In order to save time and effort, people often want to try and answer the second question first by collecting water samples and using Microscopic Particulate Analysis (MPA) to determine if pathogens (and other indicator organisms, such as living algae) are present in the collected drinking water. Unfortunately, while the presence of pathogens is sufficient to determine that a groundwater source is under the influence of surface water, the absence of pathogens in a water sample alone cannot give adequate proof that a surface water influence does not exist. This is because a surface water influence may only occur seasonally. Also, while these waterborne pathogens are commonly found in surface water, their concentrations are usually very low. Furthermore, there is no certification process for laboratories performing MPAs, and laboratory methodologies and criteria for data interpretation are difficult, expensive, subjective, and not fully reliable. Therefore, MPA samples should be taken only at the specific times when an strong surface water influence is indicated by water quality data. Only under these circumstances can the absence of GWUDI indicators in MPA samples provide reasonable proof that the ground provides adequate natural filtration to remove pathogens.Once the potential for GWUDI is established through screening, a more detailed investigation should begin. This can be either a Hydrogeological Assessment or a Water Quality Assessment. However, unless hydrogeological work is being done for another purpose (e.g. finding adequate quantities of water), this option will not often be practical due to difficulty, expense, and the need to do a Water Quality Assessment if results are inconclusive.A GWUDI Water Quality Assessment is designed to detect the rapid movement of surface water to an underground drinking water collector. These studies simply look at fluctuations in the drinking water's temperature and conductivity and try to determine if these fluctuations are overly affected by conditions measured in the surface water or the weather. For example, GWUDI conditions are often indicated by the temperature and/or conductivity of collected drinking water shifting to resembling the surface water during periods of high pumping and/or flooding. While the data collection for these assessments is relatively easy to perform, the rapid onset and seasonal nature of GWUDI conditions require a daily sampling interval, often over the course of an entire year.

As stated above, MPA samples should only be collected at times when a strong surface water influence is suspected, as indicated by water quality data. For example, if the temperature and conductivity measured in the drinking water both increase to resemble that of the surface water about 10 days after a big rain, then this is when MPA samples should be collected. The purpose of these samples is to determine if the soil provides adequate filtration at times when there is strong indication that surface water is moving quickly into a drinking water collector.