All natural water contains dissolved minerals, and there is a relationship between the amount of dissolved minerals and the ability of the water to conduct an electrical current, or electrical conductivity. Generally, the greater the concentration of dissolved minerals, the higher the conductivity.  However specific minerals also influence the conductivity value differently.

 

At high enough concentrations, dissolved minerals can have effects on benthic or aquatic life. However, the ways in which those effects may occur and the concentrations that are significant are very much dependent on what the dissolved minerals are.  Even where correlations can be made between levels of total dissolved solids (TDS) and effects on insect or other aquatic life, the degree and consequence of effect depend on both the specific dissolved constituents involved and the specific benthic or aquatic life form or genus that is impacted.

 

The type and concentration of dissolved minerals varies greatly from stream to stream and area to area, depending on the type of rock and soils in a given watershed, the land use activities, and the influences of other water bodies on a stream. These influences can result from natural dissolved constituents in a lake, bay, or lagoon that may influence an interconnected stream.

 

On April 1, 2010, the US EPA set forth “enforceable guidance,” which in effect is a regulation, but one which was not promulgated under formal rulemaking procedures. This stipulated that water discharging from central Appalachian mining disturbances must not be allowed to exceed 500 microSiemens per centimeter (µS/cm, a unit of measurement of conductivity), and that water in excess of as low as 300 µS/cm would be cause for close, critical scrutiny of the mining operation’s permit conditions.

 

These values are extremely low and difficult to achieve, thus effectively eliminating mining in that region.  EPA established these low "threshold of concern" values based solely on the inconclusive correlations between conductivity and the distributions of mayfly populations.  The EPA has ignored the overall stream function, stream use, and condition of other aquatic life in the streams.

 

A brief review of conductivity values in eastern streams outside of the coalfields area helps to put the conductivity threshold into perspective.  Programmatic stream monitoring of conductivity performed by the US Geological Survey shows that conductivity higher than 500 µS/cm is not uncommon in streams outside the coal-bearing region. In fact, it’s not unusual, and not necessarily an adverse condition, for stream water conductivity to exceed the value chosen by EPA as an enforceable limit.

Reconnaissance measurements from more localized, specific areas show that the promulgated threshold values are commonly exceeded by streams in areas far removed from mining.  For example, a reconnaissance of the College Park, Maryland, area indicates stream conductivity that typically exceeds 300 µS/cm.  At a location in Campus Creek just downstream, the value can far exceed 500 µS/cm value, yet this is disregarded at Campus Creek.

Boone, North Carolina, is a city surrounded by sparsely populated forest lands. While streams outside of or upstream of the city show less than 300 µS/cm conductivity, preliminary reconnaissance sampling in the town itself showed values approaching 500 µS/cm, and exceeding that value for at least 1,000 feet downstream of the Boone wastewater treatment plant.

In summary, electrical conductivity is a non-specific parameter that does not specifically characterize water quality or define its impact on insect or aquatic life. It is therefore an inappropriate parameter for use as a regulatory metric. Further, the conductivity value established as a threshold by the EPA is commonly found to be exceeded in other regions, where it is not regulated.