to contaminants associated wi

soils. Higher soil erodibility values indicate higher su

lower soil erodibility values indicate lower susceptibi

The potential effect of reservoirs in the watershed 

assessed by analysis of the ratio of total storage in th

to annual runoff in the watershed. High index values 

susceptibility at the intake because of reservoir storag

cial effect resulting from dilution); low index values 

increased susceptibility.

To assess a watershed's intrinsic susceptibility ass

time-of-travel, the ratio of the area of the contributing 

to the basin slope will be calculated. High size-slope 

cate longer time-of-travel and thus less susceptibility

indicate shorter time-of-travel and thus increased sus

Non-Point Source Component

 This component will involve a statewide investiga

develop statistical relations between known occurren

point source contaminants in surface water and the n

anthropomorphic factors or activities (environmental 

within the watershed. To supplement existing TNRC

Rivers Program, and USGS contaminant occurrence 

48 PWS reservoirs are being sampled during 1999-20

PWS reservoirs selected for sampling have watershed

tive of the various hydrologic conditions and land use

Samples are being collected using specialized, low-le

sampling procedures developed by the USGS and are 

selected soluble pesticides and VOCs (including MT

stated in the ground-water component, environmenta

databases to support the development of statistical re

are being compiled that include land use (percent urb

tion density, animal densities/CAFOs, agricultural cr

oil and gas production), selected natural factors (soil 

and hydrologic characteristics), and urban and agricu

cide and nutrient use. TNRCC will develop threshold 

the statistical relations.

Point Source Component

The objective of this component is to assess the su

of surface-water supplies to point source discharges d
6

Some watersheds have naturally occurring contaminants that 

render the water less desirable for human consumption. Thus, 

an analysis, both spatially and temporally, of existing surfacewater quality and PWS point-of-entry (POE) monitoring data is 

needed to determine if the occurrence of a contaminant(s) in water 

is caused by natural conditions in the watershed. Several existing 

databases contain surface-water quality data useful for this 

analysis, such as TNRCC surface-water-quality and entry-point 

databases and USGS National Water Information System databases. Using spatial analysis techniques, these data will be identified within each watershed containing a PWS intake or reservoir. 

If naturally occurring contaminants within a watershed are 

detected, the contributing watershed will be assessed as susceptible to contamination from naturally occurring contaminants. 

Contaminant detections also serve as a confirmation check of 

the methodology for assessing the degree of susceptibility of 

source water to contamination. Stream, reservoir, and entry-point 

monitoring data will be used to verify assessment decisions. If a 

surface-water source is determined to have low susceptibility to a 

particular contaminant, then monitoring data should not reveal 

detections. If monitoring data reveal detections, the assessment 

model needs to be re-evaluated. If a surface-water source is determined to have high susceptibility, data may or may not support the 

assessment. The lack of detection may only mean that the stream, 

reservoir, and PWS, or POE monitoring data were not collected at 

the appropriate “hydrologic” time, such as during or just after a 

runoff event, or during baseflow conditions.

Area-of-Primary-Influence Component

The proximity of a surface-water intake to a point source discharge, potentially adverse land use, major transportation corridor, 

or pipeline can result in the source water being susceptible to contamination. The relatively short time-of-travel of a chemical spill, 

continuous release, or runoff to the intake minimizes the opportunity for reducing a contaminant concentration or converting or 

degrading a contaminant to a less hazardous form. 

The approach will consist of compilation and/or creation of 

GIS datasets as necessary to support area-of-primary influence 

(API) assessments using software developed under a separate task. 

For intakes in reservoirs, an API will initially be defined as the 

susceptibility (decreased susceptibility to increased su

will be assigned on the basis of presence of PSOCs, p

releases or spills of contaminants, and contaminants 

with each specific PSOC in the API. The susceptibili

tion will be guided by the number of PSOC sites, the 

dedicated to activities known to generate contaminan

contaminants and amounts (if available) potentially g

the various activities within the API.

Susceptibility Summary Determination Comp

As in Ground-Water Susceptibility Assessments, t

nent will determine the cumulative susceptibility of t

each listed contaminant or contaminant group, as con

point and non-point sources. The susceptibility determ

be automated using software to populate a matrix-typ

unique codes describing the intake, hydrologic settin

and their contaminant(s), intrinsic susceptibility, etc. 

plex and extensive information will be simplified into 

ily comprehended. A detailed report, as well as a sum

for the PWS will be produced. 

— Randy L. Ulery

For more information, please contact:

Project Chief

Source Water Assessment Project

U.S. Geological Survey

8027 Exchange Dr. 

Austin, TX 78754–4733

Phone: (512) 927–3500

FAX: (512) 927–3590

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

OR

Source Water Assessment and Protection Program

Texas Natural Resource Conservation Commission

P.O. Box 13087

Austin, TX 78711–3087 

World Wide Web: http://www.tnrcc.state.tx.us/water/