July, 2001

Geochemical Analyis For Water Quality

Techniques are used by LBG to determine whether ground-water contamination resulted from naturally-occurring chemical constituents or human caused pollution.

Piper diagrams aid in determining the chemical reactions taking place in the ground water as the water moves downgradient. A Piper diagram is a three-part diagram consisting of two three-component diagrams for anions and cations, and a diamond-shaped field between them used to combine and replot the anion and cation ratios. Geochemical data are plotted as the percent of the major constituents. Chemical reactions and mineral dissolution or precipitation can be interpreted by plotting downgradient data in sequence and noting changes in relative concentrations of constituents. The inset figure is a Piper diagram showing the dissolution of gypsum (CaSO₄).

Saturation indices, the log ratio of the ion activity product and the solubility product, represent the saturation of the ground water with respect to various minerals. They show the likelihood of a mineral dissolving or precipitating from solution. Interpretation of saturation indices from geochemical data taken in a downgradient sequence can demonstrate the reactions taking place in the ground water. As a saturation index goes up, the mineral in question is being dissolved, and as a saturation index goes down, the mineral is precipitating.

Isotope analyses can absolve a suspected polluter by providing a chemical "fingerprint" of the native rock in any particular area. The isotopes present in a rock depend on the isotopes available at the time that rock was formed, so the ratios of the isotopes of an element present are unique to an area.

Ground-water samples and samples of the native rock are obtained and tested for the isotopes of the subject element. If the ratios of the subject isotopes in the ground water and the native rock do not match, human-caused pollution is suspected.

For example, western and southwestern Ohio are known to have relatively high concentrations of sulfate in the ground water. A study conducted in southwestern Ohio found high concentrations of sulfate were linked to a construction debris landfill. Piper diagrams and saturation indices produced during the study showed that the concentrations of sulfate were not consistent with the known geologic materials in the area. Isotope analyses were conducted on samples of the ground water and the native bedrock downgradient from the landfill, and the chemical "fingerprints" for sulfate did not match. The source of the sulfate was found to be gypsum wallboard buried in the landfill.

A study conducted in west-central Ohio showed that the source of high concentrations of sulfate is the limestone bedrock in the area. A transect trending downgradient was defined and ground-water samples were collected. Piper diagrams constructed from the geochemical testing results showed chemical reaction trends in the ground water that are consistent with the known geologic materials in the area. Saturation indices for gypsum and dolomite generally increased downgradient, which is indicative of the known carbonate aquifer.

Without knowing if contamination is naturally occurring, valuable resources could be spent by a potentially responsible party on a problem with no human cause.

For more than 57 years, Leggette, Brashears & Graham, Inc. has provided state-of-the-art consulting services to our clients. The bulk of our work is repeat business from customers who have benefited from the value-added services we provide. If you would like more information on how we can add value to your ground-water or environmental project, please contact your local LBG office for assistance.