ROCK-WATER INTERACTION: A SEMESTER LONG DISCOVERY BASED LABORATORY EXERCISE DESIGNED TO REINFORCE CONCEPTS OF SOLUBILITY AND EQUILIBRIUM IN AQUEOUS GEOCHEMISTRY

In north-central Pennsylvania, variable water quality is a function of the rock types to which the surface and groundwater are exposed. Within a relatively restricted area, we have watercourses ranging from ones that are highly impacted by legacy coal mines to ones that are fairly pristine. To illustrate the effects of rock types on water chemistry, we designed an experiment for our Aqueous Geochemistry class using three rock types indigenous to Northeastern Pennsylvania from areas with very different water chemistries: black shale from waste coal piles in the middle anthracite belt near Centralia, limestone from Lime Ridge (a rock commonly quarried in the area for aggregate and used in driveways, roads and ditches), and Pottsville Conglomerate, a quartz conglomerate making up the ridge between the coal mining areas to the south and the Roaring Creek reservoir water supply to the north. Large volume samples of each rock type were collected, cleaned and broken up. For each rock type, three one gallon jugs were filled about 1/5 full with the rock pieces and deionized water added to the shoulder, after which the jugs were shaken. Three jugs of pure deionized water were set up as reagent blanks. At three points during the semester (7, 56 and 81 days), students measured pH and conductivity in each bottle, then filtered an aliquot for alkalinity analysis. At 56 and 81 days, additional samples were collected and frozen for subsequent anion and metal analysis. At the end of the experiment, students graphed the data and then analyzed the trends within treatments. They were then asked to contrast the effects of limestone, coaly black shale and quartz conglomerate on the water quality in the jugs and then speculate on the effects of local rock- water interactions on local water supplies. We feel that this exercise brings a local flavor to familiar geological concepts and thereby demonstrates in a meaningful way to local students the immediate application of what they are learning. In addition, the replication in this experiment provides a platform for discussion of statistical comparison, reliability of measurements and effects of even small differences in sample treatment on water chemistry. This experiment can be tailored to suit several levels of instruction, and therefore may be useful to a wide population of environmental educators.