Rogers. The challenge would be to design a system that would be able to provide accurate assessment from multiple water sources.

In partnership with the U.S. Geological Survey (USGS), Texas began to develop a software system that could assist viable decisions with minimal human error, while ensuring ease of usability. The resulting system connects with the state’s multimillion-record drinking water source and geographical information system (GIS) databases, giving the user a three-dimensional visual snapshot of the location or area of interest. It has enabled, for example, development of the Aquifer Atlas of Texas a mapping of all of the state’s aquifers, including the multilayer aquifers that are common in Texas—derived from properties such as transmissivity, porosity, saturated thickness, and storage capacity. The system is based on accurate location information regarding the state’s numerous public wells, together with details such as a well’s depth, casing, and opening intervals, and it is flexible enough to adjust to various conditions such as a well’s intersecting the bottom of an aquifer. Using these three-dimensional data sets the Texas team was able to model the actual effects of water draw on the aquifers. Thus, TCEQ could get more detailed than the fixed radius approach of the 1986 wellhead protection program.

Surface water assessment was more challenging because of the size of the watershed, which required that Texas look at an area of primary influence (API) or an area of primary concern, noted Rogers. By using a digital elevation model to delineate the watershed of a lake, TCEQ could identify an API, in which it wanted to be aware of potentially catastrophic effects.

To fully assess source water, knowledge of point source and nonpoint source contamination is required. While recognizing that knowing the locations of leaking storage tanks, oil and gas wells, and landfills is important and should continue to be monitored, there also has been recognition that nonpoint source contamination will continue to increase in influence in the future, observed Rogers. To assess the effects of nonpoint sources, TCEQ used multiresolution land classifications (MRLCs)—based on 20 different types of land-uses—that divided Texas into approximately 700 million 100-foot squares. These classifications formed the basis for establishing statistical relationships between land-use and water quality and further allowed for predictable equations in areas where the data were incomplete.

The Texas program is a contaminant specific assessment, which requires cataloguing the locations of potential sources of contamination (e.g., airports, landfills, gas stations) and relating specific chemicals to



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