ET-1B (2/17/05 version)
Surface and Ground Water Interactions

Potential Impacts of Streams on Ground Water Quality

Vertical and horizontal migration of surface water into an aquifer must be taken into account in certain areas of Ohio. For example, in Clark County there are documented examples of streams rapidly recharging the deeper aquifers through fractures. This presents an opportunity for chemical and pathogen contaminants to enter the aquifer from the stream, especially during flood events.

Studies on riverbank filtration in sand and gravel aquifers have identified flood stage as the most likely time for pathogens to enter wells. This is due to:

  • bottom scour of the stream that removes less permeable silty stream deposits during flood events; and
  • the elevated hydrologic heads associated with flood stage that can drive surface water into the aquifer.

Although the duration of elevated flood stage is limited, the high permeability of fractured bedrock, and some sand and gravel deposits, exposes these aquifers to potential impacts from surface water. Well owners can use turbidity and discoloration as a rough indicator of impacts from surface water. If well water is discolored (or turbid) during or immediately after flood events, it can indicate that bacteria, or other potentially harmful organisms or chemicals, have entered the ground water from surface water. Well water that is discolored or turbid should be tested for total coliform and nitrate contamination.

An extreme example of surface and ground water interaction is in the karst areas of Ohio where streams have a very high connection with ground water through fractures, sink holes, and swallet holes. In some cases, small streams can completely "disappear" into swallet holes in the limestone bedrock.

Potential Impacts of Ground Water on Surface Water Quality

Ground water is almost always a contributor to surface water flow and, in some cases, can comprise a major percentage of a stream base flow. Self-sustaining stream channels, with good connection to ground water, are able to maintain base flow conditions during drought stress longer. For example, at low surface water flow rates, ground water comprises up to 75% of the Mad River volumetric base flow. The streambed and floodplain store ground water during high surface flows and release it under low surface flows. This circulation of water between ground and surface water also creates cooler flows in streams, tempering extremes in temperature during hotter months. Cooler water, with higher dissolved oxygen holding capacity, benefits more sensitive aquatic organisms. Preservation of ground water contribution to stream systems can be an important component of the overall health of the watershed, particularly in Class III primary headwater habitat.

Modified stream channels disconnected from floodplains may lose the benefit of interaction with ground water and associated nutrient assimilation. Flows captured through the use of systematic sub-surface field tile, export potential ground water recharge to surface stream flow. The combination of field tile drainage, storm water impoundments, and modified stream channels interrupt the natural cycling of water between surface and ground water. This can reduce nutrient assimilation locally, thus creating nutrient enrichment downstream. These practices may also increase the thermal stress on sensitive aquatic species.

In contrast to the positive benefits of ground water contribution to a stream, if ground water recharge is contaminated, this can adversely impact surface water quality. This is especially true in southeast Ohio, where discharges from abandoned underground coal mines can be a major cause of nonpoint source impairment to streams.