Ground Water under Direct Influence of Surface Water

The Surface Water Treatment Rule and the Interim Enhanced Surface Water Treatment Rule classify some ground-water sources as "ground water under the direct influence of surface water" (GWUDI), and ground waters classified as GWUDI must follow the same treatment guidelines as surface-water sources. The microscopic particulate analysis (MPA) method is the most widely used method to identify GWUDI; however, this method has several deficiencies. An improved protocol for classifying ground-water sources as GWUDI is presented in this paper. The procedure consists of calculating the absolute risk of illness from waterborne Giardia and Cryptosporidium in the pumped water, accounting for the uncertainty in the surface-water concentrations, hydrogeology, straining characteristics, pathogen viability in ground water, and observations of Giardia and Cryptosporidium in samples of the pumped water. In cases where the risk of illness exceeds the acceptable risk, the ground water should be classified as GWUDI. Two examples are presented to illustrate the application of the proposed method.