In the LWS March 3 blog we discussed the methodologies for the collection of representative water quality samples. The follow-on question is how do we analyze these data? There are a number of methods for evaluating water quality depending on the parameters and what you are trying to understand. Do you want to know if the sample complies with Safe Drinking Water Act because the water is going to be used for potable purposes? Do you want to know the type of water so you know its applicability for certain uses? Do you want to know if the water has been contaminated with certain constituents? Each of these questions is addressed in a different manner.

If you are evaluating the potability of a water supply, the water quality sample should be analyzed at a minimum for the parameters that have an EPA-set maximum contaminant level (“MCL”). A MCL is the maximum concentration that a constituent can have and still be considered under the Safe Drinking Water Act to be potable and, therefore, acceptable for a public water supply. There are both primary MCLs and secondary MCLs. Primary MCLs are set for constituents where exceedance of the MCL creates a potential health and safety issue if used for human consumption. Secondary MCLs are not compliance parameters, they are more guidance values related to issues such as taste, color, and odor in the water. While technically not required to be monitored for a public potable water supply, we always encourage the testing of these parameters, as they can result in objectionable water to customers. For example, high total dissolved solids in water can make it taste bitter or metallic.

There are MCLs for a wide variety of both inorganic and organic parameters and, typically, to obtain certification of a public potable water supply, all parameters regulated under the Safe Drinking Water Act need to be sampled and analyzed before connecting a new source to the distribution system. The statewide groundwater standards in Colorado can be found in Regulation 41.

The type of water is evaluated based on the concentrations of the major cations and anions in the water. The major cations are calcium, magnesium, potassium, and sodium, while the major anions are bicarbonate/carbonate alkalinity, chloride, and sulfate. Two common means to evaluate the type of water that has been analyzed are the Piper Diagram and the Stiff Diagram (Figures 1 and 2). Concentrations obtained from the analytical laboratory need to be converted to milliequivalents per liter prior to using either of these methods. This conversion is conducted by changing the concentration of the major cations and anions to an equivalent weight, based on the parameter’s atomic weight and its valence. For example, calcium has an atomic weight of 40.078 and a valence of two, so the concentration (mg/L) is divided by 20.04 to obtain its equivalent weight. To obtain the milliequivalent (meq) per unit volume for use in constructing a Stiff or Piper Diagram, the parameter concentration is then divided by this equivalent weight, with the end result being a meq/L.

Both of these methods provide visual depictions of the predominant ions in the water. This is very useful information to understand the predominant species in the water. For example, water with a high alkalinity will make the water taste bitter, while water that is high in calcium and magnesium produces hard water, which can be seen to cause scale deposits on fixtures. When evaluating the type of water using either a Piper or Stiff Diagram, it is important to conduct an ion balance to assess if there is a good balance, i.e., as a cross-check on the accuracy of the laboratory results. Milliequivalents per unit volume are also the units used to construct the ion balance. Generally speaking, there is a good ion balance if the sum of the cations compared to the sum of the anions is between 0.96 and 1.04.

If there are specific constituents that are of concern related to the geologic terrane or anthropogenic impacts, the sampling and analytical process can be tailored to specific needs. For example, in Precambrian rocks, or in alluvial sediments that have shed from mountainous regions, there may be the need to test for radionuclides. Another example would be a location adjacent to urban development where a shallow well may be proposed and there is the potential for surface, or shallow subsurface, contamination related to contaminated runoff and/or leaking underground storage tanks. Therefore, analysis of volatile and semi-volatile organics may be warranted to assess groundwater contamination risks.

If you are in need of analysis of water quality data and/or development of site-specific groundwater quality parameter suites, LWS can help with your water quality needs. Feel free to give us a call (303-350-4090) or email or contact the LWS team members listed below..

Bruce Lytle, P.E. bruce@lytlewater.com

Chris Fehn, P.E., P.G. chris@lytlewater.com

Ben Bader ben@lytlewater.com

Anna Elgqvist, EI anna@lytlewater.com