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In its recent report, “Factors Affecting Public-Supply-Well Vulnerability to Contamination: Understanding Observed Water Quality and Anticipating Future Water Quality,” the USGS provided the results of its nationwide, 10-year study on drinking water contamination and ways to identify, track and reduce pollution sources.
As we discussed yesterday, understanding the vulnerabilities of underground drinking water supplies involves several complex and interrelated aspects addressing recharge, geochemical conditions and age-mixture of the water. Another very essential aspect is that of preferential flow pathways: conduits that allow water to flow with little resistance and relatively fast speeds in the subsurface areas of groundwater. Preferential flow pathways are considered to be among the most important sources of contaminants in drinking water supplies for many reasons, including:
- They are often responsible for transporting the youngest and most contaminated water to a well, especially during storm events that increase recharge rates.
- They can originate far from the well and make it difficult to assess where the water came from and how best to reduce the threats from associated above-ground activities.
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- The unrestricted nature of preferential flow pathways and speed of transport can assist pathogenic microorganisms in reaching wells by reducing the potential for microorganism die off or natural removal by filtration or sorption to sediments or rock that occur over longer periods of time.
- In the same way, the transport speed of preferential flow pathways can also hamper the degradation rates of chemicals contaminants, such as dissolved nitrate, which normally degrades over time through denitrification.
- They can increase the likelihood that water and aquifer materials that are not in equilibrium will meet and cause chemical reactions resulting in increased levels of naturally occurring contaminants.
- They have the capacity to transport large quantities of water to wells. In one study performed in Nebraska, an estimated 35 percent of the water studied arrived at the well via preferential flow pathways.
- They are “pervasive” and, according to the USGS report, at least one preferential flow pathway affected each of the 10 different aquifers involved in the studies.
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Although preferential flow pathways are underground and thus difficult to determine and access, the USGS report describes four types that were encountered during the studies, and include both natural and manmade causes.
Fast flow pathways are the result of natural variations in the ability of aquifer materials to transmit water. One example would be the difference between clay and sand layers, where the most transmissive layer, sand, becomes the preferential flow pathway for water and associated contaminants when well pumping begins.
Natural features such as bedrock cavernous zones, fractures, bedding planes, and conduits are considered hydraulic short circuits because they conduct water and contaminants directly, and bypass normal aquifer material. These preferential flow pathways have large openings and the capability to quickly transport large quantities of water with little resistance. Unfortunately, their location and even their presence are often unknown making it difficult to quantify their impact on either flow or contamination.
Manmade features, such as wells with screens that access multiple aquifers can also act as hydraulic short circuits. In these cases, well screens can allow water to move quickly through otherwise confining layers between aquifers that would normally help to restrict the flow. The report also notes that movement of water within wellbores is not limited to multiple aquifer wells and may serve to move water and contaminants upward or downward, depending upon the hydraulic gradient, if the well screen is long enough.
In addition to measures and preferential flow pathways, the USGS report offers a wealth of site-specific information from the 10 different aquifers studied as well as new tools to assist with analysis and decision making. The entire report is available at http://water.usgs.gov/nawqa/.