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Ground Water Wells Under the Direct Influence of Surface Water


Water beneath the surface of the ground is classified as being “under the direct influence of surface water” (UDI) if it exhibits either:

  1. a significant occurrence of insects or other macroorganisms, algae or large diameter pathogens such as Giardia lamblia or Cryptosporidium, or
  2. significant and relatively rapid shifts in water characteristics such as turbidity, temperature, conductivity or pH that closely correlate to climatological or surface water conditions.

If the Department of Environmental Protection (DEP) determines that the water supplied by a water system is UDI, it means that the water could become contaminated with Giardia lamblia (picture 1, below) or Cryptosporidium (picture 2, below).

Closeup of a Giardia lamblia protozoan parasite.
Cryptosporidium parvum oocysts in wet mount slide


The purpose of the UDI program is to protect public health. Giardia lamblia and Cryptosporidium are parasites commonly found in surface water. They enter the environment through fecal contamination such as sewage or animal waste. When these parasites make their way into drinking water supplies, they can cause severe outbreaks of gastrointestinal illness including diarrhea, nausea and/or stomach cramps. Although they do not pose a grave danger for most people, they can be fatal to immunosuppressed individuals. Giardia lamblia and Cryptosporidium are extremely hard to kill with conventional amounts of chlorine; therefore, water that is classed as UDI must be treated more thoroughly than ground water.


Poor well design and construction, and vulnerable geology such as unconfined aquifers, can increase the risk that a system is UDI. EPA has developed a procedure for determining if ground waters are under the direct influence of surface water using microscopic particulate analysis. This analysis identifies organisms that occur in surface water whose presence in ground water would clearly indicate mixing of the two. If the results show that there could be a significant risk, DEP will notify the system in writing. The system must then provide additional treatment for the well, or find a new source of water that is not UDI, or identify the source of contamination and eliminate it.

Last Modified:
December 7, 2022 - 11:42am

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