A GIS Model for Segregating Populations Served by Public Water Supplies.

Allpress SA, Nuckols JR, Ward MH, Cantor KP, Skinner CC

Federal regulations for monitoring of water contaminants under the Safe Drinking Water Act only apply to public water supplies. Private drinking wells may have higher levels of nitrate and pesticides compared to public supplies because they are often located in intensely agricultural areas, frequently shallower in depth, and not routinely monitored. Because of these differences in water quality between users of public and private water supplies, it is important to be able to determine drinking water source for exposure assessment of populations in health studies.

EHASL developed a GIS-based model to estimate the geographic boundaries of populations served by a community water supply. The model uses a modified nearest-neighbor technique for selecting census blocks and classifying them as served or not served by a specified water utility. The reported population for each census block classified as "served" is added to the predicted service population by the model until it reaches a user-specified threshold. At this point, these census blocks are mapped, thus defining the geographic extent of the service area for the utility.

weld county nitrate graph The model was applied to water utilities serving three towns in Colorado. The predicted extent of the service area for each water utility was tested for validation by comparing distribution pipe network maps with census block boundary maps. Validation data was available for two of the three utilities. Predicted versus reported service populations for these two utilities was 99.8% and 99.0% for these two case studies. The results showed that the boundaries of water utility service areas can be significantly different from township boundaries for communities served by public water supplies.

After validation, the procedure was applied to over 600 counties across 13 states in conjunction with an epidemiological study being conducted by the National Cancer Institute (NCI). These estimates were used for development of population based exposure assessment of nitrates in drinking water supplies at the census block group level. CSU aggregated data to create an exposure index for each county, which is the reporting scale of the cancer registry data used by NCI.

Presented at the 1998 3rd National Conference for GIS in Public Health in San Diego, California, USA.

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