7.2 Groundwater Under the Direct Influence of Surface Water (GWUDISW)

It is widely recognized that wells producing water with a substantial component of surface water from streams, rivers or lakes present risk of harm to users who drink that water. Surface water often contains harmful biological constituents such as pathogenic bacteria including parasites and viruses. Thus, it is generally accepted that when surface water is used for drinking water it should be treated to render the water safe. In contrast, groundwater pumped from wells is generally considered to be safe for drinking unless there are extenuating circumstances associated with the wells. One such circumstance is the presence of water that comes from surface water if the potentially harmful constituents are not sufficiently attenuated as the water travels from the surface water body to the well.

The United States Environmental Protection Agency (USEPA) established a well category designated as GWUDISW in 1992 and many states in the USA, provinces in Canada and elsewhere have issued guidance documents or regulations for this category of well. These documents provide advice on how to recognize such wells and, when recognized, provide regulations concerning water treatment to make the well water safe for drinking. However, these guidelines for identification of GWUDISW wells do not include mention of any role for DOC analyses. Given the following:

  • groundwater pumped from wells used for drinking water with no indications of contamination (e.g., pristine) consistently has DOC in the range of 0.3-0.7 mg/L;
  • DOC in such wells is rarely above 5 mg/L;
  • surface waters typically show thousands of mg/L DOC; and,
  • the cost of DOC analysis is low;

it would be appropriate to include DOC as one of the criteria for identification of GUIDISW wells. The presence of anomalously high DOC is not proof of a risk from surface water but, is a sufficiently strong indicator to justify more investigation. DOC above 1 mg/L is reason for concern. DOC above 5 mg/L is reason to investigate further to determine whether other evidence indicates a GWUDISW which would require treatment if used for drinking water.

Figure 20 shows a pumping well that draws water both from recharge through the vadose zone and from a river. Whether or not the well water will have a high enough concentration of DOC to indicate a substantial contribution of river water depends on the level of DOC in the river water, the degree of degradation of the DOC in the water as it flows to the well and the relative portions of river/non-river water entering the well.

Figure showing a cross-sectional view of a pumping well near a river

Figure 20  Cross-sectional view of a pumping well near a river illustrating that water in the well is a mix of water from recharge that infiltrates through the vadose zone and water that seeps from the river into the aquifer (from Konikow and Bredehoeft, 2020).


Dissolved Organic Carbon in Groundwater Systems Copyright © 2022 by Francis H. Chapelle. All Rights Reserved.