Texas Source-Water Susceptibility Assessment

Science Center Objects

In the State of Texas, both surface water (streams, canals, and reservoirs) and ground water are used as sources of public water supply (PWS). Surface water sources of public water supply are susceptible to contamination from point and nonpoint sources. To help protect sources of drinking water and to help develop protective yet cost-effective and risk-mitigated monitoring strategies, the Texas Commission on Environmental Quality (TCEQ) and the U.S. Geological Survey (USGS) developed procedures to assess the susceptibility of PWS source waters to the occurrence of 227 contaminants.

One component of the assessments was the determination of susceptibility of surface water sources to nonpoint source contamination. To accomplish this, water quality data at 323 monitoring sites were matched with geographic information system (GIS) derived watershed characteristic data for the watersheds above the sites.

In many earlier USGS investigations, strong relations between the occurrence of agricultural chemicals in streams and their use, or land-use within the up-stream watershed have been noted. In some investigations statistical models were used to describe these relations. One type of model, logistic regression, estimates the probability that a particular chemical will be above or below a specified concentration. Chemical use patterns, streamflow, land-use within the up-stream watershed, physical characteristics of the watershed, climate, soil type, and other factors are the independent variables in these models, which are conditioned using available water-quality data. Once developed for a region and if sufficiently accurate, logistic regression models can be used to model the probability of chemical occurrence at sites where water-quality data are not available. The logistic regression models developed by RPE researchers for Texas are one part of a tool and developed by Randy Ulery and others at the Texas office of the USGS to support the Texas Natural Resources Conservation Commission (TNRCC) Source Water Assessment Program, now the Texas Commission on Environmental Quality (TCEQ), efforts to assess the susceptibility of all Texas public-water supplies to contamination.

Tractor spraying agricultural chemicals

Tractor spraying agricultural chemicals