Carly Madge HBESc thesis abstract
The purpose of this study was to use leaching test experiments to help explain the lithological controls on the groundwater geochemistry of different geologic formations in the Thunder Bay area. A secondary goal of this project was to test the effectiveness of the 15 minute USGS Field Leach Test for this purpose relative to more conventional 30-day deionized water and acetic acid tests.
Samples were taken from different sedimentary units of the Gunflint and Rove as well as from the Sudbury Impact layer, diabase, and granite formations of the Thunder Bay area. Some of the samples used in this study were collected from previously logged drill core from the MNDM library. Each sample was crushed into a fine powder. The fine powders were bottled and labeled, with half the bottles being shipped to the Geoscience Labs in Sudbury for whole rock geochemical analysis and the other half remaining in Thunder Bay to be used for leach test analysis.
Three types of leach tests were performed in this study. These included the USGS Field Leach Test, a 30-day double deionized water leach test, and a 30-day leach test using a 5% acetic acid solution. All three of the leach test types used a 20:1 leachate to sample ratio, using 2.5 grams of sample and 50 mL of leaching agent. The USGS Field Leach Test is a relatively new method developed for testing the leaching potential of different geologic and environmental materials. This testing method requires the use of the 20:1 sample to leachate ratio, and that the sample be added to deionized water, shaken for a period of five minutes, and then allowed to settle for ten minutes before being filtered and submitted for geochemical analysis. The effectiveness of this method as a way of determining the leaching potential of rocks in groundwater basins was in question because it did not seem logical that such a short-term test could provide any accurate insight into the water rock reactions that occur over long-periods of time in groundwater basins.
Whole rock and leach test geochemical data were used to produce two types of graphs. The first graph type showed a comparison of whole rock and leach attest concentrations for each element analyzed. The second graph showed the percentage of whole rock elemental content that was leached into solution for each of the three leach tests. Through analysis of each graph it was determined that calcium, magnesium, sodium, and potassium are the elements most readily leached into solution from the rock samples analyzed.
The second method used to analyze data in this study was Piper plots. Piper plots are commonly used for analyzing groundwater geochemistry. A Piper plot classifies water samples into four different types based on its concentration of major cations (Ca2+, Mg2+,Na2+, and K+) and major anions (HCO3-, CO3-, CI-, SO42-). Through analysis of each Piper plot, it was determined that samples collected from the Rove Formation tend to produce sodium bicarbonate type groundwater, whereas samples collected from the Gunflint Formation produce a variety of groundwater types.
Lastly, this study was conducted to evaluate the effectiveness of the USGS Field Leach Test as a method for determining the leaching potential of elements in groundwater analysis. Due to the difficulty of filtering the leach test solutions performed in the method specified by the USGS Field Leach Test protocols and the poor geochemical data provided by these leach test solutions, it was determined that this is not the best method that can be used for groundwater analysis.